R. Delfos
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1
In many high-tech applications, efficient thermal dissipation is necessary to maintain components within strict operational temperature limits, ensuring system reliability and performance stability. This is achieved mainly through thermal conduction, which is the dominant mechanism between contacting surfaces. Thermal contact conduction is a function of many variables, such as contact area, material properties, and surface roughness characteristics. Various studies have focused on how these variables affect the heat flow between two contacting geometries; however, the inherent variability of the surface roughness induces a significant amount of uncertainty in the measurements. This paper describes the theory and methodology followed to develop a thermo-mechanical framework that incorporates the inherent stochasticity of the surface roughness of metallic surfaces, through a probabilistic surface representation, to efficiently quantify the heat flow conductance through the calculation of the Thermal Contact Conductance (TCC). The results indicate that the proposed methodology predicts the range of thermal contact conductance with good accuracy when compared to experimental data. Lastly, the 2D implementation showed substantially higher computational efficiency compared to the 3D model while retaining comparable predictive capability.
...
In many high-tech applications, efficient thermal dissipation is necessary to maintain components within strict operational temperature limits, ensuring system reliability and performance stability. This is achieved mainly through thermal conduction, which is the dominant mechanism between contacting surfaces. Thermal contact conduction is a function of many variables, such as contact area, material properties, and surface roughness characteristics. Various studies have focused on how these variables affect the heat flow between two contacting geometries; however, the inherent variability of the surface roughness induces a significant amount of uncertainty in the measurements. This paper describes the theory and methodology followed to develop a thermo-mechanical framework that incorporates the inherent stochasticity of the surface roughness of metallic surfaces, through a probabilistic surface representation, to efficiently quantify the heat flow conductance through the calculation of the Thermal Contact Conductance (TCC). The results indicate that the proposed methodology predicts the range of thermal contact conductance with good accuracy when compared to experimental data. Lastly, the 2D implementation showed substantially higher computational efficiency compared to the 3D model while retaining comparable predictive capability.
Current wafer handling methods in the high-tech industry rely on mechanical contact to transport and position wafers. This leads to high breakage rates and contamination, ultimately reducing product yield. To overcome these challenges, TU Delft has developed several air-based contactless handling systems. However, stringent thermal requirements in the high-tech industry demand a highly uniform and stable temperature distribution across the wafer, posing significant challenges for such systems. Hence, research is performed on the thermomechanical effects of active air bearings on wafers during contactless handling.
Initially, a comprehensive theoretical analysis is performed on fluid flow and heat transfer phenomena in simplified geometries. Both rectilinear and axisymmetric models are developed to capture essential aspects of laminar thin film flows. Analytical expressions reveal the balance between expansion and viscous dissipation effects, particularly for rectilinear Poiseuille flow. The thesis further advances to complex numerical modelling using COMSOL Multiphysics, refining the geometry to better mirror actual operating conditions and incorporating the influence of mechanical wafer deformation.
In parallel, an experimental test setup is designed and implemented to replicate the operational environment of an air bearing system in a simplified form. This setup enables controlled measurement of temperature profiles and wafer deformation using complementary sensor techniques. One technique provides absolute temperature calibration, while another maps the relative temperature distribution across the wafer surface. Additionally, an optical method inspired by free-surface synthetic schlieren is used to quantify wafer deformation. This experimental setup not only serves to validate the theoretical models and numerical COMSOL simulations but also aims to explore measurement techniques applicable to operational air bearing systems. ...
Initially, a comprehensive theoretical analysis is performed on fluid flow and heat transfer phenomena in simplified geometries. Both rectilinear and axisymmetric models are developed to capture essential aspects of laminar thin film flows. Analytical expressions reveal the balance between expansion and viscous dissipation effects, particularly for rectilinear Poiseuille flow. The thesis further advances to complex numerical modelling using COMSOL Multiphysics, refining the geometry to better mirror actual operating conditions and incorporating the influence of mechanical wafer deformation.
In parallel, an experimental test setup is designed and implemented to replicate the operational environment of an air bearing system in a simplified form. This setup enables controlled measurement of temperature profiles and wafer deformation using complementary sensor techniques. One technique provides absolute temperature calibration, while another maps the relative temperature distribution across the wafer surface. Additionally, an optical method inspired by free-surface synthetic schlieren is used to quantify wafer deformation. This experimental setup not only serves to validate the theoretical models and numerical COMSOL simulations but also aims to explore measurement techniques applicable to operational air bearing systems. ...
Current wafer handling methods in the high-tech industry rely on mechanical contact to transport and position wafers. This leads to high breakage rates and contamination, ultimately reducing product yield. To overcome these challenges, TU Delft has developed several air-based contactless handling systems. However, stringent thermal requirements in the high-tech industry demand a highly uniform and stable temperature distribution across the wafer, posing significant challenges for such systems. Hence, research is performed on the thermomechanical effects of active air bearings on wafers during contactless handling.
Initially, a comprehensive theoretical analysis is performed on fluid flow and heat transfer phenomena in simplified geometries. Both rectilinear and axisymmetric models are developed to capture essential aspects of laminar thin film flows. Analytical expressions reveal the balance between expansion and viscous dissipation effects, particularly for rectilinear Poiseuille flow. The thesis further advances to complex numerical modelling using COMSOL Multiphysics, refining the geometry to better mirror actual operating conditions and incorporating the influence of mechanical wafer deformation.
In parallel, an experimental test setup is designed and implemented to replicate the operational environment of an air bearing system in a simplified form. This setup enables controlled measurement of temperature profiles and wafer deformation using complementary sensor techniques. One technique provides absolute temperature calibration, while another maps the relative temperature distribution across the wafer surface. Additionally, an optical method inspired by free-surface synthetic schlieren is used to quantify wafer deformation. This experimental setup not only serves to validate the theoretical models and numerical COMSOL simulations but also aims to explore measurement techniques applicable to operational air bearing systems.
Initially, a comprehensive theoretical analysis is performed on fluid flow and heat transfer phenomena in simplified geometries. Both rectilinear and axisymmetric models are developed to capture essential aspects of laminar thin film flows. Analytical expressions reveal the balance between expansion and viscous dissipation effects, particularly for rectilinear Poiseuille flow. The thesis further advances to complex numerical modelling using COMSOL Multiphysics, refining the geometry to better mirror actual operating conditions and incorporating the influence of mechanical wafer deformation.
In parallel, an experimental test setup is designed and implemented to replicate the operational environment of an air bearing system in a simplified form. This setup enables controlled measurement of temperature profiles and wafer deformation using complementary sensor techniques. One technique provides absolute temperature calibration, while another maps the relative temperature distribution across the wafer surface. Additionally, an optical method inspired by free-surface synthetic schlieren is used to quantify wafer deformation. This experimental setup not only serves to validate the theoretical models and numerical COMSOL simulations but also aims to explore measurement techniques applicable to operational air bearing systems.
Prediction of heat transfer phenomena and resulting temperature distributions is crucial among many appliances in the industry sector. In a vacuum, the dominant mode of heat transfer is conduction. The resulting temperature drop at the body’s interface is dependent on a parameter known as Thermal Contact Conductance (TCC) or, alternatively, Thermal Contact Resistance (TCR). During the literature study, it was found that several parameters are mainly affecting the heat transfer performance between the samples: surface roughness, contact pressure, material properties, and environmental conditions. The Thesis project is accomplished in cooperation with Philips Engineering Solutions (PES). For the Thesis, it is proposed to explore the TCC phenomenon through a combined study. The first part is investigating newly manufactured samples with a modified experimental setup in vacuum conditions. Before the Main Test Campaign started, the experimental setup and old laboratory samples were tested to mark the way. Also, several more samples were tested alongside the Main Tests to extend the TCC database. Another important study goal is to perform thorough surface analysis using a Digital Microscope. The analysis results in a surface roughness evolution study and reconstruction of the surfaces using MATLAB © software. The surfaces are then matched together to simulate and predict the parameter known as the real contact area. The study ends with a section summarizing the investigation and proposing recommendations and possible further research.
...
Prediction of heat transfer phenomena and resulting temperature distributions is crucial among many appliances in the industry sector. In a vacuum, the dominant mode of heat transfer is conduction. The resulting temperature drop at the body’s interface is dependent on a parameter known as Thermal Contact Conductance (TCC) or, alternatively, Thermal Contact Resistance (TCR). During the literature study, it was found that several parameters are mainly affecting the heat transfer performance between the samples: surface roughness, contact pressure, material properties, and environmental conditions. The Thesis project is accomplished in cooperation with Philips Engineering Solutions (PES). For the Thesis, it is proposed to explore the TCC phenomenon through a combined study. The first part is investigating newly manufactured samples with a modified experimental setup in vacuum conditions. Before the Main Test Campaign started, the experimental setup and old laboratory samples were tested to mark the way. Also, several more samples were tested alongside the Main Tests to extend the TCC database. Another important study goal is to perform thorough surface analysis using a Digital Microscope. The analysis results in a surface roughness evolution study and reconstruction of the surfaces using MATLAB © software. The surfaces are then matched together to simulate and predict the parameter known as the real contact area. The study ends with a section summarizing the investigation and proposing recommendations and possible further research.
Quality assurance of 3D prints
The development of a low cost test & Investigating the influence of temperature and humidity on print quality
Low- and middle-income countries are facing a growing burden of non-communicable diseases that require surgical interventions. However, the lack of functioning medical equipment is hindering access to surgical care in these countries. 3D printing has the potential to provide a solution to this problem, but ensuring the quality of 3D-printed medical products remains a challenge.
The objectives of this master thesis are twofold.
1. To design a low-cost and easy-to-use test that can be used in low- and middle-income countries to ensure the quality of 3D-printed parts.
