Hydrogen's competitiveness as a sustainable energy carrier depends greatly on its transportation and storage costs. Liquefying hydrogen for these purposes offers benefits like purity, versatility, and higher density. However, current industrial hydrogen liquefaction processes face efficiency and cost challenges, with a second-law efficiency below 25% and costs between 2.5-3.0 US$/kgLH2, and they are mainly limited to small-scale. Scaling up liquefaction plants is a potential solution to reduce costs, but existing models often overlook the economic and technical viability of the conceptual plants. Most reported liquefaction costs rely on "ballpark" estimation, hindering process economic comparisons between various processes. Industry-sponsored studies often use confidential data, limiting accessibility. Thus, this thesis aims to develop a comprehensive framework for modelling large-scale liquefaction processes and assessing their feasibility, focusing on the large-scale high-pressure hydrogen Claude-cycle liquefier concept. The technical analysis focuses on the preliminary designs of main equipment, such as compressors, turboexpanders, and heat exchangers, ensuring compatibility with existing technology limitations. Aspen Process Economic Analyzer (APEA) is employed to estimate the capital and operating expenditure. At 0.1 €/kWh electricity price, the techno-economic assessment of the 125 tonnes per day (TPD) Claude-cycle concept yields specific liquefaction costs (SLC) of 1.55 €/kgLH2. Sensitivity analysis shows electricity price has significant influence. Applying the established design approach, incorporating high-speed centrifugal compressors could reduce the SLC by 5.42% and potentially more. Scaling up to 250 and 500 TPD shows potential SLC reductions of 7.80% and 9.45%, respectively, at electricity price of 0.1 €/kWh. Future projections suggest a potential 12.6% SLC reduction as the Claude-cycle liquefiers matures. In an ideal scenario, with incentives and low-cost renewable electricity, costs could range from 0.87 to 1.09 €/kgLH2. Finally, a cost-scaling curve for hydrogen liquefaction plants are estimated based on the cost results from this study. The curve is comparable to existing cost curves reported by industrial and government joint research projects, validating the methodologies developed in this thesis. Moreover, an experience curve is also predicted using cost data from this study and data found in the literature. The curve indicates a 17% price decrease in hydrogen liquefiers with each doubling of global liquefaction capacity.

The main objective of this thesis was to explore the capabilities of neural networks in terms of representing governing differential equations, primarily in the purview of fluid/aero dynamic flows. The governing differential equations were accommodated within the loss functions for training the neural networks, thereby making them 'physics-informed'. Subsequently, this idea of physics-informed neural networks (PINNs) was extended to parameterized geometries generated with the help of commercial auto-encoders developed by the UK based company Monolith AI pvt. ltd. because neural networks have the capability to learn the desired PDEs over variable/parameterized geometries without the need to recompute the solution for every minor change in the input geometry, which proves out to be a huge advantage over classical numerical techniques. The advantages, limitations and scope for further research in the field of physics-informed deep learning have been discussed in the contents of the underlying thesis report.

At present, there is a worldwide awareness and concern about how global warming could impact daily lives and the need to switch to renewable sources of energy. Commercially, various green energy technologies such as hydro, wind, solar, nuclear fission, etc. are being used to harness their stored energy and convert it into electricity. However, amongst these, the natural sources of energy are limited in the operation by their intermittent nature. This potentially leads to the instability of the power grid system. Hence, it is difficult to replace the existing conventional fossil-fired energy generation systems with their renewable counterparts, unless and until, flexibility is introduced to damp the intermittency of renewable sources of energy. A Pumped Thermal Energy Storage (PTES) system, which uses a combination of a heat pump and heat engine to store electricity in form of heat and convert the heat back to electricity, could efficiently provide intraday to multiday flexibility. However, the literature reveals that this technology is not mature enough and requires enhanced research and experimental work. The scope of this thesis includes a conceptual design, steady-state modelling, and optimization of a waste heat integrated PTES, alternatively called Compressed Heat Energy Storage (CHEST) system. Two configurations of the CHEST system were analysed: (i) CHEST with latent heat storage, and (ii) CHEST with sensible heat storage. A combination of R1233zD as a working fluid and sunflower oil as a sensible thermal energy storage medium was selected and a Phase Change Material (PCM) as a latent heat storage medium. Towards this end, a sensitivity analysis is performed to examine the effect of various design parameters on the performance of the system. It is found that CHEST systems are not suitable for high-temperature storage applications (> 200°𝐶). Further, the effect of superheating in a heat pump, and the effect of waste heat source temperature on system performance is investigated. An optimization study is conducted on CHEST with a sensible storage configuration to use a single heat exchanger for the charging and discharging cycle during sensible heat transfer with a storage medium. A detailed comparison between the two configurations have been performed. Most notably a trade-off has to be set between the performance and economic viability of the system. Finally, the integration of the optimised sensible heat storage configuration with a solar-powered alkaline electrolyser is evaluated. A 30 MW of heat from the electrolyser unit is considered to be recovered with the help of the CHEST system. Towards the end, a preliminary design of the CHEST components (heat exchangers and turbomachinery) is provided to estimate the overall sizing and performance characteristics of the components.

