TIME-AVERAGED HEAT TRANSFER IN CONFINED IMPINGING QUADRUPLE SYNTHETIC JETS

The convective heat transfer between an impinging quadruple synthetic jet and a heated surface is experimentally investigated. Infrared Thermography and the heated thin foil technique are employed to determine the time-averaged Nusselt number distributions with varying impingement distance in four different configurations (monopole-like, dipolelike, quadrupole-like and 90-degrees-circularly-shifted-jets). Such configurations are obtained by varying the phase relationships between the single synthetic jets at constant values of the Reynolds and Strouhal numbers, equal to 4000 and 0.2, respectively. It is shown that the heat transfer is strictly related to the morphology and evolution, but also to the frequency of generation, of the coherent vortex structures that form after the interaction of the vortex rings inherent to the single jets. The thermal performances of the four configurations are compared by focusing on the area-averaged Nusselt number distributions. Significant differences are found at small impingement distances, where the 90-degrees-circularly-shifted-jets configuration offers the highest heat transfer rates, although with the lowest degree of uniformity, while the monopole-like jet is characterized by the lowest Nusselt number values and the highest uniformity. Instead, at high impingement distances the four configurations are equivalent from the viewpoint of both magnitude and uniformity of the heat transfer rate.