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• Pure, binary, and ternary refrigerants of 14 refrigerants are analyzed. • Different refrigeration systems are considered. • Pareto fronts of the COP and volumetric heating capacity are obtained. • Binary refrigerants have similar advantages to ternary refrigerants. Mixed refrigerants are widely used in heat pumps; thus, it is necessary to analyze the performance advantages of pure, binary and ternary refrigerants. In this paper, 14 kinds of pure refrigerants were selected, and 24,024 kinds of pure, binary and ternary refrigerants were obtained and analyzed. The refrigerant performance in terms of the COP and volumetric heating capacity were compared in normal, vapor injection with internal heat exchanger and with flash tank systems. In this paper, 40 °C was used as the temperature lift, and four cases with heating production temperatures of 70, 80, 90 and 100°C were analyzed. The Pareto fronts for the different refrigerant types and compositions were obtained by using exhaustive method. The COP and volumetric heating capacity exhibited contradictory trends in all cases. The results showed that mixed refrigerants had significant advantages over pure refrigerants in achieving the Pareto fronts. However, in all cases, when the COP was higher than 5, the binary refrigerants had similar advantages to ternary refrigerants; thus, ternary refrigerants were not necessary. When the COP was lower than 5, it was difficult to further increase the volumetric heating capacity by sacrificing the COP. [Display omitted] [ABSTRACT FROM AUTHOR]

• The two-phase distribution in the header of a microchannel heat exchanger was quantified by a flexible capacitance sensor. • Distribution of local vapor quality showed that gases were concentrated upstream of the header, while liquids were concentrated in the middle and lower reaches. • Compared with the downstream, the upstream flow had less stability with more discrete capacitance signals. • The increase of mass flux caused that both the local vapor quality maximum point and the minimum point in the header were shifted back toward the downstream. The two-phase distribution has always been one of the research focuses of microchannel heat exchangers, because the maldistribution could seriously affect the heat transfer performance. This paper developed a capacitive method to quantify the two-phase distribution of vapor-liquid flow in the aluminum header of a microchannel heat exchanger. A flexible capacitance sensor was used to quantify the two-phase distribution with R134a. The inlet mass fluxes vary from 17.47 kg m−2 s−1 to 52.42 kg m−2 s−1 and inlet vapor qualities range from 0.2 to 0.6. The normalized time-averaged capacitance signal was processed by a Gaussian function to obtain the mathematical expectation and the standard deviation. The local vapor quality in the header was solved for using a correlation derived from the pre-experiments. The distribution of local vapor quality showed that gases were concentrated upstream of the header, while liquids were concentrated in the middle and lower reaches. The local vapor quality reached a minimum and remained constant at 0.1 in the middle and lower reaches, unaffected by changes in inlet conditions. As the mass flux increased, both the local vapor quality maximum point and the minimum point in the header were shifted back toward the downstream. Additionally, the increase of mass flux caused the fluctuation of the standard deviation increased, which meant the stability of the flow decreased. The two-phase flow measurement technique based on flexible capacitance sensors proposed in this study realized the extension of the capacitive method from the experimental stage to the industrial application stage. [ABSTRACT FROM AUTHOR]

• Developed a dynamic model of a triple-evaporator domestic refrigerator/freezer. • Dynamic model validated within a 2 % deviation in average power consumption. • Implemented a two-stage economized cycle and its control scheme. • Achieved 12.9 % and 80 kWh/year reduction in power and energy consumption, respectively. • Estimated 18 TWh/year in global energy savings by 2030 with 10 % market penetration. Domestic refrigerator/freezers account for approximately 6 % of all energy consumption around the globe and mainly rely on vapor compression cycles to operate. Researchers have investigated alternative cycle architectures such as dual-loop cycles and parallel circuit cycles to improve their efficiencies. Despite the demonstrated energy saving potential of these advanced cycles, additional implementation costs are often not justifiable. However, to meet forthcoming stricter energy standards while ensuring flexible multi-temperature operation of domestic refrigerator/freezers, advanced cycle architectures are needed. In this paper, a state-of-the-art bypass circuit cycle triple-evaporator domestic refrigerator freezer with R-600a and a reciprocating compressor has been used as the baseline cycle investigate an alternative cycle configuration. Specifically, this work presents a two-stage vapor-injected cycle with a multi-evaporator system to enable energy savings and cost-effectiveness. The cycle establishes two separate evaporation temperatures to better match the cabinet temperature of fresh food and freezer compartment. The reduced difference between cabinet temperature and its evaporation temperature decreases the irreversibilities in the heat exchanger and improves overall system efficiency. Moreover, the addition of the economization line from the medium temperature evaporator reduces the compressor work. To capture the complex transient behavior of both the baseline system and proposed cycle architecture with their control strategies, a dynamic model has been developed and validated with experimental data. The validated dynamic model with the baseline cycle was modified to consider the two-stage vapor-injected cycle and its control logic, and simulation results yielded up to 13 % energy consumption reduction with respect to the baseline system. [ABSTRACT FROM AUTHOR]

The objective of this paper is to provide the reliable calculation procedure for determining heat and mass flow rates for plate finned tube heat exchangers in dehumidification regimes that can be used easily in engineering practice. For this purpose, the experiments are conducted on two heat exchangers, and datasets for six more heat exchangers are used from literature. Comprehensive database is established with total of 637 sets of data, and it gathers 365 new measurement sets and 272 sets from open literature. The new calculation procedure predicts heat transfer rate and condensate flow rate where new methodology approach (based on the porous velocity, the ratio of characteristic surfaces and hydraulic diameter) is used. Predicted results show 5–10 % deviation for heat duty and up to 20 % for mass flow rate from experimental results, which is of importance to industrial practice. [ABSTRACT FROM AUTHOR]

This paper compares the performance of a vertical-finned microchannel heat exchanger (VMHX) and a 2-row fin-tube heat exchanger (FTHX) under wet and frosting conditions. The VMHX features vertically oriented fins with an extension at the windward side and parallel microchannel tubes, allowing for efficient water drainage and better performance than conventional microchannel heat exchangers with parallel serpentine fins. However, the performance of the VMHX has not been sufficiently compared to that of FTHX. After conducting experiments under specific operating conditions, the VMHX exhibits better water drainage performance than the FTHX at higher inlet air velocities, higher maximum heat flow, and time-integrating heat flow under certain operating conditions. Although the heating time is relatively shorter than the 2-row FTHX, these results suggest that the VMHX may be a more suitable option for applications requiring efficient water drainage and heat transfer. Additionally, the operating conditions significantly affect the heat exchanger's performance. The performance of VMHX is analyzed under different inlet air velocities, inlet air temperatures, and evaporation temperatures. The results indicate that frost itself had no effect on the heat exchanger performance and that the decrease of the inlet air flow rate was the main factor that decreased the heat exchanger performance. Overall, the results of this study provide valuable insights into the performance of VMHX and FTHX under wet and frosting conditions and the impact of various operating parameters on VMHX performance. These findings can inform the development of more efficient and reliable heat exchangers for various applications. [ABSTRACT FROM AUTHOR]

Air-side fouling of heat exchangers has received much attention in the literature through both experimental and modeling studies. However, no standards currently exist that define a test procedure to experimentally foul heat exchangers and evaluate their performance in a fouled condition. This is evidenced by the variation in test procedures, fouling agents, metrics, and operating conditions reported. Based on a detailed review of past experiments, a standardized, repeatable test procedure is proposed that allows cross-comparison of data acquired in different laboratory facilities. The procedure generates time-resolved information on foulant buildup on the finned heat exchanger and the consequent impact on heat exchanger performance. Various fouling agents used in the literature and fouling deposits observed on field-installed heat exchangers are compared, and a representative fouling agent is recommended. A method is proposed to quantify heat exchanger fouling and metrics identified to quantify heat exchanger performance before and after fouling. This is the first of two companion papers on an experimental investigation of air-side fouling of heat exchangers. The companion paper presents experimental data obtained by implementation of the proposed procedure to investigate fouling of a finned microchannel heat exchanger. [ABSTRACT FROM AUTHOR]