Experiments to characterise vortices in hyperbolic shaped funnels are being conducted at the Water Application Centre (WAC) in Wetsus. These have demonstrated their higher gas transfer rates in comparison to the conventional aeration systems presently in use. Depending on the imposed flow conditions, different regimes of vortices are formed, among which the Twisted vortical structure is observed to have the highest gas dissolution rates. This has probed several questions on the physical mechanisms responsible on both micro-and macroscopic scales. The present research aims to numerically analyse the flow field organisation in these vortices to reason the observed high gas transfer rates using Computational Fluid Dynamics (CFD).  Transient simulations were performed on a three-dimensional radially structured hexahedral mesh. Multi-phase modelling was done using the Euler-Euler approach-based Volume-of-Fluid (VOF) method, while the turbulent flow was modelled using Shear Stress Transport (SST) based on k - ω equations with curvature correction. The choice of boundary conditions and their location is crucial for forming a stable vortex in the hyperbolic funnels. The position of the air-water interface from experimental results was used to validate the obtained numerical results. Two regimes of the vortex, namely the Twisted and Straight vortical structures, were evaluated for their gas transfer capabilities in terms of Hydraulic Retention Time (HRT), interfacial area and mixing in the bulk.  Instabilities arise in the secondary flow field of these vortical structures analogous to the Taylor-vortices that develop in the well known Taylor-Couette flow systems. In hyperbolic funnels, these instabilities aid in advecting the bulk of liquid to the air-water interfacial region and also enhance mixing within the bulk of water. The former enhances the gas transfer rates while the latter promotes uniform mixing. The strength of these instabilities is qualitatively analysed in terms of average vorticity per unit mass of water. This is found to be higher in the Twisted regime in comparison to other regimes. This is augmented by high air-water interfacial area making this regime possess superior gas transfer rates.  Although the gas transfer rates are high, water exiting the funnel is undersaturated at the given operating conditions. In order to further enhance the amount of gas dissolved few possibilities are qualitatively discussed at the end of this study.     

The growing environmental concerns has put a lot of political & ethical pressure on the conventional industrial practices. The research community is ever so eager to investigate new technologies that can offer an efficient and environmentally sound solution to the issues caused by the polluting industrial processes. One of the potential solution is an Organic Rankine Cycle (ORC) which is proven to be an effective technology to efficiently extract energy from low-temperature sources. The ORC is basically a Rankine cycle but employs a high molecular weight organic fluid as the energy carrier. The peculiar non-classical gas dynamics behaviour of these fluids during dry expansion close to the vapour saturation line (in the dense gas region) poses certain challenges on the turbine design. Furthermore, the low speed of sound in organic fluids and high expansion ratio required in an ORC turbine leads to highly supersonic flows in the turbine. Such high speed flows within a turbine stage are susceptible to high shock losses, if necessary corrective steps are not taken during early design phase. This has sparked a wave of interest in the scientific community to come up with an efficient supersonic ORC turbine design guidelines. The design of a supersonic turbine rotor is crucial to ensure efficient performance of the turbine. In the mid-20th century, a supersonic impulse rotor design methodology for axial turbines was proposed that boasts shock-free turning of supersonic flows using the vortex-flow theory. This procedure relies on Method of Characteristics (MOC) to solve hyperbolic governing PDEs. The method got enriched later with the capability to account for dense gas effects on the rotor geometry which is essential for ORC applications. However, there are no such design guidelines available in the literature to generate a radial rotor blade especially for supersonic flows. The conventional 1D preliminary design methods, generally available for radial rotor blades, are not very reliable in supersonic flow regime. This calls for a novel design philosophy for supersonic radial rotors that accounts for supersonic flow phenomenon early in the design phase to ensure efficient turbine performance. The objective of this thesis is to put forth a design methodology for supersonic radial rotors that acknowledges the flow equations in the design procedure. The proposed design ideology is to extend the supersonic axial rotor design procedure by including the fictitious forces' effects in the governing equations to generate a radial rotor using MOC. These pseudo forces (Centrifugal & Coriolis) are experienced by the flow in the radial runner if seen from the rotor's rotating frame of reference. Furthermore, the dense gas effects are also included in the design using the properties from CoolProp library. A design tool is developed in PYTHON that implements the aforementioned ideology in the design procedure. This thesis takes the first step towards developing a fully functional design methodology for supersonic radial rotors. Therefore, a simple case of a constant force's influence on the rotor geometry is studied in this work and compared with the CFD simulations in both perfect & dense gas region. Future research will focus on understanding the complex behaviour of organic fluids in the presence of an external force and modify the design procedure accordingly.

