Numerical research on thermal mixing characteristics in a 45-degree T-junction for two-phase stratified flow during the emergency core cooling safety injection

The thermal mixing and direct contact condensation are the most significant phenomena appeared during the Emergency Core Cooling (ECC) system safety injection process. The three-dimensional thermal mixing characteristics accompanied by complicated direct contact condensation are investigated using CFD method. Firstly, the feasibility of two-phase CFD models are validated by the experimental data obtained from the XJTU-ECC experimental apparatus. Results show that the established CFD models could predict the two-phase mixing process correctly and the errors between numerical results and experimental data are smaller than 30%. Then the two-phase thermal mixing features, including temperature fields, velocity fields and volume fraction, are achieved in detail. Moreover, the local steam condensation characteristics are studied carefully and results show that the condensation rate is proved closely related to the coolant thermodynamic ratio between the main pipe and branch pipe. Finally, an empirical correlation is proposed to predict the condensation rate in case that the thermodynamic ratio is lower than 0.55. This work provides a fundamental guideline for the ECC system structure optimization and fatigue aging in the advanced nuclear power plants.