Improvement of TRAC-PF1 interfacial drag model for analysis of high-pressure horizontally-stratified two-phase flow.

Abstract

Horizontally stratified two-phase flow is one of the important phenomena for analysis of small-break loss of coolant accidents (SBLOCAs) in a pressurized water reactor (PWR). Thus, separate effect tests have been conducted at the ROSA-IV Two-Phase Flow Test Facility (TPTF) to investigate the flow regime transitions in horizontal, co-current, steam-water two-phase flow for high pressure (3~11.9 MPa) in a large diameter (180 mm) pipe. These experiments were analyzed using the TRAC-PF1/MOD1 code to assess the adequacy of the code's flow regime transition criterion and interfacial drag models. The TRAC code predicted slug or transitional flow regime for tests conducted for relatively high flow rates, whereas, the experimental flow regime was stratified flow. This resulted in a large overprediction of the interfacial drag for these tests. The flow regime transition criterion, the Taitel-Dukler model, was modified according to the experimental flow regime transition condition, by replacing the gas velocity in the criterion by the relative velocity. However, the interfacial drag was still overpredicted for the stratified flow regime. A sensitivity study was carried out on this coefficient to find the value which provides the best agreement with respect to the test section void fraction distribution. Based on this sensitivity study, it is recommended to use an interfacial drag coefficient which is a half of the value used in the current version of the TRAC.