Effects of geometric parameters on lock exchange and buoyancy driven exchange flow

The process of lock-exchange and buoyancy driven exchange flow for fluids of differing densities is of particular interest in the postulated Depressurized Loss of Forced Convection (D-LOFC) accident scenario for the Very High Temperature Gas-Cooled Reactor (VHTR). This scenario involves the characteristic gravity-driven ingress of air into the helium filled reactor vessel, ultimately leading to a possible oxidation of graphite components in the vessel. In order to assure that the pertinent phenomena can be accounted for correctly in performing a scaled experiment using air and helium, a scoping test using water-brine as simulant fluids is performed. The test apparatus consists of two connected narrow rectangular acrylic tanks, one filled with brine and the other with water. In the present study, the effect of break orientation and the break length are investigated by employing a circular connecting pipe with a diameter of 1.905 cm to simulate a guillotine break. The break length is varied over the range of L/D = 0.63 to L/D = 5.0 while the break angle is varied from horizontal to vertical at every 15 degrees. The volumetric exchange rate is obtained by measuring the time rate of change for the density in a single compartment using a hydrometer. In general the results display similar characteristic behaviors in Froude number when compared to previous studies. However, it is found from experimental results and flow visualization that both the geometric configurations of the break and the formation of the mixture interface inside the compartment are important in determining the exchange rate. It is seen that during the initial ingress, the upper tank density decreases linearly with time, but the rate of ingress decreases significantly as the mixture interface reaches the break This causes the overall exchange rate to decrease up to 70 % of the value in the initial steady region.