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To address the curvature effect on single-row chevron-nozzle jet impingement heat transfer on concave surface, a series of experiments are conducted in the present investigation. Four concave surfaces including one semi-cylindrical concave surface and three parabolic concave surfaces with different width-to-depth ratios are tested under three typical Reynolds numbers (Re = 5000, 10000 and 15000) and several dimensionless nozzle-to-surface distances ranging from 1 to 8. The results show that the concave curvature has a clear impact on chevron-nozzle jet impingement heat transfer, tightly dependent on jet Reynolds number and impinging distance. In general, the semi-cylindrical concave surface produces the highest longitudinally-averaged Nusselt number at the leading line of concave surface. Under a low jet Reynolds number, the parabolic concave surface with a highly curved curvature produces higher longitudinally-averaged Nusselt number at the leading line and more uniform longitudinally-averaged Nusselt number distribution along the curvilinear direction. However, the longitudinally-averaged Nusselt number at the leading line of concave surface is the lowest for the highly curved surface under a high jet Reynolds number and large impinging distance. In comparison with the round-nozzle, chevron nozzle plays a more significant role on improving jet impingement heat transfer at small impinging distances. Keywords: Chevron nozzle, Concave surface, Experimental test, Heat transfer, Jet impingement

An experimental research is performed to illustrate the effects of the transmission pipe and nozzle exit shape on the synthetic jet impingement heat transfer, by using a piston-driven actuator. Three transmission pipes (namely short pipe, moderate pipe and long pipe) and two nozzle exit shapes (round and chevron) are taken into considerations. The experimental tests are conducted at five operational frequencies (f = 5 Hz, 10 Hz, 15 Hz, 20 Hz and 25 Hz) and four dimensionless jet-to-surface distances (H/d = 2, 4, 6 and 8). The results show that the heat transfer around the jet stagnation is generally decreased by the long-pipe nozzle in comparison with the orifice-nozzle case, especially at higher operational frequencies. However, at a high operational frequency and a small jet-to-surface distance, of particular interest is that the short-pipe nozzle could improve the synthetic jet impingement heat transfer when compared to the orifice-nozzle case. The chevron nozzle is confirmed to be benefit for the synthetic jet heat transfer enhancement. Its role behaves distinctly at different jet-to-surface distances and operational frequencies. At low operational frequencies, the favorable jet-to-surface distance for the synthetic jet impingement heat transfer is altered due to the presence of the chevron exit when compared to the round-exit nozzle. While at larger operational frequencies, the relationships of the circumferentially-averaged Nusselt number varying with the jet-to-surface distance in the round synthetic jet impingement remain in the chevron synthetic jet impingement.