NUMERICAL STUDY ON CONDENSATION HEAT TRANSFER OF AMMONIA OUTSIDE A MICROCHANNEL TUBE IN THE STRATOSPHERE

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.