Impingement heat transfer of quadruple synthetic jets

In this work, the heat transfer performance of impinging quadruple synthetic jets is experimentally investigated by means of infrared thermography and the heated thin foil technique. Four configurations (monopole-like, dipole-like, quadrupole-like and 90-degrees-circularly-shifted-jets), obtained by introducing different phase shifts between the single synthetic jets, are comparatively studied at constant values of the Reynolds and Strouhal numbers, equal to 4000 and 0.2, respectively. It is shown that the heat transfer behavior is strictly related to the dynamics of the coherent vortex structures that form after the interaction of the vortex rings inherent to the single jets. The signature of these vortical structures on the heat transfer distributions is evident at short impingement distances, where the most significant differences between the investigated configurations are indeed found. In the comparison, the 90-degrees-circularly-shifted-jets configuration is observed to offer 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. As the impingement distance increases, these differences are progressively smeared out and the four configurations exhibit equivalent performance from the viewpoints of both magnitude and uniformity of the heat transfer rate.