NH₃/H₂O based systems are promising for thermal energy storage and thermal energy conversion. These systems are used for absorption energy storage and Kalina cycles. This paper investigates the condensation of high-concentration NH₃/H₂O in vertically downward plate heat exchangers. A combined method is proposed by discussing the applicability of equilibrium and non-equilibrium models. Both models are necessary for zeotropic mixtures with large temperature glide. The non-equilibrium model applies where the temperature glide is non-linear or the vapor is in non-equilibrium with the liquid. The equilibrium model becomes applicable with decreasing vapor qualities. Heat transfer correlations are proposed according to the equilibrium model, which interpret convective condensation and gravity-controlled condensation. The additional heat transfer resistance is calculated considering mass transfer. The non-equilibrium model is further developed quantifying the heat and mass transfer of vapor and neglecting the mass transfer resistance of the liquid. The non-equilibrium model transforms into the equilibrium model as the concentration gradient of vapor approaches zero. Additionally, a frictional pressure drop model for separated flow conditions is proposed and quantifies the two-phase shear force.

High concentration NH₃/H₂O is suitable for Kalina cycles used for the recovery of low grade heat. Plate heat exchangers (PHEs) are compact and reduce the charge of working fluid. This paper investigates the condensation of NH₃/H₂O with NH₃ mass concentrations of 80%-96%. The vapor and liquid concentrations are close to equilibrium state, which are different from normal absorbers. The apparent heat transfer coefficients (HTCs) and frictional pressure drop are presented, covering the mass fluxes of 32–86 kgm⁻²s⁻¹, the averaged vapor qualities of 0.08–0.65 and the saturated pressure of 610 to 780 kPa. Larger mass fluxes noticeably increase the apparent HTCs and frictional pressure drop. At the mass concentrations of 96%, 91% and 88%, higher vapor qualities increase the apparent HTCs for large mass fluxes. The apparent HTCs decrease slightly with vapor qualities for 80% mass concentration. The experimental results are compared with those of pure NH₃. The flow patterns of high concentration NH₃/H₂O are considered as full film flow and partial film flow, which are the same as for NH₃. The mass transfer resistance deteriorates the heat transfer especially for partial film flow, which happens at small liquid mass fluxes. The mass transfer resistance has negligible influences on frictional pressure drop.

Energy shortage and energy related environmental problems are urgent issues to be addressed in the coming years. Low-grade heat is utilized to drive energy conversion cycle and to produce electricity, which is a renewable and sustainable approach to energy supply. These thermodynamic cycles for energy conversion require eco-friendly working fluids and highly efficient heat transfer processes. NH3 is a natural refrigerant with superior thermal properties such as large latent heat and high thermal conductivity. However, the application of NH3 is restrained due to safety issues. Plate heat exchangers have the potential to be used in the thermal facility of NH3 for the recovery of low-grade heat. These compact structures are able to transfer large heat loads with reduced charge of working fluid, thereby mitigating the safety risk. For instance, the Organic Rankine Cycles of NH3 equipped with plate heat exchangers have smaller sizes compared with the plants filledwith other refrigerants. Furthermore, plate heat exchangers have the advantage of design flexibility and easy maintenance for highly efficient heat transfer, bringing aboutwide utilization in refrigeration, pharmacy and chemical engineering. In this thesis, NH3 condensation is experimentally and theoretically investigated in plate heat exchangers. The main aim is to provide design methods of compact plate condensers used in the thermal facility of NH3, which are not available in open literature. The experiments ofNH3 condensation have been reported, but no design method is provided. The heat transfer and frictional pressure drop correlations of hydrofluorocarbons (HFCs), hydrocarbons (HCs) and hydrofluoroolefins (HFOs) are assessed making use of an experimental database. Most suitable correlations are recommended.

Heat transformers reveal significant potential for primary energy savings in domestic and industrial processes, which can use different heat sources as driving force to provide the heat or cooling. In this paper, a hybrid resorption-compression heat transformer is presented, which aims to upgrade the heat source e.g. industrial waste heat or solar energy with a large temperature lift. Performance of hybrid heat transformer is also compared with that of multi-stage sorption type. Results indicate that with heat source temperatures ranging from 40 °C to 90 °C, energy and exergy efficiencies of hybrid heat transformer decrease from 0.429 to 0.403 and from 0.8 to 0.64, respectively. Energy efficiency of hybrid type is a bit lower than that of basic resorption transformer but almost double higher than that of multi-stage cycle. For different operating parameters, mass ratio and global conversion rate have larger influences on thermal performance than isentropic efficiency of compressor. Also hybrid resorption-compression heat transformer is prospective for domestic heat application through the integration with solar photovoltaic thermal collector. When heat output temperature ranges from 50 °C to 70 °C, it could ensure that the heat density is higher than 1000 kJ·kg_am⁻¹ with an energy storage function.

This paper develops predicting models for NH₃ condensation in plate heat exchangers based on the experiments of flow patterns, heat transfer coefficients and frictional pressure drop previously reported by the authors. The aim is to provide design methods of compact plate condensers used in NH₃ systems, which are not available in open literature. The experimental data are firstly compared with selected correlations, showing a poor agreement. A heat transfer model is developed based on flow patterns, which represents the transition from convective condensation to gravity-controlled condensation. The physical interpretation of the two-phase multiplier approach and the deviation from Nusselt's theory are discussed. A transition criterion of condensation mechanisms is proposed based on the wetting characteristics. Since the flow patterns indicate separated flow, the Lockhart and Martinelli model is selected and is modified to predict the frictional pressure drop. The model is the sum of the liquid pressure drop, vapor pressure drop and interface pressure drop. The contributions of vapor pressure drop and interface pressure drop are discussed and quantified. The proposed heat transfer and frictional pressure drop models show good predictive performances. NH₃ flow has large two-phase slip because of the large density ratio. Plate heat exchangers have corrugated channels and tend to break up the liquid film. The models identify the distinct flow characteristics based on flow patterns.