Method: First, the requirements for the test method were established. Existing test methods were reviewed, after which a test set-up was designed and built. The test set-up was subsequently validated by comparing it with a conventional Zwick/Roell test. PLA specimens were produced using an Ultimaker 2Go 3D printer according to the ASTM D790 standard. Three different batches of specimens were created, divided into two groups, and tested for their bending properties using both the homemade test set-up and the conventional Zwick/Roell test.
Results: The majority (8/9) of the results obtained from the homemade test set-up and the Zwick/Roell test exhibited no significant difference. However, the t-score for the Fmax measured for specimen batch 1 showed a significant difference between the two methods. All results measured with the homemade test set-up were higher than those measured with the Zwick/Roell test, indicating that the homemade system may have been incorrectly calibrated initially. Calibration of the load cell may therefore reduce this discrepancy.
Conclusion: Depending on the desired level of measurement accuracy, the homemade test set-up appears to be a viable alternative to the Zwick/Roell test.
2. To investigate the effect of temperature and relative humidity on the quality of fused deposition modelling (FDM) printed parts.
Method: The influence of temperatures of 20°C, 35°C, and 40°C combined with relative humidity levels of 50%, 70%, and 90% was investigated under nine environmental conditions. Two situations were studied: (1) storage of the print material 24 hours prior to and during printing, and (2) storage of the final print 24 hours prior to and during testing.
In Situation 1, PLA filament was stored under one of the environmental conditions before printing. Specimens were then printed under the same conditions using an Ultimaker 2Go printer inside an Espec humidity oven. For each condition, two batches of seven specimens were produced. After printing, specimens were stored at room temperature and approximately 40% relative humidity for a maximum of five days before being tested for bending properties using a Zwick/Roell test.
In Situation 2, specimens were printed similarly, after which the final printed parts were stored under one of the nine environmental conditions for 24 hours prior to testing. The specimens were then tested for their bending properties using a Zwick/Roell test.
Results: Fmax, maximum bending strength, and elastic modulus all decreased when specimens were printed or tested at increased temperature and relative humidity. Increased relative humidity appeared to have a particularly negative effect at higher temperatures. Furthermore, higher temperature and humidity resulted in under-extrusion, lower specimen weight, and poorer surface quality.
Conclusion: Increasing relative humidity and temperature during both the printing process and storage prior to testing negatively affects the bending properties of FDM-printed PLA specimens. The findings demonstrate the importance of environmental control for maintaining the quality of 3D-printed products and suggest that a low-cost quality assurance test may provide a practical solution for resource-limited settings. ...
The objectives of this master thesis are twofold.
1. To design a low-cost and easy-to-use test that can be used in low- and middle-income countries to ensure the quality of 3D-printed parts.
Method: First, the requirements for the test method were established. Existing test methods were reviewed, after which a test set-up was designed and built. The test set-up was subsequently validated by comparing it with a conventional Zwick/Roell test. PLA specimens were produced using an Ultimaker 2Go 3D printer according to the ASTM D790 standard. Three different batches of specimens were created, divided into two groups, and tested for their bending properties using both the homemade test set-up and the conventional Zwick/Roell test.
Results: The majority (8/9) of the results obtained from the homemade test set-up and the Zwick/Roell test exhibited no significant difference. However, the t-score for the Fmax measured for specimen batch 1 showed a significant difference between the two methods. All results measured with the homemade test set-up were higher than those measured with the Zwick/Roell test, indicating that the homemade system may have been incorrectly calibrated initially. Calibration of the load cell may therefore reduce this discrepancy.
Conclusion: Depending on the desired level of measurement accuracy, the homemade test set-up appears to be a viable alternative to the Zwick/Roell test.
2. To investigate the effect of temperature and relative humidity on the quality of fused deposition modelling (FDM) printed parts.
Method: The influence of temperatures of 20°C, 35°C, and 40°C combined with relative humidity levels of 50%, 70%, and 90% was investigated under nine environmental conditions. Two situations were studied: (1) storage of the print material 24 hours prior to and during printing, and (2) storage of the final print 24 hours prior to and during testing.
In Situation 1, PLA filament was stored under one of the environmental conditions before printing. Specimens were then printed under the same conditions using an Ultimaker 2Go printer inside an Espec humidity oven. For each condition, two batches of seven specimens were produced. After printing, specimens were stored at room temperature and approximately 40% relative humidity for a maximum of five days before being tested for bending properties using a Zwick/Roell test.
In Situation 2, specimens were printed similarly, after which the final printed parts were stored under one of the nine environmental conditions for 24 hours prior to testing. The specimens were then tested for their bending properties using a Zwick/Roell test.
Results: Fmax, maximum bending strength, and elastic modulus all decreased when specimens were printed or tested at increased temperature and relative humidity. Increased relative humidity appeared to have a particularly negative effect at higher temperatures. Furthermore, higher temperature and humidity resulted in under-extrusion, lower specimen weight, and poorer surface quality.
Conclusion: Increasing relative humidity and temperature during both the printing process and storage prior to testing negatively affects the bending properties of FDM-printed PLA specimens. The findings demonstrate the importance of environmental control for maintaining the quality of 3D-printed products and suggest that a low-cost quality assurance test may provide a practical solution for resource-limited settings. ...
Low- and middle-income countries are facing a growing burden of non-communicable diseases that require surgical interventions. However, the lack of functioning medical equipment is hindering access to surgical care in these countries. 3D printing has the potential to provide a solution to this problem, but ensuring the quality of 3D-printed medical products remains a challenge.
The objectives of this master thesis are twofold.
1. To design a low-cost and easy-to-use test that can be used in low- and middle-income countries to ensure the quality of 3D-printed parts.
Method: First, the requirements for the test method were established. Existing test methods were reviewed, after which a test set-up was designed and built. The test set-up was subsequently validated by comparing it with a conventional Zwick/Roell test. PLA specimens were produced using an Ultimaker 2Go 3D printer according to the ASTM D790 standard. Three different batches of specimens were created, divided into two groups, and tested for their bending properties using both the homemade test set-up and the conventional Zwick/Roell test.
Results: The majority (8/9) of the results obtained from the homemade test set-up and the Zwick/Roell test exhibited no significant difference. However, the t-score for the Fmax measured for specimen batch 1 showed a significant difference between the two methods. All results measured with the homemade test set-up were higher than those measured with the Zwick/Roell test, indicating that the homemade system may have been incorrectly calibrated initially. Calibration of the load cell may therefore reduce this discrepancy.
Conclusion: Depending on the desired level of measurement accuracy, the homemade test set-up appears to be a viable alternative to the Zwick/Roell test.
2. To investigate the effect of temperature and relative humidity on the quality of fused deposition modelling (FDM) printed parts.
Method: The influence of temperatures of 20°C, 35°C, and 40°C combined with relative humidity levels of 50%, 70%, and 90% was investigated under nine environmental conditions. Two situations were studied: (1) storage of the print material 24 hours prior to and during printing, and (2) storage of the final print 24 hours prior to and during testing.
In Situation 1, PLA filament was stored under one of the environmental conditions before printing. Specimens were then printed under the same conditions using an Ultimaker 2Go printer inside an Espec humidity oven. For each condition, two batches of seven specimens were produced. After printing, specimens were stored at room temperature and approximately 40% relative humidity for a maximum of five days before being tested for bending properties using a Zwick/Roell test.
In Situation 2, specimens were printed similarly, after which the final printed parts were stored under one of the nine environmental conditions for 24 hours prior to testing. The specimens were then tested for their bending properties using a Zwick/Roell test.
Results: Fmax, maximum bending strength, and elastic modulus all decreased when specimens were printed or tested at increased temperature and relative humidity. Increased relative humidity appeared to have a particularly negative effect at higher temperatures. Furthermore, higher temperature and humidity resulted in under-extrusion, lower specimen weight, and poorer surface quality.
Conclusion: Increasing relative humidity and temperature during both the printing process and storage prior to testing negatively affects the bending properties of FDM-printed PLA specimens. The findings demonstrate the importance of environmental control for maintaining the quality of 3D-printed products and suggest that a low-cost quality assurance test may provide a practical solution for resource-limited settings.
The objectives of this master thesis are twofold.
1. To design a low-cost and easy-to-use test that can be used in low- and middle-income countries to ensure the quality of 3D-printed parts.
Method: First, the requirements for the test method were established. Existing test methods were reviewed, after which a test set-up was designed and built. The test set-up was subsequently validated by comparing it with a conventional Zwick/Roell test. PLA specimens were produced using an Ultimaker 2Go 3D printer according to the ASTM D790 standard. Three different batches of specimens were created, divided into two groups, and tested for their bending properties using both the homemade test set-up and the conventional Zwick/Roell test.
Results: The majority (8/9) of the results obtained from the homemade test set-up and the Zwick/Roell test exhibited no significant difference. However, the t-score for the Fmax measured for specimen batch 1 showed a significant difference between the two methods. All results measured with the homemade test set-up were higher than those measured with the Zwick/Roell test, indicating that the homemade system may have been incorrectly calibrated initially. Calibration of the load cell may therefore reduce this discrepancy.
Conclusion: Depending on the desired level of measurement accuracy, the homemade test set-up appears to be a viable alternative to the Zwick/Roell test.
2. To investigate the effect of temperature and relative humidity on the quality of fused deposition modelling (FDM) printed parts.
Method: The influence of temperatures of 20°C, 35°C, and 40°C combined with relative humidity levels of 50%, 70%, and 90% was investigated under nine environmental conditions. Two situations were studied: (1) storage of the print material 24 hours prior to and during printing, and (2) storage of the final print 24 hours prior to and during testing.
