Abstract
One of the main concerns regarding in-vessel retention (IVR) during a severe accident is guaranteeing sufficient cooling performance to avoid the melt-through of the pressure vessel. In such an event, the vessel is submerged in water, and boiling is occurred to remove the heat. However, the main problem is that there is a limit to the pool boiling heat transfer at the outer surface of the reactor vessel due to occurrence of critical heat flux (CHF) conditions. Therefore, CHF enhancement is important to increase safety margin of IVR. In previous studies, CHF was enhanced approximately twice compared to bare surface by attaching honeycomb porous plate (HPP) to the heated surface. In order to establish the IVR, the cooling technology should be considered in forced flow boiling with the downward heat transfer surface. Moreover, we proposed a two-layer structured HPP for further CHF enhancement. Therefore, in the present paper the effect of the single and two layers HPPs on the critical heat flux under flow boiling with the downward heat transfer surface was investigated experimentally. As a result, the coalescent vapor bubbles cover the heated surface for a long period depending on the flow pattern under the flow boiling condition, resulting in liquid drying inside a HPP and reduction of CHF. Even in this situation, a two-layer structured HPP can be applied to CHF enhancement in flow boiling with the downward heat transfer surface.
