Acoustic and heat transfer characteristics of an impinging elliptical synthetic jet generated by acoustic actuator

Abstract

The acoustic aspects and average heat transfer characteristics of an elliptic synthetic jet impinging on a heated flat plate were experimentally investigated and compared with circular, rectangular and square orifices having same equivalent diameter. The aim of the study is to investigate the effect of orifice shape, aspect ratio and operating parameters of the synthetic jet at which optimum acoustic and heat transfer performance is obtained. It is found that irrespective of the shape of the orifice, the synthetic jet generates the minimum sound pressure level when operated at its resonance frequency, which is equal to the diaphragm frequency in the present case. The maximum heat transfer enhancement is obtained at the resonance frequency for elliptical orifice of aspect ratio 1.4 at the jet to plate spacing (z/de) of 3, where z is the axial distance and de is the equivalent diameter of the orifice. The aspect ratio of an orifice is a sensitive parameter as with increase in the aspect ratio, reduction in the Nusselt number is observed. For low jet to plate spacing ratios (z/de < 6), elliptical orifice performs better as compared to other shapes of the orifice. This was mainly due to enhanced mixing and a large entrainment rate with elliptic orifice. For higher jet to plate spacings (z/de ⩾6), circular and square orifices outperform elliptic and rectangular orifices. This was mainly due to the high spreading rates because of which air loses its momentum before reaching the target plate. The heat transfer characteristics of circular and square shape orifices are similar due to same aspect ratio. The present results indicate a strong correlation between acoustics and heat transfer characteristics of synthetic jets and the existence of such a correlation has not been previously reported for synthetic jet flows. These results are significant from practical view point of designing synthetic jet based electronics cooling solutions.