An experimental study on the flow and heat transfer of an impinging synthetic jet

This study experimentally investigated the combined effects of the wall temperature (T_w0) and the orifice-to-wall distance (H/D) on the flow and heat transfer characteristics of an impinging synthetic jet. Thermographic phosphor thermometry (TPT) was used to measure the wall surface temperature, and the quantitative flow velocity was obtained using time-resolved particle image velocimetry (PIV). A cavity-diaphragm actuator was employed to generate a round synthetic jet to impinge onto a heated wall that was coated with Mg₄FGeO₆:Mn (MFG) for use as a temperature sensor. Three orifice-to-wall distances (H/D = 10, 15, and 20) and three wall temperatures (T_w0 = 60 °C, 90 °C, and 120 °C) were tested for comparison, whereas the the operating conditions of the incident synthetic jet were kept constant. It was found that the maximum temperature drop at the stagnation point could reach approximately 50°C although the orifice-to-wall distance was relatively large in this study, which indicated a good cooling performance of the synthetic jet. The penetration of the wall shear layer played an important role on the cooling performance of the impinging synthetic jet. For a heated wall with high T_w0, the enhanced buoyancy and thermal boundary layer resulted in the formation of a strong wall shear layer, which was more difficult for the impinging synthetic jet to affect or penetrate. For a large H/D, the vortex rings of the synthetic jet lose coherence completely before impacting the wall. Thus, they have no ability to penetrate the wall's shear layer to interact with it directly. As a result, the cooling performance of the impinging synthetic jet gradually decreased as both T_w0 and H/D increased. In particular, the maximum cooling effect by the synthetic jet impingement can achieve 64% of the theoretical maximum.