Turbulence is a commonly encountered state of fluid dynamics. Unsteady turbulent flows in pipes are present in many engineering applications and also in biological flows. However, the various processes active in such flows are not well understood. The present work employs stereo-PIV to investigate the effects of a sinusoidal pressure gradient on the various turbulence parameters, including the terms of the turbulent kinetic energy budget equation. The bulk flow rate was oscillated with a frequency of 0.5 Hz with a mean Re 26,000 and an amplitude of modulation, 0.23 times the mean value. It is seen that there is a delay in the response of turbulence to the oscillations of the bulk flow and the delay increases with increasing distance from the wall. The axial and the in-plane turbulence parameters show a difference in the delay of their responses. This delay extends into the small scales responsible for the dissipation of turbulent kinetic energy. Changes are also observed in azimuthal length scales when the flow oscillates.The effects of oscillation on the streaks of low momentum are also discussed and the structural organization in unsteady pipe flows are found to be different from that in steady pipe flows

Organic Rankine Cycle systems are gaining increasing attention as a modular, cost-effective and decentralised thermal energy recovery solution. The design of an efficient expander in the ORC system is crucial to its rapid uptake by the industry. Compared to volumetric expanders, a radial turbine provides advantages in terms of flexibility and better part-load performances. The design of ORC radial turbines is faced with two challenges. Firstly, the expansion in the stator occurs close to the vapour-liquid dome and the critical point, where the ideal gas assumption is no longer valid. Such expansions fall under the Non-ideal thermodynamic regime. Secondly, due to the lower speed of sound in the working fluid, the expansions are supersonic and are accompanied by compressible flow features such as shock waves, expansion fans which generate entropy and reduce the performance of the expander. There exists significant validated design methodologies and empirical relations for conventional turbomachinery. However this is not the case for unconventional turbomachinery where established loss correlations are not applicable. Hence, there is a knowledge gap in validated design tools for unconventional turbomachinery like ORC radial turbines. The thesis forms a part of the validation campaign of the open-source flow solver, SU2 and approaches the exercise by studying expansions in two paradigmatic test cases, namely (i) a linear stator cascade and (ii) a converging - diverging nozzle. The study of expansions through a linear cascade is a preceding step to the study of a rotating radial turbine. Consequently, a RANS simulation on the single channel blade passage is studied under the validated assumption of flow periodicity. The expansion takes place from inlet total conditions of 18.4 bara and 525 K (Z =0.558) to a static pressure of 1.95 bara (Z = 0.951) and exit flow of Mach 2. Two types of response quantities are studied namely direct and system response quantities. Direct response quantities are those that can be measured directly such as the pressure, Mach, density. System response quantities are derived from direct measurements and provide information to characterise the performance of the stator. The selected system response quantities are: (i) Pressure loss coefficient (Cp= 0.074), (ii) Kinetic energy loss coefficient (ζKE= 0.115), (iii) Entropy loss coefficient (ζs=0.118), (iv) Base pressure loss coefficient (Cpb= -0.065), (v) Standard deviation of exit flow angle (σβ2 = 1.244) and (vi) Standard deviation of exit Mach (σM2= 0.033). Experimentally, it is possible to measure the pressure loss coefficient, base pressure loss coefficient and the flow uniformity at the outlet using a combination of pressure and direct velocity measurements. Through a Design of Experiments approach, the sensitivity of the flow to input uncertainties was studied through a stochastic collocation based forward propagation of the uncertainty. The input uncertainties considered are the inlet total pressure, fluid viscosity and critical point properties. The Sobol indices from the uncertainty quantification indicate the more dominant influence of the critical point properties over other inputs considered. The results also validate the use of a constant viscosity assumption for the RANS simulation. The subsequent planned unit test case was to characterise the expansion through an optimised stator blade row. To this end, a deterministic adjoint based optimisation was performed with the objective function of minimising entropy generation. The resulting geometry was studied and resulted in a pressure loss coefficient improvement of around 4%, although stator flow uniformity at the exit was compromised. The uncertainty quantification performed on the optimised blade geometry yielded robustness improvements on the pressure loss coefficient and reduction in uncertainties associated with direct response quantities, although no strong correlations can be drawn between the improvements in uncertainty and the deterministic optimisation. Given current machining tolerances, the realisation of such a negligible geometry change is not viable from an experimental perspective. The second section of the thesis deals with experimental investigation of expansions in a converging-diverging nozzle through a matrix of isobars with increasing degree on non-ideality. Two isentropes with inlet pressure of 2.73 bara (Z = 0.9526) and 6 bara (Z = 0.901) with pressure ratio of 8.76 were performed with recording of pressure, temperature, flowrate and density measurements along the ORCHID. The flow field was visualised using the schlieren method using a z-type layout. The thesis reports the first measurements of total pressure before the nozzle inlet and vapour density and flowrate measurements. The experimental data was post-processed to identify steady-state and quantify the Type A and Type B uncertainties. The schlieren images were used to extract information on the Mach distribution along the nozzle mid-plane using an inhouse line extraction tool. Lastly, a 2.5o wedge at the exit of the nozzle is used to generate oblique shock waves that are then manually measured. The flow field at the exit of the nozzle is in the ideal region where the shock angle only depends on upstream Mach number. Hence, the experimentally observed oblique shock angles for the off-design case are close to on-design (Inlet pressure = 18.4 bara) predictions.

