Numerical study of convective heat transfer in randomly packed beds of low aspect ratios
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abstract
The development of fluoride-cooled high-temperature reactors has greatly heightened the need for a thorough comprehension of heat transfer in packed beds that are cooled by liquid salts. The presence of restricting walls has a significant impact on the arrangement and movement of particles in packed beds with modest D/dp ratios (3.756). This study examines the changes in pressure drop and heat transfer between particles and fluid in a flow with a high-Prandtl number fluid (FLiBe), a molten-salt made from a mixture of lithium fluoride (LiF) and beryllium fluoride (BeF2) in randomly packed beds of 200 pebbles, under conditions of significant wall interference and high Reynolds number. We analyze the effects of different ratios of D/dp and evaluate the suitability of various empirical correlations by utilizing the k shear-stress transport (SST) turbulence model in conjunction with the discrete element approach for generating random packing. The results of our study demonstrate that decreasing the D/dp ratios leads to a decrease in pressure drops. These pressure drops are strongly influenced by the porosity of the bed. Notably, even minor variations in porosity can lead to substantial changes in pressure drop. The Wakao and Bird correlations demonstrated superior accuracy in predicting the Nusselt number compared to other empirical correlations. The Nusselt number analysis also demonstrated consistent heat transfer behavior in both the inner and outer regions at lower D/dp ratios, reflecting a more uniform thermal distribution. However, with increasing D/dp ratios, the outer region experienced a significant reduction in Nusselt number compared to the inner region, highlighting the need to account for spatial thermal variations when assessing packed bed performance. At low D/dp ratios, the wall's influence extended throughout the bed, dampening radial velocity and temperature fluctuations. This effect was observed across all cases, suggesting that channeling flow near the wall should not be overstated in analyses, as it could misrepresent transport phenomena in systems with very low D/dp ratios. These findings provide fresh perspectives on addressing the impact of enclosing walls in densely packed beds with small D/dp ratios.