In Situation 1, PLA filament was stored under one of the environmental conditions before printing. Specimens were then printed under the same conditions using an Ultimaker 2Go printer inside an Espec humidity oven. For each condition, two batches of seven specimens were produced. After printing, specimens were stored at room temperature and approximately 40% relative humidity for a maximum of five days before being tested for bending properties using a Zwick/Roell test.
In Situation 2, specimens were printed similarly, after which the final printed parts were stored under one of the nine environmental conditions for 24 hours prior to testing. The specimens were then tested for their bending properties using a Zwick/Roell test.
Results: Fmax, maximum bending strength, and elastic modulus all decreased when specimens were printed or tested at increased temperature and relative humidity. Increased relative humidity appeared to have a particularly negative effect at higher temperatures. Furthermore, higher temperature and humidity resulted in under-extrusion, lower specimen weight, and poorer surface quality.
Conclusion: Increasing relative humidity and temperature during both the printing process and storage prior to testing negatively affects the bending properties of FDM-printed PLA specimens. The findings demonstrate the importance of environmental control for maintaining the quality of 3D-printed products and suggest that a low-cost quality assurance test may provide a practical solution for resource-limited settings.
This thesis aims to investigate the effects of contact pressure and type of deformations on the thermal contact conductance(h’) for dry metal-metal contacts. Also, an attempt is made to develope a thermal contact conductance estimation model for the same. The materials used in this thesis are metals (aluminium and tin). Several experiments are performed on two different test setups. The first setup is situated in the process and energy department[P&E] of TU Delft where tin material is used to investigate deformation effects on h’ with respect to contact pressure ranging till 4MPa. The sample is in a shape of a wire with a diameter 2mm and placed between two copper cylinders for evaluating h’. The second setup is situated in Philips Engineering Solutions [PES] where flat aluminium samples are used with different roughness under high contact pressures (up to 25MPa). Later, the results from both experimental setups are compared to the prediction from literature models to brief their general applicability. The increase in contact pressure over a surface roughness, first deforms a surface elastically and then plastically. Plastic deformation increases the actual contact area and conducts more heat than elastic with sample contact pressure.
Oxide layer formation over time is critical for aluminium surfaces and so aging may cause differences in contact conduction. In conclusion, there is a large difference between the experimental results and prediction by literature models. Only when a lot of effort is put in keeping orientation exactly the same, reproducibility is good, rest the thermal contact conduction is irreproducible. The development of an entirely new semi-empirical model is very complex. There is a need to be aware of validity ranges of thermal contact conductance literature models as they quickly deteriorate outside their application ranges and the usage of available models should be done critically. The real contact area plays a major role in estimating h’. ...
Oxide layer formation over time is critical for aluminium surfaces and so aging may cause differences in contact conduction. In conclusion, there is a large difference between the experimental results and prediction by literature models. Only when a lot of effort is put in keeping orientation exactly the same, reproducibility is good, rest the thermal contact conduction is irreproducible. The development of an entirely new semi-empirical model is very complex. There is a need to be aware of validity ranges of thermal contact conductance literature models as they quickly deteriorate outside their application ranges and the usage of available models should be done critically. The real contact area plays a major role in estimating h’. ...
This thesis aims to investigate the effects of contact pressure and type of deformations on the thermal contact conductance(h’) for dry metal-metal contacts. Also, an attempt is made to develope a thermal contact conductance estimation model for the same. The materials used in this thesis are metals (aluminium and tin). Several experiments are performed on two different test setups. The first setup is situated in the process and energy department[P&E] of TU Delft where tin material is used to investigate deformation effects on h’ with respect to contact pressure ranging till 4MPa. The sample is in a shape of a wire with a diameter 2mm and placed between two copper cylinders for evaluating h’. The second setup is situated in Philips Engineering Solutions [PES] where flat aluminium samples are used with different roughness under high contact pressures (up to 25MPa). Later, the results from both experimental setups are compared to the prediction from literature models to brief their general applicability. The increase in contact pressure over a surface roughness, first deforms a surface elastically and then plastically. Plastic deformation increases the actual contact area and conducts more heat than elastic with sample contact pressure.
Oxide layer formation over time is critical for aluminium surfaces and so aging may cause differences in contact conduction. In conclusion, there is a large difference between the experimental results and prediction by literature models. Only when a lot of effort is put in keeping orientation exactly the same, reproducibility is good, rest the thermal contact conduction is irreproducible. The development of an entirely new semi-empirical model is very complex. There is a need to be aware of validity ranges of thermal contact conductance literature models as they quickly deteriorate outside their application ranges and the usage of available models should be done critically. The real contact area plays a major role in estimating h’.
Oxide layer formation over time is critical for aluminium surfaces and so aging may cause differences in contact conduction. In conclusion, there is a large difference between the experimental results and prediction by literature models. Only when a lot of effort is put in keeping orientation exactly the same, reproducibility is good, rest the thermal contact conduction is irreproducible. The development of an entirely new semi-empirical model is very complex. There is a need to be aware of validity ranges of thermal contact conductance literature models as they quickly deteriorate outside their application ranges and the usage of available models should be done critically. The real contact area plays a major role in estimating h’.
Energy efficiency is a key goal for the Quooker system. To further increase the energy efficiency of the Quooker system, this study investigates a possible heat integration system where the waste heat from the refrigeration cycle for chilled drinking water is implemented to preheat water before entering the boiling water reservoir.
Due to the intermittent nature of both the supply of heat, which is coupled to the control scheme of the chilled water reservoir and the demand for heat, which is determined by the user, a thermal storage system is required. Literature showed that the best refrigeration system for this application was a vapour compression system using a natural refrigerant such as isobutane. The most efficient heat storage could be done using organic phase change materials (PCM). Using a PCM allows the system to retain more energy in the same volume due to the latent heat in the system. To enhance the heat transfer between the fluid streams and the PCM, a fin-and-tube heat exchanger concept was designed. This concept, coupled with existing Quooker system demands, leads to a preliminary set of design requirements as well as a set of variables left to be optimised by modelling. The heat transfer from the refrigerant to the tubes was modelled using known correlations for condensation in tubes. The heat transfer between tubes and fins was modelled using a two-dimensional finite difference scheme. The heat transfer between the tubes and the water was modelled using forced convection models. The models gave dimensions for the size of the PCM container, the number of passes for each fluid stream and the thickness and spacing of the heat transfer fins. An experimental setup based on the optimal design was created to validate the models. The results showed that PCM storage is an effective manner to store thermal energy. Heat transfer was significant in the regions surrounding the tubes. Further away from the tubes, the fins did not provide enough heat transfer to utilise the whole storage capacity effectively. In its current design state, the system would have an economic payback time of around 20 years. With small design improvements, such as increasing the fin thickness and decreasing the fin distance, the payback period can be brought down significantly. The added product value from being a more efficient product makes the concept promising for future implementation.
...
Due to the intermittent nature of both the supply of heat, which is coupled to the control scheme of the chilled water reservoir and the demand for heat, which is determined by the user, a thermal storage system is required. Literature showed that the best refrigeration system for this application was a vapour compression system using a natural refrigerant such as isobutane. The most efficient heat storage could be done using organic phase change materials (PCM). Using a PCM allows the system to retain more energy in the same volume due to the latent heat in the system. To enhance the heat transfer between the fluid streams and the PCM, a fin-and-tube heat exchanger concept was designed. This concept, coupled with existing Quooker system demands, leads to a preliminary set of design requirements as well as a set of variables left to be optimised by modelling. The heat transfer from the refrigerant to the tubes was modelled using known correlations for condensation in tubes. The heat transfer between tubes and fins was modelled using a two-dimensional finite difference scheme. The heat transfer between the tubes and the water was modelled using forced convection models. The models gave dimensions for the size of the PCM container, the number of passes for each fluid stream and the thickness and spacing of the heat transfer fins. An experimental setup based on the optimal design was created to validate the models. The results showed that PCM storage is an effective manner to store thermal energy. Heat transfer was significant in the regions surrounding the tubes. Further away from the tubes, the fins did not provide enough heat transfer to utilise the whole storage capacity effectively. In its current design state, the system would have an economic payback time of around 20 years. With small design improvements, such as increasing the fin thickness and decreasing the fin distance, the payback period can be brought down significantly. The added product value from being a more efficient product makes the concept promising for future implementation.
...
Energy efficiency is a key goal for the Quooker system. To further increase the energy efficiency of the Quooker system, this study investigates a possible heat integration system where the waste heat from the refrigeration cycle for chilled drinking water is implemented to preheat water before entering the boiling water reservoir.
Due to the intermittent nature of both the supply of heat, which is coupled to the control scheme of the chilled water reservoir and the demand for heat, which is determined by the user, a thermal storage system is required. Literature showed that the best refrigeration system for this application was a vapour compression system using a natural refrigerant such as isobutane. The most efficient heat storage could be done using organic phase change materials (PCM). Using a PCM allows the system to retain more energy in the same volume due to the latent heat in the system. To enhance the heat transfer between the fluid streams and the PCM, a fin-and-tube heat exchanger concept was designed. This concept, coupled with existing Quooker system demands, leads to a preliminary set of design requirements as well as a set of variables left to be optimised by modelling. The heat transfer from the refrigerant to the tubes was modelled using known correlations for condensation in tubes. The heat transfer between tubes and fins was modelled using a two-dimensional finite difference scheme. The heat transfer between the tubes and the water was modelled using forced convection models. The models gave dimensions for the size of the PCM container, the number of passes for each fluid stream and the thickness and spacing of the heat transfer fins. An experimental setup based on the optimal design was created to validate the models. The results showed that PCM storage is an effective manner to store thermal energy. Heat transfer was significant in the regions surrounding the tubes. Further away from the tubes, the fins did not provide enough heat transfer to utilise the whole storage capacity effectively. In its current design state, the system would have an economic payback time of around 20 years. With small design improvements, such as increasing the fin thickness and decreasing the fin distance, the payback period can be brought down significantly. The added product value from being a more efficient product makes the concept promising for future implementation.
