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.

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.

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.

This study investigates the in-orbit liquid hydrogen management capability of screen channel tanks in cryogenic propulsion systems by developing a three-dimensional multiphysics model that integrates filling ratios (5%-50%) and microgravity disturbances (10^-3 g). The competition mechanism between capillary and inertial forces, as well as the fluid retention stability during tank reorientation, was systematically analyzed. The key findings include the following. The fluid retention capability was attributed to the structural synergy between liquid collection channels and tank walls, ensuring continuous liquid coverage at the channel inlets under all operating conditions. At low filling ratios, surface tension dominated the phase distribution with a quasi-static interfacial evolution. Increased filling enhanced inertial forces, inducing phase oscillations via momentum transport. The directional sensitivity analysis revealed that bottom acceleration induced the largest centroid depression, top acceleration had a minimal impact on the relative centroid height, and lateral disturbances caused larger centroid oscillation amplitudes and higher frequencies than oblique lateral disturbances.

Improving indoor air quality in homes requires fresh air; however, this is a strain on air conditioning systems. To address this issue, the use of energy-efficient fresh air units equipped with exhaust air heat recovery is recommended. These units include both passive and active types, with prominent examples being air-to-air enthalpy heat exchangers and heat pump units. Currently, the evaluation of the energy efficiency in fresh air units predominantly revolves around air-to-air enthalpy heat exchangers, rendering the commonly used heat exchange efficiency inapplicable to heat pump units. The concept of exhaust air heat recovery is perplexing and contradictory. Furthermore, the assessment of fresh air units primarily focuses on the units themselves, without considering their impact on air conditioning units and the overall system performance once combined. This study aims to establish a unified definition of exhaust air heat recovery for fresh air units, elucidating its intrinsic meaning. Additionally, it proposes a comprehensive energy efficiency evaluation method for a combined fresh air and air conditioning system. Through a case study of seven existing fresh air unit types, the necessity of exhaust air heat recovery is highlighted, and the energy efficiency levels of different unit types are compared.

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.

Cryogenic liquid fuel launch vehicles encounter longitudinal unstable vibrations during flight, which is a serious threat to the normal operation of rockets. Such vibrations exhibit typical low-frequency characteristics and often occur during the jet condensation of cryogenic liquid oxygen in propellant pipelines. To solve this problem at the source, the characteristics of the jet condensation oscillation and flow pattern transition must be investigated. Based on the height function method, a modified mass transfer model was used to dynamically capture the interfacial curvature. The relationship between the condensation pulsation frequency and two-phase interfacial curvature was established, and the frequency of the pressure oscillation was found to be 9.8-10.6 Hz. The results indicated that three typical types of jet condensation oscillations exist: stable pulsation, gas plume oscillation, and suck-back flow. The pressure amplitude of the suck-back and oscillation flows was up to 130 kPa, whereas that of the stable pulsation was only 1-3 kPa. From the dimensional analysis, the transition threshold of the flow pattern was Jc^*=7.3 when dimensionless structure parameter L^*=2.2. When Jc^*>7.3, a suck-back oscillating flow pattern appeared. The dimensionless criterion could precisely predict the condensation flow pattern. This provides a theoretical basis and technical support for the design of cryogenic liquid fuel rockets.

In cryogenic hydrogen storage systems, an accurate calculation method for the heat release from different ortho-para hydrogen catalytic conversions is important to determine the load of hydrogen storage systems. The objective of this study is to present a precise calculation method for the conversion heat of ortho-para hydrogen. By establishing a conversion model, the methods of using a smoothing spline curve to fit the experimental data and energy balance calculations are used to derive the released heat in adiabatic conversion multistage converters and the released heat during continuous conversion. The heat release properties of different conversion methods are analyzed in this study. Notably, the heat release amount of continuous conversion is the smallest, that of isothermal conversion is the largest, and the conversion heat of adiabatic conversion is in between, which is related to the number of conversion stages. In addition, a specific method and procedure are programmed to solve the implicit differential formula for continuous conversion. Finally, the calculation results of the different methods are consistent with a maximum deviation of only 0.22%, indicating that the calculations in this study are valid and accurate.

The current greenhouse effect and the "dual-carbon" goal have set off a new wave of refrigerant substitution, requiring new refrigerants to achieve a comprehensive balance between environmental protection, safety and thermal properties. However, there are currently no ideal substitutes for these refrigerants. Most environmentally friendly refrigerants, such as R290, R32, and R1234yf, which have low ozone depletion potential (ODP) and Global Warming Potential (GWP), are flammable and pose safety risks. Identifying suitable flame retardants for environmentally friendly flammable refrigerants has emerged as a crucial focus of current research on refrigerant alternatives. This article provides a recent overview of research progress on the compatibility of flame retardants with flammable refrigerants. The main focus was categorizing the various flame retardants and assessing their efficacy across different flammable substances. It also discusses their performance, effects, mechanisms, and environmental impacts. Furthermore, the article analyzes their potential applications and development trends and recommends flame retardants compatible with flammable substances.

Propane (R290) is a potential refrigerant substitute for household air conditioners. However, its flammability limits its application. In this study, the flammability limits of R290, 2,3,3,3-tetrafluoropropene (R1234yf), trans-1,3,3,3-tetrafluoropropene (R1234ze(E)), R1234ze(E)/R290, and R1234yf/R290 were determined according to the ASHRAE 34-2022 standard. The effects of R1234yf and R1234ze(E) on the flammability of R290 were analyzed, and the inhibiting abilities of R1234yf, R1234ze(E), R32, R13I1, and R134a on the flammability of R290 were compared. In addition, the refrigeration cycle performance of R290 mixtures with different compositions was simulated. The results showed that both R1234yf and R1234ze(E) exhibited limited flame inhibition capabilities for R290. When the mass fractions of R1234yf and R1234ze(E) reached 80%, the lower flammability limit of the mixture increased by approximately 1.0%. The experimental data were correlated using the Le Chatelier model, which resulted in an average absolute deviation of 0.57% between the calculated and experimental results. Compared with R290, the energy efficiency ratio of the two refrigerants and R290 mixture decreased by less than 1%, and the volumetric cooling capacity increased by less than 0.4%. The flame inhibition effect of the refrigerant on R290 decreased in the following order: R13I1>R134a>R32>R1234ze(E)/R1234yf.