Convective Cooling of Simulated Core Debris Beds

Natural convection heat transfer in simulated core debris beds has been examined. The debris beds are simulated using electrically heated packed tube bundles arranged in either a square or staggered lattice with porosities varying between 0.31 and 0.95. The effects of bed height, heat generation rate, particle size, porosity, overlying liquid layer height, and top surface boundary condition on the downward and upward power fractions and Nusselt numbers have been determined. Flow patterns within the bed and overlying fluid region have been visualized using particle tracing techniques. Correlations for the downward and upward Nusselt numbers, Nu_B and Nu_T, as functions of the internal Rayleigh number have been developed. In all cases, the beds are bounded from below by a cooled isothermal surface. When the overlying fluid is bounded from above by a cooled solid isothermal surface, the Nusselt numbers are given by Nu_B = 0.424 Ra^0.226 and Nu_T = 1.61 Ra^0.220. When the upper surface of the overlying fluid is free, the downward Nusselt number is given by Nu_B = 0.503 Ra^0.180. These correlations are valid for the ranges 10² ≤ Ra ≤ 10⁷ and 0.1 ≤ η ≤1.0, where η is the ratio between the heights of the overlying fluid layer and the bed.