The combination of a super-long gravity heat pipe and a heat pump system for harvesting deep geothermal heat has the advantages of low cost, high efficiency, and no groundwater contamination. Direct heat exchange between the evaporator of the heat pump system and the condenser of the gravity heat pipe can simplify the heat exchange process and improve the heating efficiency of the system. Therefore, a U-shaped evaporator-condenser was developed, and its heat transfer performance was studied by building an experimental platform combining a heat pump and a heat pipe. Notably, the heat transfer coefficient of the U-shaped evaporator-condenser reached 2 037.92 W/(m²·℃) when the working fluid on the heat pump side passed through the tube. Based on the homogeneous flow model, a one-dimensional steady-state evaporator-condenser heat transfer model was established by integrating the mass, energy, and momentum conservation equations with empirical formulas for condensation outside the tube and boiling heat transfer inside the tube. Using Python, the simulation results were compared with experimental data. Notably, the average deviation of the heat transfer in the evaporator-condenser was 18.91%, confirming the accuracy of the model and providing a theoretical calculation method for designing an efficient evaporator-condenser.

To investigate electrostatic accumulation induced by cryogenic liquid hydrogen (LH₂) flow in pipelines, a test system was constructed with LH₂ as the primary working medium. Using vacuum insulation, insulated connections, electrostatic shielding, and other measures, as well as the application of the leakage charge method, safe and accurate measurement of extremely low-level charge quantities generated by LH₂ flow under cryogenic conditions was achieved. The charge accumulation characteristics under multiple flow conditions with Reynolds numbers (Re) below 2×10⁵ were analyzed. The experimental results indicated that notable flow charging phenomena occurred during LH₂ flow with extremely low electrical conductivity in pipelines. Furthermore, charge accumulation demonstrated a linear growth during the test period. Within the range of pipe lengths and Reynolds numbers covered by the experiment, the average charge density decreased with an increase in the flow velocity; however, the rate of decrease gradually diminished. The average charge density of flow decreased with increasing pipe diameter. The developed electrostatic accumulation test system for low-temperature LH₂ pipe flow provided an important platform support for conducting LH₂ electrostatic tests. This study validated the feasibility of the electrostatic measurement method for LH₂, providing design guidance for exploring the electrostatic laws of LH₂ and the boundary of safe flow velocity.

Frosting is one main adverse factor hindering the application of finned tube evaporators in refrigeration systems. To accurately predict the growth characteristics of the frost layer, its growth behavior on the surface of a three-dimensional finned tube under forced convection conditions was numerically simulated based on the coupled VOF (volume of fluid) multiphase flow and phase change mass transfer rate model method. The maximum difference between the simulated frost layer thickness and the experimental results was within 15%. The estimated value of the frost layer density was within a confidence interval of up to 90%; this is in good agreement. By building a visualization experimental platform for frosting on the surface of the finned tube of the evaporator, the influences of ambient temperature, relative humidity, and frontal wind speed on frost growth characteristics under single-factor changes were analyzed. The results showed that the thickness of the frost layer gradually decreased from front to back in the direction of the airflow. The lower the air temperature, the greater the wind speed, the higher the relative humidity, the larger the thickness of the frost layer, the more frost, and the larger the thermal resistance of the frost layer. The thickness of the frost layer reached its highest value of 2.344 mm at 85% relative humidity. According to the Morris sensitivity analysis, the relative humidity had the greatest influence on the frost layer thermal resistance, and the sensitivity coefficient reached 2.41. The correlation of the frost layer thermal resistance with different environmental parameters was obtained using the least squares regression method.

With the development of computer technology and the application of artificial intelligence, electronic chips are becoming increasingly miniaturized and integrated, leading to a rapid increase in their volumetric heating power, thus affecting their normal operation. To address this problem, a heat sink with an array of finned porous microjets was designed, and HFE-7100, which has good thermal stability and electrical insulation, was selected as the cooling medium. Through a combination of numerical simulations and experimental research, the influence of factors such as the longitudinal aspect ratio of the slotted fins, inlet subcooling, inlet volumetric flow rate, and jet Reynolds number on the heat transfer process of microjet boiling was investigated. The results showed that the optimized structure with an aspect ratio of 0.5 met the requirements of chip cooling and had a better cooling effect. In the single-phase convection heat transfer stage, under the same working condition, the inlet subcooling degree had little effect on heat transfer, and increasing the volume flow rate or jet Reynolds number could strengthen the convection heat transfer, and the maximum heat transfer coefficient could reach 15 724.40 W/(m²·K). However, in the jet boiling stage, the heat flux corresponding to the onset of nucleate boiling (ONB), and it decreased with a decrease in the inlet subcooling degree. Increasing the inlet volume flow rate or jet Reynolds number inhibited the occurrence of boiling, thus weakening the heat transfer. However, compared with the single-phase convective heat transfer stage, the heat transfer coefficient increased by 20.6%.

Mechanical vapor compression (MVC) systems are energy-saving technologies that recover and reuse low-temperature waste heat resources, achieving energy conservation and carbon reduction. As the core equipment in MVC systems, the compressor directly affects the overall performance of the system. This article primarily reviews the thermodynamic and structural performance of vapor compressors, proposes relevant enhancement suggestions and improvement ideas, and provides a reference and assistance for the subsequent optimization of vapor compressor performance.

