SINOPSIS

The present work compares the results on large eddy simulation (LES) and Reynolds averaged Navier-Stokes (RANS) modeling of turbulent jet and coflow mixing of incompressible fluid (Schmidt number Sc ≈ 1000) in a coaxial mixer representing a cylindrical channel with a diameter D placed coaxially to a tube with an inner diameter d. Two different mixing regimes can be observed: (1) with a recirculation zone to develop just behind the tube at D/d > 1 + Q and (2) without a recirculation zone at D/d > 1 + Q. Here D/d is the diameter ratio and Q is the coflow-to-jet flowrate ratio. Turbulent transfer of inert passive admixture is considered to verify LES and RANS mixing models by comparing our numerical results and the available experimental data. For turbulent mixing to be described, the conservative scalar theory is adopted to calculate the averaged mixture fraction and its variance. The chemical source term in the transfer equation for reagent concentration is closed by the eddy dissipation concept and the presumed β-PDF of mixture fraction. LES is performed by invoking two subgrid scale (SGS) models: a dynamic variant of the Smagorinsky model proposed by Germano et al. (1991) and a dynamic mixed model extended to scalar fields. These models are adopted to study turbulent mixing with a fast chemical neutralization reaction. Complete analysis is made of the numerical results obtained.