Due to the intermittent nature of both the supply of heat, which is coupled to the control scheme of the chilled water reservoir and the demand for heat, which is determined by the user, a thermal storage system is required. Literature showed that the best refrigeration system for this application was a vapour compression system using a natural refrigerant such as isobutane. The most efficient heat storage could be done using organic phase change materials (PCM). Using a PCM allows the system to retain more energy in the same volume due to the latent heat in the system. To enhance the heat transfer between the fluid streams and the PCM, a fin-and-tube heat exchanger concept was designed. This concept, coupled with existing Quooker system demands, leads to a preliminary set of design requirements as well as a set of variables left to be optimised by modelling. The heat transfer from the refrigerant to the tubes was modelled using known correlations for condensation in tubes. The heat transfer between tubes and fins was modelled using a two-dimensional finite difference scheme. The heat transfer between the tubes and the water was modelled using forced convection models. The models gave dimensions for the size of the PCM container, the number of passes for each fluid stream and the thickness and spacing of the heat transfer fins. An experimental setup based on the optimal design was created to validate the models. The results showed that PCM storage is an effective manner to store thermal energy. Heat transfer was significant in the regions surrounding the tubes. Further away from the tubes, the fins did not provide enough heat transfer to utilise the whole storage capacity effectively. In its current design state, the system would have an economic payback time of around 20 years. With small design improvements, such as increasing the fin thickness and decreasing the fin distance, the payback period can be brought down significantly. The added product value from being a more efficient product makes the concept promising for future implementation.
Boiling Heat Transfer with Nanofluids
An Experimental Study
Recent advancements in the field of nanotechnology have proven to offer viable alternatives for energy production, transport, and storage. As far as thermal energy is concerned, nanofluids have emerged as a novel method to enhance heat transfer. Indeed, nanofluids exhibit superior thermal capabilities, which may be able to meet the requirement of high heat dissipation rate in limited space advanced by various high-tech industries.
In particular, boiling heat transfer is an efficient heat removal mechanism that may be further improved by using nanofluids. Indeed, it has been reported that nanoparticles play a crucial role in affecting the parameters which have major impact on the boiling process (i.e. thermophysical properties of the fluid, heating surface morphology, near-surface hydrodynamics). Being boiling very sensitive to surface characteristics, the latter factors have been found to have a significant influence on the boiling heat transfer coefficient. Hence, the aim of the present research is to elucidate the physical mechanisms underlying pool boiling of nanofluids.
Based on this framework, a pool boiling test facility has been designed and validated, thus enabling to conduct a comparative study on boiling of a pure fluid (water) and a water-alumina 0.1% da nanofluid. The pool boiling experiments were performed on six aluminium samples, which were characterized by SEM (scanning electron microscopy) and WLI (white light interferometry) before and after boiling in order to highlight the change in surface topography.
The research efforts were targeted at correlating the trend of the boiling curves and the surface parameters of the corresponding sample. Nonetheless, due to the limited dataset and the inconsistencies in the behaviour of the tested nanofluid, further investigation is required to assess the potential of nanofluids as more efficient heat transfer media. ...
In particular, boiling heat transfer is an efficient heat removal mechanism that may be further improved by using nanofluids. Indeed, it has been reported that nanoparticles play a crucial role in affecting the parameters which have major impact on the boiling process (i.e. thermophysical properties of the fluid, heating surface morphology, near-surface hydrodynamics). Being boiling very sensitive to surface characteristics, the latter factors have been found to have a significant influence on the boiling heat transfer coefficient. Hence, the aim of the present research is to elucidate the physical mechanisms underlying pool boiling of nanofluids.
Based on this framework, a pool boiling test facility has been designed and validated, thus enabling to conduct a comparative study on boiling of a pure fluid (water) and a water-alumina 0.1% da nanofluid. The pool boiling experiments were performed on six aluminium samples, which were characterized by SEM (scanning electron microscopy) and WLI (white light interferometry) before and after boiling in order to highlight the change in surface topography.
The research efforts were targeted at correlating the trend of the boiling curves and the surface parameters of the corresponding sample. Nonetheless, due to the limited dataset and the inconsistencies in the behaviour of the tested nanofluid, further investigation is required to assess the potential of nanofluids as more efficient heat transfer media. ...
Recent advancements in the field of nanotechnology have proven to offer viable alternatives for energy production, transport, and storage. As far as thermal energy is concerned, nanofluids have emerged as a novel method to enhance heat transfer. Indeed, nanofluids exhibit superior thermal capabilities, which may be able to meet the requirement of high heat dissipation rate in limited space advanced by various high-tech industries.
In particular, boiling heat transfer is an efficient heat removal mechanism that may be further improved by using nanofluids. Indeed, it has been reported that nanoparticles play a crucial role in affecting the parameters which have major impact on the boiling process (i.e. thermophysical properties of the fluid, heating surface morphology, near-surface hydrodynamics). Being boiling very sensitive to surface characteristics, the latter factors have been found to have a significant influence on the boiling heat transfer coefficient. Hence, the aim of the present research is to elucidate the physical mechanisms underlying pool boiling of nanofluids.
Based on this framework, a pool boiling test facility has been designed and validated, thus enabling to conduct a comparative study on boiling of a pure fluid (water) and a water-alumina 0.1% da nanofluid. The pool boiling experiments were performed on six aluminium samples, which were characterized by SEM (scanning electron microscopy) and WLI (white light interferometry) before and after boiling in order to highlight the change in surface topography.
The research efforts were targeted at correlating the trend of the boiling curves and the surface parameters of the corresponding sample. Nonetheless, due to the limited dataset and the inconsistencies in the behaviour of the tested nanofluid, further investigation is required to assess the potential of nanofluids as more efficient heat transfer media.
In particular, boiling heat transfer is an efficient heat removal mechanism that may be further improved by using nanofluids. Indeed, it has been reported that nanoparticles play a crucial role in affecting the parameters which have major impact on the boiling process (i.e. thermophysical properties of the fluid, heating surface morphology, near-surface hydrodynamics). Being boiling very sensitive to surface characteristics, the latter factors have been found to have a significant influence on the boiling heat transfer coefficient. Hence, the aim of the present research is to elucidate the physical mechanisms underlying pool boiling of nanofluids.
Based on this framework, a pool boiling test facility has been designed and validated, thus enabling to conduct a comparative study on boiling of a pure fluid (water) and a water-alumina 0.1% da nanofluid. The pool boiling experiments were performed on six aluminium samples, which were characterized by SEM (scanning electron microscopy) and WLI (white light interferometry) before and after boiling in order to highlight the change in surface topography.
The research efforts were targeted at correlating the trend of the boiling curves and the surface parameters of the corresponding sample. Nonetheless, due to the limited dataset and the inconsistencies in the behaviour of the tested nanofluid, further investigation is required to assess the potential of nanofluids as more efficient heat transfer media.
Conventionally, the geometry of the Fri-Jado open vertical refrigerated display cabinet design is optimized by drawing and producing the new components and testing the new configuration in a climate chamber, which is known to be an iterative process that consumes a large amount of time. To speed up the optimization process, the application of computational fluid dynamics (CFD) was proposed. For this thesis, a case study was performed on a specific Fri-Jado cabinet that required many iterations for optimizing the geometry, using the commercial Simscale CFD package. In addition, experimental tests were performed on the Fri-Jado cabinet to compare with the simulation results.
During the experimental testing and literature review, it was found that the entrainment of ambient air was the main component of the heat load for the cabinet. By designing and validating a simplified 2D k-ω SST simulation of the air curtain and external environment, an optimization study was performed. In this optimization study, it was found that for an offset angle of 10° and throw angle of 25° in combination with a stepped velocity profile, minimal thermal and mass entrainment ratios were reached.
...
During the experimental testing and literature review, it was found that the entrainment of ambient air was the main component of the heat load for the cabinet. By designing and validating a simplified 2D k-ω SST simulation of the air curtain and external environment, an optimization study was performed. In this optimization study, it was found that for an offset angle of 10° and throw angle of 25° in combination with a stepped velocity profile, minimal thermal and mass entrainment ratios were reached.
...
Conventionally, the geometry of the Fri-Jado open vertical refrigerated display cabinet design is optimized by drawing and producing the new components and testing the new configuration in a climate chamber, which is known to be an iterative process that consumes a large amount of time. To speed up the optimization process, the application of computational fluid dynamics (CFD) was proposed. For this thesis, a case study was performed on a specific Fri-Jado cabinet that required many iterations for optimizing the geometry, using the commercial Simscale CFD package. In addition, experimental tests were performed on the Fri-Jado cabinet to compare with the simulation results.
During the experimental testing and literature review, it was found that the entrainment of ambient air was the main component of the heat load for the cabinet. By designing and validating a simplified 2D k-ω SST simulation of the air curtain and external environment, an optimization study was performed. In this optimization study, it was found that for an offset angle of 10° and throw angle of 25° in combination with a stepped velocity profile, minimal thermal and mass entrainment ratios were reached.
During the experimental testing and literature review, it was found that the entrainment of ambient air was the main component of the heat load for the cabinet. By designing and validating a simplified 2D k-ω SST simulation of the air curtain and external environment, an optimization study was performed. In this optimization study, it was found that for an offset angle of 10° and throw angle of 25° in combination with a stepped velocity profile, minimal thermal and mass entrainment ratios were reached.
