Scalar Dissipation Measurements in Turbulent Jet Diffusion Flames of Air Diluted Methane and Hydrogen

Abstract

Simultaneous two-dimensional Rayleigh and fuel Raman images have been collected in air-diluted methane and hydrogen jet diffusion flames. Temperature, fuel mass fraction and mixture fraction images are derived by a two-scalar approach based on one-step chemistry and equal species diffusivities. This enables calculation of two components of the scalar dissipation rate x-The inherently weak Raman signal has been maximised by intra-cavity measurements, using a flashlamp-pumped dye laser. In addition, the Raman signal-to-noise ratio is drastically improved by a novel contour-aligned smoothing technique which exploits the high correlation between the Rayleigh and Raman signals. Quantitative measurements of scalar dissipation are presented, including probability density functions for components of x- Profiles of mean and rms mixture fraction show the usual features already documented in other published results for this type of flame. Probability density functions of ξ are close to Gaussian on the axis, and tend to bimodal at the edge of the flame. Results for the CH4 flames indicate that the mean of x shows little change with Reynolds number. In the H2 flame, mean values for the axial and radial components of the scalar dissipation rate, x are nearly the same, indicating a more isotropic structure than in the CH4 flames. For both fuels, the pdf of ln(x) on the axis is more peaky than a lognormal distribution and somewhat skewed. The profiles of ( x\\ 𝛈 show a nonlinear dependence on mixture fraction and have no clear resemblance to the skewed, monomodal shapes seen in cold flows. In the H2 flame there is a strong correlation between instantaneous, local values of scalar dissipation and the departure from equilibrium, as measured by temperature depression.