Prandtl Number Dependent Natural Convection Heat Transfer in a Rectangular Pool

During a severe accident in a nuclear reactor, the core may melt and be relocated to the lower plenum to form a hemispherical pool. Should there be no effective cooling mechanism, the core debris may heat up and the molten pool run into natural convection due to decay heat. Natural convection tests were performed in SIGMA RP (Simulant Internal Gravitated Material Apparatus Rectangular Pool). The SIGMA RP apparatus comprises the test section, heat-exchanger, cartridge heaters, thermocouples and the data acquisition system. The internal heater heating method was used to simulate uniform heat source which is related to the modified Rayleigh number (Ra). The test procedure started with air and water, the working fluid, filling the test section. The working fluid Prandtl number (Pr) ranged from 4 to 8 for water, and 0.7 for air. Particular attentions were paid to the Pr influence on natural convection heat transfer in the pool. There were two boundary conditions: one dealt with both walls being cooled isothermally, while the other had to with only the upper wall being cooled isothermally. The heat exchanger was used to maintain the isothermal boundary condition. Four side walls were surrounded by the insulating material to maintain the adiabatic condition and to minimize the heat loss. In the top and bottom boundary cooling condition, the upward Nusselt number (Nuup) was greater than the downward Nusselt number (Nudn). For a lower Pr fluid, the thermal boundary layer at the upper wall is greater than for a larger Pr fluid. Note that the heat transfer is inversely proportional to the thermal boundary layer thickness. This implies that high Pr increased the upward heat transfer Nuup. The influence of Pr on the fluid behavior grows with the increase in Ra