The pvTx properties of refrigerants are the fundamental parameters for the design and performance optimization of refrigeration equipment. However, experimental data for key mixtures such as R125 + R1234yf, R125 + R1234ze(E), and R143a + R1234yf remain scarce, especially at high pressures. In this work, the pvTx properties of these three mixtures were measured using a high-precision isochoric apparatus. The measurements cover temperature and pressure ranges of 237.02 - 310.17 K and 1.3 - 13.6 MPa, respectively. The average absolute relative deviation (AARD) of the multi-parameter equation of state in REFPROP 10.0 from the experimental data is 0.28% for the R125 + R1234yf mixture, 0.37% for the R125 + R1234ze(E) mixture, and 0.49% for the R143a + R1234yf mixture. The measured pvTx data were correlated using the modified Tait equation, and the AARDs of this equation for the three mixtures are 0.12%, 0.10%, and 0.04%, respectively. The modified Tait equation shows better agreement with experimental data and exhibits no obvious systematic deviations.

This study presents a systematic experimental investigation on the condensation heat transfer, frictional pressure drop, and entropy generation characteristics of zeotropic mixtures R1234yf/R601a in a horizontal mini-channel with an inner diameter of 2 mm. Experiments were conducted over a mass flux range of 200–600 kg·m−2·s−1, saturation pressures from 0.5 to 1.5 MPa, and across the entire vapor quality range. The effects of mass flux, vapor quality, saturation pressure, and mixture composition were systematically analyzed. Results show heat transfer degradation, which becomes more pronounced at higher vapor qualities. Frictional pressure drop generally increases with vapor quality but decreases beyond a vapor quality of 0.75 under certain conditions. Total entropy generation increases first and then decreases with mass flux. New heat transfer and frictional pressure drop correlations were developed, showing mean absolute relative deviations of 9.63% and 12.67%, respectively, for R1234yf/R601a mixtures. These correlations also predict 85.36% of heat transfer data and 87.70% of pressure drop data from literature within ±30% error. The total entropy generation calculated using the new correlations agrees well with experimental results.

The problems of overpressure and overheating in the gasbag of a stratospheric airship can be effectively alleviated by introducing ammonia as a phase change working gas and utilizing its significant volume change during the gas-liquid transition. However, the convective heat transfer of the heat exchanger is significantly reduced at low pressure, seriously affecting the phase change efficiency of ammonia. This study explores the key factors affecting ammonia condensation heat transfer outside the microchannel tube under different pressures, focusing on surface tension, subcooling, and inlet velocities, while also analyzing the underlying mechanisms using the droplet heat transfer model. The results show that the condensation heat transfer of ammonia outside the microchannel tube is significantly weakened under low pressure because the increase in the vapor-liquid interface resistance. The average heat flux decreases by 60.5%, and the liquid film thickness increases by 3.9 times at the pressure of 6.5 kPa compared to 100 kPa. Increasing the inlet velocity within a certain range reduces the liquid film thickness and enhances condensation heat transfer, while increasing the subcooling leads to a greater improvement in heat transfer performance than that of velocity. Optimizing subcooling and inlet velocity enhances condensation heat transfer to provide an effective strategy for improving the heat exchanger performance and increasing the operational efficiency of the stratospheric airship.

In this paper, a concentration-adjustable Linde-Hampson system was proposed, which can actively adjust the component concentrations according to the change of refrigeration conditions. The thermodynamic performance of the proposed and conventional Linde-Hampson systems was evaluated and compared over a refrigeration temperature range of -120 degrees C to -60 degrees C. A genetic algorithm in MATLAB, coupled with Aspen HYSYS, was used to optimize the system parameters to maximize the COP. The results show that the proposed system outperforms the conventional system in operating performance. At a refrigeration temperature of -120 degrees C, the proposed system achieves a COP of 0.31, corresponding to a 78.4% enhancement relative to the conventional system. The core of the performance enhancement lies in the reduction of exergy loss in the recuperative heat exchanger and the throttle valve. Exergo-economic analysis reveals that despite a higher initial investment cost of the proposed system, its total exergy cost is superior when exergy loss cost is considered, with a 33.9% reduction at -120 degrees C compared with the conventional system.

