Abstract>Tunnel ground source heat pump is one of the effective technologies to solve the problem of subway tunnel thermal pollution. At present, this technology has not formed a systematic design method. The capillary heat exchanger (CHE) is used as the front-end heat exchanger of the tunnel ground source heat pump system because of its corrosion resistance, easy bending, good heat transfer performance and space saving. This study intends to analyze the heat transfer performance of CHE in tunnel lining under intermittent operation. Firstly, the CHE fluid-thermal coupling heat transfer model was established based on the demonstration project. Secondly, the CHE simulation model was built on the TRNSYS platform. Finally, the heat transfer performance of CHE under continuous and intermittent operating conditions was compared and analyzed. The results show that intermittent operation can significantly improve the heat transfer performance of the heat exchanger and help alleviate the deterioration of tunnel surrounding rock environment. The research can provide a theoretical basis for engineering design and application of tunnel ground source heat pump technology.
Abstract>Electrocaloric effect is a kind of novel solid-state refrigeration technology, which is considered one of the options to replace traditional compressor refrigeration due to its advantages such as low cost, high COP and miniaturization. The main problem that hinders the application of electrocaloric refrigeration devices nowadays is the insufficient temperature change of single chip electrocaloric effect material. In device design, using a regenerative or series structure can amplify the temperature change of materials to the level of engineering applications. This paper introduces the concept, performance indicators, and current research achievements of thermal switch devices in series electrocaloric refrigeration devices. Based on the principle of electrocaloric effect, the adaptability of different types of thermal switches in electrocaloric refrigeration devices is analyzed, and the research results of regenerative and series of electrocaloric refrigeration prototype devices are introduced. Simulation and analyses of the impact of the thermal switching devices on the performance of the refrigeration device based on the series of electrocaloric refrigeration were carried out. .The authors believe that the series structure is theoretically more suitable for electrocaloric refrigeration devices. The use of the fast response speed and high thermal resistance of the thermal switch device driven by an electric field to drive the fluid medium can help the series electrocaloric refrigeration devices break through the current technological bottleneck, and thus unleash the technical potential of electrocaloric refrigeration technology in various refrigeration system fields.
Abstract>In order to investigate the vapor injection performance of the scroll compressor with short profile, a three-dimensional transient numerical model of vapor-injection scroll compressor for electric vehicle with the displacement of 38 cm3/rev is established and verified with a deviation within 9.5%, and the effects of pressure and temperature of vapor injection on the performance of the scroll compressor are studied by the model. The results show that under the circumstance of a constant superheat, with the increase of injection pressure, the discharge temperature of the compressor initially decreases and then gradually increases, and the heating capacity shows a trend of increase, and the maximum increase of heating capacity can reach 20.5% and 17.1% respectively at the rotary speed of 5000 r/min and 6000 r/min respectively. In addition, the heating COP gains an upward trend at the first stage and then declines. Moreover, the maximum value of the heating COP with vapor injection under the former speeds is 3.9% and 2.3% separately higher than that without vapor injection. The compressor efficiency shows the same tendency as heating COP, and the volume efficiency gradually decreased. The compressor discharge temperature grows somewhat with constant injection pressure, whereas the compressor power, the heating capacity, and the heating COP remain nearly constant as the injection temperature increases. The injection pressure has a stronger impact on the scroll compressor compared to the injection temperature.
