HYDROXYL TIME-SCALE CORRELATIONS IN TURBULENT COUNTERFLOW NONPREMIXED FLAMES

Abstract

Time-series measurements of OH concentration have been obtained in turbulent counterflow nonpremixed H₂/CH₄/air flames to study the influence of turbulence parameters on the dynamics of such flames by independently varying the Reynolds number (Re) and global strain rate (SR). The autocorrelation functions from the measured time series are found to be self-similar and can be fully characterized by their integral time scales. The integral time scale displays a power law dependence on both Re and SR. At lower strain rates, fluctuations in the OH layer are nearly independent of Reynolds number. An increase in SR changes the spatial displacement of OH, paving the way for faster fluctuations of the OH layer at higher Reynolds numbers. Both the bulk strain rate and Reynolds number scale linearly with the jet exit velocity (U). Furthermore, the combined effect of SR and Re on the integral time scale behaves approximately as U^−1.35 in these counterflow nonpremixed flames. Hence, the Re^−1.4 dependency previously observed for the OH integral time scale in jet diffusion flames is likely due to a combined effect of jet exit velocity on both strain rate and Reynolds number. Finally, stochastic time-series simulations are used to examine the relationship between time scales for OH and mixture fraction in turbulent nonpremixed flames. We thus demonstrate that integral time scale for mixture fraction is considerably greater for opposed-jet as compared to free-jet flames.

Keywords:

• Counterflow nonpremixed flames
• OH concentrations
• Time-series

This work was supported by the National Science Foundation, with Dr. Linda Blevins serving as technical monitor. We thank Dr. Andreas Dreizler and Dr. Dirk Geyer (Technische Universität, Darmstadt) for providing us with the counterflow burner used for the measurements reported in this study.