Thermal conductivity and viscosity are key transport properties in heat transfer and mass transfer processes. In this work, we introduce a practical and simplified model to predict these properties for liquid fluids using only density as input. By incorporating critical parameters and the acentric factor, new reduced thermal conductivity and reduced viscosity are proposed. Based on hidden scale invariance, the logarithm of the reduced thermal conductivity exhibits a linear correlation with the reduced density, while that of the reduced viscosity follows a combined exponential-linear relationship with the reduced density. For different fluids, the functional form remains identical, differing only in parameters. The model requiring only two parameters for thermal conductivity and three for viscosity. It was validated against extensive experimental data: 30,112 data points for thermal conductivity (114 fluids) and 18,442 data points for viscosity (113 fluids), covering nonpolar, weakly polar, and some polar fluids. The results demonstrate average absolute relative deviations of 3.2% for thermal conductivity and 3.8% for viscosity, with almost no systematic bias observed for the vast majority of fluids. It provides a simplified engineering tool for rapid and reliable estimation of transport properties, facilitating more efficient thermal system design and analysis.

A low-temperature and low-pressure phase change experimental system for regulating the buoyancy of stratospheric airships has been designed and tested. The system is highly compact, and all components are integrated into a modular structure for easy installation and control. The setup operates based on a phase change working gas circulation system using ethylene as the refrigerant and ammonia as the phase change working gas. A microchannel tube bundle heat exchanger was designed as the core of the liquefaction chamber to enhance the condensation heat transfer process under stratospheric conditions. The system was tested for extended periods under simulated stratospheric conditions (- 57 degrees C, 6.5 kPa@19 km) using an environmental simulation chamber. A maximum liquefaction rate of 20.3 m3/h was achieved with a corresponding cooling capacity of 502.2 W and a coefficient of performance of 1.84. Additionally, a maximum vaporization rate of 25.2 m3/h was achieved with the heat rejection rate of 617.9 W and a COP of 2.08. The system can be applied to high-altitude airship platforms for buoyancy control and is also suitable for experimental studies of heat transfer processes in low-temperature, low-pressure environments.

Effective thermal management is vital for the stable operation and long-endurance flight of high-altitude platforms, but the low-pressure environment severely limits heat dissipation. To address this challenge, a novel mini-channel tube bundle heat exchanger (MCTBHE) coated with a high infrared emissivity coating was proposed. The coating, primarily composed of graphite sheets and SiO2, exhibited a high mid-infrared emissivity (epsilon = 0.93). Heat dissipation performance of the proposed exchanger was experimentally evaluated in a low-pressure wind tunnel, with the pressure reduced to 5.5 kPa. The results indicated that the high infrared coating synergistically enhanced the convective and radiative heat dissipation performance of the MCTBHE. Compared with the uncoated counterpart, the coated exchanger achieved a 7.8% higher heat transfer coefficient and a 4% improvement in overall thermohydraulic performance, despite a 10.5% increase in pressure drop. Notably, under lowpressure conditions, its heat transfer coefficient was 2.6 times that of conventional designs reported in the literature while maintaining comparable resistance. This study contributes to the experimental understanding of MCTBHEs under low-pressure conditions and demonstrates the potential of coating-based approaches for improving thermal management in high-altitude platforms.