• A multi-objective optimization method for COP and volumetric heating is proposed. • The performance of ternary refrigerants in different systems is analyzed. • Optimal solution under the set conditions is obtained. • The proposed method has high scalability. Vapor injection two stage cycles proved to be an effective method for heat pumps to cope with extreme conditions. COP (coefficient of performance) and volumetric heating capacity usually contradictory as two important indicators. It is necessary to provide system designers a variety of solutions to select to adapt different working conditions. This paper proposed a method to obtain the Pareto front of COP and volumetric heating capacity by exhaustive method and non-dominated sorting. This paper sets a series of equivalent conditions and range conditions to evaluate the performance of different combinations. Variables include system form, types of mixed refrigerants, mixed refrigerants components, ratio of injection. The results show that IHX (internal heat exchanger) and ternary refrigerants have advantages under the set conditions. An example and an extended case study were used to illustrate the application method of a series of optimization schemes in the Pareto front. The method proposed in this paper is not limited to the set working conditions and can be applied to other working conditions. At the same time, the Pareto front is not limited to one solution, and the system designer can select the best solution according to the actual needs from a series of solutions. [ABSTRACT FROM AUTHOR]

• Spray evaporation of L-GWP refrigerant on enhanced tube bundles. • Influence of miscible oil on heat transfer performance. • Foaming characteristics of refrigerant/oil mixtures. • Impact of lubricant oil on spray evaporation of enhanced surfaces. • Comparison between square and triangular configurations of tube bundles. Lubricating oil is used in compressors of vapor compression-based refrigeration systems. The oil that escapes from the compressors inevitably circulates in the heat exchangers, affecting their thermal performances. This paper presents a comprehensive study on the impact of lubricating oil on the heat transfer performance of spray evaporator tube bundles used in air conditioning and refrigeration systems. The research includes valuable data on the heat transfer coefficient of tube bundles when subjected to refrigerant and lubricant mixtures of R134a (HFC) and R1234ze(E) (HFO) with synthetic polyolester (POE) lubricant. The investigation delves into the influence of three crucial factors: heat flux, oil circulation ratio (OCR), and saturation temperature. The saturation temperature was examined across a range of −6.7 to 10 °C (20–50 °F), while the heat flux was varied from 6 kW/m2 to 17 kW/m2. OCR was controlled at 1.2 %. The effects of oil on spray evaporation of R1234ze(E)/POE mixture on enhanced tube bundle was studied by varying tube surface structure and bundle layout, that is, square and triangular configurations. The bundle heat transfer coefficient decreased by at least 10 % when lubricant was present in the mixture but, in some cases, varied by 50 % or more compared to the performances with refrigerant only (i.e., no lubricant). The data suggested that the oil penalized the heat transfer coefficient of the bottom tube rows of the bundle, which in many conditions, starved and experienced a partial-dry out state. Externally enhanced boiling-type tubes were more sensitive to the presence of oil than externally enhanced condenser-type tubes. [ABSTRACT FROM AUTHOR]

In a vapor compression refrigeration system, the oil enhances the efficiency of the compressor but impairs the performance of the heat exchanger. This paper focuses on the experimental study of the influence of oil quantity on the operational stability of CO 2 air conditioning system. The operational stability of the system under different oil charge quantity and operating conditions is studied. The experimental results indicate that when the oil circulation rate increases to a certain value, the adjustment of electronic expansion valve (EXV) opening or the compressor speed will cause system fluctuation and the outlet of evaporator gets close to no superheat. The critical oil circulation rate of the experimental system is about 9.2 %. The analysis of the fluctuation shows that it becomes more severe with the increasing of the refrigerant mass flow rate. When the mass flow rate increases from 210 kg∙ h −1 to 250 kg∙ h −1 at 7000 r min−1 of compressor speed, the fluctuation frequency increases from 0.072 Hz to 0.095 Hz. The occurrence of unstable phenomena has remarkable impact on system performance, the maximum difference of this system in instantaneous cooling capacity and COP during the fluctuation period is respectively 0.5 kW and 0.13. [ABSTRACT FROM AUTHOR]

In this paper, an experimental study of a magnetic refrigerator was carried out to identify static and dynamic models, which can be used for proposing and tuning control strategies. For the experimental evaluation, a 40 liter thermally insulated compartment was coupled to a magnetic refrigeration system equipped with an active magnetic regenerator. The influence of fluid flow rate, blow fraction, and magnetic cycle frequency on the temperature behavior inside the refrigerated compartment and on the power consumption of the system was evaluated. The results of the characterizations show that it is possible to reach equivalent cabinet temperatures by changing the flow rate of the thermal fluid or the cycle frequency. However, the former has a greater influence on the temperature of the cabinet, and the latter has a greater impact on the energy consumption and is associated with slower transients. Additionally, the temperature of the cabinet at steady state showed a parabolic behavior with respect to the fluid flow rate, demonstrating that there is an optimal flow rate for a given operating condition. • A magnetic refrigerator is experimentally evaluated at different operating conditions. • A thermally insulated compartment and real heat exchangers are employed. • Control-oriented dynamic and static models based on experimental data are provided. • Fluid flow rate, blow fraction, and magnetic cycle frequency are manipulated. • Temperature of the cabinet and energy consumption are measured. [ABSTRACT FROM AUTHOR]

• Novel mathematical model to predict the heat transfer rate of skin condensers. • Experimental validation with eigth hot-wall condenser designs. • Adhesive tape has the strongest impact on the heat exchanger performance. This paper comes out with an investigation of the thermal performance of skin-type hot-wall condensers for household refrigeration applications. Eight different hot-wall condensers were manufactured and installed in identical refrigerators. The condenser samples were constructed by varying the following design parameters: (i) adhesive tape (aluminum or polyethylene), (ii) tube O.D. (4.00 or 4.76 mm), (iii) total length (10 or 11.5 m) and (iv) condenser positioning. The refrigerators were placed inside a climate-controlled chamber and tested in steady-state conditions. A mathematical model that takes into account the heat transfer not only to the surroundings but also to the refrigerated compartments was also put forward. The model predictions were compared to the experimental data when deviations within the ±10% bounds were achieved. It was found that the adhesive tape thermal conductivity and the tube-outer sheet contact area have the strongest impact on the condenser performance. It was also found that the polyethylene tape increases the condensing pressure, due to its low thermal conductivity and thus to the smaller useful heat transfer area. It was thus concluded that the refrigerator foaming process must be carried out with an extreme care to avoid any negative impact on the tube-outer sheet contact resistance. Finally, it was noted that the heat transfer to the refrigerated compartments is directly affected by the condenser positioning and the thermal insulation thickness. [ABSTRACT FROM AUTHOR]

• The exergy efficiency of the hybrid absorption cycle with Ammonia-Water is investigated. • The exergy efficiency of the hybrid absorption cycle can be 0.20% than the ARS cycle. • The hybrid absorption generation temperature is 24 °C lower than the ARS cycle. • The evaporation temperature can be reduced around 19 °C when generator pressure of 5.94 bar is used. • The Generator, absorber and the compressors are the main components that increase the relative exergy loss. This paper presents an exergy analysis considering different operating conditions of an alternative configuration of an ammonia-water hybrid absorption refrigeration system, where part of condensation heat is recovered and supplied to generator (RCHG-ARS). In order to identify and quantify the irreversibilities in each component of the cycle and its variations, the exergy efficiency and the relative exergy loss of the system were evaluated. Moreover, the results obtained were compared with those obtained by the conventional ARS cycle. Furthermore, the effect of the internal heat exchanger effectiveness on the exergy efficiency and relative exergy loss was analyzed. The results indicate that the generation temperature and the exergy efficiency of the RCHG-ARS cycle decrease in 24 °C and 30%, respectively, it in comparison to ARS cycle at the conventional generation pressure (11.67 bar). However, to decrease the generator pressure at 5.94 bar, the evaporation temperature for the RCHG-ARS decreases in 19.3 °C and the exergy efficiency can be 0.20% higher than the obtained with the ARS cycle. It is also revealed that the exergy efficiency for the RCHG-ARS cycle become more significant at lower condensing and evaporating temperatures as well as lower generator pressures and higher internal heat exchanger effectiveness respect to ARS cycle. Moreover, the minimum values of relative exergy losses are obtained at higher generator temperatures or lower evaporating and condensing temperatures in the RCHG-ARS cycle. Finally, it can be concluded that the main components that increase the relative exergy destruction in the RCHG-ARS cycle are the generator, absorber and the compressors. [ABSTRACT FROM AUTHOR]

