IMPINGEMENT FLOW AND HEAT TRANSFER WITH FRACTAL GRIDS

Jet impingement heat transfer is widely acknowledged as an effective approach for localized cooling applications. A new passive method to augment jet-impingement heat transfer based on inserting a fractal-grid (FG) at the jet-nozzle exit is explored in this paper. Both the flow behavior and heat transfer characteristics are investigated experimentally for two fractal grids (FG1 and FG2) placed immediately downstream of the jet-array and with different jet-to-target plate spacings (H/D). Results are compared with those using a standard grid with round-holes (RG) and same blockage ratio. The results indicate that the FG2 provides significant heat transfer enhancements for the smaller H/D values. An enhancement of 57% and 34% is obtained respectively for spanwise-averaged and spatially-averaged Nusselt number for FG2 case at H/D=2 compared to the baseline no-grid case. The flow measurements indicate that the turbulence intensity and spectral characteristics downstream of the FG are different than those of the baseline and RG cases and are the primary drivers for the heat transfer enhancements. These differences generally reduce with increasing distances of the target surface from the jet-nozzle indicating that highest heat transfer enhancements are achieved for the lowest H/Ds. This is confirmed by the Nusselt number results.