Effect of Flow Pulsations on the Cooling Effectiveness of an Impinging Jet

An experimental investigation has been performed to study the effect of flow pulsations on local, time-averaged convective heat transfer to an impinging water jet. Sinusoidal and square-pulse waveforms were considered. For the square waveform, the flow was completely halted for a portion of the pulsation cycle. Hot-film anemometry was used to characterize both the steady and the pulsating flows with regard to turbulence level and the spatial uniformity in the velocity profile across the nozzle width in order to assess separately the influence of flow pulsation on convective heat transfer. Pulse magnitude, which was defined as the ratio of the mean-to-peak velocity change to the mean flow velocity, was varied from 0.5 to 100 percent. Pulse frequencies ranged from 5 to 280 Hz, which corresponded to Strouhal numbers based on jet width and velocity of 0.014 to 0.964. Observed effects on convective heat transfer are explained in terms of nonlinear dynamic responses of the hydrodynamic and thermal boundary layers, boundary layer renewal, and bulges in the jet free surface.