Time-resolved gas temperatures in the oscillating turbulent flow of a pulse combustor tail pipe

Abstract

The cyclic behavior of the gas temperature in the oscillating turbulent flow in a pulse combustor tail pipe was studied using two-line atomic fluorescence. In this flow, the oscillations result from an acoustic resonance, and have amplitudes of up to 5 times the mean velocity. Oscillation frequencies were varied from 67 to 101 Hz. Spatially resolved temperature measurements were made to within 400 μm of the wall, providing cycle-resolved profiles of the temperature and the random temperature fluctuations. The combustor-cycle phase relationships among the gas temperature, random-temperature-fluctuation intensity, velocity, and combustion chamber pressure, are compared. Velocity field effects dominated the cyclic behavior of the gas temperature, over the effects of isentropic compressive heating and the convection of hot pockets of gas from the combustion chamber. Cycle-resolved profiles show the gas temperature to be constant across the tail pipe, except for a boundary layer region, at all times during the cycle. Although cyclic temperature oscillations of more than 200 K were observed, the thermal boundary layer was well developed at all times during the cycle. The gas temperature was greater than the wall temperature at all cycle times, unlike the reversing velocity field, indicating that Reynolds analogy between momentum and thermal transport is not valid in this flow. Time-resolved wall heat flux was also measured and its fundamental oscillation is found to be in phase with that of the gas temperature.