Flow boiling critical heat flux at sudden expansion flow path under low pressure condition

Abstract

In severe accident condition of nuclear power plant, cladding can be deformed by rapid change of pressure and temperature. Cladding deformation, especially for ballooning, can change the flow path of coolant, which changes the flow and heat transfer characteristics. Effect of flow path change toward critical heat flux (CHF) was experimentally investigated to ensure safety of long-term core cooling phase with deformed channel. From the experimental results, location of CHF was varied by expansion ratio and inlet mass flux. In experiments with either a low expansion ratio ($\epsilon<1.5$) or a low mass flux ($<150$ $kg/m^2s$), CHF occurred at the test section outlet with values similar to results from a plain tube with the same inlet diameter. Core condition during long-term core cooling phase was in this range, so deformed cladding showed negligible effect on CHF for present reactor design. On the other hand, in experiments with a high expansion ratio ($\epsilon<1.5$) and high mass flux ($<150$ $kg/m^2s$), CHF was detected at the sudden expansion point. In order to analyze this phenomenon, flow velocity was obtained from the CFD and force balance was analyzed by that result. The reason for transition of CHF location was suggested that local bubble stagnation induced by a balance between drag and buoyancy forces on the bubbles resulted in local flooding in the downward flow.