AN EXPERIMENTAL STUDY OF FREELY PROPAGATING TURBULENT PROPANE/AIR FLAMES IN STRATIFIED INHOMOGENEOUS MIXTURES

Abstract

Experiments of freely propagating turbulent flames in nonhomogeneous propane/air mixtures are performed for various initial distributions of heterogeneities and compared with homogeneous cases. For various flame propagation times, laser tomography and planar laser-induced fluorescence on acetone are performed simultaneously to characterize the flame-front structure as well as the local mixture fraction in front of the flame. The two-dimensional scalar fields are characterized by a mean stratified mixture fraction field, symmetrical around ignition, and by a fluctuation field. The scalar scales are determined from the spatial autocorrelation coefficients and are compared with the turbulence scales, because the turbulence grid is responsible for the mixing. From the flame contours, global parameters (flame radii and wrinkling ratio) and local curvature distributions are also studied to evaluate the contribution of heterogeneities on the topology of the turbulent flame front. At the early stages of flame propagation, the flame structure is weakly affected by the local heterogeneities. Then, at higher propagation times, the heterogeneities tend to smooth the wrinkling produced by the turbulent flow field. A characteristic scale of flame curvature is then compared with the integral length scale of the flow field, L _T , and scalar scale L _Z . For heterogeneous conditions, the flame front presents small wrinkles with positive and negative curvatures equally distributed all around the flame with a characteristic size strongly depending on L _Z .

Keywords:

• partially premixed combustion
• optical diagnostics
• local flame structure

This work is supported by the ECODEV/CNRS program and by Peugeot S. A. and Renault, as part of the ARC (Action de Recherche Concertée) Combustion Stratifie´e research program.

Notes

U is the mean velocity, u′ the velocity RMS, L _T the integral length scale, λ the Taylor scale obtained from the osculating parabola of the autocorrelation coefficient, τ _T the eddy turnover time L _T /u′, Re _T the turbulent Reynolds number u′L _T /ν, Re _λ the Reynolds number based on the Taylor length scale u′L _λ/ν, k the turbulent kinetic energy, and ε the turbulent dissipation rate.