Instantaneous convective heat transfer to pulsating submerged jets

An investigation was carried out to study the effect of flow pulsation on the heat transfer characteristics of a planar airjet impinging normally onto a heated surface. A detailed boundary-layer model was implemented to ascertain the influence of pulse shape, frequency, and amplitude on instantaneous and time-averaged convective heat transfer in the planar stagnation region. Interactions between low-frequency/high-amplitude flow pulsations and the nonlinearities in the governing equations led to reductions in time-averaged Nusselt number up to 16%. High-frequency/low-amplitude pulsations in the incident flow led to small (;The experimental set-up was designed to produce pulsating planar air jets of adjustable flow rate and pulse characteristics. The electrically heated impingement plate carried a surface-mounted microsensor that provided highly localized and essentially instantaneous heat flux and temperature signals. Flow measurements were made with a single-axis hot-film probe. Experiments were performed for jets of Reynolds numbers of 1000, 5500, and 11000. Pulsations had frequencies up to 82 Hz (corresponding Strouhal numbers below 0.13) and amplitudes at the nozzle exit up to 50% of the mean flow velocity. Flow measurements in non-impinging jet flows characterized the formation and interaction of coherent flow structures, for which Strouhal number was found the most influential parameter. Effects of Reynolds number, pulse frequency, and pulse amplitude on instantaneous and time-averaged heat transfer distributions were assessed. Nusselt numbers at the stagnation line increased by up to 12% above the steady-state values due to surface renewal effects of impinging large coherent flow structures associated with high-amplitude/high-frequency pulses. For intermediate Strouhal numbers, Nusselt numbers decreased by up to 8% due to boundary layer nonlinear dynamics effects, as predicted by the theoretical model. Away from the stagnation line, high-amplitude/high-frequency pulses promoted the transition to turbulence in the boundary layer, causing enhanced heat transfer. Pulses of small-amplitude and low-Strouhal number induced a quasi-steady behavior for the jet, with no effect on the time-averaged Nusselt number.