• Dehumidification performance of polymer composite desiccants were compared. • PVA/PEO/LiCl ratio and thickness with the greatest performance were mastered. • Cycling performance of PVA/PEO/LiCl remains unchanged after numerous tests. • Mechanism of dehumidification of PVA/PEO/LiCl has been discovered. Composite desiccants have led to a growing interest because of their eco-friendliness, sustainability, and excellent dehumidification properties. In this paper, we present a method for constructing a dehumidification composite of PVA/PEO/LiCl that is eco-friendly and recyclable. The results show that PVA/PEO/LiCl has better dehumidification performance than SG, SG/LiCl, PVA/PEO, and PVA/PEO/CaCl 2 as coated heat exchangers desiccants. The addition of LiCl caused the internal hydrophilic polymeric hygroscopic carriers to work in concert with the membrane porosity. This resulted in a considerable increase in the membrane's hygroscopic rate and capacity, as well as improved mechanical qualities. As the concentration of LiCl was increased, the surface sealing of the PVA/PEO polymer improved dramatically, and the shape of the polymer-formed film changed from loose to firmly packed. The structural shift toward densification boosted the strength of the co-blended dehumidified films. With a PVA/PEO/LiCl ratio of 1:1:2.5 and the coating thickness of 10–15 µm, the greatest dehumidification effectiveness is achieved. During five subsequent sorption and desorption procedures, the sorption of PVA/PEO/LiCl remained between 70 and 75%, demonstrating its outstanding reusability and cycling stability. [Display omitted] [ABSTRACT FROM AUTHOR]

• The environmental and thermophysical characteristics of the investigated working fluids were comprehensively evaluated. • The research focuses on mapping the minimum superheat for various selected refrigerants. • The favourable impact of mapping the minimum superheat on both energetic and exergetic performance. • The potential replacements for HFC-245fa and HFC-365mfc are HCFO-1233zd(E) and HFO-1336mzz(Z), respectively. Suitable low global warming potential (GWP) refrigerants that conform to F-gas regulations are fundamental to the operation and future development of high-temperature heat pumps (HTHPs) used for industrial processes and waste heat recovery. This paper presents the results of a theoretical simulation to investigate a range of low-GWP refrigerants and their suitability to supersede refrigerants HFC-245fa and HFC-365mfc. A steady-state thermodynamic model of a single-stage HTHP with an internal heat exchanger (IHX) was developed to assess system cycle characteristics and performance at temperature setpoints at 60 and 70 °C heat source, 90 and 140 °C heat sink, at 30 and 70 K lift. This study focuses on energetic and exergetic efficiencies within the system and the impact of regulating superheat to optimise performance. Based on energetic and exergetic theoretical results, a trade-off between COP, VHC, and exergetic efficiency indicates HCFO-1233zd(E) and HFO-1336mzz(Z) as the most likely replacements for HFC-245fa and HFC-365mfc respectively. The refrigerant HC-601, followed by HFO-1336mzz(Z) and HCFO-1233zd(E), exhibited the lowest exergetic destruction within test conditions. Mapping the minimum superheat indicated optimum performance for HCFO-1233zd(E) between 5 and 8 K and HFO-1336mzz(Z) between 17 and 22 K, depending on temperature lift. Validation of the theoretical results with experimental data indicates that simulated COP closely matches empirical values. This work provides a method to optimise refrigerant selection in HTHPs based on operational indicators to maximise overall system performance. [ABSTRACT FROM AUTHOR]

• A BOG buffer tank and a control strategy is proposed to optimize the liquefaction process and can maintain the stability of the system. • The inlet gas volume increases by 12.2% and system specific energy consumption is reduced by 5.7% compared with the reference process. • The results of the simulation data provide an identifiable scale demarcation for the control strategy. • The optimal intake ratio exists which can be more responsively adjusted and adapted to process fluctuations in actual operation. The re-liquefaction of BOG is a necessary part of the safety and security of LNG cargo ships. And the volatility of BOG will fluctuate with the sea conditions of ship transportation, so how to deal with the fluctuation of the re-liquefaction system and improve the liquefaction capacity is a problem that cannot be ignored in the operation of the liquefaction system; In this paper, a BOG buffer tank is added to the Reverse Brayton Process. Moreover, a control strategy is proposed to optimize the liquefaction process. The proposed four possible operating conditions' efficiency,the specific energy consumption (SEC), liquefaction rate and main heat exchanger parameters were simulated and analyzed by using Aspen HYSYS software. According to the results, the specific energy consumption of the basic re-liquefaction process is 1.329 kWh/kg LNG , which is 5.7% less than that of the reference process; In the study of flow fluctuation, after adopting the control strategy, the specific energy consumption of the whole process is 1.29 kWh/kg LNG , with 15.6% efficiency and 99.5% liquefaction rate. The whole system can be maintained at a low specific energy consumption level when the feed gas volume is fluctuating. The analyses of various BOG composition and the parameters of the main heat exchanger show that the regulation of the inlet gas volume can make the primary re-liquefaction process flexible and maintain the heat exchanger in a good working condition; Even if the system is in a worse working condition, the inlet gas volume can be improved by 12.2% compared with the reference process. [ABSTRACT FROM AUTHOR]

An experimental test protocol for simulating the air-side fouling of heat exchangers, as well as metrics to characterize the extent of fouling undergone by the heat exchanger and its performance in clean and fouled conditions, were proposed in a companion paper. In this study, the air-side fouling of a finned microchannel heat exchanger is experimentally investigated according to the proposed protocol. Key test parameters influencing heat exchanger fouling are identified based on studies in the literature, and their impact is experimentally investigated. The effectiveness of in situ cleaning methods is also experimentally evaluated. Transient measurement data and photographs taken during the fouling process reveal the nature of fouling, while steady-state data quantify the degradation in heat exchanger performance due to fouling. This two-part study defines a generalized experimental approach to enable characterization and comparison of heat exchanger surfaces on a standardized basis, and provides detailed experimental data for modeling heat exchanger fouling that includes all necessary information to allow for model validation. [ABSTRACT FROM AUTHOR]

This paper discusses the design procedure and first results of a large-scale magnetic air-conditioning system prototype demonstrated in a relevant environment compatible with the Technology Readiness Level (TRL) 6. The prototype, which contains all components of a functional cooling system, was experimentally evaluated in an especially developed calibrated calorimeter designed according to international technical standards for testing split-type air-conditioning units. The proposed experimental analysis is one of the first to quantify the impact of external losses associated with the heat exchangers on the cooling system performance. For outdoor and indoor temperatures fixed at 35 and 27 °C, respectively, the coefficient of performance disregarding thermal, flow and mechanical losses beyond the limits of the active magnetic regenerator was 5.72, corresponding to a second-law efficiency of 20.7%. When these losses and other essential operating variables, such as the heat exchanger fan power and the valve power consumption, are included, one-order-of-magnitude reductions are observed in the performance parameters, indicating that they should not be neglected in realistic assessments of not-in-kind cooling technologies. • Design and evaluation of a TRL-6 magnetic air-conditioning prototype is presented. • Tests conducted in a calibrated calorimeter for split-type air-conditioning units. • Impact of heat exchanger losses on the performance parameters is quantified. • The AMR coefficient of performance and second-law efficiency are 5.72 and 20.7%. • Internal and external losses are highly detrimental to the system performance. [ABSTRACT FROM AUTHOR]

• A desiccant coated humidification/dehumidification heat pump system is established. • The feasibility of the system has been theoretically analyzed. • The COP of the system for humidification and dehumidification are both more than 5. • The performance of the system under different operation conditions is tested. Widely used vapor-compression heat pump system can provide both heating and cooling capacity, considering it cannot realize humidification and efficiency of condensation dehumidification is relatively low. In this paper, a fresh air humidification and dehumidification system based on desiccant coated heat pump is established. In which the condenser and evaporator of the heat pump system are replaced by desiccant coated heat exchangers. By using the 4-ventilation valve, the system is very compact. The feasibility of humidification and dehumidification of the system has been theoretically analyzed. The performance of the system under GB Shanghai summer conditions and GB winter conditions is experimentally verified. The results show that the system can realize dehumidification and humidification in summer and winter efficiently and the COP under these two conditions is 5.16 and 5.06 respectively. The dehumidification and humidification rates of the system in the two chosen conditions are 2.99 L h −1 and 1.99 L h −1 respectively. The results also show that under Shanghai summer conditions, by changing the compressor frequency, the system can get a dehumidification of 2.40 L h −1 to 3.05 L h −1 with a COP of 7.80 to 3.56. The optimized switchover period is 6 min. [ABSTRACT FROM AUTHOR]

