The effects of microgravity on mini-channel flow boiling and CHF behavior

Subcooled flow boiling is a promising technology for space applications since it can control the motion of bubbles near the heated surface and enhance nucleate boiling. However, a lack of data and predictive models on gravity's role hinders the use of flow boiling in a microgravity (μg) environment. We address this knowledge gap by performing flow boiling experiments on Earth and the International Space Station. This study investigates the effect that gravity has on flow boiling of n-Perfluorohexane in a rectangular channel of 5.0 × 2.5 mm² cross-sectional area and 114.6 mm heated length. The comparison of results of μg and 1 g experiments, demonstrates that when the vapor quality is low, gravity plays only a small role in the mechanisms behind flow boiling, and when the vapor quality is high, a larger difference can be seen, where a 15.06% drop in the Nusselt Number at critical heat flux is observed in μg. Gravity does not appear to affect when critical heat flux occurs. It was found that critical heat flux occurs at a Boiling Number of 1.53⋅10⁻³ and 1.43⋅10⁻³ in 1 g and μg, respectively, corresponding to a heat flux of 13.4 W/cm² and 12.4 W/cm², respectively. At Reynolds Numbers below 4000, there is a performance drop in μg. For Reynolds numbers above 4000, the two systems perform similarly. At Reynolds Number of 2440, a 54.96% drop in the Nusselt Number was observed. A correlation linking the wall superheat temperature to the total heat duty, was shown to be statistically significant.