Low-Reynolds-number conditions in near space can markedly degrade the aerodynamic performance of transonic centrifugal compressors, yet the coupled loss evolution between the impeller and diffuser remains insufficiently understood. A three-dimensional steady numerical model of the NASA CC3 compressor, including the impeller and vaned diffuser, was established and validated against experimental data. Simulations were performed for inlet conditions corresponding to altitudes of 0, 10, 20, and 25 km at the same corrected rotational speed. Stage characteristics, component entropy generation, boundary-layer transition, low-speed regions, vortex-core structures, and diffuser-inlet flow angles were analyzed jointly. As the reference Reynolds number decreased from 5.70 × 10⁵ to 2.28 × 10⁴, the maximum isentropic efficiency decreased by 12.9%, while the stage dimensionless entropy-generation parameter increased by 13.3%. The diffuser dominated stage loss at sea level, whereas the impeller contribution increased with altitude and became comparable to that of the diffuser above 20 km. The reduced Reynolds number suppressed boundary-layer transition, promoted low-momentum-fluid accumulation and local separation near the casing-side suction surface, and expanded the vortex-core coverage within the impeller. These changes intensified impeller-exit flow distortion, increasing the negative-flow-angle area at the diffuser inlet from 9.32% to 15.92% and reducing the minimum flow angle from -20.3° to -64.5°. The results reveal a loss-transfer pathway from transition suppression and near-casing separation to impeller-exit distortion and diffuser performance deterioration.

Elastocaloric cooling is considered a leading alternative to traditional vapor-compression refrigeration. While current research has primarily focused on room-temperature applications, elastocaloric technology holds significant potential for low-temperature solid-state refrigeration. This study develops the first elastocaloric refrigerator operating below −60 °C. The refrigerator features a compact design driven by a single power source that manages the loading and unloading of shape memory alloy ribbon. A synchronized linkage mechanism regulates contact between the ribbon and the heat source/sink, facilitating efficient solid–solid contact heat transfer. By employing a TiNiCuNb quaternary alloy specifically designed for low-temperature superelasticity and elastocaloric response, with the ambient environment precooled from room temperature to −60 °C in an environmental simulation chamber, the system achieves a temperature span of 8.5 K under adiabatic conditions and a pull-down temperature of 4.8 K when rejecting heat at −60 °C, with a maximum cooling power of 5.9 W at zero temperature span. These mechanical innovations demonstrate the viability of elastocaloric cooling at low temperatures, offering valuable insights for future advancements in elastocaloric refrigeration technology.

In this work, compressed liquid density for the CO2 + R1234yf and CO2 + R1243zf binary mixture was measured based on a newly established isochoric apparatus. The measured density data of propane in the compressed liquid phase are used to calibrate the volume of the sample container, and the liquid phase density data of R1234yf are used to verify the reliability of the volume measurement. 18 data points for the CO2 + R1234yf mixture (xR1234yf = 0.747) and 17 data points for the CO2 + R1243zf mixture (xR1243zf = 0.827) are obtained, which cover the temperature and pressure ranges of 233.15-313.15 K and 1-15 MPa. The combined standard uncertainties of temperature, pressure, density, and composition are 90 mK, 25 kPa, 0.2 %, and 0.006. The experimental density data for the compressed liquid phase were correlated using the Peng-Robinson equation of state (EoS) with van der Waals (vdW) mixing rules. Compared with PR + vdW model and REFPROP 10.0, the average absolute relative deviation (AARD) for the CO2 + R1243zf mixture is 1.12 % and 0.86 %, and the AARD for the CO2 + R1234yf mixture is 0.21 % and 0.18 %. The data from other literature is used to compare the PR + vdW model based on experimental data from this work, and shows good consistency.

Thermal conductivity is a key thermophysical parameter in the energy transfer process and is important for the design and optimization of heat transfer equipment. This work monotonically linearizes the logarithm of the reduced thermal conductivity with respect to the reduced density of 95 pure fluids and 66 mixtures (including ternary and quintuple mixtures) based on hidden scale invariance. Quasiuniversality is used to make the images of different fluids as a function of the reduced thermal conductivity vs the reduced density coincide. A generalized predictive model for the liquid thermal conductivity is constructed. The average absolute relative deviation of the 26 327 pure fluid thermal conductivity experimental data is 3.6%, and the average absolute relative deviation of the 7033 mixtures experimental data points is 4.8%. In addition, the critical enhancement of thermal conductivity is considered. The new model contains only four global parameters, no specific fitting parameters for pure fluids and no interaction parameters for mixtures.

