Vortex dynamics and mechanisms of heat transfer enhancement in synthetic jet impingement

When confined flow is oscillated from and into a quiescent volume, a periodic coherent flow that resembles that of a conventional jet can be generated. Such a jet, termed a synthetic jet, has been investigated for thermal management by causing it to impinge onto a heated surface. Because of its fluctuating nature, the impinging jet thus formed is dominated by vortices that are advected towards the surface. This surface-vortex interaction is key to understanding the fundamental mechanisms of convective heat transfer by the impinging synthetic jet, which motivates this work along with the search for the improvement of the system thermal efficiency. A canonical geometry was developed to investigate the flow and heat transfer of a purely oscillatory jet that is not influenced by the manner by which it is produced. The unsteady two-dimensional Navier-Stokes equations and the convection-diffusion equation were solved using a finite volume approach in order to capture the complex time dependent flow field. The Q-criterion (Hunt et al., 1988), which defines vortices as connected fluid regions with positive second invariant of the velocity gradient tensor was utilized to identify vortices without ambiguity. A definition of the jet characteristic velocity was developed rigorously based on the vortex dynamics produced by the jet. It is equivalent to the common definition accepted in the literature, which has been successfully used to match the dynamics of synthetic and steady jets, but which was developed using heuristic reasoning. When the primary vortex advects in a direction parallel to the target surface it gives rise to a secondary vortex with opposite net vorticity. This secondary vortex is largely responsible for enhancement of the heat transfer within the wall jet region. Under certain conditions, vortex coalescence occurs, leading to degradation in the heat transfer enhancement due to the reduction in the number of secondary Vortices interacting with the heated surface. By understanding, quantifying and predicting the mechanisms that drive the phenomenon of vortex merging, optimum conditions of operation are demonstrated, ultimately leading to higher efficiencies by maximizing the heat transfer at similar pumping costs. (C) 2016 Elsevier Masson SAS. All rights reserved.