Comparison of OH time-series measurements and large-eddy simulations in hydrogen jet flames

Hydroxyl time-series measurements have been obtained using picosecond time-resolved laser-induced fluorescence in a turbulent H₂/N ₂ jet diffusion flame. The flame is well characterized, as it is part of the Turbulent Nonpremixed Flame Workshop. Large-eddy simulations (LES) of the same flame were used to predict OH, density, and mixture fraction temporal statistics. The two sets of results are for the first time compared to examine in detail the LES-predicted time scales for OH fluctuations. These time scales come from complete time series and cannot be recovered from more common single-time measurements or time-averaged simulations. The measured and predicted power spectral densities for OH are found to collapse to the same shape when normalized by the local integral time scale. Hence, LES predicts the correct scale distribution for scalar fluctuations. The measured time scales range from 0.4 to 1.1 ms along the jet centerline. The LES results are quantitatively low (predicted fluctuations are too fast) by a factor of two. Grid resolution, the chemical submodel, and procedures for time-series processing are found to have insignificant effects on the computed time scales. However, the turbulent fluctuations prescribed on the inflow plane strongly influence scalar time scales, indicating that separate validation of mixture fraction and velocity time scales are necessary for further model development.