Heat transfer in multiphase flows plays an important role in many industrial applications. For instance, particle-based solar receivers utilize the high absorptivity and heat capacity of dispersed phase in a carrier fluid to improve efficiency and heat transfer. This dispersed phase generally consists of a large number of small particles. It is, therefore, difficult to completely resolve such flows considering their finite size. Usually, these particles are so small, that they can be treated as point particles. This considerably reduces the computational effort, while preserving the essential characteristics of the particle-laden flows. In this thesis, the focus is on heat transfer modulation in a particle-laden turbulent channel flow using direct numerical simulations. In flows with temperature gradients (for example, channel flow between hot and cold wall), particles absorb heat from hotter regions and release heat to colder regions, thereby enhancing heat transfer through particle feedback flux. On the other hand, presence of particles leads to decay in turbulence resulting in lower turbulent heat transfer. The interplay of this two phenomena can either increase or decrease the overall heat transfer based on Stokes number and thermal Stokes number (ratio of thermal response time to characteristic time scale of the flow). To investigate the heat transfer modulation in particle-laden channel flow, the existing DNS code developed by Boersma [5] has been modified to include particle transport and heat transfer. The point-particle approach with two way coupling is implemented using trilinear interpolation scheme and 3 rd order Runge-Kutta time marching scheme. The implemented code is validated using the results from literature [20] for a flow with no external heating. With the developed code, cases with no external source term and external source term with different optical thickness of the fluid have been analyzed. In order to focus only on the fluid-particle interaction, the effect of gravity is neglected and the flow is considered to be incompressible. It has been observed from these simulations that particles play an important role in modulation of heat transfer in such flows. Mean temperature profiles, heat flux mechanisms, temperature variance and budgets of temperature variance are studied extensively in order to understand the underlying phenomenon. It is found that the particle feedback heat transfer is the dominating mode in particle-laden flows.

In order to reach the goals of the Paris Agreement, global emissions must decrease at a high rate. Looking at the industrial sector, the largest share of energy is used for process heating, with fossil fuels as its primary source. A way to improve the energy efficiency and reduce the emissions of process heating is by the integration of heat pump technology. However, operational costs are still a limiting factor in the widespread uptake of this technology. A potential of industrial heat pumps that will have a positive effect on the operational costs, as well as its general applicability, is the possibility to operate the equipment in a flexible manner. At this point, the potential to respond with high temperature industrial heat pumps to heat demand, the electricity market or to grid congestion, is not yet unlocked. The focus in this study is on gaining more insight into the dynamic characteristics of industrial heat pump operation. The approach is based on the modeling of a 2 MW high temperature industrial heat pump in the Dymola simulation environment. Based on the system dynamics, a suitable control method is selected and the controller settings are optimized. With this control system in place, the dynamic limitations are studied. The limiting factor in the considered economizer based heat pump cycle is the ability of the control system to keep the superheat at the screw compressor injection port within acceptable limits during load level changes. The heat pump is found to be able to ramp up and down at a maximum rate of approximately 20%/min. This level of flexibility allows use of the heat pump for both heat demand and electricity price response, as well as for participation in grid load balancing pools. On a higher level, based on these results flexible heat pump operation can be considered possible.

Over the past decades, engineers have focused on a common goal - to reduce emissions by making industrial processes more efficient and by utilizing renewable energy sources. One possibility to reach this goal can be achieved by employing processes with non-ideal fluids, such as fluids at supercritical conditions in refrigeration, heat pump cycles and power cycles. Most of the flows in industry are turbulent and hence the need to study turbulence in non-ideal fluids arose. Turbulence in non-ideal fluids is extremely challenging since there are many complex effects at play. One such effect is caused by variations in properties, which also occurs in compressible flows, flows with high concentration gradients, or flows in heat exchangers. This thesis presents a review of the existing theory on semi-local scaling for variable property flows with the aim to take it one step further and apply it to turbulent heat flux modeling. Two types of variable property cases are analysed in this thesis; (1) low-Mach number flows with uniform pseudo-heating sources, (2) high-Mach number flows with non-uniform viscous heating. For the former, a Direct Numerical Simulation (DNS) data-base, already available at TU Delft, has been post-processed with the main goal to investigate if the semi-local theory can also be applied to thermal turbulence and its modeling. For the latter, additional DNS simulations of high-Mach number channel flows have been performed to investigate how the viscous heating and its correlations can be accounted for in the semi-local scaling framework. Using a 2-equation heat flux model, we find that modeling thermal turbulence in semi-local scales considerably improves the results for low-Mach number flows. However, for high-Mach number flows, additional unknown (closure) terms arise due to fluctuations in the viscous heating source. A model for the source term in the enthalpy variance equation is successfully proposed. In addition, the DNS study of two high-Mach number flows with constant semi-local Reynolds number profiles shed light on the importance of a newly defined parameter (modified Eckert number) and also unveils one of the most important conditions in which semi-local theory can be compromised, e.g. extreme density gradients.