Mixed refrigerant throttle refrigeration holds significant application potential in space, such as for Martian carbon dioxide freeze-trapping and cryogenic freezers on space stations. These scenarios impose stringent requirements on temperature control precision and temperature uniformity. Mixed refrigerants exhibiting vaporliquid-liquid equilibrium have fewer degrees of freedom, enabling more precise control over refrigeration temperatures. Furthermore, their phase change process closely approximates isothermal evaporation, resulting in a more uniform temperature distribution. Tetrafluoromethane (R14)+difluoromethane (R32), a nonpolar-polar mixture, is a promising VLLE working fluid, but its practical utilization is limited by scarce experimental phase equilibrium data. In this study, the vapor-liquid (liquid) equilibrium characteristics of the R14+R32 binary mixture were experimentally investigated at 173.150-213.150 K. The experimental data were fitted and the binary interaction parameters were regressed using two thermodynamic models: the Peng-Robinson equation of state combined with the van der Waals mixing rule (PR-vdW), and the PR equation of state combined with the modified Huron-Vidal second-order mixing rule and the nonrandom two-liquid activity coefficient model (PRMHV2-NRTL). For the PR-vdW model, the maximum average absolute relative deviation of pressure (AARDp) and average absolute deviation of vapor-phase composition (AADy) were 3.03% and 0.0092, respectively. For the PR-MHV2-NRTL model, the corresponding values were 1.65% and 0.0103. Further investigation was conducted on the refrigeration performance of R14+R32. The results showed that the mixture achieved a maximum exergy efficiency of 44%. This highlights its potential for high-precision cryogenic systems and extraterrestrial applications, such as stratospheric and Martian environments. This work provides the first comprehensive experimental dataset and thermodynamic assessment for the R14+R32 mixture, enabling accurate design of next-generation cryogenic refrigeration systems.

During stratospheric super-pressure balloon missions, the balloon is highly likely to encounter cold clouds, which can cause severe supercooling of the helium inside the helium gasbag, potentially leading to a rapid drop in flight altitude. This paper first investigates the mechanism by which atmospheric cold clouds affect the thermal characteristics of the super-pressure balloon. It establishes a coupled dynamic and thermodynamic model for the super-pressure balloon, along with an atmospheric cold cloud model. The thermal radiation model of the super-pressure balloon is then modified according to the relative positions between the cold clouds and the balloon. Subsequently, using measured atmospheric data, simulations are conducted to analyze parameters such as helium temperature, internal and external pressure difference, and flight altitude under impacts of cold clouds, demonstrating the impacts of cold clouds on the long-duration station-keeping of super-pressure balloons. Finally, a cold cloud mitigation method based on phase change regulation is proposed. A backstepping sliding mode controller is designed to cyclically control the volume of the phase change gasbag, and comparisons are made with the traditional ballast release regulation. Simulation results show that the phase change regulation approach has significant advantages in resisting impacts of cold clouds.

The over-pressure and over-heating problems of the gasbag in the stratospheric airship can be solved by introducing ammonia as a phase change working gas through its significant volume change in the gas-liquid and liquid-gas phase change process. However, the thermal convection is greatly reduced at low pressure, which becomes the main challenge of this method in controlling buoyancy. This study investigates the enhancement of ammonia condensation heat transfer in fin-and-tube heat exchangers at low pressure using numerical simulations of rectangular winglet vortex generators (RWVG). With the height of RWVG increases, the Nusselt number and condensation mass flow rate first increase and then decrease, reaching the maximum value at the height of 2.5 mm. The RWVG structure significantly improves heat transfer performance by optimizing the flow structure and enhancing fluid mixing. The average heat transfer coefficient at the height of 2.5 mm is 25.2% higher than the flat case under 6.5 kPa, the condensation mass flow rate increases by 23.8%, and the thermal performance factor increases by 13.5%. The ammonia condensation efficiency under low-pressure conditions can be significantly improved by precisely adjusting the RWVG structure, providing a solution for improving the heat transfer efficiency, and achieving long-term station-keeping of the stratospheric airships.

