Effect of Acoustic Excitation on the Heat Transfer to an Impinging Air Jet

Impinging air jets are known as a method of achieving particularly high heat transfer coefficients and are employed in many applications including the cooling of electronics, manufacturing processes such as grinding, etc. The current investigation is concerned with acoustically exciting an impinging air jet to enhance its overall cooling capacity. Distributions of the heat transfer to an axially impinging air jet for a range of Reynolds numbers (Re) from 10000 to 30000, non-dimensional nozzle to impingement surface heights (H/D) from 0.5 to 2 and excitation frequencies (f) that range from 0.5 to 1 times the natural frequency of the jet are presented. For this low range of nozzle to impingement surface spacings it has been shown that the heat transfer distribution exhibits a peak at the stagnation point and secondary peaks at a radial location that is both excitation frequency and Reynolds number dependent. Distributions of the fluctuating component of the heat transfer coefficient are also presented for the range of parameters tested. These have been used, along with spectral analysis of the heat flux signal, to discern whether local variations in heat transfer are due to changes in the local vortex flow or to changes in the mean flow structure of the impinging jet.