Effects of acoustic excitation positions on heat transfer and flow in axisymmetric impinging jet: main jet excitation and shear layer excitation

An experimental study is conducted to investigate the flow and heat transfer characteristics of an impinging jet with acoustic excitations. Two different acoustic excitation methods based on the locations of the actuator are tested and compared: one is a main jet excitation and the other is a shear layer excitation. Effects of excitation level on the heat transfer and flow characteristics are also investigated. Local Nusselt number distributions are measured on the impingement surface and velocity and turbulence intensity distributions are also measured. The forcing Strouhal numbers (excitation frequency, St) are 1.2, 2.4, 3.0 and 4.0 and the excitation level varies from 80 to 100 dB. When the vortex pairing is promoted by the excitation St of 1.2 with the main jet excitation, low heat transfer rates are obtained at the large nozzle-to-plate distances. For the excitation St of 2.4 with the main jet excitation, high heat transfer rates are obtained at the large gap distances due to the extended potential core length. The main jet excitation method shows similar heat and flow characteristics to the shear layer excitation although the basic flow schemes and the location of the forcing actuator are different. The effects of the acoustic excitation to the jet increase as the excitation level increases. Therefore, the excitation frequency and the excitation level are both important factors in the acoustic excitation.