Vapor–liquid critical properties of mixtures are key parameters in the petrochemical industry and supercritical technology. Experimental measurements and theoretical calculations are the primary methods for determining the critical parameters of mixtures. However, existing empirical correlations to quickly predict the critical temperatures and pressures of mixtures are limited by critical volume data for pure substances. In this work, improved methods of Li method and Kreglewski–Li (KL) method are proposed. Improved methods do not require critical volume data for pure substances, but replace it with acentric factors, normal boiling points, or critical temperatures of pure substances that are easier to obtain and more accurate. About 9,000 critical temperature and critical pressure data points for binary and ternary mixtures were collected to compare and evaluate the Li method, KL method, and improved methods. Notably, the improved methods are only applicable to the class I and II mixtures according to the classification of Van Konynenburg and Scott. Overall, compared with the original method, both Improvement 3 (critical volumes for pure substances of Li method and KL method are replaced with critical temperatures of pure substances) and Improvement 4 (critical volumes for pure substances of Li method and KL method are replaced with normal boiling points of pure substances) greatly improve the accuracy. Meanwhile, when predicting critical temperatures and critical pressures, Improvement 3 not only reduces the input thermophysical property parameters but also improves the prediction accuracy. Among the improved methods, Improvement 4 shows the highest prediction accuracy. The average absolute relative deviation (AARD) and average absolute deviation (AAD) of Improvement 4 for predicting the critical temperatures of binary and ternary mixtures are 1.88

In recent years, whole-body cryotherapy has garnered increasing interest within the health and medical fields, with the trend shifting from liquid nitrogen-based systems towards fully electric equipment due to its safety and low operating costs. This study focuses on the design, construction, and testing of an electric whole-body cryotherapy chamber based on a Joule-Thomson refrigeration system. The developed mixed-refrigerant refrigeration prototype demonstrated cooling capacities of 500 W, 1000 W, 1500 W, and 2000 W at temperatures of 131.6 K, 132.2 K, 134.3 K, and 142.5 K, respectively, with a relative Carnot efficiency of 23.9 % at 142.5 K. A multi-person electric cryotherapy chamber was developed based on the prototype, featuring an internal volume of approximately 3.2 m3. The chamber can be cooled to a minimum temperature of 136 K, requiring around 95 min. The testing results indicate the excellent performance of the electric-powered cryotherapy chamber at low temperatures, demonstrating its promising potential as a viable alternative to traditional liquid nitrogen-based cryotherapy chambers.

In the field of condensation research, accurate and efficient numerical simulation models are highly desired. One of the widely used phase change mass transfer models is known as the Lee model. However, this model has limitations due to uncertainties in the mass transfer intensity factor, which can lead to simulation failures and impede large-scale engineering applications. This study presents an improved mass transfer model which employs a tuning function to dynamically and accurately modify the mass transfer intensity factor in each two-phase cell via feedback adjustment. This approach enables immediate and customized adjustment of the factor for each grid within the limit of the tuning function, significantly reducing the errors caused by repeated artificial adjustments and related uncertainties. The adjustment capabilities of the hyperbolic tangent and softsign functions are evaluated. The softsign function is superior in both regulation efficiency and accuracy. The model's accuracy is verified by comparing with previous experiments and simulations, as well as by the one-dimensional Stefan problem. The proposed model achieves high prediction accuracy, reduces the risk of simulation dispersion, and significantly enhances computational efficiency with the iteration number reduced by nearly half under certain conditions compared to the Lee model. Furthermore, the new model shows robustness as the prediction accuracy is insensitive to variations in critical parameters. The improved model is expected to provide valuable assistance for future research and applications in flow condensation with outstanding computational accuracy, efficiency, and stability.