Numerical investigation of the effect of surface roughness on flow and heat transfer characteristics of single sphere particle in supercritical water

Supercritical water fluidized bed is a novel gasification reactor which can achieve efficient and clean utilization of coal. The rough surface of particle produced during grinding and thermochemical conversion processing will deeply affect supercritical water-particle two-phase flow and heat transfer characteristics. In this paper, fully resolved numerical simulation of supercritical water flow past single rough sphere particle with the Reynolds number ranging from 10 to 200 was carried out to investigate the effect of surface roughness. The simulation results show that as roughness increases, the separation bubbles generated in the dimple enhance the flow separation but has no significant effect on the drag coefficient. Particle surface-average Nusselt number decreases with an increase of roughness and surface enlargement coefficient due to the isolation effect at low Re and local separation bubbles in the dimple at high Re. Furthermore, the effect of surface enlargement coefficient on heat transfer efficiency factor for supercritical water near the critical point is greater than that under constant property condition and has a higher dependence on Re.