One-dimensional raman scattering for determination of multipoint joint scalar probability density functions in turbulent diffusion flames

Abstract

Joint probability density functions (PDFs) are one approach to describe turbulent reacting flows. One-dimensional spontaneous Raman scattering (1D-SRS) allows the simultaneous detection of the scattered light from numerous chemical species in a reacting flow at different points along a line, thereby providing all necessary information for multipoint joint scalar PDFs. The flow analyzed in this work with the help of 1D-SRS is a turbulent methane diffusion flame with a coflow of oxygen at moderate turbulence intensities. The simultaneous measurements of different species concentrations at several points determine multipoint joint scalar PDFs. From these PDFs, statistical moments such as mean values and standard deviations can be derived, as well as scalar spatial correlations and scalar length scales. The mapping of turbulent structures in the reacting flow is possible with all the statistical information contained in multipoint joint scalar PDFs. The chemical reactions in the methane flame correspond to a global two-step mechanism. This can be seen from the map of the mean mole fractions and is confirmed by the map of species correlations. The scalar length scales calculated from the space-correlation functions of concentrations show the general course of length scales for velocities in the similarity regions of the jet. However, length scales will be different for different chemical species and they deviate from length scales for velocities in regions of the jet where high turbulent reaction rates prevail. Scalar spatial correlations and length scales are directly reflected by the multipoint scalar PDFs that may serve as a data basis for mixing models in PDF transport equations.