Numerical analysis of a compressed air-cycle refrigeration system with indirect evaporative gas coolers under reduced ambient-pressure conditions

Air-cycle refrigeration uses air as an environmentally benign working fluid, but its performance is often limited by high-temperature-side heat rejection. This study couples indirect evaporative gas coolers with a two-stage compressed air-cycle refrigeration system and evaluates its performance under reduced ambient-pressure conditions. A segmented model of the gas cooler is combined with a system model to analyze the effects of ambient pressure reduction, ambient temperature, and ambient relative humidity on the gas-cooler outlet state, expander inlet state, cooling capacity, and COP. The results show that pressure reduction produces competing effects: it increases the evaporation driving force but decreases the mass-transfer coefficient. Under the baseline condition, increasing ambient pressure reduction from 0 to 20 kPa increases the evaporative latent heat transfer rate from 10.16 kW to 10.34 kW, while reducing the mass-transfer coefficient from 45.71 to 39.79 g/(m²·s). At the system level, the expander inlet temperature decreases from about 27.22 to 24.15 °C, and the specific cooling capacity increases from 14.35 to 17.18 kJ/kg. These system-level changes indicate an evaporation-driven compensation effect, in which the increase in specific cooling capacity partly offsets the reduction in circulating-air mass flow rate. As a result, the total cooling capacity can be maintained or even increased under high-temperature or high-humidity conditions, while the COP improves throughout the investigated range. The results suggest that indirect evaporative gas cooling can enhance high-temperature-side heat rejection and mitigate capacity degradation in air-cycle refrigeration systems operating under reduced ambient-pressure conditions.