This paper presents the design of a testing facility at Oklahoma State University that enables performance testing of commercial-sized air to refrigerant heat exchanger coils. The closed-loop facility contains the necessary equipment to condition the air supplied to the tested coil. This includes the glycol conditioning system for the air conditioning heat exchangers, fans, electric heaters, and a steam humidification system. The design is compliant with multiple ASHRAE standards; the airflow measurement apparatus is compliant with both, Standard 41.2 (1987 and 2018). The test section's cross section is 2.13 m wide by 2.44 m tall with a maximum airflow rate of 3.78 m3 s−1. The thermal model presented in this paper predicts a maximum capacity of 80.9 kW for air heating coils (e.g. refrigerant condensers) at an air inlet temperature of 19.4 °C. The specified air flowrate and facility's maximum flowrate limit the capacity of air cooling (e.g. refrigerant evaporators) to a range of achievable values. The facility will operate over an air temperature range of −17.8 °C to 60 °C. Future work includes completion of construction of the facility including physical assembly, assembly of psychrometers, and commissioning of the developed facility control software. [ABSTRACT FROM AUTHOR]

• Experimentally measured falling-film evaporation heat transfer coefficients. • Effects of temperature, flow rate, tube spacing, tube row, and heat flux discussed. • Newly observed heat transfer coefficient trend that is not seen in round tubes. • New trend due to film breakdown quantified and supported by force balance. An experimental study on falling-film evaporation on horizontal rectangular tubes, the external geometry of extruded flat, microchannel tubes, was conducted. Previous falling-film evaporation studies have focused on horizontal and vertical round tubes, and the usage of microchannel tubes provides a promising new heat exchanger design option. The test sections used in this study have external dimensions of 203 × 1.42 × 27.4 mm (length × width × height) and were tested with water as the refrigerant. Heat transfer coefficients were measured for saturation temperatures from 10 to 30°C, test section spacings from 5 to 15 mm, heat fluxes from 10 to 20 kW m−2, and film Reynolds numbers of 48 to 544. This paper, Part I, reports the experimental methodology and the results, while in the companion paper, Part II, models for predicting heat transfer are presented. [ABSTRACT FROM AUTHOR]

• The discharge temperature of LTC increased first and then decreased during the pull-down process. • The maximum value of LTC discharge temperatures increased with the rise of startup temperature. • The volumetric efficiency of HTC compressor deteriorated more obviously than that of LTC during the pull-down process. This paper presents an experimental investigation on the pull-down performance of a -80°C ultra-low temperature freezer. Experiments were conducted in a two-stage cascade refrigeration system with an inner recuperative heat exchanger in low temperature cycle (LTC). In addition, the pull-down performances of the freezer were discussed on different startup temperatures ranging from -20°C to 24°C when the ambient temperature was fixed at 26°C. In this paper, the cooling process was divided into early start-up phase, rapid cooling phase and stable cooling phase, and it was found that thermal parameters changed obviously with the startup temperatures, especially in the rapid cooling phase. The experimental results showed that there were maximum discharge temperatures of LTC compressor during pull-down process, and the maximum values increased with the rise of startup temperatures. When the startup temperature was -20°C, the maximum discharge temperature of LTC compressor was 8.1% (11.6°C) lower than that when the startup temperature was 24°C. As two factors affecting the discharge temperature of LTC significantly, the superheating degree was the main influencing factor in rapid cooling phase, while the decrease of the evaporation and condensation pressures had a more significant effect in the stable cooling phase. Besides, the volumetric efficiency of high temperature cycle (HTC) compressor deteriorated more obviously during the pull-down process, and had higher potential for performance improvement compared with that of LTC. [ABSTRACT FROM AUTHOR]

• A Stirling heat pump with a temperature gradient in the heat exchanger is studied. • The heat exchanger with a temperature gradient achieves cascade output of energy. • Negative effect due to the mix of gases at different temperatures is insignificant. • Considering economy and efficiency, single large heat pump is also a good choice. The free-piston Stirling heat pump (FPSHP) is efficient, environmentally friendly and has a wide available temperature range, making it a promising device for building heating. The temperature of the external heat-carrying fluid is different at the inlet and outlet of the heating heat exchanger, so there is a temperature gradient in the heating heat exchanger. The larger the scale of the FPSHP, the more pronounced the temperature gradient. However, the influence of such a temperature gradient on system performance has not been investigated so far. This paper therefore presents a detailed study of the FPSHP with a circumferential temperature gradient in the heating heat exchanger. The calculation results show that a cascade output of energy can be achieved when there is a circumferential temperature gradient in the heating heat exchanger. With the same outlet temperature, the larger the temperature difference between the inlet and outlet of the heating heat exchanger, the higher the COP. However, due to the mixing of internal working gases with different temperatures in the compression space, part of the high temperature heat is directly transferred to the low temperature module and output at a lower level, adversely affecting system efficiency. Fortunately, this adverse effect is found to be insignificant when compared with multiple independent small FPSHPs without circumferential temperature gradients. Considering the higher investment cost of multiple small FPSHPs, a single large FPSHP is also a good choice for large heating capacity demand. [ABSTRACT FROM AUTHOR]

• Analyzing the NH3/CO2 cascade refrigeration system with ejector with advanced exergy method. • Put forward the improvement direction of each component. • The low-temperature compressor has great potential to improve itself. • The ejector fully reduces the expansion loss, and the expansion process has no areas for improvement. Aiming at the NH 3 /CO 2 Cascade Refrigeration System with ejector (CRSE), this paper establishes the mathematical model of the refrigeration cycle and compares it with the literature based on thermodynamic analysis. Advanced exergy analysis on the CRSE is carried out and the exergy destruction of components in the system are divided into two ways: avoidable/unavoidable exergy destruction and endogenous/exogenous exergy destruction, and combine the two ways to clarify the types and values of exergy destruction in components, which provides a direction for the improvement of the system. The analysis shows that the avoidable exergy destruction of the system accounts for 38.674% of the total exergy destruction, and the proportion of endogenous exergy destruction and exogenous exergy destruction in the total exergy destruction of the system is 77.772% and 22.228% respectively. The improvement potential of the system is very high, and the low-temperature compressor, condenser, and intermediate heat exchanger should be paid special attention to in the improvement. [ABSTRACT FROM AUTHOR]

• Refrigerant charge amount needed throughout the year is nearly constant in GSHP. • In low-charge systems, the refrigerant dissolved in oil is a substantial amount. • Heating the crankcase reduces the tot refrigerant charge amount needed in the system. • Domestic space heating with less than 150 g of R290 is achievable. This paper presents experimental results from a brine-to-water heat pump used for space heating at low temperature, with a low refrigerant charge of R290 (propane). Performance and refrigerant distribution were analysed in every test condition studied. Performance results show a declared heating capacity of 9.5 kW , obtaining a specific capacity (C c) of 48.7 kW rm kg − 1 and a seasonal coefficient of performance of 4.01. In terms of refrigerant distribution, at the nominal point, 41.5% of the refrigerant is located in the compressor, mainly dissolved in the oil, and the rest is separated almost evenly in both heat exchangers, 23.9% in the condenser and 27.8% in the evaporator; having the lines and accessories (pneumatic-ball valves) the remaining 6.8%. Due to this fact, reducing oil solubility by heating the crankcase or increasing the superheat (SH) has a positive impact on the refrigerant charge reduction, but it also affects the coefficient of performance. [ABSTRACT FROM AUTHOR]

• Three mean-line methods for the design of turbo-coolers are evaluated. • A coupling model is established to predict the performance and volume size. • Matching characteristics of three coaxial-impellers was quantitatively described. • SS-CR, PR and VC approach gives 84.0%, 83% and 80.9% efficiency respectively. • The system volume is the lowest at the aspect ratio of 1.5 of the heat exchanger. High-speed turbo-cooler of coaxial expander-compressor is a promising solution for reverse Brayton cycle refrigeration systems to improve compactness, power recovery and efficiency. The performance curves of coaxial impellers are crucial for their matching characteristics as well as the system efficiency under off-design conditions. In this paper, three mean-line methods for the design of high-speed turbo-coolers are evaluated by comparing the matching characteristics of three coaxial-impellers, including the specific speed and characteristic ratio (SS-CR) approach, the power ratio (PR) approach and velocity coefficient (VC) approach. A coupling model is established to predict the off-design performance of turbo-cooler and the system size. The results show that the rotating speed and mass flow rate by all three methods lie within 2% of each other. At the design point, SS-CR approach gives 84.0% expander efficiency compared to 83.0% by PR approach and 80.9% by VC approach, but the PR approach has better performance at speeds below 35 krpm. The system volume is the lowest at the aspect ratio of 1.5 of the heat exchanger designed with all the three methods. However, SS-CR gives 6.0% advantage in system volume 5.6% over VC approach and 3.7% over PR approach. When the cooling capacity is controlled at 2000 W, SS-CR approach gives 79.3% expander efficiency compared to 78.9% by PR approach and 76.4% by VC approach. [ABSTRACT FROM AUTHOR]

