New turbulence modeling for simulation of Direct Contact Condensation in two-phase pressurized thermal shock

Injection of Emergency Core Cooling System (ECCS) water into the primary loops of the Pressurized Water Reactors (PWRs) leads to rapid cooling of Reactor Pressure Vessel (RPV) inside wall after Loss Of Coolant Accident (LOCA). This condition, known as Pressurized Thermal Shock (PTS) intensifies the propagation of the RPV structural defects and would be considered as an ageing mechanism. For structural and fracture analysis of RPV wall, thermal-hydraulic analysis of PTS should be accomplished to obtain the steam/water flow characteristics in the downcomer. For this purpose, simulation of steam/water stratified flow (due to density difference) after the injection point should be done by Computational Fluid Dynamics (CFD) methods. In this region, steam condensation over water layer is considered as the only heat source and controlled by turbulence eddy motion near the steam/water interface. Based on Surface Renewal Theory (SRT), Heat Transfer Coefficient (HTC) would be calculated by evaluation of turbulence length and velocity. Therefore, prediction of turbulence characteristics plays a significant role for estimation of interfacial mass transfer and temperature profile. High gradient of velocity and Turbulence Kinetic Energy (TKE), and interfacial mass and momentum transfer at the steam/water interface needs some modifications for application of traditional turbulence models. Implementation of damping function is one of the common solutions to overcome the overestimation of TKE at the steam/water interface. Although, this function improves flow characteristics of smooth stratified flow, investigation of conservation equations and experimental data implies that the other source function is needed when the flow regime changes to wavy flow. In this paper, a new source function of TKE based on variations of turbulence characteristics is proposed for steam/water interface leading to a special boundary condition of turbulence. To investigate the effects of this modification, simulation of air/water and steam/water stratified flow in three different test facilities is performed. The results show that the implementation of the source function of TKE improves the prediction of turbulence characteristics at the interface of isothermal stratified flow. Also condensation rate and temperature gradient of steam/water stratified flow have a better agreement with experimental data.