In this thesis a method of combined PIV and LIF experiments on the Turbulent/Non-Turbulent interface (TNTI) of a round jet is presented. By varying the distance from the nozzle in the measurements, the Kolmogorov scale of a large range of Reynolds number (2×103 ≤ Re ≤ 48×103) is kept constant, allowing for a constant resolution. The large range in Reynolds number is needed to identify a difference in scaling of the TNTI, which is difficult to capture due to the weak dependency of the separation of Kolmogorov and Taylor scales on the Reynolds number (λ/η ∼ Re1/4). A factor of over 2.5 is achieved in the current work. An experimental setup for these measurements is presented, as well as the design of experiments for seven Reynolds numbers based on the boundary and initial conditions of the setup. A design for measurements with a camera moving with the flow is presented as well. Conditional statistics are used on the jet to look at the change of span-wise vorticity over the interface. For the detection of the interface scalar images from LIF are used, where the interface is detected using a threshold of dye concentration (Rhodamine B). All Reynolds numbers show self-similarity within the measured range, comparable to literature and with a spatial resolution better than 4η. Some small differences between the low and high Reynolds number measurements can be seen. They are not expected to have significant impact on the results and can likely be solved by slight changes in the experimental setup. The influence of the threshold on the shape and width measurements of the mean conditional vorticity profile is examined, indicating a significant dependency on chosen threshold. Current results indicate a scaling of the interface thickness with the Taylor length scale, around a value of 0.5-0.6λ, or 10-15η This is in accordance to the literature for low Reynolds numbers. Due to this significant influence of the threshold on the outcome, these results cannot be seen conclusive. For this, further investigation is needed on the dependency of the width on the threshold, and validation of the jet contour. Universal ways to compute the threshold, jet contour and mean conditional vorticity profiles are currently missing, while they have shown to be potentially of significance on the outcome. Another method of studying TNTI scaling is via the local velocity scales, which can be found via current data, which could provide a comparison of methods. The measurements with the moving camera could help identifying the link between physical processes and the scaling of the interface.

With increasing discussion and legislation around emissions, gas turbines have evolved to limit their emissions, especially NOx emissions. The way of operating the gas turbine, with lean premixed combustion reduces the flame temperature in the combustion chamber which results in low NOx emissions. The downside of this way of operating is the increased sensitivity of the combustion system to thermo-acoustic instabilities. These instabilities arise due to an interaction between unsteady heat release at the flame and the acoustics of the combustion system. At lean premixed conditions, a fluctuation in equivalence ratio results in a large fluctuation in flame speed which is related to the heat release. This makes the lean premixed combustion very susceptible to acoustic pressure waves travelling through and reflecting inside the combustion system, as these pressure waves can influence the mass flow of gas and air in the premixing duct and thus changing the composition of the fuel mixture that is convected towards the flame. An additional effect to the unsteady heat release are fluctuations of the total mass flow at the burner outlet, which are convected towards the flame front, an increase in combusted fuel will result in an increase in heat release.To analyze the sensitivities of the combustion system, a thermo-acoustic model will be constructed. The model will be a one-dimensional and linearised system, where conservation equations across the (thermo-)acoustic elements of the combustion system are rewritten into transfer matrix form. Validation will be done by analyzing operational data from the HW09 power plant and comparing this with the model response. The combustion dynamics are analysed for frequencies in the range 0 - 300 Hz.It was found that the HW09 is susceptible to combustion instabilities when the pressure drop across the premix gas nozzles is low, which results in a decrease in fuel supply impedance. This increases the pressure amplitude in the 90 Hz range, this behaviour corresponds to the modelled results when changes to the gas preheating temperature or fuel nozzle compositions are made in the thermo-acoustic model, which influence the pressure drop over the gas nozzles. This is accompanied with a decreased amplification at the critical 120 Hz range. This is a known side-effect, as the 90 Hz needs to remain dominant for a stable combustion system. This means the pressure drop over the gas nozzles balances on a fine line, too low will result in high 90 Hz pressure waves and too high will result in a more unstable 120 Hz frequency. Several of the hybrid burners in the annular combustion chamber are fitted with a cylindrical burner outlet. This changes the flow field out of the burner and is used to allow a higher maximum load for the gas turbine. The inclusion of this burner ring makes the combustion system upstream of the burner outlet more resistant to acoustic pressure fluctuations in the frequency range up to 120 Hz. The burner system does become more sensitive to acoustic pressure fluctuations for frequencies higher than 120 Hz. Fuel composition influences the combustion behaviour of a fuel mixture. Low-calorific fuel shows to be inherently more unstable in this gas turbine configuration as opposed to high-calorific fuel. If hydrogen is added to high-calorific fuel, the flame speed is increased as is the influence of both equivalence ratio perturbations and velocity perturbations on unsteady heat release.The increase in flame speed results in a reduction of flame length, which changes the convective time delay and shifts the instability of the combustion system from the 120 Hz area towards the 180 Hz area.The thermo-acoustic model responds to changes of operational parameters in an equal manner as was found from data analysis. The results are comparable up to 200 Hz, after which the flame can no longer be assumed to be compact, which means compressibility, 2D and 3D effects have to be taken into account. Fuel composition analysis shows the importance of flame length and convective time delay on combustion stability. For hydrogen addition it can shift the critical frequency towards the second harmonic frequency of the combustion chamber, this will result in rapidly increasing pressure waves and high accelerations. With increased knowledge of the effects of hydrogen addition or low-calorific fuel on the flame dynamics, the model can be a predictive tool to investigate the combustion behaviour when the change to this fuel mixture will be made. The ability to predict the influence of different operational parameters or changes to burner geometry on the stability of the combustion system can be very beneficial for analysis of future changes to the gas turbine and its operation.

