Pulse Combustor Tail-Pipe Heat-Transfer Dependence on Frequency, Amplitude, and Mean Flow Rate

A commonly cited advantage of pulse combustors is a high rate of heat transfer in the tail pipe. Past research on these rates of heat transfer is inconclusive regarding the amount of heat transfer enhancement and how various flow parameters affect this enhancement. This article reports an experimental heat transfer study in the tail pipe of a pulse combustor. The pulsation frequency, pulsation amplitude, and mean flow rate were varied systematically, and their effects on the heat transfer rates assessed. Spatially averaged Nusselt numbers were obtained from thermocouple measurements using a standard log-mean heat exchanger calculation. The Nusselt number was found to increase with both pulsation amplitude and frequency, with a maximum enhancement of 2.5 times that of steady flow at the same mean Reynolds number. The Nusselt number enhancement decreased with increasing mass flow rate for a given combustor pulsation frequency and amplitude. Independent axially resolved heat flux and gas temperature measurements confirmed the large Nusselt number increase with pulsations and demonstrated that entrance effects, although present, were small compared to the Nusselt number enhancement due to the pulsations. The data are compared with quasi-steady theory, which is the only available theory in the literature for this problem. Quasi-steady theory does not account for frequency effects and is not adequate for describing the data from this study.