Flow instability and heat transfer enhancement of unsteady convection in a step channel

The formation of flow instability and heat transfer enhancement of unsteady convection is studied for backward-facing step flow, forward-facing step flow and combined step flow. The governing equations are solved by a program of FORTRAN code. The numerical method is validated by PIV and heat transfer experiments. The effects and the affecting factors of flow instability are investigated. The results show that the flow instability caused by the reattachment flow has a positive effect on heat transfer. In combined step flow, the formation of this flow instability is mainly affected by the bottom wall length and the critical Reynolds number. The minimum critical Reynolds number of the transition from steady to unsteady case is between 200 and 250. The fundamental flow pattern results show that 7 typical flow characters appear in different step channel. A new concept named the vortex length ratio (LRv) is proposed to distinguish the vortex shape. The vortex with the smaller LRv is more effective in heat transfer enhancement mainly because of the increase of vortex attack angle and vortex rotating speed.