Effect of surface orientation on nucleate boiling and critical heat flux of dielectric fluids

Experiments were performed to analyse the effects of surface orientation on nucleate boiling of dielectric fluids from a smooth copper surface. Pool boiling data at atmospheric pressure were obtained with both saturated FC-72 and HFE-7100. For both fluids, the orientation angle of the copper surface from the horizontal plane was varied from 0°(upper surface) to 175°, with regular increments. For each orientation, the boiling curve was plotted up to thermal crisis conditions. The results obtained confirmed some of the effects of heater orientation on heat transfer coefficients reported in the literature: in the low-heat-flux nucleate boiling region, the heat transfer coefficient increases markedly with orientation angle; for higher heat flux values, the effect is evident only for angles greater than 90° and the heat transfer coefficient diminishes as the angle increases. The critical heat flux CHF decreases slightly as the orientation angle increases from 0° to 90°, while for downward-facing surfaces, the CHF decreases rapidly as the orientation angle increases towards 180°. CHF values obtained for both fluids and for different angles, normalized to the maximum value at 0°, showed good agreement with several literature correlations. The heat transfer coefficients and CHF values of HFE-7100 were found to be higher than those of FC-72. Finally, new correlations for heat transfer coefficients and CHF values as a function of inclination angle have been proposed.