Conserved scalar probability density functions in a turbulent jet diffusion flame

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Abstract
The first four moments of conserved scalar probability density functions (p.d.f.'s) measured by Raman scattering in an H2 turbulent jet diffusion flame are analysed and compared with those found by Pitts & Kashiwagi (1984) in a non-reacting CH4 jet. The measurements are in good agreement, indicating that heat release and combustion have little effect on p.d.f. shapes. However, the measured p.d.f.'s are not qualitatively similar to the simple forms often assumed in combustion modelling. A three-zone model by Effelsberg & Peters was used to separate the experimental p.d.f.'s into a delta function (non-turbulent zone), a Gaussian (turbulent zone) and the remainder (interface zone). The interface zone contributed as much as 90% of the total p.d.f. in both the H2 flame and the non-reacting CH4 jet. A physical interpretation for the existence of broad interface zones in reacting and non-reacting turbulent jet flows is suggested based upon large-scale structures.