Effects of acoustic actuation frequency and nozzle geometry on heat transfer and flow characteristics of an impinging confined water jet

An experimental study is performed to investigate the effects of acoustic actuation on heat transfer and flow characteristics of an impinging confined water jet. The water, collected in a reservoir, is accelerated by means of free fall and impinged on a smooth plate at which constant heat flux is applied. Two different nozzle geometries were investigated: sudden and smooth contracting nozzle. The flow is actuated with a loudspeaker at different Strouhal numbers in the range of 0~1, where nozzle-to-plate spacing of 6 nozzle diameters are tested while Reynolds number is kept constant at 10000 to study the effects of other parameters.. Flow visualization, turbulence, and surface heat transfer measurements are performed in the scope of this work. A sharp, continuous and significant roll-up evidence is seen in the smooth contracting nozzle, with the actuation frequency of St=0.175, at the axial location of about z/D~2 where the acoustic actuation has caused a decrease in heat transfer. It is observed that turbulence measurements with hot film anemometry are also in consistency with the surface heat transfer measurements. Acoustic actuation is found to be ineffective both in generating new flow structures or enhancing heat transfer in the sudden contracting nozzle.