SCALAR DISSIPATION MEASUREMENTS IN TURBULENT JET DIFFUSION FLAMES OF AIR DILUTED METHANE AND HYDROGEN

Simultaneous two-dimensional Rayleigh and fuel Raman images have been collected in air-diluted methane and hydrogen jet diffusion flames. Te mperature, 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 scal ar dissipation rate chi. The inherently weak Raman signal has been max imised by intra-cavity measurements, using a flashlamp-pumped dye lase r. In addition, the Raman signal-to-noise ratio is drastically improve d by a novel contour-aligned smoothing technique which exploits the hi gh correlation between the Rayleigh and Raman signals. Quantitative me asurements of scalar dissipation are presented, including probability density functions for components of chi. Profiles of mean and rms mixt ure fraction show the usual features already documented in other publi shed results for this type of flame. Probability density functions of xi 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 chi shows little change with Reynolds number. In the H-2 flame, mean valu es for the axial and radial components of the scalar dissipation rate, chi, are nearly the same, indicating a more isotropic structure than in the CH4 flames. For both fuels, the pdf of ln(chi) on the axis is m ore peaky than a lognormal distribution and somewhat skewed. The profi les of (chi/eta) show a nonlinear dependence on mixture fraction and h ave no clear resemblance to the skewed, monomodal shapes seen in cold flows. In the H-2 flame there is a strong correlation between instanta neous, local values of scalar dissipation and the departure from equil ibrium, as measured by temperature depression.