CHF Measurement for Downward-Facing Stainless Steel and Carbon Steel Flat Plates under Pool Boiling Condition

Abstract

When comparing downward-facing condition to upward case, it shows some unique characteristics; Generated bubbles are hard to escape, and thus, residing time & covering area is larger. In addition, there are macroscopic natural circulation effect and dimensional effects according to the previous researches. Due to lack of academic observation and approaches, there still is room for further approaches. In this study, two materials have been considered (SA 508 & SS 304) to consider material effect. SA 508 is normally utilized in RPV (Reactor Pressure Vessel) in nuclear power plants and sometimes for core catcher. Therefore, material effect had to be considered, but in most of preceding studies, stainless steel or copper have been adopted. Furthermore, dimensional effect (especially for width effect) have been assessed under atmospheric and 2 bar conditions. Heater widths have been chosen based upon preceding observations. Coalesced bubble motions were analyzed to explain the width effects. According to the results, SA 508 tended to show equivalent or sometimes enhanced results compared with stainless steel heaters. According to the preceding researches, enhancement of SA 508 was mainly explained by wettability effect, but there was no effective oxidation occurred on SA 508 surfaces. Static contact angles were measured before and after the experiments which could explain it. Also, concentration of dissolved oxygen has been measured, and several boiling scheme have been utilized to assess the oxidation effect.