Differential Diffusion in Jets using Joint PLIF and Lorenz-Mie Imaging

Abstract

Differential diffusion between species often substantially effects the results of laboratory-scale experiments in turbulent combustion. The present work documents these effects for a simple jet flow by simultaneous 2D imaging of LIF from biacetyl and Lorenz-Mie scattering from submicron TiO₂ particles, at low to moderate Reynolds numbers. This novel technique, involving a frequency-tripled Nd:YAG laser and two slow-scan CCD cameras, captures directly the structural differences that arise when two species with widely different Schmidt numbers mix in a classical jet How where molecular diffusion makes a substantial contribution to overall mixing. Matched instantaneous image pairs of biacetyl and particle concentration show differences, with the particle images exhibiting sharper contours and a more convoluted structure with finer detail. The fluorescence images show a “superlayer” similar to that found in Rayleigh images in flows of this kind. Subtraction of one (normalized) image from the other in each pair reveals greater diffusive spreading of the biacetyl gas, a direct indication of its molecular contribution to total mixing. These findings support the argument that molecular diffusivity in general, and differential diffusion between species in particular, must be accounted for in the interpretation of laboratory flame results that are used to validate and test combustion models.

Key Words:

• Differential Diffusion
• Laser Diagnostics