The impact of inclination on flow boiling inside a 7 mm outer diameter microfin tube is experimentally investigated and visualized for the low global warming refrigerant R515B. The heat transfer coefficient is evaluated at mass fluxes of 50, 100, 200, and 400 kg m–2 s–1, heat fluxes of 10, 30, and 50 kW m–2, at the saturation temperature of 30 °C, for vapor qualities ranging from 0.1 to 0.9. The investigated inclinations vary from +60° to –60° including the horizontal one. Under conditions of nucleate boiling dominance with the absence of strong convective boiling, the downward configurations experience an elevation of the heat transfer coefficient while the upward configurations have a lower heat transfer coefficient. Increasing the mass flux and heat flux to 400 kg m–2 s–1 and 50 kW m–2 nullifies the effect of inclination for all inclinations except for –60°. A noteworthy aspect with respect to –60° was the occurrence of thermal crisis at high vapor qualities, a unique phenomenon that did not occur at other inclinations at similar operating conditions, which was ascribed to the nature of the flow patterns. Two correlations (Irannezhad et al. (2024) and Mehendale (2017)) developed for flow boiling inside horizontal microfin tube were compared with the results which resulted in mean absolute error of 21.4% and 25.2% respectively. A dimensionless parameter, which is correlated with inclination factor (IF) was proposed, which could be utilized for further studies as a tool to predict the heat transfer coefficients for various inclinations.
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