The effect of jet location and duty cycle on the fluid mechanics of an unconfined free jet and its heat transfer on an impinging plate

Unsteady heat transfer caused by an unconfined impinging air jet is experimentally studied. Four test cases are considered, two pulse frequencies and two duty cycles. The outcome of this experimental study is compared to that of steady jet impinging heat transfer relations and data. Characterization of the pulsed air jet is conducted using a hot wire anemometer at twelve discrete locations for each pulse case considered. The local instantaneous velocities collected were then used to calculate time averaged mean velocity, and turbulent intensity. A power spectral distribution was produced for each of the jet flows to acquire insight toward the most effective jet flow characteristics for heat removal on an impingement surface. An infrared (IR) camera was used to determine a full field instantaneous heat transfer coefficient for each of the pulse cases and a nozzle to plate setback distances. All data was collected over a Reynolds number range of 866 and 3776. The jet characterization suggested that a setback distance of three hydraulic diameters (as opposed to one-half or six) produces the most effective power spectral density given a pulse frequency of 5 Hz and a duty cycle of 50%. The infrared camera data collected argues against the conclusions drawn from the hot wire data suggesting that the most effective setback distance is six hydraulic diameters and with a frequency of 10 Hz and a duty cycle of 50%. All data provides similar evidence that the duty cycle over a pulse period has a larger impact on the heat transfer of an impinging air jet than that of pulse frequency. Published by Elsevier Ltd.