Time-series analysis and measurements of intermediate-species concentration spectra in turbulent non-premixed flames

The ability to make frequency-resolved measurements of minor-species concentrations in turbulent flames has recently been demonstrated by picosecond time-resolved laser-induced fluorescence (PITLIF). However, few previous studies have focused on the power spectral density (PSD) of minor species in reacting flows. What little information is available suggests that strong interactions between velocity fluctuations and scalar distributions complicate the PSD, making interpretation of flame data difficult. In this work, time-series analysis, previously applied in flame-radiation studies, is extended to the study of temporal fluctuations in minor-species concentrations. In particular, an assumed PSD model for mixture fraction (Z) is used to simulate its time series. State relationships for narrowly distributed minor species as a function of mixture fraction are applied to develop concentration time series. Extension of this laminar-flamelet approach to minor species requires additional consideration of the effect of flame strain rate on the width of a minor-species profile in Z space as well as its peak concentration. The resulting minor-species concentration PSDs are computed and studied as a function of profile width (in Z) and fluctuation intensity (ZRMS). These simulations compare favorably to previously reported CH and OH PSDs in a low-Reynolds-number, methane diffusion flame. Similarly, new PITLIF measurements of OH time series in a hydrogen diffusion flame at several Reynolds numbers also demonstrate favorable comparisons to the PSD simulations.