The secondary deterioration phenomenon of heat transfer performance of supercritical CO2 in horizontal tube under different gravity conditions

Abstract

Supercritical CO₂ (SCO₂) is a promising aerospace coolant. However, spacecraft often operate under abnormal gravity. Quantifying SCO₂ heat transfer characteristics under different gravity is crucial for enhancing aviation thermal control system precision and stability. Therefore, this study employs simulation methods to explore the influence mechanisms of gravity on the heat transfer performance in macrochannel and microchannel. Furthermore, an evaluation system is constructed to quantify the degree (P_up) and range (D) of localized heat transfer deterioration caused by gravity. Finally, the variations in evaluation factors under different gravity (G) and operational conditions are investigated. The results show that in both channels, near-wall high-temperature SCO₂ gas flows upward under gravity, forming blocking film near the top baseline. This self-circulation phenomenon deteriorates the heat transfer performance near the top baseline, while enhancing it along the rest of the channel wall. For the macrochannel, as the fluid advances, localized reverse-circulation occurs near the top baseline, further worsening heat transfer in this region, defined as the secondary deterioration phenomenon. However, in microchannel, the top and bottom baselines heat transfer gradually converges, and no secondary deterioration is observed. As G increases, P_up increases, D first decreases and then increases. The novelty of this work is the construction of the evaluation system and the comprehensive assessment of the SCO₂ heat transfer characteristics in horizontal tubes under anomalous gravity. The paper provides scientific basis for designing aerospace thermal control system. Additionally, it offers reference for heat transfer characteristics of other supercritical fluids in horizontal tubes under abnormal gravity.