Six-Field Governing Equation Development for Advanced System Codes

System codes are used to analyze nuclear reactor systems during steady state and transient operations. These codes are able to predict pressure drop, void fraction distributions and temperature distributions for various coolants, heated flow geometries, and heat configurations. Advanced system codes are able to model two-phase flow regimes using built-in models and correlations to account for the heat transfer, drag, and interactions between phases. In spite of this, extreme flow conditions can tax current code capabilities, particularly those flows that involve significant phase change. Current system codes (such as RELAP5-3D, RELAP-7, and TRACE) have a six-equation model that includes mass, momentum, and energy conservation equations for two fields (liquid and vapor). Work on the conservation equations in a few of these codes has included development of a separate droplet field from the continuous liquid. This is in line with a trend toward the inclusion of more fields in system codes. The representation of two phase flow phenomena is improved by increasing the number of fields. Conservation equations based on six fields (liquid, vapor, small bubble, large bubble, small droplet and large droplet) are derived in this work.