Experimental and numerical study of a highly diluted turbulent diffusion flame close to blowout

Abstract

The nature of stability, finite rate chemistry, and differential diffusion inherent in a highly diluted turbulent non-premixed hydrogen flame close to blowout is investigated. Major-species concentrations and temperature are measured by Raman/Rayleigh spectroscopy. OH concentration is simultaneously measured by laser induced fluorescence (LIF), and laser Doppler velocimetry (LDV) is used to determine velocities. The experimental data are used to assess the performance of a probability density function (PDF) simulation with a five-step reduced chemistry mechanism and differential diffusion effects. Comparison is made in terms of averaged quantities, scatter plots, and conditional averaged data. First- and second-order statistics of predictions and experimental data are found in reasonably good agreement. The significant amount of finite rate chemistry observed in the flame as well as the relaxation to equilibrium with downstream distance is well reproduced by the PDF calculation. PDF model predictions of differential diffusion compare favorable with experimentally determined values. Numerical results reveal that differential diffusion may have a major impact on the flame stability, although the average deviation from the equal diffusivity limit is small. Computed laminar extinction limits of laminar flame calculations with multicomponent diffusion and equal diffusivity are in agreement with this observation.