Abstract>Two-phase flow pattern of hydrocarbon working fluids in the shell side of a helically baffled heat exchanger for liquefied natural gas determines the heat transfer performance of the heat exchanger. In this study, the two-phase flow patterns of propane and ethane/propane mixtures in the shell side of a helically baffled heat exchanger were tested using a visualization experimental method. The test results show that with the increase of vapor quality, the experimental observations sequentially included stratified flow, stratified-spray flow, and spray flow; as the mass flux of propane increases from 20 kg/(m2·s) to 40 kg/(m2·s), the transition vapor quality from stratified flow to stratified-spray flow decreases from 0.7 to 0.3, while the transition vapor quality from stratified-spray flow to spray flow decreases from close to 1 to 0.7; when the proportion of ethane increases from 0% to 50%, the transition vapor quality from stratified flow to stratified-spray flow increases from 0.3~0.45 to 0.43~0.55, while the transition vapor quality from stratified-spray flow to spray flow increases from 0.69~0.85 to 0.83~close to 1. The existing flow pattern map for water-air mixtures was found to be inadequate for predicting the flow patterns of hydrocarbon working fluids. A new set of flow pattern transition criteria was established, with prediction deviations of approximately 6.5%, 5.5%, and 4.2% for the experimental stratified flow, stratified-spray flow, and spray flow, respectively.
Abstract>The Centrifugal Reverse Brayton Cycle (CRBC) is a novel refrigeration cycle that harnesses the conversion of inertial potential energy to pressure energy, enabling efficient compression and expansion processes through the centrifugal and centripetal flow of the working fluid within a rotating tube. This offers a promising potential for improving the refrigeration efficiency of conventional gas refrigeration cycles. Building upon earlier research, this study employs a thermodynamic model to conduct a parametric analysis of the CRBC, with the aim of exploring the cycle"s thermodynamic efficiency and loss distribution, providing a theoretical foundation for system evaluation and enhancement. This research reveals that, under the same inlet air temperature, there exists an optimization possibility for the heater inlet temperature. The system experiences its maximum exergy loss rate during the centrifugal isothermal compression flow, adiabatic compression, and centripetal adiabatic expansion flow, each approximately at 20%. Conversely, the exergy loss rates during the air cooling process and centrifugal adiabatic compression flow are comparatively lower and demonstrate an inverse relationship with the inlet temperature of gas heater. The exergy efficiency of the CRBC reaches an impressive 19.2%, significantly surpassing the 8.1% for CO2 Reverse Brayton Cycle, 4.9% for open air Reverse Brayton Cycle, and 2.3% for closed air Reverse Brayton Cycle.
Abstract>In recent years, the research of regenerative heat pump has been widely concerned. The combination of phase change heat storage technology and heat pump can improve the performance of heat pump and the utilization rate of renewable energy, but it still needs further cost reduction and efficiency increase. Therefore, the present study reviews the progress of solid-liquid phase change regenerative heat pumps, and summarizes the applicable conditions and characterization methods of phase change materials applied to heat pumps. The optimization approaches of the performance of the regenerative heat pump system are summarized, including the selection and improvement of phase change materials, the optimal setting of heat exchanger, and the dynamic optimization control strategy of the system. The outstanding performance of regenerative heat pump with cascade heat storage in improving the supply-side comfort and improving the utilization rate of renewable energy indicates the broad prospect of cascade heat storage applied to heat pump energy storage system, and the non-eutectic mixed phase change materials are proposed as alternative materials for cascade heat storage. It is pointed out that it is necessary to summarize and develop new methods to adjust the thermophysical properties of phase change materials for energy storage, so that make the selection and improvement of phase change materials adapt to the optimization research of thermodynamic theory of cascade heat storage device, and further improve the heating decarbonization ability of latent heat storage heat pump.
Abstract>The vehicle mounted cold storage plate is an important part of a refrigerated vehicle, and shortening the charging time of the cold storage plate plays an important role in improving the efficiency of cold chain logistics. This study proposes a heat pipe heat exchange and cold storage plate, and simulates the freezing performance of the cold storage plate, such as the freezing time. Through experimental research and comparative analysis, the reliability of the theoretical model is verified. The influence of heat pipe diameter, tube spacing, refrigerant channel flow rate and refrigerant channel Reynolds number on the charging performance of the cold storage plate was studied by orthogonal test method. The results show that the optimal diameter of the heat pipe of the cold storage plate is 7 mm, the tube spacing is 39 mm, the flow rate of the refrigerant channel is 1.2 m/s, and the Reynolds number of the refrigerant channel is 3.89. Comparing the freezing performance of the thermal storage plate under the optimal structure with the results reported in relevant literature, the freezing time of the thermal storage plate has been significantly shortened.
