Pulse combustion: The importance of characteristic times

Abstract

The response of a valved pulse combustor to changes in the relative timing between the resonant pressure wave and the instantaneous energy release rate has been examined. Experiments were designed to examine the pulse combustor's response to independent changes in the experimental conditions that resulted in nearly independent changes in the fluid dynamic species mixing time, the fluid dynamic mixing time of cold reactants with hot products, the characteristic chemical kinetics time, and the characteristic resonance time. The time scales considered in this study were adjusted independently to modify the coupling between the instantaneous energy release rate and the resonant pressure wave, thereby modifying the magnitude of the pressure oscillations and altering the frequency of operation. All of these experimental observations of the pulse combnstor response to variations in characteristic time scales are interpreted in terms of Rayleigh's criterion.

Pulse combustion: The quantification of characteristic times

“…Since N2 is already the major constituent of air, small amounts of N2 will cause negligible changes in the physical properties of the reactants; therefore only "/'kinetic (and therefore the total cycle time ~'to~) will be altered. Following the work of Keller et al [2], Nz was chosen as a diluent to control 7"kineti c for this study.…”

“…The tailpipe is made of a variable number of 200 mm sections, allowing the total length of the tailpipe to be altered. The tailpipe length for this work was chosen so that the domain of obtainable total ignition delay times was centered about the delay time corresponding to maximum reso-nance (see Keller et al [2]); this criterion yielded a tailpipe length of approximately 600 mm.…”

“…Recall that Keller et al [2] established that the influence of these two characteristic times on the total ignition delay time is, at least to first order, independent of the other characteristic times in the system. Prom these studies information was obtained to calibrate the mixing model under conditions of constant chemical kinetic time and to calibrate the kinetics model under conditions of constant mixing times.…”

“…calc was also made. In the reference case the instantaneous energy release rate occurs early in the cycle, prior to the peak in the pressure root mean square about the mean, P~s (see Keller et al [2]). The kinetic ignition delay time is then retarded by adding N2 while maintaining constant mixing characteristics.…”

“…However, the precise role of mixing has never been quantified experimentally or theoretically. Keller et al [2] established the concept of individual characteristic delay times for pulse combustors (for example, time required to mix fuel and oxidizer, time required to mix the reactants with the hot products, time required for chemical kinetics, and the natural resonance time) and the idea that these times contribute nearly independently to a total ignition delay time. This work found that it is only their contributions to the total delay time that affected the phase relationship between the resonant pressure wave and the instantaneous energy release rate.…”