Interfacial drag force model based on the drag coefficient approach for dispersed gas-liquid flows in rod bundles

TRAC-BF1/MOD1 is commonly employed for the safety analysis of the boiling water reactors (BWRs). TRAC-BF1/MOD1 uses the two-fluid model, requiring the interfacial drag force model in the phasic momentum equations. The interfacial drag force influences the prediction of the void fraction, a critical parameter in the safety assessment. This study aimed to develop an interfacial drag force model for dispersed flows in rod bundles using the drag coefficient approach with improved interfacial area concentration and shape factor models. This study first evaluated the TRAC-BF1/MOD1 interfacial area concentration model, showing the mean absolute relative error (MARE) of 849%. The primary reason for the extremely high MARE was that the TRAC-BF1/MOD1 adopted a critical Weber number approach. The TRAC-BF1/MOD1 utilized the relative velocity as the characteristic velocity scale in N We c which would be suitable for aerodynamic droplet breakup. However, the relative velocity would not be a suitable characteristic velocity scale for bubble coalescence and breakup induced by the liquid turbulence. This study developed an improved interfacial area concentration model in which the critical Weber number was defined by the turbulent fluctuation velocity. Next, this study identified that the assumption of a shape factor equal to 1 used in TRAC-BF1/MOD1 was inappropriate. This study demonstrated that the shape factor could be significantly less than unity and developed a shape-factor model. The MAREs for the developed interfacial area concentration, shape factor, and interfacial drag force models were 27.6%, 25.2%, and 14.3%, respectively, using the air-water data in rod bundles. The proposed interfacial drag force model was evaluated using boiling flow data from rod bundles at elevated pressures up to 12.0 MPa, yielding an MARE of 15.2% across databases covering void fractions up to 0.920 in rod bundles.