Due to the global energy crisis, there is a growing need to make industrial processes more efficient and clean. One such method is to use effective and renewable working fluids in power and refrigeration cycles. Research in this field during the past decade has placed supercritical CO2 (SCO2) in the forefront due to its non-ideal properties and abundance in nature but, to fully understand their heat transfer properties, insight into boundary layer flow and hydrodynamic instabilities are crucial. Experimental setups such as closed loop wind tunnel contraction facilities are needed which provide a laminar steady flow to take boundary layer measurements. However, using a contraction causes various problems such as formation of secondary flows like boundary layer separation, Gortler vortices, cross-flows and non-uniformity’s. In this study a novel 1D contraction shape with curvature on only one side is pro- posed. This will completely eliminate the risk of Gortler vortices on the bottom plate and make it a suitable candidate for boundary layer experiments. Nevertheless, the risk of the other secondary flows and Gortler vortices on the top wall disturbing this laminar flow remains. Hence, the present research aims to optimise this 1D contraction to reduce the risk of such secondary flows and analyse the growth of Gortler vortices on the top wall using numerical simulations. Steady state laminar simulations were performed to optimise the design of the 1D wind tunnel contraction. Optimum contraction and settling chamber lengths were found by identifying the risk of flow separation and boundary condition effects respectively. For the optimisation of the contraction wall shape, a family of transformed fifth order polynomial curves with varying inflection point distances were selected. To identify the curve with least risk of boundary layer separation and lowest flow non-uniformity, a multi- objective optimisation procedure was implemented. The procedure found that a curve with inflection point of 101mm downstream of contraction inlet gives the best performance. To analyse the effect of Gortler vortices on outlet uniformity, unsteady laminar simulations were performed on the contraction by forcing sinusoidal perturbations of different wavenumbers. This study found that perturbations form symmetrical steady steam-wise Gortler vortices in the contraction. The most unstable wavenumber was found to be 𝜆=83.33 𝑚-1 which formed secondary vortices of high vorticity. However it was also seen that for all wavenumbers, the vortices lose energy as they exit the contraction and do not affect the bottom boundary layer. Further, the effects of a side wall on the formation of Gortler vortices were also investigated. The results showed that the Gortler vortex closest to the wall is absorbed into high vorticity corner vortices, while those close to the centreline develop into steady Gortler vortices. No effect on the centreline velocity profile was seen due to the vortices making the 1D contraction suitable for boundary layer experiments. Finally, it was also seen that the 1D contraction shape produces asymmetrical Gortler vortices due to alternative perturbation methods such as random inlet perturbations.

The time a naval combatant can be deployed for a mission is limited by its dependency on supplies. At a certain point the ship needs to leave the area of operations to be replenished in a harbour or by a support ship. Moreover, the Royal Netherlands Navy has a societal obligation to reduce its environmental impact, in particular on global warming. Therefore, the Royal Netherlands Navy (RNLN) wants to reduce the fossil fuel dependency of its fleet significantly [50]. One of the methods to reduce fossil fuel dependency of ships is to reduce their energy requirement. The operational profile of fast naval combatants for the RNLN requires that the ships operate on the diesel engines for 90 percent of the time, often in part load. In part load, the turbocharger cannot supply the engine with the right amount of charge air. This results in a limited operating envelope for the diesel engine, and a decreased efficiency in part load. This is caused by the matching of a turbocharger, which is a compromise between high efficiency in the design point, and off design performance. However, in part load, advanced charge air configurations can potentially resolve this and improve the results as shown by Grimmelius et al. [15] and Zhang et al. [56] This study investigates the effect of advanced charge air configurations on the efficiency and acceleration performance of diesel engines in hybrid configurations aboard fast naval combatants. First, two mean value first principle diesel engine models based on the work of Geertsma et al. [12] were used to model the diesel engine. Next, the models were partly validated with ship data. We found that an approach using compressor maps and a motion based turbocharger model was most accurate. Then, a parallel-sequential turbocharger and a hybrid electric turbocharger were incorporated into the model. A hybrid turbocharger is a turbocharger with an electric machine coupled to the turbocharger shaft. The electric machine can increase the turbocharger speed to boost the charge air pressure in motor mode. Also, in generator mode excessive power from the turbocharger shaft can be taken out and utilized elsewhere. It was concluded that the application of advanced charge air configurations can significantly improve the engine efficiency in part load. For example, in a diesel hybrid propulsion configuration with power take-off this can lead to an efficiency increase of almost 10% at 20% load in comparison with a single charged engine. Furthermore, hybrid turbocharging enables extending the operating envelope of a parallel-sequential turbocharged engine with up to 25% at 60% engine speed. This enables the engine to deliver constant torque from 600 to 1000 rpm. With these concepts therefore, both improved efficiency and improved acceleration performance can be achieved.