Bachelor thesis
(2021)
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F.J.M. van Eijnatten, F. Uijtewaal, F.J. van Buren, K.W. Verloop, B.J. Boersma, R. Delfos
To improve the efficiency of domestic heat energy use, a heat battery was added to a boiler. This heat battery contains a phase change material (PCM) characterized by low thermal conductivity and high specific heat, enabling efficient storage of residual heat energy. Paraffin wax was selected due to its high latent heat storage capacity; however, its low thermal conductivity posed challenges regarding storage time. To address this, a finned tube was introduced, and the feasibility of the heat battery for domestic applications was investigated. The effectiveness of the finned tube was evaluated using both a numerical model and an experiment. In the experiment, a temperature- and flow-adjustable water source was connected to a plain tube and a finned tube, with temperature and heat flux measurements recorded to assess the impact of the fin structure. These conditions were replicated in the numerical model for comparison. The results indicated that heat extraction from the PCM using the finned tube required approximately 60 minutes, compared to nearly five hours with the plain tube. These findings suggest that while the method shows promise, further development is needed before it is suitable for domestic implementation.
...
To improve the efficiency of domestic heat energy use, a heat battery was added to a boiler. This heat battery contains a phase change material (PCM) characterized by low thermal conductivity and high specific heat, enabling efficient storage of residual heat energy. Paraffin wax was selected due to its high latent heat storage capacity; however, its low thermal conductivity posed challenges regarding storage time. To address this, a finned tube was introduced, and the feasibility of the heat battery for domestic applications was investigated. The effectiveness of the finned tube was evaluated using both a numerical model and an experiment. In the experiment, a temperature- and flow-adjustable water source was connected to a plain tube and a finned tube, with temperature and heat flux measurements recorded to assess the impact of the fin structure. These conditions were replicated in the numerical model for comparison. The results indicated that heat extraction from the PCM using the finned tube required approximately 60 minutes, compared to nearly five hours with the plain tube. These findings suggest that while the method shows promise, further development is needed before it is suitable for domestic implementation.
Predicting Infection Using Infrared Thermography in Premature Infants
Quantifying the Interaction Between Infrared Thermography and a Neonatal Incubator
Predicting when a neonate will fall victim to an infection or a disease allows prevention through early medicine administering. Such physiological conditions can be made visible using infrared thermography (IRT). This is a technique for measuring heat emitted in the infrared spectrum and transforming them into visible signals that can be recorded photographically. This thesis will contribute to the prediction of infection in (pre)term neonates by quantifying the interaction between IRT and a neonatal incubator without (and with) a neonate in it. A system was designed that consisted of three modules: a measurement (incubator and IRT camera), back-end (embedded system and server), and front-end module. The scope of this thesis is limited to the measurement module and the embedded system of the back-end module. Minimum camera requirements were set up which required the camera to: be inexpensive (i.e. ≤ €1000,-), be mobile, be open-source (for Linux), have a minimum frames-per-second of 5, have a resolution of at least 160x160 pixels with a field of view (FOV) of 27°, sensitivity of < 0.1°C, and safe to the patient. Such a camera was found in the FLIR One Pro. For this thesis a different FLIR camera was used due to lack of budget, namely the FLIR A305sc, which was already available at the TU Delft. The A305sc is not open-source, which required a work-around. The Aravis Open Source Project allowed for communication with the camera. Internal camera parameters had to be determined to calculate temperature based on analogue-to-digital values. ExifTool was used on a file stored by the camera to extract these parameters. This calculated temperature was compared to the temperature as determined by FLIR’s software and led to a difference in the range of 1·106°C. An open-source application was written that can connect with this IRT camera that has a GenICam interface using Aravis. Additionally, this application implemented the temperature calculation based on the internal camera parameters. The hood of the incubator is opaque to infrared, which required the design of a measurement setup to circumvent this. Three different setups were discussed, with the final choice falling on placing the camera in front of an opened incubator porthole on a tripod, and sealing this porthole with high or low density ethyl polyethylene (HDPE/LDPE). Regular H/LDPE used for construction site was found to have a attenuated transmissivity as found in literature. To quantify the interaction between IRT and a neonatal incubator, the IRT measurements were to be compared against the current golden standard sensor, namely thermistors. These sensor values were to be read out from the incubator as this would also be used in the final product. Code was written which allows for automatic detection between the GE GiraffeTM Omnibed and the Dräger Caleo® incubator, automatic connecting, and manipulation of all sensors values to a standard string which allows for easy uploading to the InfluxDB database on the server. To be allowed to perform measurements on human subjects, approval had to be acquired by the human research ethics committee (HREC) of the TU Delft and the respective hospital. A “non-wet medisch wetenschappelijk onderzoek met mensen” (nWMO) request was submitted and approved, which resulted in 25 recorded sick and healthy neonates in incubators divided over two hospitals (the JKZ in The Hague, and the RDGG in Delft), with over 25 hours of recording material. Simultaneously, measurements were performed on an empty incubator to gain an understanding in the behaviour of an incubator when actors from outside interacted with the internal environment. Measurements that were performed included determining the reflected apparent temperature (RAT) for every possible opened porthole and for both incubator types. The RAT for the Caleo was found to be higher for every measurement for the GE. The accuracy of the IRT and hospital skin temperature sensors was compared against a calibrated Pt-100 sensor, which show that the Pt-100 sensor measures an equal value as the hospital skin temperature sensor, whereas The IRT camera measured .6°C higher. The effect of changing the distance on IRT values was measured, which shows that for a distance of 0.2m to 1.2m the accuracy of the IRT camera is within the specified accuracy. Finally, the effect of opening additional portholes on IRT was measured, the effect of the airboost setting on IRT, and the measurement of opening additional portholes was repeated with a different IRT camera. Overall the IRT camera measures a higher temperature than the hospital skin temperature sensors, but follows the skin temperature sensors’ pattern. ...
Predicting when a neonate will fall victim to an infection or a disease allows prevention through early medicine administering. Such physiological conditions can be made visible using infrared thermography (IRT). This is a technique for measuring heat emitted in the infrared spectrum and transforming them into visible signals that can be recorded photographically. This thesis will contribute to the prediction of infection in (pre)term neonates by quantifying the interaction between IRT and a neonatal incubator without (and with) a neonate in it. A system was designed that consisted of three modules: a measurement (incubator and IRT camera), back-end (embedded system and server), and front-end module. The scope of this thesis is limited to the measurement module and the embedded system of the back-end module. Minimum camera requirements were set up which required the camera to: be inexpensive (i.e. ≤ €1000,-), be mobile, be open-source (for Linux), have a minimum frames-per-second of 5, have a resolution of at least 160x160 pixels with a field of view (FOV) of 27°, sensitivity of < 0.1°C, and safe to the patient. Such a camera was found in the FLIR One Pro. For this thesis a different FLIR camera was used due to lack of budget, namely the FLIR A305sc, which was already available at the TU Delft. The A305sc is not open-source, which required a work-around. The Aravis Open Source Project allowed for communication with the camera. Internal camera parameters had to be determined to calculate temperature based on analogue-to-digital values. ExifTool was used on a file stored by the camera to extract these parameters. This calculated temperature was compared to the temperature as determined by FLIR’s software and led to a difference in the range of 1·106°C. An open-source application was written that can connect with this IRT camera that has a GenICam interface using Aravis. Additionally, this application implemented the temperature calculation based on the internal camera parameters. The hood of the incubator is opaque to infrared, which required the design of a measurement setup to circumvent this. Three different setups were discussed, with the final choice falling on placing the camera in front of an opened incubator porthole on a tripod, and sealing this porthole with high or low density ethyl polyethylene (HDPE/LDPE). Regular H/LDPE used for construction site was found to have a attenuated transmissivity as found in literature. To quantify the interaction between IRT and a neonatal incubator, the IRT measurements were to be compared against the current golden standard sensor, namely thermistors. These sensor values were to be read out from the incubator as this would also be used in the final product. Code was written which allows for automatic detection between the GE GiraffeTM Omnibed and the Dräger Caleo® incubator, automatic connecting, and manipulation of all sensors values to a standard string which allows for easy uploading to the InfluxDB database on the server. To be allowed to perform measurements on human subjects, approval had to be acquired by the human research ethics committee (HREC) of the TU Delft and the respective hospital. A “non-wet medisch wetenschappelijk onderzoek met mensen” (nWMO) request was submitted and approved, which resulted in 25 recorded sick and healthy neonates in incubators divided over two hospitals (the JKZ in The Hague, and the RDGG in Delft), with over 25 hours of recording material. Simultaneously, measurements were performed on an empty incubator to gain an understanding in the behaviour of an incubator when actors from outside interacted with the internal environment. Measurements that were performed included determining the reflected apparent temperature (RAT) for every possible opened porthole and for both incubator types. The RAT for the Caleo was found to be higher for every measurement for the GE. The accuracy of the IRT and hospital skin temperature sensors was compared against a calibrated Pt-100 sensor, which show that the Pt-100 sensor measures an equal value as the hospital skin temperature sensor, whereas The IRT camera measured .6°C higher. The effect of changing the distance on IRT values was measured, which shows that for a distance of 0.2m to 1.2m the accuracy of the IRT camera is within the specified accuracy. Finally, the effect of opening additional portholes on IRT was measured, the effect of the airboost setting on IRT, and the measurement of opening additional portholes was repeated with a different IRT camera. Overall the IRT camera measures a higher temperature than the hospital skin temperature sensors, but follows the skin temperature sensors’ pattern.
