Heat transfer of impinging sweeping jets: Influence of nozzle-to-target spacing and feedback channel minimum passage width

Abstract

Infrared Thermography and the heated thin foil heat flux sensor are employed to experimentally investigate the heat transfer characteristics of sweeping jets impinging on a flat surface. Multiple nozzle-to-target spacings $H$ ($H/w=0.5,1,1.5,2,4,6,8$ and 10, where $w$ represents the width of the square exit nozzle throat) and feedback channel minimum passage widths $g$ ($g/w=1,0.83,0.67,0.50,0.33,0.17$ and 0) are analyzed to evaluate the effects of these two parameters on the convective heat transfer of the investigated jets. The Reynolds number is set to $1.47\times 10^4$ for all the experiments performed. To assess the heat transfer behavior of the studied sweeping jet device, both time-averaged and phase-averaged analyses are conducted. This study demonstrates that the convective heat transfer of the impinging sweeping jet affects a broader area of the foil as the nozzle-to-target spacing increases, whereas the opposite effect is observed when reducing the minimum passage width of the feedback channels. Furthermore, the time-averaged analyses reveal that for $0.5\le H/w\le 1.5$, compared to the corresponding steady jet, sweeping jets enhance the convective heat transfer close to the impingement center of the target surface; instead, for $H/w>2$ the steady jet exhibits superior heat transfer performance near the stagnation region, while the sweeping jets generate a more uniformly distributed region of maximum convective heat transfer. Additionally, the analysis of the phase-averaged Nusselt number distributions across the target surface reveals that the maximum convective heat transfer region is situated on its uphill side, close to the stagnation point, resembling the behavior of inclined impinging jets.