A generalized isentropic gas model is derived following earlier work by Kouremenos et al. in the 1980s, by replacing the traditional adiabatic exponent γ by the real exponents γPv, γTv, and γPT, describing the isentropic pressure-volume, temperature-volume, and pressure-temperature relations respectively. The real adiabatic exponents are expressed as functions of state variables to take into account compressibility effects on the isentropic behavior of substances. Due to the implicit analytical nature of the real exponents, any equation of state or thermodynamic library can be used for their evaluation. The theoretical limits and overall behavior of the real isentropic gas model are explored for a Van der Waals substance. In the two opposing physical limits, the model is shown to reduce to the incompressible substance model for liquid densities and the ideal gas model as the temperature increases or the pressure goes to zero. The relation of the generalized isentropic gas model with other thermodynamic properties is explored, leading to the development of specific heat relations and other thermodynamic properties in terms of the real exponents γPv, γTv, and γPT. Besides providing alternative schemes for their evaluation, special features of thermodynamic properties such as the state of maximum density and inversion temperature may be related to the value of the isentropic exponents determined by the local compressibility of the substance. Due to the exact definitions of the real adiabatic exponents at a state point, the relations between properties is thermodynamically consistent – another physical requirement. The generalized isentropic gas model is then applied to isentropic flows to derive traditional gas dynamic relations such as speed of sound, stagnation properties, and choked flow conditions for non-ideal compressible fluid flows. Exact solutions are provided for Prandtl-Meyer expansion fans, and approximate Rankine-Hugoniot jump conditions are explored for real gases. Finally, attributes of the fundamental derivative of gas dynamics are explored under the generalized isentropic gas model to gain new insights into its mathematical properties. Under the generalized isentropic model, the fundamental derivative is shown to satisfy both liquid and gaseous physical limits. Non-classical behavior is attributed to higher-order derivatives of the real exponents. The application of the generalized isentropic gas model is demonstrated and validated for use in non-ideal compressible fluid dynamic (NICFD) codes by simulation of the one-dimensional Euler equations for a standard shock tube problem. Several numerical schemes for the evaluation of thermodynamic properties of varying levels of accuracy are presented for evaluation of the isentropic gas model. In the application of the shock tube problem, the general equation for the speed of sound is demonstrated to be equivalent to the speed of sound of a Van der Waals gas, proving the validity of the model.

In response to climate change, the marine sector is increasingly focused on the energy transition. New marine power plants operating more efficiently and on cleaner fuels are receiving significantly more attention. While the transition to different power plants and fuels results in reduced fossil fuel consumption and emissions, waste heat recovery systems can aid in obtaining both as well. Current as well as future marine power plants do not convert all of the energy contained in the fuel into useful power, with significant amounts of energy being wasted as heat. Waste heat recovery technologies can be applied to recover some of this energy and generate additional power, effectively boosting total system efficiency, with the resulting benefits of improved fuel economy and reduced specific emissions. This study first provides an extensive summary of all the different marine power plants, fuels, and waste heat recovery technologies, of both the present and future. One type of power plant that could be an advantageous alternative for future marine propulsion is the solid oxide fuel cell (SOFC) due to its high efficiency and potential to run on clean fuels. SOFCs generate electricity from chemical energy using a high-temperature electrochemical process, resulting in high-temperature waste heat being expelled. Therefore, the potential for a waste heat recovery technology to boost system efficiency is high, and it is chosen to investigate several power cycles for the waste heat recovery of a case study vessel powered by a 2 MW SOFC system. The aim of this study is to develop and execute an approach to evaluate these waste heat recovery technologies regarding their efficiency, size, and associated cost. While numerous power cycles for waste heat recovery are in existence, a selection of potentially suitable systems with a number of different setups is made to investigate further. Thermodynamic models are created for the selected power cycles to determine and compare their theoretical efficiencies. Subsequently, the size of the heat exchangers are calculated for the evaluation of system size, considering the size of other components such as turbomachinery as well. Two types of heat exchangers are considered in this study: the compact and innovative printed circuit heat exchanger (PCHE) and the classic but commonly large shell and tube heat exchanger (STHE). Finally, the investigated systems are subjected to an economic analysis based on the cost associated with the main components, with again considerations being made regarding excluded additional components. The results indicate that various configurations of the (transcritical) Rankine cycle operating on steam and CO2, as well as the (supercritical) Brayton cycle operating on CO2 and air, present with significant theoretical efficiencies ranging from 41 to 52% and electrical power outputs ranging from approximately 530 kWe to over 670 kWe. From the evaluation of the system size it is concluded that the smallest systems are those operated on CO2 equipped with PCHEs, while the largest are the air Brayton cycles equipped with STHEs. The economic analysis revealed that the systems with the lowest costs are the configurations of the (transcritical) Rankine cycle operating on steam, as well as certain air Brayton cycles equipped with PCHEs. The systems with the highest cost are found to be the air Brayton cycles equipped with STHEs, due to the significant sizes of the required heat exchangers. In general, it is concluded that no system outperforms the others simultaneously across all three investigated aspects of efficiency, size, and cost, and trade-offs will be required when selecting a waste heat recovery technology for the presented case study vessel. Nonetheless, a detailed process-oriented approach has been developed and executed to allow various waste heat recovery solutions to be compared, and it is proven that a significant amount of power can be produced from recovered waste heat. The results from this study can be directly consulted by ship owners and designers considering the application of waste heat recovery to an SOFC powered vessel. Furthermore, the developed approach can also be applied to waste heat recovery in other industries and power generation systems, as it provides a step-by-step guide on relevant calculations and considerations.

