EXPERIMENTAL AND NUMERICAL STUDY OF TWO-PHASE FLOW INSTABILITIES IN NATURAL-CIRCULATION BOILING CHANNELS

摘要

Experimental and numerical studies are carried out toinvestigate the fundamentals of the geysering and the densitywaveinstabilities in natural circulation boiling channels underlow pressure and low heat flux conditions, which are of concernto the start-up process of a natural-circulation BWR or SBWR.The experiment is conducted with a test facility that consists ofheated boiling channels inserted into bulk water. The numericalanalysis is conducted using a simple, one-dimensionalhomogeneous two-phase flow model with the effect of flashingtaken into consideration. Through comparison of the numericalresults with experimentally recorded void-distribution and flowrate oscillations, the different mechanisms of the geysering andthe density-wave instabilities are revealed. The geyseringinstability is found to occur when the effect of flashing isstrong enough to form a large, rapidly-growing slug bubble thatcan rapidly accelerate the liquid plug above it. An expulsion atthe channel outlet actually starts at the moment when the liquidplug is pushed out of the channel.