Nonequilibrium and Differential Diffusion Effects in Turbulent Hydrogen Diffusion Flames

Abstract

The prediction of thermal NO formation in turbulent hydrogen - air diffusion flames is studied using the laminar flamelet concept. Each of the laminar flamelets is subject to a different state of nonequilibrium. The theoretical parameter for this effect is the scalar dissipation rate. In the literature, this parameter sometimes has been interpreted as the strain rate. Recent results imply that only by using the scalar dissipation rate can the experimentally established scaling behavior of the NO emission index be predicted. Detailed comparisons between flamelet-based predictions and measurements of temperature, major species, and NO for diluted turbulent hydrogen diffusion flames are presented. These comparisons show that differential diffusion effects in the laminar flamelets are very important and lead to overpredictions of NO in the far field of turbulent hydrogen flames. Suppressing differential diffusion by assuming unity Lewis numbers for all species is shown to reduce the far-field overpredictions, but it leads to an underestimation of the near-field NO mass fractions. Velocity and scalar (including temperature and NO mass fractions) predictions agree relatively well in the near field of the investigated diluted hydrogen flames.