Experimental Characterization of Heat Transfer to Vertical Dense Granular Flows Across Wide Temperature Range

Particle-based heat transfer fluids for concentrated solar power (CSP) tower applications offer a unique advantage over traditional fluids, as they have the potential to reach very high operating temperatures. Gravity-driven dense granular flows through cylindrical tubes demonstrate potential for CSP applications and are the focus of the present study. The heat transfer capabilities of such a flow system were experimentally studied using a bench-scale apparatus. The effect of the flow rate and other system parameters on the heat transfer to the flow was studied at low operating temperatures (<200 °C), using the convective heat transfer coefficient and Nusselt number to quantify the behavior. For flows ranging from 0.015 to 0.09 m/s, the flow rate appeared to have negligible effect on the heat transfer. The effect of temperature on the flow's heat transfer capabilities was also studied, examining the flows at temperatures up to 1000 °C. As expected, the heat transfer coefficient increased with the increasing temperature due to enhanced thermal properties. Radiation did not appear to be a key contributor for the small particle diameters tested (approximately 300 μm in diameter) but may play a bigger role for larger particle diameters. The experimental results from all trials corroborate the observations of other researchers; namely, that particulate flows demonstrate inferior heat transfer as compared with a continuum flow due to an increased thermal resistance adjacent to the tube wall resulting from the discrete nature of the flow.