Flat plate heat exchangers are widely used in industrial and domestic applications. Industrial plate type heat exchangers generally operate in the turbulent flow regime. Although, increase in flow speeds leads to higher transport of heat, it also causes a rise in the pressure loss which is undesirable. Therefore, to combat the problem of high pressure drop, this thesis explores the use of a passive enhancement technique to improve heat transfer. The aim of this thesis is to experimentally investigate the effect of dimple-protrusion surfaces in a counter flow type heat exchanger. The flow behavior is studied using Laser Doppler Anemometry and the local heat transfer characteristics are investigated with the help of Infrared thermography. The average Nusselt number and friction factor data is compared with those of a flat plate and it is found that the use of dimple-protrusion surfaces provide maximum improvement in the performance of the heat exchanger by 21% in the laminar-to-turbulent transition regime, at Reynolds number of approximately 2900.
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Flat plate heat exchangers are widely used in industrial and domestic applications. Industrial plate type heat exchangers generally operate in the turbulent flow regime. Although, increase in flow speeds leads to higher transport of heat, it also causes a rise in the pressure loss which is undesirable. Therefore, to combat the problem of high pressure drop, this thesis explores the use of a passive enhancement technique to improve heat transfer. The aim of this thesis is to experimentally investigate the effect of dimple-protrusion surfaces in a counter flow type heat exchanger. The flow behavior is studied using Laser Doppler Anemometry and the local heat transfer characteristics are investigated with the help of Infrared thermography. The average Nusselt number and friction factor data is compared with those of a flat plate and it is found that the use of dimple-protrusion surfaces provide maximum improvement in the performance of the heat exchanger by 21% in the laminar-to-turbulent transition regime, at Reynolds number of approximately 2900.
Heatpipes for geothermal heat extraction
A study on film heat transfer in heatpipes
Large quantities of heat stored in geothermal aquifers can be of interest to satisfy above-ground heating demands. With the use of heatpipes, placed into the aquifers, heat may be passively extracted. Vertical two-phase thermosiphons are considered in this study. As a result of a local increase in the saturation temperature in liquid pools, heat can only be transferred in the falling liquid film. The aim of this work is to investigate film heat transfer and its limitations at varying boundary conditions in the evaporator of a vertical experimental setup. An initially homogenized film is studied in a separate film heating section, in which a specific distribution of controlled band heaters and temperature transmitters enables the sensing of local film heat removal behaviour in the heatpipe. With increasing degrees of wall superheating, the mean film heat removal rate has been found to increase. Locally, beyond some degree of superheating, the heat transfer rate stagnates with increasing Jakob numbers, possibly as a result of film dry-out. As a result of such local behaviour, the slope of the mean heat transfer rate curve decreases with increasing Jakob numbers. The onset of dry-out, characterized by an increase in the intermittency of the results, has been studied by varying the initial liquid flow rate to the heated section. Both the initial film flow rate and the degree of superheating have been found to be of significance for the onset of film dry-out. Film dry-out takes place at flow rates less than some critical flow rate. The local heat removal rate decreases significantly if the initial film flow rate is decreased beyond the critical flow rate. The critical flow rate has been found to increase with increasing rates of superheating. The significance of the saturation temperature on the film heat transfer rate has also been studied in this work. The film heat transfer rate has been found to decrease with decreasing saturation temperature, for all in this study considered degrees of superheating. The steepest decline in heat transfer rates with decreasing saturation temperature is found for the smallest degree of superheating in the analysis. At last, the possibility of film re-distribution following dry-out has been considered. Melamine foam homogenizer rings, inserted in the film flow path, have been found to decrease the degree of film thickness variation in initially inhomogeneous films to some extent. Homogenized films were found to be able to transfer a greater amount of heat than films that had not been re-distributed.
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Large quantities of heat stored in geothermal aquifers can be of interest to satisfy above-ground heating demands. With the use of heatpipes, placed into the aquifers, heat may be passively extracted. Vertical two-phase thermosiphons are considered in this study. As a result of a local increase in the saturation temperature in liquid pools, heat can only be transferred in the falling liquid film. The aim of this work is to investigate film heat transfer and its limitations at varying boundary conditions in the evaporator of a vertical experimental setup. An initially homogenized film is studied in a separate film heating section, in which a specific distribution of controlled band heaters and temperature transmitters enables the sensing of local film heat removal behaviour in the heatpipe. With increasing degrees of wall superheating, the mean film heat removal rate has been found to increase. Locally, beyond some degree of superheating, the heat transfer rate stagnates with increasing Jakob numbers, possibly as a result of film dry-out. As a result of such local behaviour, the slope of the mean heat transfer rate curve decreases with increasing Jakob numbers. The onset of dry-out, characterized by an increase in the intermittency of the results, has been studied by varying the initial liquid flow rate to the heated section. Both the initial film flow rate and the degree of superheating have been found to be of significance for the onset of film dry-out. Film dry-out takes place at flow rates less than some critical flow rate. The local heat removal rate decreases significantly if the initial film flow rate is decreased beyond the critical flow rate. The critical flow rate has been found to increase with increasing rates of superheating. The significance of the saturation temperature on the film heat transfer rate has also been studied in this work. The film heat transfer rate has been found to decrease with decreasing saturation temperature, for all in this study considered degrees of superheating. The steepest decline in heat transfer rates with decreasing saturation temperature is found for the smallest degree of superheating in the analysis. At last, the possibility of film re-distribution following dry-out has been considered. Melamine foam homogenizer rings, inserted in the film flow path, have been found to decrease the degree of film thickness variation in initially inhomogeneous films to some extent. Homogenized films were found to be able to transfer a greater amount of heat than films that had not been re-distributed.
Master thesis
(2019)
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Paul-Jan Hogendoorn, Rene Delfos, Bendiks Jan Boersma, Mark Tummers, Aris Twerda
Heat pipes are typically used in the semiconductor industry. This means that the scale of these heat pipes is typically in the order of centimeters. Zijm suggests that heat pipes could be used for geothermal applications, but literature is lacking. To further investigate the geothermal application of heat pipes, a large scale heat pipe is built. This thesis gives an insight in the typical design challenges that one faces when constructing a heat pipe of this scale. The heat pipe that is constructed can support a heat flow of 10 kW. The heat pipe is constructed from mainly 54 millimeter copper and glass pipes. The evaporator section is 1.5 meters long and facilitates controlled electric heating. Then follows a 4 meter long adiabatic section. The condenser section is 2.5 meters long. The total length of the heat pipe is 9 meters. The individual sections are held together by EPDM connectors. The dimensions of the heat pipe are compared to the operational limits posed by the Engineering Sciences Data Unit. Then the thermal resistances of the heat pipe sections are calculated and afterwards validated. The interfacial thermal resistance between the electric heaters and evaporator wall was reduced by applying thermal conduction paste to the band heaters. It was found that at coolant flows of 1000 l/h and higher, the vapour temperature in the heat pipe drops significantly. The drop in temperature facilitates a higher heat flow through the heat pipe. Also, the resistance across the evaporator and the condenser section gets smaller for higher coolant flows. The results found are supported by theory and formulae from the Engineering Sciences Data Unit.
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Heat pipes are typically used in the semiconductor industry. This means that the scale of these heat pipes is typically in the order of centimeters. Zijm suggests that heat pipes could be used for geothermal applications, but literature is lacking. To further investigate the geothermal application of heat pipes, a large scale heat pipe is built. This thesis gives an insight in the typical design challenges that one faces when constructing a heat pipe of this scale. The heat pipe that is constructed can support a heat flow of 10 kW. The heat pipe is constructed from mainly 54 millimeter copper and glass pipes. The evaporator section is 1.5 meters long and facilitates controlled electric heating. Then follows a 4 meter long adiabatic section. The condenser section is 2.5 meters long. The total length of the heat pipe is 9 meters. The individual sections are held together by EPDM connectors. The dimensions of the heat pipe are compared to the operational limits posed by the Engineering Sciences Data Unit. Then the thermal resistances of the heat pipe sections are calculated and afterwards validated. The interfacial thermal resistance between the electric heaters and evaporator wall was reduced by applying thermal conduction paste to the band heaters. It was found that at coolant flows of 1000 l/h and higher, the vapour temperature in the heat pipe drops significantly. The drop in temperature facilitates a higher heat flow through the heat pipe. Also, the resistance across the evaporator and the condenser section gets smaller for higher coolant flows. The results found are supported by theory and formulae from the Engineering Sciences Data Unit.
In industry, fired heaters are used to heat process fluids and to generate steam. The efficiency of these devices is increased with air preheating. Gnerally, the flow through the air preheaters is driven by a centrifugal fan, via forced draft. Through a widening transition duct (large angle diffuser), air flows from the fan towards the air preheater. In terms of fluid mechanics, the geometry of such a duct is often unfavorable.
This research is aimed at improving the flow in large angle diffusers. A test platform was built with which the flow in diffusers of various geometries can be studied. ...
This research is aimed at improving the flow in large angle diffusers. A test platform was built with which the flow in diffusers of various geometries can be studied. ...
In industry, fired heaters are used to heat process fluids and to generate steam. The efficiency of these devices is increased with air preheating. Gnerally, the flow through the air preheaters is driven by a centrifugal fan, via forced draft. Through a widening transition duct (large angle diffuser), air flows from the fan towards the air preheater. In terms of fluid mechanics, the geometry of such a duct is often unfavorable.
This research is aimed at improving the flow in large angle diffusers. A test platform was built with which the flow in diffusers of various geometries can be studied.
This research is aimed at improving the flow in large angle diffusers. A test platform was built with which the flow in diffusers of various geometries can be studied.