• The performance of transcritical CO 2 ejector-expansion heat pump is investigated. • The performance is compared with emphasis on the exergy analysis. • The exergy destruction of the throttling value is significantly reduced. • The cost rate with exergy destructions in throttling valve is reduced by 86.80%. • The specific exergo-environmental cost is reduced with the proposed system. Nowadays, the transcritical CO 2 heat pump water heater is widely used for hot water production. However, the larger pressure difference of the basic system leads to the huge throttling loss, which will significantly worsen the system performance. The ejector-expansion method accompanied by the internal heat exchanger is an effective way to reduce the throttling irreversibility. Nevertheless, the CO 2 ejector-expansion system is seldom studied with the exergy analysis, including the exergo-economic and exergo-environmental analysis. In this paper, this system is theoretically evaluated from comprehensive perspectives with emphasis on the exergy analysis and the sensitivity analysis of ejector cost and electricity price is also conducted. Results show that with the ejector and internal heat exchanger, the coefficient of performance is improved by 7.38% and pressure ratio will reduce to 2.66 with the ejector efficiencies of 0.90. Furthermore, the exergy destruction of the throttling value could be reduced by 95.38%-98.76%, leading to the increased exergy efficiency. The largest temperature matching factor is achieved by the basic system with the internal heat exchanger. The cost rate of the throttling valve and cost per unit of exergy production is reduced by 85.65% and 0.42 CNY⚫MJ−1, respectively. Moreover, the maximum pollutant emission reduction is 115.41 tons, which results in the lowest specific exergo-environmental cost of 1.95 CNY⋅(kW⋅h)−1 in the proposed system. [ABSTRACT FROM AUTHOR]

• Heat transfer and pressure drop calculations of fins at multiple velocities. • Porous media model simplifies the flow resistance characteristics of the fins. • Interpolation and fitting for functions between heat transfer, velocity and height. • Multi-level performance evaluation combining air distribution and fins. • Slit fins perform better than wave fins and plain fins. The heat transfer capacity of air conditioning outside units is inextricably related to fins and airflow distribution, and research on the combination of the two is lacking. To analyze the fins and fin-and-tube heat exchanger, three-dimensional numerical models of the fins and flow field of the outdoor unit are created in this paper. On the one hand, the heat transfer and fluid flow characteristics of the fins are quantitatively evaluated and analyzed. On the other hand, the resistance coefficients of the porous media are numerically fitted to study the uniformity of the air velocity distribution on the surface of the heat exchanger. Two indexes, the total integrated heat capacity and JF-factor, are applied to evaluate the performance of the fins and fin-and-tube heat exchanger on the overall and specific levels, respectively. The results indicate that slit fins perform better than wave fins at heat transfer, whereas plain fins perform the worst. However, improved heat transfer performance results in increased pressure drop, which has a two-sided effect. It causes both a decrease in air volume flow rate and an increase in air distribution uniformity. Comparing the comprehensive performance of different fins, JF-factors of slit fins and wave fins are on average 20.95% and 14.36% higher than plain fins. In terms of overall performance, slit fins and wave fins have higher integrated heat exchange capacities than plain fins by 18.0% and 9.33%, respectively. [ABSTRACT FROM AUTHOR]

• Six supercoolers with icephobic coatings were tested for ice slurry generators. • Three different chemical families were assessed at different mass flow rates. • Coated supercoolers achieve average supercooling degrees in the range of 3 K to 4 K. • Best icephobic coatings increase the supercooling degree by 30 % to 40 %. A supercooler is a heat exchanger that allows to supercool water avoiding ice nucleation on the heat exchanger surface. In this paper, six supercoolers with icephobic coatings were tested for supercooling ice slurry generators. As reference, a supercooler without any coating was used. For coating comparison, three different chemical families were tested at different mass flow rates. Experiments of coated heat exchangers show average supercooling degrees in the range of 3K to 4K. All icephobic coatings improved the supercooling degree respect to the reference non coated case by 7% to 44% at 1000 kg h − 1 mass flow rate with supercooling powers up to 5 kW. For a mass flow rate of 2000 kg h − 1 a supercooling power of about 6kW has been reached for five coatings at a temperature of − 2.5 ∘ C and hold constant for a time span of 3h. Some coatings could reach lower temperatures at this mass flow, but the experimental setup was limited to this supercooling power. The two coatings performing best in the heat exchanger tests were both from the family class hybrid organic-inorganic silane sol-gel with improvements in the degree of supercooling respect to the reference uncoated heat exchanger of 30% to 40%. The supercooling degrees achieved in the present work are up to two times larger compared to results published in scientific journals until this date using industrially relevant heat exchangers. [ABSTRACT FROM AUTHOR]

Regulatory changes worldwide are requiring air conditioning and heat pump equipment manufacturers to switch refrigerants to alternatives with reduced global warming potential (GWP). Design of air-to-refrigerant fin-and-tube heat exchangers (FTHXs) is critical to overall performance of this equipment and is not currently well understood with low-GWP refrigerants. This paper presents a simulation study exploring the impact of design changes for a FTHX used in an indoor residential evaporator originally designed for R410A. Simulation results are used to generate design guidelines for FTHXs using HFO refrigerants R1234yf and R1234ze(E), evaluated using cooling capacity, refrigerant-side pressure drop and ratio of the two as critical metrics. When evaluated at identical operational conditions, with fixed refrigerant outlet superheat, it was found that for a fixed slab size, the FTHX was most sensitive to number of circuits, followed by tube diameter, number of tubes in the FTHX, and fin density. For the low-GWP HFOs, circuitry modifications led to maximum increase in FTHX capacity of 20%, and maximum decrease in refrigerant-side pressure drop of 88.6%. Based on simulation results, it is concluded that additional coil face area is required to match both capacity and refrigerant pressure drop, compared with R410A, this is demonstrated with two proposed FTHX designs. One design prioritizes increased FTHX capacity, and the other a reduction in refrigerant pressure drop. All these results culminated in design guidelines for HFOs that suggest it will be challenging to achieve both the same capacity and pressure drop characteristics as R410A without modifications to FTHX size. [ABSTRACT FROM AUTHOR]

• An ammonia-water absorption refrigerator without distillation column is proposed. • Such cycle can be applied under a very small temperature lift applications. • COP of such cycle is enhanced by 42.6% through heat exchanger network analysis. • The improvement in COP is the combination of different internal heat recoveries. • The payback period of the cycle with optimum COP is 4.28 years. The water content in the refrigerant may undermine the cooling effect in ammonia water absorption refrigeration systems. Distillation column is normally implemented. The use of distillation columns, however, may increase system complexity, especially in practical operation. In fact, the distillation column can be eliminated under particular operating conditions. This paper presented an ammonia water absorption refrigeration cycle without distillation column, which is replaced by two separators and one heater. The resulting vapor is nearly pure with an ammonia mass fraction of 0.998. Heat exchanger network analysis is carried out for searching all the potential heating-saving possibilities. The thermal performance was improved by the combination of solution recirculation and absorption heat recovery, and the results indicated that the thermal performance has been increased by 42.6% than that of the basic cycle at certain working condition. The proposed cycle has limited ranges of working conditions operated under small temperature lifts,and the payback period is 4.28years. [Display omitted] [ABSTRACT FROM AUTHOR]

• The theoretical model of transcritical CO 2 heat pump with IHX was developed. • The effect of IHX with different water temperature was investigated. • The influence of IHX length with fixed water temperature was studied. • The conflict between discharge temperature and COP improvement was researched. Many studies have investigated the effect of internal heat exchanger (IHX) on the transcritical CO 2 heat pump (TCHP) system. However, little information is available regarding the length of IHX under various water temperature conditions. In this paper, the theoretical model was established to compare the heating performance with and without IHX. A prototype TCHP test-rig was conducted to validate the model, and the experimental results showed a good agreement. The results show that the effect of IHX on the coefficient of performance (COP) is related with inlet water temperature. The IHX is beneficial for the COP with higher temperature 40 °C. Meanwhile, the optimal discharge pressure is reduced by more than 1 MPa with the IHX length increasing from 0 m to 4 m. Although the longer length is profitable, the discharge temperature will tend to rise 25 °C and easily exceed 145 °C when the ambient temperature depresses to 2 °C. Therefore, the length in this paper is recommended to be approximately 2.5 m considering the conflict between the discharge pressure and discharge temperature. [ABSTRACT FROM AUTHOR]

