Insights into the flow and heat transfer aspects of single and multi-orifice synthetic jets

The study deals with understanding the flow and heat transfer aspects of a multi-orifice synthetic jet (SJ), and the results are compared with a single orifice SJ. The effect of actuation frequencies and amplitude is studied for a single orifice SJ, to understand the formation and the development using smoke-wire visualization. The generation of vortex structure is visualized using the smoke to understand the difference between the formation of single and multi-orifice SJ. Further, a particle image velocimetry (PIV) technique is applied to study the evolution of the free SJ. The mixing and spreading characteristics of SJ are investigated using vorticity and mean velocity profiles in the flow field for single and multi-orifice cases. Heat transfer experiments on a flat surface are carried out for both the orifice types. The results of the study show that large-sized vortices are produced with a decrease in the frequency and an increase in the amplitude of the SJ actuator. While increasing the frequency, the vortices are more closely spaced in the flow field, and with an increment in amplitude, vortices are placed at larger distances. The PIV results indicate that the multi-orifice produces strong mixing and vorticity in the near-field region, while it loses strength quickly after a certain distance downstream. For the single-orifice case, the development and mixing of the jet are fairly gradual compared to the multi-orifice case. All these flow characteristics point towards higher heat transfer rates at lower surface spacings, which is reinforced by the heat transfer results. The multi-orifice configuration provided up to 30 % improvement in the average heat transfer from the surface compared to the single-orifice case of equivalent diameter. This work adds to our understanding of multiple orifice and impinging jets and provides a link between heat transfer and fluid flow, leading the way for thermal management systems in confined spaces.