Enhancement and reversal heat transfer by competing modes in jet impingement

A transient numerical analysis for fluid flow and heat transfer from a planar jet impingement on a finite thickness substrate is performed. A discrete heating boundary condition is applied to the substrate’s under side; by including the effect of buoyancy, some assisting or opposing mixed convection configuration can be modelled and regions of momentum dominated, buoyancy dominated and unstable flows can be monitored. For low volumetric flows and small temperature differences, different competitive heat transfer modes can be detected, as conduction may affect heat transfer away from the impact site in the initial times, and flow pattern is driven by the ruling convective mechanism, whether forced or natural. The related flow field and local heat transfer rate are investigated as a function of a variety of geometry configurations, material coupling and thermal-fluid driving factors, for the unitary value of the mixed convection parameter Ri (transitional mixed convection).