Master thesis
(2019)
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Peter Zijm, Bendiks Jan Boersma, Rene Delfos, Pejman Shoeibi Omrani, Aris Twerda
Heatpipes are promising devices for geothermal energy extraction owing to their
effectiveness to transport heat. The goal of this research is to validate an analytical heatpipe model with experiments and to investigate the difficulties in designing and constructing geothermal heatpipes. There is a lack of literature and research concerning the operation, performance limits and construction of heatpipes suitable for geothermal heat extraction. A prototype heatpipe is designed based on specifications for geothermal energy extraction and constructed in a laboratory set-up with sensors and data acquisition. The prototype setup collects experimental data and is used to evaluate important parameters, requirements and practical design difficulties. This research shows the difficulties in designing a geothermal heatpipe taking into account fluid choice and physical limitations as well as complications in constructing a properly sealed heatpipe under the influence of repeated heating and cooling. Furthermore it shows the limitations of the analytical model by comparing the model predictions with experimental data. ...
effectiveness to transport heat. The goal of this research is to validate an analytical heatpipe model with experiments and to investigate the difficulties in designing and constructing geothermal heatpipes. There is a lack of literature and research concerning the operation, performance limits and construction of heatpipes suitable for geothermal heat extraction. A prototype heatpipe is designed based on specifications for geothermal energy extraction and constructed in a laboratory set-up with sensors and data acquisition. The prototype setup collects experimental data and is used to evaluate important parameters, requirements and practical design difficulties. This research shows the difficulties in designing a geothermal heatpipe taking into account fluid choice and physical limitations as well as complications in constructing a properly sealed heatpipe under the influence of repeated heating and cooling. Furthermore it shows the limitations of the analytical model by comparing the model predictions with experimental data. ...
Heatpipes are promising devices for geothermal energy extraction owing to their
effectiveness to transport heat. The goal of this research is to validate an analytical heatpipe model with experiments and to investigate the difficulties in designing and constructing geothermal heatpipes. There is a lack of literature and research concerning the operation, performance limits and construction of heatpipes suitable for geothermal heat extraction. A prototype heatpipe is designed based on specifications for geothermal energy extraction and constructed in a laboratory set-up with sensors and data acquisition. The prototype setup collects experimental data and is used to evaluate important parameters, requirements and practical design difficulties. This research shows the difficulties in designing a geothermal heatpipe taking into account fluid choice and physical limitations as well as complications in constructing a properly sealed heatpipe under the influence of repeated heating and cooling. Furthermore it shows the limitations of the analytical model by comparing the model predictions with experimental data.
effectiveness to transport heat. The goal of this research is to validate an analytical heatpipe model with experiments and to investigate the difficulties in designing and constructing geothermal heatpipes. There is a lack of literature and research concerning the operation, performance limits and construction of heatpipes suitable for geothermal heat extraction. A prototype heatpipe is designed based on specifications for geothermal energy extraction and constructed in a laboratory set-up with sensors and data acquisition. The prototype setup collects experimental data and is used to evaluate important parameters, requirements and practical design difficulties. This research shows the difficulties in designing a geothermal heatpipe taking into account fluid choice and physical limitations as well as complications in constructing a properly sealed heatpipe under the influence of repeated heating and cooling. Furthermore it shows the limitations of the analytical model by comparing the model predictions with experimental data.
The production process of milk powder consists of multiple stages. This report focuses on the falling film evaporator, which role in the production process is to evaporate the water content from the milk. A falling film evaporator is a large vertically-placed vessel whereby the inside is filled with smaller tubes. Steam enters the vessel and heats up the outside of the smaller tubes. The milk flows in a thin layer only along the inside perimeter of the smaller tubes, so these tubes are not completely filled. The advantage of this flow is that a thin layer of liquid is continuously in contact with the wall so the heating process is equal along the tube. The thin layer is called a falling film because the thickness compared to the length of the flow is very small.
The objective of this research is to determine theoretically the local overall heat transfer coefficient of the falling film and to investigate experimentally the applicability of heat flux sensors by determining the local overall heat transfer coefficient. By investigating the falling film, it is not allowed to disturb the falling film. Once a falling film is disturbed, the falling film will proceed at a different path. Heat flux sensors allow for local non-intrusive measurements. The overall heat transfer coefficient gives information about the thickness of the falling film. Theoretically the overall heat transfer coefficient is calculated by taken the thermal resistances of each component. Experimentally, the overall heat transfer coefficient is measured by the heat flux and the temperature difference between the bulk temperature of the fluid and the sensor at the outside of the tube.
Before the heat flux sensors are used in practice on the falling film evaporator, a setup has been built to experimentally determine the applicability of the heat flux sensor. This setup has been made for a tube filled with water and to create a falling film. The tube filled with water is well-described in theory and used as a reference.
The results of the experimental setup show that the Danfoss 'Koperpasta tube AT' is in good comparison with the theoretical approach. The theoretical overall heat transfer coefficient difference, caused by the mass flow difference of 0.01 kg/s in the falling film evaporator, can be detected by the heat flux sensors taking into account the error margin of the heat flux sensor and temperature sensor.
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The objective of this research is to determine theoretically the local overall heat transfer coefficient of the falling film and to investigate experimentally the applicability of heat flux sensors by determining the local overall heat transfer coefficient. By investigating the falling film, it is not allowed to disturb the falling film. Once a falling film is disturbed, the falling film will proceed at a different path. Heat flux sensors allow for local non-intrusive measurements. The overall heat transfer coefficient gives information about the thickness of the falling film. Theoretically the overall heat transfer coefficient is calculated by taken the thermal resistances of each component. Experimentally, the overall heat transfer coefficient is measured by the heat flux and the temperature difference between the bulk temperature of the fluid and the sensor at the outside of the tube.
Before the heat flux sensors are used in practice on the falling film evaporator, a setup has been built to experimentally determine the applicability of the heat flux sensor. This setup has been made for a tube filled with water and to create a falling film. The tube filled with water is well-described in theory and used as a reference.
The results of the experimental setup show that the Danfoss 'Koperpasta tube AT' is in good comparison with the theoretical approach. The theoretical overall heat transfer coefficient difference, caused by the mass flow difference of 0.01 kg/s in the falling film evaporator, can be detected by the heat flux sensors taking into account the error margin of the heat flux sensor and temperature sensor.
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The production process of milk powder consists of multiple stages. This report focuses on the falling film evaporator, which role in the production process is to evaporate the water content from the milk. A falling film evaporator is a large vertically-placed vessel whereby the inside is filled with smaller tubes. Steam enters the vessel and heats up the outside of the smaller tubes. The milk flows in a thin layer only along the inside perimeter of the smaller tubes, so these tubes are not completely filled. The advantage of this flow is that a thin layer of liquid is continuously in contact with the wall so the heating process is equal along the tube. The thin layer is called a falling film because the thickness compared to the length of the flow is very small.
The objective of this research is to determine theoretically the local overall heat transfer coefficient of the falling film and to investigate experimentally the applicability of heat flux sensors by determining the local overall heat transfer coefficient. By investigating the falling film, it is not allowed to disturb the falling film. Once a falling film is disturbed, the falling film will proceed at a different path. Heat flux sensors allow for local non-intrusive measurements. The overall heat transfer coefficient gives information about the thickness of the falling film. Theoretically the overall heat transfer coefficient is calculated by taken the thermal resistances of each component. Experimentally, the overall heat transfer coefficient is measured by the heat flux and the temperature difference between the bulk temperature of the fluid and the sensor at the outside of the tube.
Before the heat flux sensors are used in practice on the falling film evaporator, a setup has been built to experimentally determine the applicability of the heat flux sensor. This setup has been made for a tube filled with water and to create a falling film. The tube filled with water is well-described in theory and used as a reference.
The results of the experimental setup show that the Danfoss 'Koperpasta tube AT' is in good comparison with the theoretical approach. The theoretical overall heat transfer coefficient difference, caused by the mass flow difference of 0.01 kg/s in the falling film evaporator, can be detected by the heat flux sensors taking into account the error margin of the heat flux sensor and temperature sensor.
The objective of this research is to determine theoretically the local overall heat transfer coefficient of the falling film and to investigate experimentally the applicability of heat flux sensors by determining the local overall heat transfer coefficient. By investigating the falling film, it is not allowed to disturb the falling film. Once a falling film is disturbed, the falling film will proceed at a different path. Heat flux sensors allow for local non-intrusive measurements. The overall heat transfer coefficient gives information about the thickness of the falling film. Theoretically the overall heat transfer coefficient is calculated by taken the thermal resistances of each component. Experimentally, the overall heat transfer coefficient is measured by the heat flux and the temperature difference between the bulk temperature of the fluid and the sensor at the outside of the tube.
Before the heat flux sensors are used in practice on the falling film evaporator, a setup has been built to experimentally determine the applicability of the heat flux sensor. This setup has been made for a tube filled with water and to create a falling film. The tube filled with water is well-described in theory and used as a reference.
The results of the experimental setup show that the Danfoss 'Koperpasta tube AT' is in good comparison with the theoretical approach. The theoretical overall heat transfer coefficient difference, caused by the mass flow difference of 0.01 kg/s in the falling film evaporator, can be detected by the heat flux sensors taking into account the error margin of the heat flux sensor and temperature sensor.
Master thesis
(2018)
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Keerthivasan Rajamani, Rene Delfos, Bendiks Jan Boersma, Mark Tummers, Andries van Eckeveld
A liquid film falling vertically along a wall results in the formation of waves at the liquid-gas interface. The principal forces in these flows are gravity, viscosity, and surface tension, which are characterized by the Reynolds number, Re and the Kapitza number, Ka. Experimental characterization of the film flows for high viscosity liquids (Ka in the order of 1) at low Re » in the order of 10 was performed for flow along the inner circumference of a vertical pipe. The three-dimensional liquid-gas interfacial profile is quantitatively reconstructed using the laser induced fluorescence technique. The waves observed had higher surface steepness at the front when compared with the back of the wave crest. The film thickness at the back of the wave crest is higher than at the front, resembling the streak-like waves observed in the literature for low-viscosity liquids (Ka in the order of 1000) at comparable Re. When the lateral surfaces of the waves are in contact, merging in the transverse direction is observed. The experimentally determined mean film thickness was approximately 15 % lower than the Nusselt’s flat film thickness, while the experimental values of average wave velocity was approximately 4 times higher than the Nusselt’s velocity for the corresponding film thickness. On the time-averaged film thickness field, a transverse variation in the film thickness, called as ridges and valleys, were observed. These transverse variations are found to be unsteady in their behaviour. Probability density distributions of the film thickness measurements showed two distinctive time-dependant patterns for the distribution of film thickness values in the ridges.
