Enhancement of jet impingement heat transfer using surface roughness elements at different heat inputs

Experiments have been performed to characterise the heat transfer to a jet impinging normally on a heated surface, modified with multi-protrusions or V-grooves. The jet flow is in turbulent regime with Reynolds number lying in the range of 10,000–27,500. The dimensions of multi-protrusions and V-grooves are selected to be of the order of laminar sublayer thickness. Heat transfer enhancement for the surfaces modified by roughness elements is compared with the results for a smooth flat surface. The average Nusselt number is reported as a function of jet Reynolds number, nozzle-to-plate spacing, roughness type, and nozzle diameter for 60 W and 90 W heat inputs. The Nusselt number increase with surface roughness elements is partly due to area increase and partly due to turbulence enhancement. Well-separated multi-protrusions cause higher heat transfer enhancement than closely spaced V-grooves. For higher heat input (90 W) and longer stand-off distances (Z/d = 10), it is observed that the effects of weak impingement flow field are counter-acted by natural convection plume flow emanating from the heated surface. Hence, in such cases, impingement cooling is not effective.