Experimental Study for Development of Wall and Interfacial Friction Models in Two-dimensional Film Flow

Abstract

Recently, high-precision analyses of multi-dimensional thermal hydraulic phenomena in nuclear power plants have been conducted using state-of-the-art techniques. One of the multi-dimensional phenomena is multi-dimensional two-phase flow in the upper downcomer, which is due to an interaction between a downward liquid and a transverse gas flow in the reflood phase of large break loss of coolant accident (LBLOCA). It is important issue because it determines the bypass flow rate of the emergency core coolant (ECC) and subsequently, the reflood coolant flow rate. At present, some thermal-hydraulic analysis codes which incorporate multi-dimensional modules for the nuclear reactor safety analysis were used to simulate the multi-dimensional phenomena. However, their prediction capability for the two-phase cross flow in the upper downcomer has not been validated sufficiently against experimental data based on local measurements. For this reason, an experimental study was carried out for the two-phase cross flow to clarify the hydraulic phenomenon and provide local measurement data for the validation of the computational tools. The experiment was performed in 1/10 and 1/5 scale unfolded downcomers of Advanced Power Reactor 1400 (APR1400). Pitot tubes and ultrasonic thickness gauge were applied for local measurement of the air velocity and the liquid film thickness, and a depth-averaged PIV method was developed to measure the averaged liquid film velocity in the depth direction. The uncertainty of the depth-averaged PIV method with validation experiment was discussed. The local experimental data of two-dimensional film flow were used to assess the wall and interfacial friction models in the multi-dimensional module of MARS-MultiD and the assessment results indicated the wall shear stress was underestimated. To modify the problem of previous MARS-MultiD, wall and interfacial friction factors from two-dimensional, two-phase momentum conservation equations were used to investigate the reason of problem. And the new wall and interfacial friction models were developed based on the wall and interfacial friction factors. The modified MARS-MultiD with newly developed models produced improved results. The modification of code capability was also confirmed with MIDAS experiment which is steam-water separated effect test to simulate the multi-dimensional two-phase flow in the upper downcomer with 1/5 scaled test facility. Finally, the new wall and interfacial friction models can be used to simulate ECC bypass phenomenon more accurately.