Heat-transfer enhancement in the oscillating turbulent flow of a pulse combustion tail pipe

Heat transfer rates in pulse combustor tail pipes and in other reversing, oscillating, turbulent flows have been found to be much higher than those of steady turbulent flow. In this paper, to elucidate the mechanisms of the enhancement, the temperature and velocity fields, measured with two-line atomic fluorescence (TLAF) and laser Doppler velocimetry (LDV), respectively, are compared. Time-averaged temperature profiles and Nusselt numbers are found to exhibit a frequency dependence, not seen in the time-averaged velocity and wall shear stress data, although both the Nusselt number and wall shear stress are increased by the oscillations. Time-resolved wall heat fluxes are also presented, and combined with the gas temperature data to determine time-resolved Nusselt number. Large increases in the Nusselt number are found to occur twice each cycle at times of flow reversal. Possible causes for the heat-transfer enhancement in oscillating flows are reviewed and discussed in view of the data presented in this paper and the recent literature