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LES VERSUS RANS MODELLING OF TURBULENT MIXING INVOLVING CHEMICAL REACTING IN A CO-AXIAL JET MIXER

DOI: 10.1615/ICHMT.2008.CHT.960
24 pages

Andrei Chorny
Turbulence Laboratory, A. V. Luikov Heat & Mass Transfer Institute, P. Brovka Str. 15, Minsk, 220072, Belarus

Johann Turnow
Chair of Modeling and Simulation, Department of Mechanical Engineering and Marine Technology, University of Rostock, Albert-Einstein-Str. 2, 18055 Rostock, Germany

Nikolai Kornev
Chair of Modeling and Simulation, Department of Mechanical Engineering and Marine Technology, University of Rostock, Albert-Einstein-Str. 2, 18055 Rostock, Germany

Egon Hassel
Institute for Technical Thermodynamics Faculty of Mechanical Engineering and Marine Technology University of Rostock Albert-Einstein-Str. 2, D-18059 Rostock, Germany

Abstrakt

The present work compares the results on LES and RANS modelling of turbulent jet and co-flow mixing of incompressible fluid (Schmidt number Sc ≈ 1000) in the co-axial mixer representing a cylindrical channel with a diameter D placed co-axially 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 co-flow-to-jet flowrate ratio. The 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 [Zhdanov etal 2006]. For turbulent mixing to be described, the conservative scalar theory is adopted to calculate 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 [Fox 2003]. LES is performed invoking two SGS models: a dynamic variant of the Smagorinsky model proposed by Germano et al. [Germano et al 1991] and a dynamic mixed model extended to scalar fields [Kirkpatrick et al 2003]. These models are adopted to study turbulent mixing with a fast chemical neutralization reaction. A complete analysis is made of the numerical results obtained. The decay of the averaged mixture fraction and its variance by the RANS model with the mechanical-to-scalar time ratio R and the turbulent Schmidt number Scσ in the transfer equation for variance as a function of Ret is similar to the one by LES with the dynamic mixed model and from experiment. All RANS models overestimate the generation rate of a reaction product and have a poor agreement with the LES results and the experimental data for the flow region with intense chemical reacting.

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