Experimental investigation and modelling of hydrodynamics and heat transfer in flow boiling in normal and microgravity conditions

The development of long-term space thermal management systems has informed research into the influence of gravity on boiling. This work explored the influence of gravity on the hydrodynamics and heat transfer of boiling flow. Experiments were carried out using two test loops each consisting of a 6 mmID transparent cylindrical test section. Upward ($+1░g$) and downward ($-1░g$) flow boiling experiments were carried out in the laboratory while microgravity ($\mu g$) experiments were carried out during a parabolic flight campaign. The results of flow visualisation showed significant influence of gravity on the flow patterns and the influence of gravity was generally limited to mass flux, $G\le 400kg/m^{2}s$ and/or vapor quality, $x\le 0.35$. In all three gravity conditions, the measured heat transfer coefficient was influenced by heat flux, mass flux and/or vapor quality. For liquid Reynolds number, $R{e}_{lo}\le 2000\left(G\le 150kg/m^{2}s\right)$ and boiling number $Bo<0.002$ the measured heat transfer coefficient was highest in $-1g$ flow and lowest in $\mu g$ flow but becomes comparable at $Bo>0.002$. A correlation for predicting microgravity heat transfer coefficient was proposed in this work and the proposed correlation predicted 100 % of the $\mu g$ data in the current work within $\pm 20\%$, predicted nearly 100 % of the $\mu g$ data of Ohta et al. (2013) within $\pm 30\%$ and around 85 % of the $\mu g$ data of Narcy (2014) within -20 % to +50 %. A correlation for predicting the gravity dependent regime as it relates to heat transfer coefficient in $+1g$ and $\mu g$ flows was also proposed in this work. The proposed criterium correctly predicted over 85 % of the gravity-dependent heat transfer coefficient in the current work and the works of Lebon et al. (2019), Narcy (2014), Ohta et al. (2013).