Experimental study of the aspect ratio effect of a rectangular impinging synthetic jet on flow and heat transfer characteristics

Abstract

The present study aims towards studying the impinging rectangular synthetic jet, with orifice AR of 2 and 6 and the effect of zd<inf>e</inf>= 3 and 8 using local measurements. We have employed the two-dimensional planar particle image velocimetry and infrared thermography techniques for velocity and temperature, respectively. Results show the existence of axis-switching phenomena for the case when zd<inf>e</inf> is 8 and AR is 6, and were absent in the case when zd<inf>e</inf> is 8 and AR is 2, as well as at a lower zd<inf>e</inf> of 3. A vortex pair of counter-rotating nature formed in the impingement region for the R-2-8 case, but is absent for the R-6-8 case. The analysis of the wall jet region shows similar flow structures near the wall for different AR of the orifice when the zd<inf>e</inf> is 3. The maximum wall jet velocity for the R-6-3 case is greater than the R-2-3 case, while for larger impingement distances, this trend is reversed, with the smaller AR (R-2-8) showing a higher maximum wall jet velocity as compared to the larger AR (R-6-8). The POD technique confirms the phenomenon of axis-switching, providing a more precise understanding of the jet's dynamic behaviour. The heat transfer characteristics of the asymmetric jet reveal a higher heat transfer rate and an increased heat transfer ratio for smaller zd<inf>e</inf>, indicating more efficient thermal exchange in this configuration. However, for larger zd<inf>e</inf>, the trend reverses, and a lower heat transfer rate is observed, suggesting reduced interaction between the jet and the surface. This contrasting behaviour in heat transfer is well explained by the steady flow dynamics, which indicate that the flow characteristics, including vortex structures and velocity profiles, significantly influence the performance of the heat transfer characteristics at various AR of the asymmetric jet. The results of this study highlight the significance of axis-switching phenomena observed in jets issuing from rectangular orifices with certain aspect ratios. © 2025 Elsevier Masson SAS.