Development of wall and interfacial friction models for two-dimensional film flow with local measurement methods

Based on the advancement of computational capability, high-precision analyses of multi-dimensional thermal hydraulic phenomena in nuclear power plants have been carried out. One of the multi-dimensional phenomena is two-phase flow in the upper downcomer. The quantification of momentum transfer between a downward liquid flow and a transverse gas flow in the reflood phase is significant because it determines the bypass flow rate of the emergency core coolant (ECC). Multi-dimensional modules of thermal-hydraulic analysis codes have been used to simulate this multi-dimensional two-phase flow. However, the model and correlations incorporated in the multi-dimensional modules are ones developed from one-dimensional experimental results. Because the characteristics of multi-dimensional phenomena are different from one-dimensional conditions, the implemented models and correlations might produce inaccurate results. Therefore, new models based on multi-dimensional experimental data should be developed. This study involved the twodimensional, two-phase experiments performed using 1/10 and 1/5 scale facilities. These were simplified unfolded downcomers of the Advanced Power Reactor 1400 (APR1400). The local measurement methods, pitot tubes, ultrasonic thickness gauge, and depth-averaged PIV, were applied to measure local variables: air velocity, liquid film velocity, and liquid film thickness. Local experimental data on the two-dimensional film flow were used to produce the wall and to inform interfacial friction factors from two-dimensional, two-phase momentum conservation equations. New wall and interfacial friction models were developed incorporating newly proposed physical concepts.