Downstream Development of Premixed Turbulent V-Flames

Abstract

reactants from products. Within the flamelet chemical reactions that are coupled to thermal conduction and species diffusion convert reactants to products and chemical bond energy to thermal energy (heat release). As a consequence, the flame moves relative to the reactant gases, and the speed of this motion, the flame speed, SL, is an important property of premixed flames: the product of the flame speed and the reactant gas density is proportional to the rate per unit flamelet area of both product formation and heat release. Stereo particle image velocimetry (SPIV) images are used to study reactant velocity fields and flamelet wrinkling in nine rod-stabilized premixed, methaneair turbulent V-flames. Flame conditions are varied by changing a combination of equivalence ratio, flow velocity and turbulence generating grid. Components of mean velocities and r.m.s. turbulent velocities as well as Taylor microscales and turbulence Reynolds numbers are extracted from the images. By treating the SPIV images as if they were laser tomography images and analyzing as suggested in Ref. 7, mean progress variable fields and PDFs of flamelet normal vectors also are extracted. At low to moderate turbulence levels flamelets are wrinkled and contorted by turbulence and their internal structure, e.g., species concentration and temperature gradients, is perturbed 1, 2 . Still it is possible to identify a flamelet surface and a flame speed such that the mean rate of product formation per unit volume, ω, (and of heat release) is proportional to the product of the mean flamelet surface area to volume ratio (the mean surface density, Σ) and the reactant density times the ensemble mean flame speed of the flamelet as perturbed by the turbulence, ρr 1, 2, 3 : The turbulence Reynolds is found to vary over a range from 14 to 24 for the 9 flames. R.m.s. turbulent fluctuations are anisotropic near the turbulent flame brush and approach isotropic conditions away from it. The flamelet normal vector PDF is defined by a single fit parameter, ζ. ζ is found to vary from flame to flame and to increase approximately linearly with downstream distance in a flame. This rate of increase also varies from flame to flame. Analysis of these variations suggest that the magnitude of ζ and its variation with downstream distance depend on Re, the magnitude of r.m.s. turbulent velocity fluctuations scaled by the unstrained laminar flame speed and the angle of the turbulent flame brush with respect to the vertical. . Σ