Measurements of scalar dissipation in the reaction zones of turbulent nonpremixed H 2-air flames

Abstract

A new diagnostic system has been developed to obtain spatially and temporally resolved simultaneous multiple point measurements of species concentrations and temperatures in turbulent flames. The diagnostic approach is based on pulsed Raman scattering induced by a KrF excimer laser. The system has been applied in the reaction zones of turbulent nonpremixed H 2-air flames to obtain statistics of species concentrations, temperature, and mixture fraction (ξ). Results of means, and fluctuations of the measured quantities do not differ markedly from the published point measurements in similar flames, validating the diagnostic approach. Instantaneous, one-dimensional (radial) scalar dissipation rates (χ) are measured in the reaction zones of the nonpremixed H 2-air flames using the multipoint, multispecies detectivity of the diagnostic system. Radial profiles and probability density functions (pdfs) of scalar dissipation are reported. The pdfs of scalar dissipation are log-normally distributed and negatively skewed. The radial profiles of mean scalar dissipation are similar to ξ rms 3, consistent with the k-ε models of turbulent combustion. Measurements of χ are compared to laminar flamelet model calculations. Laminar flame strain rate parameters (a) estimated from the scalar dissipation measurements range from a = 50–1000s −1, much lower than the extinction strain rates ( a ≈ 12,000s −1) of H 2-air flames.