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% 𝑤𝑡 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.

Stratified turbulent flows abound in environmental and industrial settings. Examples are atmospheric boundary layer flows, the transport of nutrients and organisms and the mixing of heat and salinity in the oceans, fluid flow in heat exchangers, and the transport of reactants and products in chemical reactions. These examples and many others consider stratified wall-bounded turbulence, in which the creation of turbulence by mechanical processes contends with its dissipation due to buoyancy effects. These flows are said to be stably stratified as these are inherently stable flows and are averse to mixing. The buoyancy effects alter the structure of the flow, and consequently the dynamics of mass, heat, and momentum transport. As density fluctuations become more severe, the Oberbeck-Boussinesq approximation becomes inaccurate and the resulting dynamics are not correctly predicted. In the current work, we developed and validated a numerical solver for direct numerical simulations (DNS) of turbulent flows featuring strong property variations. More precisely, we solve the Navier-Stokes equations in the limit of vanishing Mach number (so-called low-Mach number limit), with the fluid density given by the ideal gas law, and the dynamic viscosity and thermal conductivity also expressed as functions of temperature. Our numerical solver is used to study stably-stratified turbulent channel flow under non-Oberbeck-Boussinesq conditions. The simulations are carried out at friction Reynolds number of  180, Prandtl number of 0.71, and friction Richardson number of the O(10). These non-dimensional numbers are the governing parameters and are defined based on the prescribed pressure drop and properties of the fluid at the reference temperature. Stratification is achieved by imposing constant temperature boundary conditions, with a high upper-to-lower wall temperature ratio (larger than 2), resulting in strong density variations in the flow. We will vary the temperature ratios and adjust gravity to maintain a similar Richardson number between cases, thereby isolating the effects of strong property variations in the flow dynamics. We will analyze the dynamics of heat and momentum transport under strong stratification for these conditions, also in light of DNS data of the same system under the Oberbeck-Boussinesq regime.

Radiative heat transfer has a large influence in engineering systems, especially when the temperature involved is elevated. For this reason a correct assessment of heat transfer in presence of radiation is of great importance in high temperature and pressure equipment such as combustion chambers, heat exchangers as well as reentry vehicles and rockets. This thesis presents the results of innovative coupled radiative heat transfer and turbulence simulations, which aim at investigating turbulence-radiation interactions and the effect of radiation in the turbulent heat transfer process. The simulations are performed employing heterogeneous high performance computing systems in which the radiative heat transfer is solved on graphical processing units while the fluid flow is solved on CPUs. @en

The global temperature rise, which directly results from greenhouse gases emitted by burning fossil fuels, requires humanity to harness renewable energy sources at an increased rate. However, renewable energy sources are either highly intermittent, such as wind and solar radiation, or available at low temperatures leading to low efficiencies of current thermal conversion systems. Two exciting technologies that can alleviate the low thermal conversion efficiencies of power plants with low-temperature heat sources are supercritical carbon dioxide (s-CO2) and organic Rankine cycles (ORCs). Compared to conventional power cycles, ORCs and s-CO2 power cycles have different workingmedia (e.g., CO2 and hydrocarbons), such that the working fluid provides an additional degree of freedom to better adapt to low-grade heat sources. As a consequence, the power cycles have higher thermal efficiency and a more compact design. However, the main difficulty in designing highly efficient components of these nonconventional power plants lies in the fact that the heat exchangers and the turbines operate either with fluids of high molecular complexity or with fluids in highly non-ideal thermodynamic conditions. These complexitiesmake it challenging to accurately design efficient components with computational fluid dynamic (CFD) software that can reliably predict heat transfer and pressure losses in heat exchangers, and aerodynamic performance parameters in turbomachinery equipment.@en