This paper investigates NH₃ condensation in a plate heat exchanger by visualizing the flow patterns and measuring heat transfer coefficients and frictional pressure drop. Visualization experiments were conducted between 20 and 100 kgm⁻²s⁻¹. Full film flow takes place at large mass fluxes and intermediate mass fluxes of low vapor qualities, while partial film flow occurs at small mass fluxes and intermediate mass fluxes at high vapor qualities. The heat transfer and frictional pressure drop experiments cover the mass fluxes of 21~78 kgm⁻²s⁻¹, the averaged vapor qualities of 0.05~0.65 and the saturated pressure of 630 to 930 kPa. Vapor qualities have significant influences on heat transfer and frictional pressure drop. In the tested ranges, the effect of mass fluxes is noticeable on frictional pressure drop, but is moderate on heat transfer. The impact of saturated pressure is small. The heat transfer reflects the change of flow patterns. The frictional pressure drop shows the characteristics of separated flow.

The corrugation channels of plate heat exchangers enhance the heat transfer and complicate the prediction of heat transfer coefficients and frictional pressure drop. This paper reviews the heat transfer and frictional pressure drop correlations for condensation in plate heat exchangers, and classifies the correlations into basic forms. An experimental database is developed including the data of HFCs, hydrocarbons, HFOs and CO ₂ . The mass fluxes are in the range of 2–150 kg·m ⁻² ·s ⁻¹ . The chevron angles and hydraulic diameters are distributed in 25.7°–70° and 3.23–8.08 mm. The saturated temperatures are −34.4 to 72.1 °C, while the reduced pressures are from 0.03 to 0.49. Eight heat transfer correlations are assessed with the database. The correlation of Longo et al. [48] predicts the experimental data best, while the correlation of Kuo et al. [16] shows the second best performance. Six frictional pressure drop correlations are compared with the database. The prediction of frictional pressure drop is relatively poor, and a new correlation is developed using multi-variable regression analysis with non-dimensional numbers. This new correlation predicts 87.5% of the experimental data within ±50%.

The heat transfer coefficients (HTCs) of NH₃ are higher than of HFCs and hydrocarbons during the condensation within plate heat exchangers (PHEs) mainly due to its favorable transport properties. NH₃/H₂O has a large temperature glide, and its heat transfer is dependent on mass transfer. Few models have been specifically developed for NH3 and NH₃/H₂O condensation in PHEs. This paper presents heat transfer and pressure drop experiments for partial condensation. The calculation method for condensation HTCs and frictional pressure drop is introduced. The working fluids are pure NH₃ and NH₃/H₂O with a weight concentration of 96%. For a mass flux of 62 kgm^-2s^-1, the HTCs of NH3 increase from 10 to 20 kWm^-2K^-1 with vapor quality. The apparent HTCs of NH₃/H₂O are significantly lower than NH₃ at high vapor qualities because of mass transfer resistance. Condensation pressure level has a slight influence on HTCs and frictional pressure drop.

Most plate heat exchangers (PHEs) have hydraulic diameters in the range of 2∼5 mm and show characteristics of both macro-channels and micro-channels. Both gravity and surface tension have non-negligible influences and determine the heat transfer and frictional pressure drop.

This paper investigates NH3 condensation in a PHE with a hydraulic diameter of 2.99 mm. The large surface tension of NH3 enhances the micro-channel characteristics. The heat transfer coefficients (HTCs) are compared with homogeneous and separated models, respectively. Both models have been previously compared with the experimental data of HFCs, hydrocarbons and HFOs. The prediction for NH3 is generally good since the deviations are small, while the sensitivity to mass fluxes and vapor qualities cannot be estimated properly. The data of frictional pressure drop are predicted by a correlation of two-phase Fanning friction factor, which is based on homogeneous flow and includes the influences of mass fluxes, vapor qualities, hydraulic diameters, chevron angles, etc. The fluid properties of NH3 are significantly different between liquid and vapor phases, and the averaged density derived from homogeneous flow is under-estimated. The prediction is improved by calculating the averaged density from the void fraction models of separated flow.

This study presents a literature review of work related to the two-phase flow patterns of vertical downward flow in plate heat exchangers with corrugated chevron plates. An understanding of these flow patterns is crucial for developing accurate models of plate heat exchangers functioning as condensers or absorbers. Flow pattern maps of the previous studies are combined and translated to dimensionless forms. One of the proposed flow pattern maps is based on Re_L versus FrTP,hor/Λ0.5 and performs better than other representations. This map is compared with the map of tubes and shows general agreements in terms of the pattern positions, but the separating lines between flow patterns fit poorly. Influencing factors of condensation mechanisms are presented, among which mass flux and vapor quality are dominant. The preferred flow pattern map explains the transition of condensation mechanisms qualitatively when variations of mass flux and vapor quality are considered. Recommendations are given to come to more uniform flow pattern maps in plate heat exchangers with chevron corrugations.

The availability of open source experimental data of NH3 condensation within plate heat exchangers (PHEs) is very limited. This work attempts to extend the database. For this purpose step by step experiments are carried out in a setup capable of local measurements. Temperature sensors have been installed along a PHE to obtain the temperature profile. The local heat flux and the heat transfer coefficient (HTC) are then calculated from the energy balances. Condensation of NH3 has been investigated with varying vapor quality at different mass fluxes. The experimental results are compared with the heat transfer models developed for the condensation of NH3 in tubes and for the condensation of HFCs in PHEs.