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A liquid film falling vertically along a wall results in the formation of waves at the liquid-gas interface. The principal forces in these flows are gravity, viscosity, and surface tension, which are characterized by the Reynolds number, Re and the Kapitza number, Ka. Experimental characterization of the film flows for high viscosity liquids (Ka in the order of 1) at low Re » in the order of 10 was performed for flow along the inner circumference of a vertical pipe. The three-dimensional liquid-gas interfacial profile is quantitatively reconstructed using the laser induced fluorescence technique. The waves observed had higher surface steepness at the front when compared with the back of the wave crest. The film thickness at the back of the wave crest is higher than at the front, resembling the streak-like waves observed in the literature for low-viscosity liquids (Ka in the order of 1000) at comparable Re. When the lateral surfaces of the waves are in contact, merging in the transverse direction is observed. The experimentally determined mean film thickness was approximately 15 % lower than the Nusselt’s flat film thickness, while the experimental values of average wave velocity was approximately 4 times higher than the Nusselt’s velocity for the corresponding film thickness. On the time-averaged film thickness field, a transverse variation in the film thickness, called as ridges and valleys, were observed. These transverse variations are found to be unsteady in their behaviour. Probability density distributions of the film thickness measurements showed two distinctive time-dependant patterns for the distribution of film thickness values in the ridges.
The freeze plug is a key safety component of the molten salt fast reactor (MSFR), one of six next-generation nuclear reactor technologies being developed under the Generation IV International Forum (GIF). It should be designed to melt if an accident occurs, allowing the MSFR to drain before it incurs structural damage.
Two freeze plug concepts have been considered in recent years, in which the plug is melted either through the decay heat produced in the core, or through heat generated by special heating rings and stored in steel blocks adjacent to the freeze plug. Variations consisting of both a single freeze plug, and multiple smaller plugs contained in a metal plate, have been proposed. This work seeks to evaluate the feasibility of these designs and study how parameters such as the sub-cooling of the plug affect melting times. Additionally, an alternative, wedge-shaped freeze plug design is proposed for increased reliability.
Simulations performed in COMSOL showed that the decay heat plug melts within 600 s only if placed within 0.01 m of the mixed core flow. Because such a placement makes the plug vulnerable to temperature and velocity fluctuations in the core during regular operation of the reactor, this design is considered unfeasible and is not recommended for further study. On the other hand, melting times under 600 s were possible with the heating ring design for a range of sub-cooling amounts and plug configurations, suggesting that this
design is promising. A thin frozen layer was shown to form on top of the metal grate in the multi-plug configurations, preventing heat transfer through the top of the plate. Although the melting behavior of this layer warrants further investigation, its insulating effect was found to generally cause the single-plug designs to melt faster than the multi-plug designs.
A simplified, isothermal model of the wedge-shaped plug was simulated using the enthalpy-porosity approach to account for convection. To model the sinking of the wedge, an extended Darcy term approach was developed based on an analytical solution which was validated experimentally, with good agreement. This model shows that melting of the wedge is unsteady, and that melting times depend linearly on the wedge angle and sub-cooling. Unfortunately, melting times of the wedge plug could not be estimated with realistic, non-isothermal, time-dependent boundary conditions. For future study, a customizable numerical solver such as OpenFOAM is recommended, which would allow the sinking of the solid phase to be modeled more robustly through an immersed boundary method. ...
Two freeze plug concepts have been considered in recent years, in which the plug is melted either through the decay heat produced in the core, or through heat generated by special heating rings and stored in steel blocks adjacent to the freeze plug. Variations consisting of both a single freeze plug, and multiple smaller plugs contained in a metal plate, have been proposed. This work seeks to evaluate the feasibility of these designs and study how parameters such as the sub-cooling of the plug affect melting times. Additionally, an alternative, wedge-shaped freeze plug design is proposed for increased reliability.
Simulations performed in COMSOL showed that the decay heat plug melts within 600 s only if placed within 0.01 m of the mixed core flow. Because such a placement makes the plug vulnerable to temperature and velocity fluctuations in the core during regular operation of the reactor, this design is considered unfeasible and is not recommended for further study. On the other hand, melting times under 600 s were possible with the heating ring design for a range of sub-cooling amounts and plug configurations, suggesting that this
design is promising. A thin frozen layer was shown to form on top of the metal grate in the multi-plug configurations, preventing heat transfer through the top of the plate. Although the melting behavior of this layer warrants further investigation, its insulating effect was found to generally cause the single-plug designs to melt faster than the multi-plug designs.
A simplified, isothermal model of the wedge-shaped plug was simulated using the enthalpy-porosity approach to account for convection. To model the sinking of the wedge, an extended Darcy term approach was developed based on an analytical solution which was validated experimentally, with good agreement. This model shows that melting of the wedge is unsteady, and that melting times depend linearly on the wedge angle and sub-cooling. Unfortunately, melting times of the wedge plug could not be estimated with realistic, non-isothermal, time-dependent boundary conditions. For future study, a customizable numerical solver such as OpenFOAM is recommended, which would allow the sinking of the solid phase to be modeled more robustly through an immersed boundary method. ...
The freeze plug is a key safety component of the molten salt fast reactor (MSFR), one of six next-generation nuclear reactor technologies being developed under the Generation IV International Forum (GIF). It should be designed to melt if an accident occurs, allowing the MSFR to drain before it incurs structural damage.
Two freeze plug concepts have been considered in recent years, in which the plug is melted either through the decay heat produced in the core, or through heat generated by special heating rings and stored in steel blocks adjacent to the freeze plug. Variations consisting of both a single freeze plug, and multiple smaller plugs contained in a metal plate, have been proposed. This work seeks to evaluate the feasibility of these designs and study how parameters such as the sub-cooling of the plug affect melting times. Additionally, an alternative, wedge-shaped freeze plug design is proposed for increased reliability.
Simulations performed in COMSOL showed that the decay heat plug melts within 600 s only if placed within 0.01 m of the mixed core flow. Because such a placement makes the plug vulnerable to temperature and velocity fluctuations in the core during regular operation of the reactor, this design is considered unfeasible and is not recommended for further study. On the other hand, melting times under 600 s were possible with the heating ring design for a range of sub-cooling amounts and plug configurations, suggesting that this
design is promising. A thin frozen layer was shown to form on top of the metal grate in the multi-plug configurations, preventing heat transfer through the top of the plate. Although the melting behavior of this layer warrants further investigation, its insulating effect was found to generally cause the single-plug designs to melt faster than the multi-plug designs.
A simplified, isothermal model of the wedge-shaped plug was simulated using the enthalpy-porosity approach to account for convection. To model the sinking of the wedge, an extended Darcy term approach was developed based on an analytical solution which was validated experimentally, with good agreement. This model shows that melting of the wedge is unsteady, and that melting times depend linearly on the wedge angle and sub-cooling. Unfortunately, melting times of the wedge plug could not be estimated with realistic, non-isothermal, time-dependent boundary conditions. For future study, a customizable numerical solver such as OpenFOAM is recommended, which would allow the sinking of the solid phase to be modeled more robustly through an immersed boundary method.
Two freeze plug concepts have been considered in recent years, in which the plug is melted either through the decay heat produced in the core, or through heat generated by special heating rings and stored in steel blocks adjacent to the freeze plug. Variations consisting of both a single freeze plug, and multiple smaller plugs contained in a metal plate, have been proposed. This work seeks to evaluate the feasibility of these designs and study how parameters such as the sub-cooling of the plug affect melting times. Additionally, an alternative, wedge-shaped freeze plug design is proposed for increased reliability.
Simulations performed in COMSOL showed that the decay heat plug melts within 600 s only if placed within 0.01 m of the mixed core flow. Because such a placement makes the plug vulnerable to temperature and velocity fluctuations in the core during regular operation of the reactor, this design is considered unfeasible and is not recommended for further study. On the other hand, melting times under 600 s were possible with the heating ring design for a range of sub-cooling amounts and plug configurations, suggesting that this
design is promising. A thin frozen layer was shown to form on top of the metal grate in the multi-plug configurations, preventing heat transfer through the top of the plate. Although the melting behavior of this layer warrants further investigation, its insulating effect was found to generally cause the single-plug designs to melt faster than the multi-plug designs.
A simplified, isothermal model of the wedge-shaped plug was simulated using the enthalpy-porosity approach to account for convection. To model the sinking of the wedge, an extended Darcy term approach was developed based on an analytical solution which was validated experimentally, with good agreement. This model shows that melting of the wedge is unsteady, and that melting times depend linearly on the wedge angle and sub-cooling. Unfortunately, melting times of the wedge plug could not be estimated with realistic, non-isothermal, time-dependent boundary conditions. For future study, a customizable numerical solver such as OpenFOAM is recommended, which would allow the sinking of the solid phase to be modeled more robustly through an immersed boundary method.
Doctoral thesis
(2012)
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MJW Harleman, Jerry Westerweel, Thomas van Terwisga, D Lohse, Wim Uijttewaal, JJH Brouwers, EK Longmire, Rene Delfos, Gijs Ooms