Effect of Wettability and Capillary Wicking Changes Induced by Oxidation on the Pool Boiling Critical Heat Flux

초록

For the successful application of the In-Vessel Retention-External Reactor Vessel Cooling (IVR-ERVC), defining the Critical Heat Flux (CHF) on the Reactor Pressure Vessel (RPV) outer surface is a key issue to secure the integrity of the RPV. Since the CHF is strongly affected by heating surface condition, if any surface treatments such as nano-fluids boiling or surface coating are not applied to the RPV outer surface, corrosion can dictate the effects on the CHF. This is because corrosion occurs for long time on the RPV, which is made of low carbon steel alloy of low corrosion resistance material and is irreplaceable for reactor operation time. Therefore, in this study, pool boiling heat transfer experiment was conducted with oxidized metal plates in the heater orientations of 0 and 180 degrees in atmospheric saturated water. Heater specimens made of stainless steel grade 316 (SS316) were prepared and oxidized at several distinctive periods. The oxidized surface became more wettable and rougher than bare specimens. Moreover, the heater surfaces oxidized for a short period experienced local oxide deposition, whereas the heater surfaces oxidized for a long period exhibited fairly flat oxide layer thickness. The contact angle measurement confirmed low contact angle of 15 degrees for all the oxidized surfaces. However, the maximum CHF enhancement was measured with the heaters of fully flat continuous oxide layer. Surface examination using the SEM suggests that existence of continuous porous channel in fully flat oxide layer may act as capillary wick capable of absorbing liquid effectively in superheated region and then delaying vapor coalescence. Although enhanced wettability was obtained with the local oxidation, its effect on the CHF was insignificant, which, in turn, proves that wettability may not be a predominant parameter in determining the CHF enhancement.