Numerical Analysis of Flow Instabilities in Natural Circulation Two Phase Flows

This paper deals with fundamentals of the geysering and density-wave instabilities that may occur during the start-up process of a natural-circulation BWR, with the focus placed on revealing the difference in mechanisms of the two instabilities. Numerical analysis is conducted using a simple, one-dimensional homogeneous two-phase flow model with the effect of flashing taken into consideration. Through comparison of the numerical results with experimentally recorded void-distribution and flow rat6 0scillations, the different mechanisms of the geysering and the density-wave instabilities are revealed. The geysering instability is found to occur when the effect of flashing is strong enough to form a large, rapidly-growing bubble that can rapidly accelerate the liquid plug above it. An expulsion at the channel outlet actually starts at the moment when the liquid plug is pushed out of the channel