As a clean energy source, LH₂ is poised to play a pivotal role in future energy supplies. Currently, international hydrogen liquefaction facilities suffer from high energy consumption, high liquefaction costs, and low exergy efficiencies. In contrast, the development of cryogenic hydrogen liquefaction technologies and equipment in China is still in its infancy, significantly lagging behind advanced global standards. Against this backdrop, this paper summarizes the recent research advancements in hydrogen liquefaction technology, encompassing both process design and practical facilities. It delves into the latest developments in steady-state process simulation and dynamic characteristic studies, evaluating performance metrics across various liquefaction processes. Additionally, this paper provides an overview of the technical features and equipment layouts of large-scale hydrogen liquefaction plants and small-scale laboratory setups. Finally, it consolidates the key development priorities and future directions for hydrogen liquefaction technology, aiming to provide valuable guidance for technological progress and accelerate the widespread adoption of hydrogen energy.

Ortho-para hydrogen conversion in the hydrogen liquefaction process is significant for the long-term storage and long-distance transportation of liquid hydrogen. This paper outlines the differences in the properties of orthohydrogen and parahydrogen, reviews the research progress on the physical mechanisms and reaction kinetic models of the ortho-para hydrogen catalytic conversion process, and summarizes the performance of common catalysts. Finally, three mainstream schemes for ortho-para hydrogen conversion are compared. Research on the internal physical mechanisms and reaction kinetic models explores the conversion process from microscopic and macroscopic perspectives, respectively. Owing to the lack of experimental data, scholars have not yet formed a unified explanation for the surface characteristics of catalysts, which must be quantitatively validated. Furthermore, although nickel-based catalysts have higher catalytic efficiency, iron hydroxides and oxide catalysts are the main catalyst choices for ortho-para hydrogen conversion, considering the preparation, activation, and deactivation of catalysts and the characteristics of the liquefier. Among the three mainstream ortho-para hydrogen conversion schemes, the hydrogen liquefaction process with continuous conversion has the lowest energy consumption and is the future direction. Relevant research in China is still in its early stages and has great potential for development. This study provides theoretical guidance for the design and construction of ortho-para hydrogen catalytic conversion test benches.

Existing thermal management schemes struggle to actively and efficiently create a low-temperature heat sink in a limited enclosed space. Hence, a composite thermoelectric refrigeration thermal management system based on flat heat pipes is proposed in this study. A numerical simulation model of the composite system was developed, and an experimental platform for the composite thermoelectric refrigeration thermal management system was established to verify the accuracy of the model. The results showed that the proposed composite thermal management system provided a low-temperature heat sink for the entire thermal management system in a limited space and solved the problem of heat accumulation at the hot end of the thermoelectric refrigeration module by coupling with the plate heat pipe. The thermoelectric refrigeration system based on a flat-plate heat pipe was considerably better than that based on aluminum fins in terms of 1-12 A working current. The cooling capacity and COP (coefficient of performance) of a single thermoelectric module plate were effectively increased by 38.35% and 14.81%, respectively, under the best working conditions.

Seasonal thermal energy storage (STES) can effectively mitigate the supply and demand imbalance of solar energy between winter and summer. Large-scale water pit thermal storage systems require efficient and accurate computational simulations to avoid investment waste. This study proposes a simplified numerical analysis method and establishes a cylindrical underground pit with a total volume of 11 304 m³ to describe the operation of a STES system. The model establishes a one-dimensional heat transfer model for the water body and a two-dimensional heat transfer model for the soil, separately solving for the water and the soil temperature field. The two models are connected through the temperature boundary at the pool wall to simulate the entire system. To comprehensively verify the accuracy of the numerical simulation model, validation was conducted under standby, charging, and discharging modes. The results indicate that the developed model has good accuracy and reliability. Under the standby mode, the temperature error of the five water layers in the sandbox test is less than 10%, with the highest accuracy in the middle and lower-middle water layers, with an average absolute error of 1.75% and 1.24%, respectively. Under the charging mode, the average relative error is 1.57%, and the average temperature error is 0.44 ℃. Under the discharging mode, the average relative error is 0.46%, and the average temperature error is 0.24 ℃.

A high-efficiency condensation dehumidification system utilizing copper foam driven by a Stirling refrigerator was developed to address the demands for high-efficiency heat transfer and a compact lightweight design in space stations. An experimental study was conducted to investigate its heat and mass transfer characteristics under various conditions. The experimental parameters were set as follows: air inlet temperature ranging from 20 ℃ to 30 ℃, relative humidity between 50% and 80%, cold plate temperature from 8 ℃ to 13 ℃, and inlet wind speed from 0.4 m/s to 1.4 m/s. The results indicated a positive correlation between the increase in the air inlet temperature and the enhancement of both the heat and mass transfer coefficients. Specifically, when the air inlet temperature increased from 20 ℃ to 30 ℃, the heat transfer coefficient increased by 10.5%, whereas the mass transfer coefficient exhibited a more substantial increase of 57.1%. Furthermore, variations in the relative humidity of the air inlet distinctly affected the heat and mass transfer coefficients: the heat transfer coefficient decreased by 31.6% with an increase in the relative humidity, whereas the mass transfer coefficient increased by 11.4%. Although reducing the temperature of the cold plate can effectively improve heat transfer, it leads to the accumulation of condensate water and reduces the efficiency of heat and mass transfer. Therefore, an appropriate cold plate temperature must be selected. Additionally, the efficiency of heat and mass transfer was markedly enhanced with increasing inlet wind speed. However, a continuous increase in wind speed resulted in higher system energy consumption. Thus, a balance between efficient heat transfer and high system energy consumption was essential. Based on extensive experimental data, the heat transfer model was refined using regression analysis. The standard deviation between the theoretical and experimental values was 8.21%, and the maximum deviation was 19.76%, demonstrating the strong predictive accuracy of the model.