A comparative study of coaxial and conventional synthetic jet heat transfer

This study presents a coaxial orifice synthetic jet (SJ) designed to reduce recirculation at smaller orifice-to-surface spacing (z/d) compared to conventional single-orifice SJs. The heat transfer characteristics and flow behavior of the jet are investigated using infrared (IR) imaging and smoke wire visualization techniques, respectively. The equivalent diameter of the jet is kept constant (d = 12 mm) for single and coaxial orifices. In order to get a higher heat transfer rate, a different combination of inner and outer diameter of the coaxial orifice jet is examined. The effect of the amplitude (2-6 V), jet Reynolds numbers (3490,5830,7360), and orifice-to-surface spacing on flow and heat transfer characteristics is also investigated. The results show that the coaxial SJ improves heat transfer by up to 34.8 % at z/d = 1, but its performance declines at larger spacings (z/d > 6). The effectiveness of the coaxial jet decreases as the inner diameter increases. At small surface spacings, the inner and outer jets act independently on the surface, enhancing heat transfer due to velocity differences and vortex interactions. However, at large surface spacing (higher z/d ratios), the vortex structure formation becomes an important factor in changes in the heat transfer rate. Smoke wire visualization shows that coaxial jets spread more effectively at lower spacings, improving heat transfer. This observation underscores the advantages of coaxial jets in enhancing heat transfer efficiency. Thus, coaxial jets outperform conventional jets at lower spacings by mitigating flow recirculation but lose this advantage at higher spacings due to a breakdown of larger size vortical structure with the coaxial jet. The research findings indicate that coaxial SJ offers a promising approach for improved heat transfer, providing valuable insights for optimizing jet design based on specific applications and desired characteristics.