Effects of surface orientation on wall heat flux partitioning during nucleate pool boiling of saturated water at atmospheric pressure

Orientation of the wall exerts a strong influence on the bubble-scale parameters during nucleate boiling and the resulting wall heat flux. A number of mechanistic models have been developed for the prediction of wall heat flux and partitioning in nucleate boiling. The mechanistic model by Kurul and Podowski (usually called as the RPI model), is widely employed in two-phase computational fluid dynamic and thermal-hydraulics codes for nuclear reactor system analysis. However, the RPI model was not developed with taking effects of surface orientation on the bubble-scale parameters and wall heat flux of nucleate boiling into account. This study aims at experimentally examining effects of the surface orientation on the bubble-scale parameters and wall heat flux of nucleate boiling and improving the prediction accuracy of the RPI wall heat flux partitioning model. In this study, nucleate boiling experiments of saturated water under atmospheric condition were conducted on a wall with a constant surface temperature of 107.5 °C. Orientation of the boiling wall changed from 0 (upward-facing horizontal) to 30, 60, 90° (vertical). A unique optical setup integrating infrared thermometry, total reflection, and shadowgraph techniques in the present study permitted to simultaneously acquire quantitative data on all the bubble-scale parameters related with the sub-models, including nucleation site density, bubble departure diameter and frequency, bubble wait time. The bubble parameters and total wall heat flux obtained from experiment and the RPI model are compared with regard to surface orientation. There was a big difference in them between measured data and prediction data by RPI model. The main cause of that was the effect of bubble merging; the effect of bubble sliding was negligible. The evaporation was a dominant contributor for heat flux with surface orientation of 0, 30, 60, 90.