Scalar time-series simulations using flamelet state relationships for turbulent non-premixed flames

Abstract

Simulations of scalar (OH, CH, and number density) time series and comparisons to experimental data are presented using a laminar flamelet model. Realistic time series for mixture fraction (Z) were constructed by employing measured Z mean and rms values in conjunction with realistic power spectral densities (PSDs), probability density functions (PDFs), and integral time scales. A unique procedure was implemented to permit simultaneous specification of both the PSD and PDF shapes for Z. These Z time series were mapped to other scalar time series by using flamelet state relationships from a strained laminar flame code. The simulated statistics are compared to recent data in hydrogen/methane/nitrogen flames. The predictions of OH and CH time scales directly depend on the time scale for Z; however, they are not identical because of the narrow state relationships for reactive scalars. The model successfully captures complicated features in the radial distribution of OH integral time scales. In a separate inverse calculation, the simulation is used to estimate Z time scales from each of the measured OH, CH, and number density data. In each case the Z time scale is found to be ∼0.75 ms on the jet centerline. In contrast to time scales from non-reacting jet studies, this Z time scale is nearly invariant with axial height and at low axial heights varies only slowly with radial location, implying that convective scaling (jet width/local velocity) may be insufficient for the accurate description of mixing time scales in jets with heat release.