(An) experimental study on the effect of the heated surface orientation on the critical heat flux

Abstract

This study has been accomplished to investigate the effect of the heated surface orientation on the Critical Heat Flux (CHF) in the pool of saturated water at atmospheric pressure. An experimental loop is constructed under atmospheric pressure and experiments have been performed for various orientations using two test sections. One has 0.02 m width, 0.20 m length, 0.0005m thickness and the other has 0.025 m width, 0.20 m length, 0.0005m thickness. Test sections are made up of stainless steel 304 considering the corrosion in the water pool and rectifier capacity. CHF data are obtained for various orientations, $180^\circ$ (downward facing), $178^\circ,\;176^\circ,\; 174^\circ,\;150^\circ\;\mbox{and}\; 130^\circ$, using two test sections. Experimental results show that (1) CHF decreases as the heated surface orientation increases. CHF gradually decreases till certain degree(transition degree) as the degree of the heated surface increases and rapidly decreases after that. This transition degree decreases as the width of the test section increases. For all orientations, measured CHF is much lower than those predicted by typical pool boiling CHF correlations. (2) CHF decreases as the width of heated surface increases. Above two results can be inferred by following reasons. Generated bubbles are stayed at the heated surface for a long time as the degree and the width of heated surface increase and water is not contact with the heated surface. Thus, heat generated at the surface is not sufficiently cooled by the water. Therefore, CHF is occurred at low heat flux. (3) There are two mechanisms in the heat transfer of inclined heated surface. They are bubble agitation mechanism and evaporative mechanism. Bubble agitation mechanism seems to be most dominant at low heat flux and evaporative mechanism appears to be dominant at high heat flux.