Flow and heat transfer experiment in a RPV with direct safety injection

An experimental investigation was performed to study the thermal hydraulic behavior in the RPV downcomer when the direct vessel injection (DVI) was adopted to enhance the effectiveness of the safety injection. Flow and heat transfers were studied experimentally on a 1/10-scaled RPV model at low temperature and pressure. Three experiments, including flow visualization, local convective heat transfer coefficient and temperature transient were performed at four possible conditions during safety injection. Both single-phase and two-phase flows were investigated. The vapor is simulated by air based on void fraction similarity in two-phase flow condition. The visualization experiment revealed the flow pattern was determined by the safety injection velocity USI and downcomer flow velocity UDC when the inlet flow rate of the cold leg was more than 0 m/s at single-phase test. The injection impinges the barrel and expands along the downcomer at stagnant flow condition. When the downcomer is filled with air, the injection flow is attached on the barrel at most of cases. The stronger turbulence was found in two-phase than that in single-phase flow. Heat transfer at some points in the RPV has monotonous ascend, but at other points, it may ascend, then fall and then ascend again. The transient temperature is determined by the injection velocity, downcomer flow rate and the distance from the nozzle. The local convective heat transfer coefficient was the largest at the zone near the nozzle and the temperature transient here was fast too. When the point was very far from the nozzle, the heat transfer coefficient and temperature were mainly controlled by the downcomer flow rate. The local convective heat transfer coefficient is linear to the raise of injection velocity except that very near to the nozzle, which heat transfer coefficient is special and complex since it is affected by impingement strongly.