Surface orientation effects on heat transfer performance of spray cooling

•Surface temperature distribution and liquid flow are similar for different orientations.•Vapour–spray (or surface) interaction accounts for surface orientation effects on CHF.•A macrolayer model introduced allows predicting the CHF for different orientations. In this study, the heat transfer performances of spray cooling for upward, vertical, and downward surfaces under different nozzle-to-surface distances, inlet pressures, nozzle types, and surface areas were investigated experimentally. Almost no difference in heat transfer performance can be found among the different orientations in single-phase and two-phase regimes. A considerable orientation dependency of the critical heat flux (CHF) can be observed in medium-spray volumetric fluxes: the CHF of the vertical surface is higher than that of the upward and downward surfaces. The orientation dependency of the CHF reduces as the inlet pressure increases, implying that an increase in droplet diameter or decrease in droplet velocity would enhance this dependency. The surface orientation has no significant effect on surface liquid film flow and temperature distribution. In this paper, interaction between the vapour and spray (or surface wall) is proposed and verified to be the main mechanism that affects orientation dependency by intermittent spray cooling. A macrolayer model for the CHF is semi-empirically introduced to account for the orientation effects and provide a quantitative evaluation of these effects. Finally, based upon this model, a general correlation for the CHF is proposed.