Effects of Variable Solids Loading on Gas-Solid Heat Transfer in a Staggered Tube Array

Abstract

For staggered tube arrays subject to gas-solid cross flows with low solids concentrations, the overall heat-transfer performance can be reduced by the presence of the solids in the flow. This paper investigates the effect of varying solids concentration on cross flow heat transfer with a view to identifying the dominant heat-transfer mechanisms for higher solids concentrations. Measurements are obtained for tubes located within a staggered tube array subject to mass loading ratios of up to 2.2 kg/kg of particles of 58 and 46 μm mean diameter at flow Reynolds numbers of 3000 and 10 000. From the results it is found that, although heat transfer in the wake of a tube is commonly reduced by the particles, a significant increase in the overall Nusselt number can be expected. The reduction in heat transfer in the wake region is attributed to a combination of delayed boundary layer separation and turbulence suppression by the solids, whereas the very significant enhancement of heat transfer measured over the front of the tubes is considered to result mainly from the increased thermal capacity of the mixture.