| Abstract: | Quantitative measurements of scalar dilution transients within a scaled model of a PWR downcomer are presented. The injection transients correspond to the case of a single pump start-up injecting into an initially stagnant system. Detailed results of the flow path and velocity field are presented for a nominal case where a neutrally buoyant front of dilute water is injected into the downcomer. Additional tests are made for a finite-sized slug of dilute water with a volume approximately equal to that of the downcomer and the lower plenum. The results of the tests indicate that the flow is dominated near the injection leg by an impinging jet, which is bifurcated into two primary streams by the cylindrical geometry of the downcomer. The two streams exit the downcomer on the opposite side of the injection leg. The splitting of the streams creates a bell-shaped recirculation region in the lower downcomer beneath the injection leg, which is the last volume to be displaced by the injected fluid flowing up from the lower plenum. A third plume sometimes forms at the boundary between the streams and the recirculation region, introducing mixed fluid into parts of this relatively stagnant area at earlier times than is typically observed. The results are discussed in the context of computational fluid dynamics codes that are starting to be used to simulate the mixing phenomena encountered in such complex systems. |
