Heat transfer of granular flow around aligned tube bank in moving bed: Experimental study and theoretical prediction by thermal resistance model

Gravity-driven granular flow has been the heat transfer mediums in moving bed heat exchangers, which play vital roles in industrial heat saving and solar energy utilization. However, the heat transfer mechanism of granular flow is still worth of investigating around the tube bank. In this work, the aligned tubes were arranged in the moving bed, and the heat transfer of metallurgical slag was experimentally tested. It was found that, the heat transfer coefficients are significantly sensitive to the flow rate in the slow granular flow. The smooth flow with particle mixing can improve the heat transfer, while the local particle lagging will worsen the heat transfer around aligned tubes. To predict heat transfer coefficients of granular flow, the contact thermal resistance and the penetration thermal resistance are built with empirical factors (χ, a and η). In the near-wall region, x represents the sensitivity of the contact thermal resistance to particle sizes. Meanwhile, the contact time can be modified by a and η for the penetration thermal resistance in the funnel flow structure. The values of a and η are affected by the granular flow pattern, which tend to decrease in the smooth flow with strong particle mixing. Verified with the present experimental results and the previous literature data, the thermal resistance model has great potentials to explain the heat transfer around the aligned tube bank.