Statistical Interpretation of Scalar Time-Series Measurements in Turbulent Partially Premixed Flames

Abstract

Abstract : Systematic comparisons have been obtained between hydroxyl time series as measured by picosecond time-resolved laser-induced fluorescence (PITLIF) and as predicted by large-eddy simulations (LES) for turbulent non-premixed H2/N2 jet diffusion flames. The measured and predicted autocorrelation functions for OH were found to collapse to the same shape when normalized by their integral time-scales. However, the time-scales predicted by LES were systematically low by a factor of 2-3 as compared to the PITLIF measurements. Time series for hydroxyl and total number density were also obtained using (PITLIF) and laser Rayleigh scattering (LRS), respectively, in non-premixed and partially premixed turbulent H2/CH4/N2-air flames having similar Rayleigh cross-sections for air and the exhaust gases. The measured PSDs for OH and number density were found to collapse to the same shape when normalized by their respective integral time-scales. The density to OH time-scale ratio was typically 2-4, with larger ratios prevailing nearer non-premixed conditions owing to thinner OH flamelets. Finally, a novel PITLIF methodology has been developed which is capable of simultaneous two-point, time-series measurements of OH concentrations in turbulent flames. Two-point time-series data obtained in a standard H2 diffusion flame at Re = 10,000 have been used to compute space-time correlations, spatial autocorrelation functions, and integral length-scales. Preliminary two-point results are discussed with a focus on implications for future measurements in gas-turbine combustors and thrust augmentors.