Ammonia/water absorption–resorption refrigeration systems are an interesting technological alternative to conventional absorption systems thanks to their additional degree of freedom, which makes operation more flexible and can significantly reduce the typically high working pressure, so cheaper assembly materials can be used. This paper aims to review the theoretical and experimental works carried out so far on these systems, paying special attention to the experimental plants currently operating and recent progress in improving system control. This paper reports how to determine the operational pressure range when the temperatures of the main components of the ammonia/water single-stage cycle are fixed and how the pressure range changes when these working temperatures are varied. As a result of the theoretical study of the base case selected, the cycle performance was evaluated without rectification after the generator in order to reduce the size and cost of the system without reducing the performance. Finally, it should be pointed out that the effectiveness of the resorption solution heat exchanger plays an important role in the performance and operating conditions of the cycle because the cycle cannot work below certain values of effectiveness. [ABSTRACT FROM AUTHOR]

• Four kinds of NH 3 /LiNO 3 +H 2 O solutions with different water mass fractions was selected. • A prototype of an absorption refrigeration system driven by exhaust heat was introduced. • Experimental performance evaluation of varying operating conditions was conducted. • Experimental results were compared to evaluate the influences of water mass fraction. • Large water mass fraction leads to good performance and low required driven temperature. Regarding the working fluids of absorption refrigeration systems, compared with the binary ammonia/water or ammonia/salt solution, the ternary ammonia/lithium nitrate+water solution has some advantages including small viscosities, good heat-transfer efficiency and low crystallization risk. In this paper, four kinds of NH 3 /LiNO 3 +H 2 O solutions with different water component proportions are selected as working fluids, which is No.1(9.6%) < No.2(18.6%) < No.3(27.6%) < No.4(36.9%), and the experimental performance evaluation was conducted on a prototype of absorption refrigeration system driven by exhaust heat. First, experimental tests were carried out with varying operating conditions to evaluate their influences on performance parameters; the detailed experimental parameters operating with No.2 and No.4 working fluids are presented, and the reasons of the experimental results are explained. Then, the experimental results of four ternary working fluids and binary working fluid were compared and analyzed to evaluate the influences of water mass fraction. Compared with NH 3 /LiNO 3 , the cooling capacities of No.1, No.2, No.3, No.4 increased by 10.6%, 17.8%, 24.8%, 26.4%, respectively; the logarithmic mean temperature difference of the solution heat exchanger decreased by 1.2 K, 2.0 K, 2.6 K, 3.9 K, respectively. The comparison results indicate that, for the working fluid with larger water mass fraction, it has better heat and mass transfer characteristics, the system can produce larger cooling capacities and has lower required driven temperatures of heat sources, but the overall coefficients of performance hardly changed with the water mass fraction. The experiments and comparison results provide significant references in practical applications for the selection of working fluids of the absorption refrigeration systems. [ABSTRACT FROM AUTHOR]

• Thermal characteristics in loop thermosyphon with serpentine tube heat exchanger are studied. • Wide filling ratios of 10.4%∼99% and input heat flux of 16.7∼266.7 W•cm−2 are examined. • Flow instability and transient response are studied. • The serpentine tube condenser provides a small thermal resistance. The loop thermosyphon, which has an excellent heat transfer performance, can effectively solve the problem of high heat flux heat transfer in outdoor high-power electronic equipment. In order to solve the problem of high heat flux heat transfer, the researchers focused on the new evaporator, but the impact of the condenser on the entire system cannot be ignored. This paper designed an air-cooling type loop thermosyphon with a condenser composed of a serpentine copper tube and corrugated aluminum fins. The experiment confirmed the serpentine tube condenser is a condenser with excellent performance. It can be used in conjunction with the loop thermosyphon to realize the 266.7 W•cm−2 heat flux transfer. Intriguingly, using a serpentine tube evaporator will bring about two kinds of flow instability, and the filling ratio is the key to restraining the flow instability. A high filling ratio is a suitable choice, which avoids flow instability and has a fast transient response. Moreover, the air temperature has little effect on the thermal resistance and the maximum temperature difference of the loop thermosyphon; therefore, this loop thermosyphon can be used in various weather conditions. However, extreme weather conditions (extremely high/ low temperature) may affect the performance of the loop thermosyphon. Consequently, the loop thermosyphon with serpentine finned tube condenser at an optimized high filling ratio is worthy of being employed in outdoor high-power electronic equipment. [ABSTRACT FROM AUTHOR]

• The refrigerant mass distribution in an air-to-water heat pump has been studied. • Experimental tests have been performed during cooling and heating operations. • A model, for charge prediction and performance estimation, has been described and validated. • The effect of the maldistribution in the finned coil heat exchanger has been addressed. • Existing void fraction correlations have been assessed against experimental data. This paper presents the results of refrigerant mass distribution measurements carried out in an invertible air-to-water heat pump. The unit works with refrigerant R32 and produces a nominal cooling capacity equal to 60 kW. Experimental tests have been performed during both cooling and heating operations using the quick-closing valves technique to isolate the charge within the components of the system. A mathematical model, developed to predict the refrigerant charge in the heat exchangers of the heat pump and its performance, is described and validated. The model includes a physical description of the heat transfer mechanisms in the condenser and in the evaporator. The model employs void fraction correlations to calculate the refrigerant mass in the two-phase regions of the heat exchangers. In the finned coil heat exchanger, the model can account for the effects of the airflow maldistribution, which has been also experimentally measured. When the not uniform air velocity is considered, the refrigerant charge in the heat exchangers is modelled within ±10%. The results of the model showed that the airflow maldistribution can increase the total refrigerant charge in chiller mode by 12.7% and decrease the performance of the system by 11%, compared to the case of uniform air velocity. [ABSTRACT FROM AUTHOR]

This paper presents the air-side pressure drop and overall heat transfer coefficient characteristics for serpentine-louvered-fin, microchannel heat exchanger in periodic frosting. It focuses on quantification of the effects of louver angle on heat transfer and pressure drop and on defrost and refrost times. Nine heat exchangers differing in louver angle and fin pitch (i.e. louver angle 15° to 39° and fin pitch 12 to 18 fpi) are studied. The face velocity was 3.5 m s −1 and inlet air relative humidity of 70% and 80%. The major finding of this paper is seen in the proof that it is necessary to perform experiments in several consecutive cycles to be able to understand the situation in operation of the heat exchangers and that geometry that is the best in first frosting test does not stay the best in real operation where multiple consecutive frosting occurs. [ABSTRACT FROM AUTHOR]

• A mechanistic model is developed to predict two-phase flow distribution in BPHEs. • Vapor distribution is predicted by the phase separation models of two flow stages. • Liquid distribution is predicted by the pressure balance. • The developed model is validated by the experimental results. This paper presents a mechanistic model that predicts two-phase refrigerant distribution in brazed plate heat exchangers. The modeling of two-phase distribution is based on the flow visualization in the inlet header, in which the flow regime is identified to be periodic and one cycle includes two or three flow stages, given a fully developed stratified/stratified-wavy flow is present at the heat exchanger entrance. To predict the vapor refrigerant distribution, this model considers two different flow stages in the inlet header: for the top corner vapor flow, the vapor refrigerant distribution is predicted by the balance between the suction force (radial pressure gradient) and the axial momentum of each phase; for the vapor jet flow, the vapor refrigerant is assumed to be distributed evenly among the channels. The time ratio of the two flow stages is estimated by assuming the irregular large-amplitude rolling waves in the feeding tube, which lead to the vapor jet flow in the header, to have the most dangerous wavelength and their traveling time can be related to the liquid slug frequency. The apparent vapor refrigerant distribution is the time average of the vapor distributions at the two stages. For the liquid refrigerant distribution, it is predicted by imposing an equal total pressure drop for each flow path. The proposed model is validated against the experimental results: quantified two-phase refrigerant distribution, heat exchanger capacities, pressure profiles in the headers, and surface temperature of the sidewalls. [ABSTRACT FROM AUTHOR]

