COMPARISON OF FLOW INSTABILITY CRITERIA AND THRESHOLD CALCULATION METHODS OF DENSITY WAVE OSCILLATIONS IN A VERTICAL BOILING CHANNEL

Abstract

Two-phase flow instabilities have aroused considerable attention and been extensively studied in nuclear engineering. Among them, density wave oscillations are the foremost ones possibly encountered in boiling water reactors and steam generators. Because reaching flow instability is usually a gradual process with increasing oscillation amplitude, a variety of flow instability criteria have been put forward from different perspectives. With various flow instability threshold calculation methods, flow instability boundaries with considerable discrepancy could be obtained. In this work, density wave oscillations in a vertical single boiling channel with constant pressure drop are simulated by RELAP5/MOD4.0 code. The cross-sectional areas of timedependent volume components are varied to ensure valid pressure boundary conditions. Different heating power growth rates are tested to achieve flow instabilities by increasing heating power linearly in order to check whether the onset of the flow instabilities is influenced by different disturbances introduced. Flow instability boundary data obtained from the combinations of three typical flow instability criteria and three flow instability threshold calculation methods are compared to systematically investigate the influence of these two factors. Finally, the effect of inlet subcooling is studied to explain the above observations. This work could serve as a reference for the selection of flow instability criteria and flow instability threshold calculation methods to obtain accurate and conservative flow instability boundaries.