The rapid development of spaceflight, especially manned spaceflight and deep space exploration, puts forward more stringent requirements for space heat transfer technology in terms of power, heat flux density and transmission distance, and the multiphase heat fluid system based on latent heat release of gas-liquid phase change has become the main direction of the development of widely valued space advanced heat transfer technology due to its efficient heat transfer performance. The flow characteristics and heat transfer performance of gas-liquid two-phase caused by condensation in different gravity environments are one of the key issues in the space application of multiphase thermal fluid systems, which determine the heat transfer capacity and system stability of the system. When the working medium is a deep and low temperature working medium, the lower surface tension will lead to the instability of the gas-liquid interface, and this trend is superimposed on the change of gas-liquid interface distribution pattern caused by the difference in gravity, which will further strengthen the difference between condensation flow and heat transfer characteristics in the deep and low temperature working fluid pipe. The detailed analysis of the gas-liquid interface behavior and its influence on heat exchange performance during the condensation process of deep and low temperature working fluid flow under different gravity has great practical value for the application of space two-phase system, especially the design and optimization of condenser, one of the key components.
Recently, the research team of Zhao Jianfu, Key Laboratory of Microgravity of the Institute of Mechanics, has made progress in the heat transfer characteristics of condensation flow and the influence of gravity level in neon working fluid tubes in the 35K temperature zone. The team constructed a numerical model of condensation flow and heat transfer in deep and low temperature working fluid tubes, and fully verified the reliability of the numerical model by comparing the experimental data of ground constant gravity neon working fluid condensation heat transfer and the observation results of nitrogen working fluid condensation flow pattern with the experimental data of ground constant gravity neon working fluid and the observation results of nitrogen working fluid condensation flow pattern by Professor Wang Wen of Shanghai Jiao Tong University. Using the verified numerical model, the effects of different pipe diameters, mass fluxes and gravity levels on the gas-liquid distribution pattern and heat transfer characteristics of neon working fluid condensation flow were systematically analyzed by system simulation, and the formation mechanism of heat transfer intensification, heat transfer deterioration and gravity irrelevant in the condensation heat transfer process of deep and low temperature working fluid was revealed, and the local liquid film distribution pattern determined by the competition between inertial force, gravity and surface tension in the flow was revealed. Among them, gravity has a significant thinning effect on the liquid film in the upper part of the horizontal pipe section, and also causes condensate to converge at the bottom to form a so-called “liquid pool”. The thin liquid film has small thermal resistance and high heat transfer efficiency; However, the large thickness of the liquid pool leads to a significant increase in thermal resistance and low heat transfer efficiency. The decrease in the thickness of the liquid film in the upper part of the horizontal pipeline section caused by gravity and the decrease in the circumferential proportion of the thin liquid film will exist at the same time, and its comprehensive effect depends on the specific flow state, and then presents three different manifestations of heat transfer intensification, heat transfer deterioration or gravity irrelevance depending on the specific situation. For deep and low temperature working fluids, the surface tension is often low, and when the gravity decreases, the condensation interface is more likely to flow instability, causing fluctuations in the gas-liquid interface, and even causing the liquid film to rupture and produce droplet entrainment. The fluctuation of the gas-liquid interface will increase the heat exchange area, and the droplet entrainment will reduce the thickness of the liquid film of the attached wall, which can enhance the condensation heat transfer effect.
The relevant research results have been published in Applied Thermal Engineering (2023, 233: 121162) and Journal of Engineering Thermophysics (2023, 44(3): 684-689), the first author is He Falong, a doctoral student of the Institute of Mechanics, Du Wangfang, a researcher of the Institute of Mechanics, a researcher Zhao Jianfu, and Miao Jianyin, a researcher of the Beijing Space Vehicle General Design Department of the Chinese Academy of Space Technology, are co-corresponding authors. Some of the results were communicated by He Falong at the first International Conference on Multiphase Transmission and Energy Conversion and Utilization (MTCUE-2022) and won the “Best Paper Award”.
The related work has been supported by the National Natural Science Foundation of China (11972040) and the National Key Research and Development Program of China (2022YFF0503502). (Source: Institute of Mechanics, Chinese Academy of Sciences)
Related paper information:https://doi.org/10.1016/j.applthermaleng.2023.121162
http://dspace.imech.ac.cn/handle/311007/91903
Fig. 1 Numerical model verification results of condensation heat transfer in deep and low temperature working fluids
Fig. 2 Effect of gravity level on gas-liquid distribution patterns of neon working fluid sections with different dryness
Fig. 3 Effect of gravity level on condensation heat transfer coefficient of neon working fluid flow under different mass flow rates
Fig.4 Gas-liquid distribution patterns of different dryness positions in 1mm and 2mm horizontal tubes (a) x=0.9; (b) x=0.7; (c) x=0.5; (d) x=0.3
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