• Two-phase flow visualizations of evaporator inlet under normal and microgravity. • Consistent increase of condensation temperature at the onset of microgravity captured. • Mild evaporator oscillations observed through five independent measurements. • Categorization of cycle response by refrigerant-side cooling capacity changes to flight maneuvers. Propositions of vapor compression cycles for microgravity applications have a long history. In contrast, experimental efforts and implemented applications are rare in general and in recent years even less than in the 1980s and 1990s. The paper shows experimental results from a vapor compression cycle on parabolic flights. The focus lies on the effects of varying gravity forces on the condenser and evaporator. Flow visualizations clearly showed a change to an annular or slug-annular flow regime at the evaporator inlet shortly after the onset of gravity. The discharge pressure was found to consistently increase sharply by 1–3% across widely varying operating conditions. Oscillations were observed in the evaporator for some parabolas due to microgravity. Overall, the cycle response to varying gravity based on the cooling capacity can be categorized in four different groups, which appear to have a dependence on both the mass flow rate and the evaporator superheat. [ABSTRACT FROM AUTHOR]

• The model obtained CO 2 outlet temperature with an absolute mean error of 1.14 K. • The heat transfer rate of fluted tube-in-tube gas cooler was generally higher. • The improvement of maximum COP could be up to 29.41% by using the fluted tube. • The COP improvement could be impaired to 6.30% at water inlet temperature of 40 °C. The utilization of fluted inner tube in tube-in-tube heat exchangers is an effective way to improve the heat transfer performance. Based on the segmented model, the performance of a fluted tube-in-tube gas cooler (FTGC) in the transcritical CO 2 heat pump water heater was studied in this paper. The simulation model was validated and adopted to numerically investigate the effects of different parameters on the heat transfer performance of the gas cooler, which was compared to the smooth tube-in-tube gas cooler (STGC) with the same geometric parameters. The results showed that the FTGC had higher heat transfer capacity in most conditions. The heat transfer rate first increased and then decreased as the CO 2 inlet pressure increased. The temperature difference at the pinch point in the FTGC was lower than that in the STGC. In the FTGC, the pinch point position was mainly influenced by the CO 2 inlet pressure and water inlet and outlet temperatures. Regarding the system COP, the improvement of maximum COP could be up to 29.41% by using the FTGC compared to STGC when the water inlet temperature was 10 °C and the water outlet temperature was 80 °C. However, when the water inlet temperature was 40 °C, the improvement of COP could be impaired to 12.98%. Through changing the designs of the FTGC with the fixed total heat transfer area, when the water inlet and outlet temperatures were 10 °C and 60 °C, the highest maximum COP could be obtained with the design used in the experiments. [ABSTRACT FROM AUTHOR]

• Drop-in comparison of R134a, R450A and R513A. • R450A heating capacity is within − 15.02 % and − 6.59 % compared to that of R134a. • R450A COP is within − 2.48 % and + 2.99 % with respect to that of R134a. • R513A heating capacity is within − 8.78 % and + 3.16 % compared to that of R134a. • R513A COP is within − 7.48 % and 2.22% with respect to that of R134a. This paper discusses the results of an experimental analysis of the use of R450A and R513A as drop-in alternatives to R134a in a water-to-water heat pump. The heat pump is tested setting the rotational frequency of the compressor shaft, the vapour superheating at the evaporator outlet and the mass flow rates and temperatures of the secondary fluids at the heat exchangers outlets to the desired conditions. Compared to R134a, the use of R450A leads to a capacity reduction, that lays in the range 6.59% to 15.02%, and to a COP variation, that is within the range − 2.48% to +2.99%, whereas with R513A the variations of the heating capacity and COP lay in the range − 8.78% to +3.16% and − 7.48% to +2.22% respectively. The increase in the rotational frequency of the compressor shaft to bring the heating capacity back to the R134a values is discussed too. The required increase is in the range 10% to 16% with R450A and 2% to 8% with R513A, but a COP reduction in the range 0.14% to 5.22% with the former refrigerant and 2.34% to 6.48% with the latter consequently arises. [ABSTRACT FROM AUTHOR]

• A correlation that considers the effects of ambient and structural parameters has been established to predict frost thickness growth on microchannel heat exchanger. • Effective blockage rate based on frost thickness calculation of heat exchanger surface was first put forward in this paper. • An effective guideline for electric vehicle heat pump system defrosting start-time optimization is provided. Automotive air conditioning heat pump systems draw increasing attention as an alternative to conventional electric heaters. Once the air temperature falls below the freezing point, frost begins to form on the surface of the evaporator. The accurate defrost strategies could avoid heat exchange efficiency reduction and prevent heating capacity from decaying, which are the main factors affecting the energy efficiency of electric vehicle heat pump systems. The frost characteristic on micro-channel heat exchanger of the electric vehicle heat pump system is tested, and a novel defrosting strategy which is based on the dimensionless analysis method is introduced too. Conclusions could be drawn as following: a correlation that considers the effects of ambient parameters and structural parameters has been developed to predict the frost thickness on fins with dimensionless analyses. Based on this correlation, the relative error and the RMS error between the experimental data and the predicted value are 8.44% and 12.54%, respectively; the effective blockage ratio (EBR), a parameter that accurately reflects the blockage ratio and directly linked to the system operation state, is first proposed. Through the analysis between the EBR and system heating capacity, it can be concluded that the optimal defrosting start-point is identified as the EBR reaches 30%. With this method, the defrosting start-time can be found accurately and effectively to avoid unnecessary defrost cycles, thus leading to a highly efficient operation and low energy consumption for the electric vehicle heat pump system. [ABSTRACT FROM AUTHOR]

Helical condenser coiled tube is widely used in refrigeration machines for water heating due to its higher heat transfer, compact structure, high energy efficiency, ease of manufacture and arrangement. In addition, developing and optimizing the helical condenser coil designs are considered a sustainable solution for improving the performance of the system. Based on this vision, a particular difference of this study with other studies performed on similar researches is the influence of different shapes of condenser coil designs on heat transfer and temperature distribution during heating process. Therefore, the main objective of this paper is to improve the thermal performance of a helical coil by modifying the coil shape. Based on the results, the average heat transfer coefficient and average COP of variable pitch coil were increased by 36.48% and 16.17%, respectively, compared with normal coil. Moreover, the obtained results indicated that the water temperature, the water velocity, the heat transfer and the performance of the helical condenser with variable pitch coil were improved in comparison with that of normal coil. However, this investigation could provide some guidance to obtain the optimum coil structure of refrigeration machines for domestic hot water production. [ABSTRACT FROM AUTHOR]

• Developed a nonlinear model order reduction framework to generate reduced order heat exchanger models. • Applied the proposed model order reduction scheme to a complete VCC system. • Validated and compared with a high-fidelity FV model and experimental measurements. • Reduced order VCC model leads to an 80% computation time reduction compared to a standard FV approach. A computationally efficient but accurate dynamic modeling approach for vapor compression systems is important for many applications. Nonlinear model order reduction techniques which generate reduced order models based on high fidelity vapor compression cycle (VCC) models are attractive for the purposes. In this paper, a number of technical challenges of applying model order reduction methods to VCCs are described and corresponding solution approaches are presented. It starts with a reformulation of a standard finite volume heat exchanger model for matching the baseline model reduction structure. Reduced order models for evaporator and condenser are constructed from numerical snapshots of the high fidelity models using Proper Orthogonal Decomposition (POD). Methodologies for system stability and numerical efficiency of POD reduced order models are described. The reduced order heat exchanger models are then coupled with quasi-static models of other components to form a reduced order cycle model. Transient simulations were conducted over a wide range of operating conditions and results were compared with the full order model as well as measurements. The validation results indicate that the reduced order model can execute much faster than a high-fidelity finite volume model with negligible prediction errors. [ABSTRACT FROM AUTHOR]

Due to the great potential of propane in environmental protection and thermodynamic properties, the promotion of propane room air conditioner will make a great contribution to environmental protection. The defrosting behaviors of propane room air conditioner system and rotary compressor were studied by experiments in this paper. Experimental analysis redefines and distinguishes defrosting time and frost melting time, and their relationship with condition temperature is different. The experimental results show that the lower the ambient temperature was, the longer the defrosting time was, but the shorter the frost melting time on the outdoor heat exchanger was. The state of the oil sump and foam layer in propane rotary compressor using Mineral Oil (MO) was recorded and the variation reason for liquid level was analysed during defrosting. Combined with the change of viscosity and liquid level of the oil sump, the experimental results show that the compressor has abnormal oil discharged, and the lower the ambient temperature was, the larger the oil discharge was under three conditions during defrosting process. The total mass of the mixture in the oil sump under the three working conditions is 275.18 g, 289.10 g and 276.41 g, the lubricating oil is 202.97 g, 200.23 g and 189.71 g, and the refrigerant mass is 72.21 g, 88.87 g and 86.71 g respectively when the defrosting process ended. This study will provide an experimental basis of the reliability study and application of propane air conditioners. [ABSTRACT FROM AUTHOR]