Abstract>Microencapsulation technology based on sodium alginate hydrogels can be used to optimize freezing and rewarming procedures and to reduce cryo-damage to cells and tissues. In this paper, the morphology of oocytes in different concentrations (0.5%, 1%, 1.5%, and 2%) of sodium alginate hydrogels was first observed to determine the safe concentration for their encapsulation. Secondly, the crystallization temperature and crystallization behavior of sodium alginate hydrogels with different concentrations (0.5%, 1%, 1.5%, 2%) were systematically investigated using cryo-microscopy, and the morphology and crystallization of oocytes were compared when they were cooled down/retempered in the base solution, cryoprotectant solution. Finally, the freezing effect of sodium alginate antifreeze hydrogel encapsulated oocytes prepared by solvent replacement type and physical mixing type were compared. The results showed that oocytes maintained their overall morphology and volume better in sodium alginate gels at 0.5% and 1% concentrations. Furthermore, oocytes in both the 1% sodium alginate group and the cryoprotectant solution group of 12.5% DMSO + 12.5% EG + 0.5 M trehalose did not produce intracellular ice during the cooling process；Additionally, compared with the physical mixture, oocytes in the solvent replacement group of hydrogels did not produce intracellular ice during the cooling process, and the cells retained their normal morphology after rewarming.
Abstract>In a occupied enclosure space formed by multiple opposed jet outlets, understanding the vortex structures plays a crucial role in controlling the spread of viruses and pollutants. This study constructed a scaled model of a occupied enclosure space with opposing jet flows, incorporating a heated floor as a heat source to create thermal plumes under cooling conditions. Particle Image Velocimetry (PIV) system was employed to measure the flow field under isothermal and cooling conditions. Studying the Turbulent Characteristics of the Flow Field from the Vortex Perspective, comparing the identification performance of different vortex identification algorithms. The Liutex vortex identification method was applied to analyze the vortex motion within one oscillation period, revealing distinct strengths in counterclockwise and clockwise directions, with maximum intensities of 50 and 110 s?1, respectively. The study concluded that the motion, merging, and annihilation of vortices influence the flow field structure, with a jet oscillation period of approximately 3.67 seconds. Due to the trapping effect of vortexes on pol-lutants, areas of pollutant accumulation can be represented through the vortex core relative frequency distribution maps. It is observed that, despite the flow field being unsteady, the distribution of vortexes remains relatively stable. Specifically, on the left side of the CS4 cross-section, the vortex core appeared at the same point up to 21 times. Under cooling con-ditions, pollutants are confined to smaller regions, which aids in containing the spread of pollutants.
Abstract>The air source heat pump capillary radiant floor heating system directly heats the floor with refrigerant as the heat medium, which has the advantages of simple system and good heat transfer effect. In this paper, an experimental device of capillary floor radiant heating with air source heat pump is built, and the heating experiment of parallel capillary floor radiant ends is carried out at different outdoor ambient temperatures. The experimental results show that the lower the outdoor temperature, the longer the time required for the temperature stability of the capillary floor radiation end. When the outdoor temperature is -5 °C, the stability time is 120 min; the temperature difference between the exhaust inlet end and the condensate outlet end on the same capillary wall is large. The temperature difference on the capillary wall can reach 6.40 °C, and the temperature difference on the surface of the 20 mm thick cement floor can reach 4.20 °C. The vertical temperature difference from the capillary wall to the cement floor surface is small, not exceeding 0.40 °C, and the vertical heat transfer effect of the floor is good. The temperature difference at the same position of different capillaries is within 0.80 °C, and the temperature uniformity is good. When the outdoor temperature is-5 °C, the heating COP of the unit can reach 4.6 with a good heating performance.
