PULSED AIR-JET IMPINGEMENT HEAT-TRANSFER

Impingement heat transfer from a pulsing jet was studied experimentally. The pulsing jet was generated using a valve similar to a ball valve in design, which was rotated at a known frequency. Temporal average heat transfer characteristics of the jet were quantified using a thermographic infrared imaging technique, while the temporal characteristics of the flow were measured using hot-wire anemometry. Jet Reynolds numbers (based on time-average flow rate) in the range 4000-40,000 were investigated for pulsing jet frequencies ranging from the steady jet condition (zero frequency) to approximately 200 Hz. The experimental results indicate that, for the configuration studied, the heat transfer is degraded at all frequencies, generally in the range 0-20% relative to that of the steady jet at the same time-average flow rate. This is in spite of the significant increase in turbulence intensity for the pulsing jet. The power spectral density function for the pulsing jet reveals a dominant peak at the pulsing frequency, with secondary peaks believed to be due to natural frequency of the fluid delivery tubing downstream of the valve. The degradation in heat transfer for the pulsing jet is believed to be due to relatively small turbulent fluctuations superimposed on the instantaneous periodic flow.