• Structural factors are investigated from different parts of the distributor. • The flow process in the distributor is deeply summarized. • The influence of mal-distribution on the evaporator is calculated. • Uniformity is improved by enhancing the mixing and backflow effects in the cavity. • Mal-distribution reduces by 4.6% and 36.5% by optimization. In the residential air-conditioning system, the mal-distribution of the two-phase refrigerant deteriorates the performance of the multi-channel evaporator, especially in distributors. In this paper, the distribution uniformity and structural optimization of a type of air-conditioning distributor are investigated with simulation models and experiments. Two types of optimized distributors are proposed through the study on multiple structural factors, the Taguchi method and the summary of flow mechanism. Furthermore, the relevance between the mal-distribution and the performance of the evaporator is established by a heat exchanger design software. The results show that the distributor can alleviate the uneven phase distribution at the inlet through the following three effects: throttling of the orifice, diffusion and backflow in the cavity. Improvements are proposed based on these points, including enlarging the space of the mixing cavity and applying a hemispherical surface. It is confirmed that the optimized distributors have better performance compared to the original sample by simulation evaluations and experimental verifications. The non-uniformity index of optimized distributors reduced by 4.6% and 36.5%, respectively. [ABSTRACT FROM AUTHOR]

• An analytical solution is proposed for a pin fin heat/mass exchanger sorber bed. • Graphite flake additives in CaCl 2 -silica gel composite sorbents. • ANOVA is utilized to understand the contribution of each parameter on performance. • Parametric study is carried out to examine the effect of key design parameters. • Transient measurement using gravimetric large temperature jump test bed. This paper proposes a novel closed-form analytical model to predict the sorption performance of a pin fin heat/mass exchanger (PF-HMX) prototype, using the Eigenfunction expansion method to solve the governing energy equation. The proposed transient 2-D solution includes all salient thermophysical and sorption properties, sorbent geometry, operating conditions, and the thermal contact resistance at the interface between the sorber bed heat exchanger and sorption composite. An analysis of variance (ANOVA) method is utilized to understand the percentage contribution of each parameter on specific cooling power (SCP) and coefficient of performance (COP). It is shown that the amount of graphite flake, sorbent thickness, and fin radius on one hand and cycle time and graphite flake content on the other have the highest level of contribution to the COP and SCP, respectively. Moreover, a parametric study found that HMX geometry, sorbent properties, and cycle time counteract effects on COP and SCP, which should be optimized simultaneously to build an optimal design. The analytical model was validated successfully using the sorption data from a custom-built gravimetric large temperature jump (G-LTJ) testbed. The experimental results show that the present PF-HMX design with a relatively low mass ratio (MR) can achieve an SCP of 1160 W kg−1 and a COP of 0.68 which are higher than the previously published results in the literature. [ABSTRACT FROM AUTHOR]

• Boiling of ammonia in a vertical dimpled tube bundle with round PVC nonconductive rods. • Study of wide range of experimental parameters. • Enhanced heat transfer coefficient compared to previous studies. This paper presents the experimental results of saturated two-phase flow ammonia in a vertical shell and tube heat exchanger comprising of seven dimpled enhanced tubes with solid PVC nonconductive round rods inserted in each tube creating an annular section. The source of heat was hot ethylene glycol/water solution flowing on the shell side. The range of saturation temperature for ammonia was –20 °C ≤ T sat ≤ –2 °C, heat flux range 5 kW m−2 ≤ q ˙ ≤ 45 kW m−2, exit vapor quality range 0.1 ≤ x ≤ 0.8 and mass flux 76.9 kg m−2 s−1≤ G ≤ 145.7 kg m−2 s−1. For each saturation temperature, the effects of heat flux and mass flux were studied while the exit vapor quality at each mass flux varied between 0.1 and 0.8. Two phase heat transfer coefficient increased with saturation temperature and heat flux. For all test runs, typical thermal performance was observed, i.e., initial increase and then a drop at the onset of dry out. Annular flow boiling showed better heat transfer coefficient compared to the dimple tube for quality x < 0.5. [ABSTRACT FROM AUTHOR]

• Two-phase flow visualization is achieved by a clear window on the inlet header. • Flow regime in the inlet header is periodic and one cycle has two or three stages. • Flow regime in the inlet header significantly affects the two-phase distribution. • Effects of mass flux, inlet vapor quality, and the number of plates are examined. This paper presents an experimental study of two-phase R134a flow in the inlet header of brazed plate heat exchangers and its effect on flow distribution among the channels. Visualization of the two-phase flow is accomplished through a 3-D printed transparent window on the inlet header of the heat exchangers. The two-phase flow distribution among channels is quantified based on infrared images of the heat exchanger sidewalls. At the test conditions, the observed flow regimes in the inlet header are periodic and two or three stages are identified in one cycle: top corner vapor flow, vapor jet flow, and (conditional) liquid blockage flow. Among them, the top corner vapor flow affects the distribution most, in which the vapor refrigerant is mainly present at the top corner of the header and branches out through the first several channels, leaving the liquid refrigerant to occupy the rest flow area of the inlet header and present a single-phase like distribution profile. When the inlet vapor quality increases, the distribution of the liquid refrigerant is improved since the vapor refrigerant can reach more downstream channels to help balance the total pressure drop. With the mass flux increases, the maldistribution caused by the header induced pressure drop is more significant, which compromises the benefit brought by the higher vapor momentum. When the number of plates is increased, the liquid refrigerant distribution is worse due to an increased pressure drop in the headers. [ABSTRACT FROM AUTHOR]

• A cascade system with a CO 2 sublimation cycle in the lower stage is experimentally investigated. • Continuous operation below the triple pressure of CO 2 has been realized. • The transient behaviors and blocking issues as an example case are discussed. • In addition, HFE 7500 is used as carrier-fluid to reduce the blockages. • The performance of the system both with and without the carrier-fluid is investigated at different sublimation temperatures. • Optimization potentials of the system are analyzed. Some studies in recent years have shown the possibility of extending the application temperature of the natural refrigerant carbon dioxide (CO 2) below its triple point. Thus, CO 2 could be a potential alternative refrigerant for R-23 in ultralow-temperature (ULT) refrigeration systems. This paper introduces a special cascade system, whose lower stage can be switched between a CO 2 sublimation cycle and a conventional R-23 cycle, so that both cycles can be compared directly under the same ambient conditions. The sublimation system features a unique sublimation heat exchanger, which allows the operation with an extra carrier fluid in order to improve the heat transfer and reduce the blockage by solid CO 2. In this study, experiments were carried out using HFE-7500 as the carrier fluid ("wet sublimation") as well as without the carrier fluid ("dry" sublimation). The system was able to be operated continuously in both operating mode. A refrigerating capacity of 600 W was achieved at a sublimation temperature of -79°C. The transient behavior and blocking issues of the system are discussed. The efficiency of the sublimation system during stable operations are compared with the result of the reference R-23 system. It shows that there is still large potential for improvement on the system performance of the CO 2 sublimation system. [ABSTRACT FROM AUTHOR]

• A method to quantify two-phase refrigerant distribution in BPHE by infrared image. • The developed quantification method is validated by the experimental results. • The developed method is applicable when all liquid refrigerant evaporates in BPHE. This paper presents a method to quantify two-phase refrigerant flow distribution in brazed plate heat exchangers (BPHEs) from infrared images. In this method, the two-phase length of the refrigerant channels is first estimated from the IR images of the BPHE sidewall. Then, the refrigerant distribution is adjusted in a BPHE evaporator model, to have the predicted two-phase length of the channels match the identified results from the IR images. The overall performance of the BPHE is also predicted in the quantification process. The proposed quantification method is validated by the experimental data: heat exchanger capacities, pressure profiles, and surface temperature profiles of the sidewall. This non-intrusive method can effectively quantify two-phase refrigerant distribution in BPHEs with the premise that all liquid refrigerant has evaporated in the heat exchangers. [ABSTRACT FROM AUTHOR]