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Investigation of turbulent reactingmixing processes at high Schmidt numbers
5th OpenFOAM Workshop, Chalmers, Gothenburg, Sweden, June 21-24, 2010
10010-12
chemistry time scale
mixing time scale
time (seconds)
mesh macro-mixing
subgrid
micro-mixing model
residence time
Sc = 1Sc > 1Sc < 1
spectral regimes of scalar field
ln (E)
1. Motivation
− mixing in (chemically reacting) turbulent flows is determined by the transport ofthe medium caused by vortices (macro-mixing) and the molecular mixing(micro-mixing)
− the molecular mixing is strongly influenced by the dissipation scale of the scalarfield (Batchelor scale )
− in flows with high Schmidt numbers ( ), the Batchelor scale is muchsmaller than dissipation scale of the velocity field (Kolmogorov scale )
− high Schmidt numbers are typical for liquids (pure water ~ 1000)− the correlation of both scales is determined from
Problem: is extremely small in turbulent liquid reacting flows , however all interactions on this scale have to be captured for a correct predictionof the mixing and the reaction rates
5th OpenFOAM Workshop, Chalmers, Gothenburg, Sweden, June 21-24, 2010
15 mm 55 mm
reactionzone
3. Implementation and numerical setup
− finite rate Eddy Dissipation Model for chemistry-turbulence interactionimplemented:
• in principle detailed reaction kinetics for is possible• for gaseous reactions (Sc ~ 1) the mixing rate is proportional to the dissipation of turbulent
kinetic energy modifications necessary to take better high Schmidt numbers into account• for liquid reactions (Sc >> 1) the mixing rate must be related to the dissipation of the scalar
• reaction rate
• scalar dissipation rate (Girimaji)
• scalar variance by a scale similarity model (Cook and Riley)
• finite rate EDM implemented using mixture fraction and reaction progress variables to simplifythe reaction kinetics, algebraic expressions for calculation of concentrations
04.06.2010
12
5th OpenFOAM Workshop, Chalmers, Gothenburg, Sweden, June 21-24, 2010
− Re ~ 12000, Sc ~ 1000− some comments on OpenFOAM
• chemistry-turbulence models in OpenFOAM require modifications to take high Schmidt numbers into account
• liquid-liquid reactions have still not properly considered in OpenFOAM• sophisticated models like FDF/transported PDF or ILDM/REDIM with presumed PDF are still
not available in OpenFOAM
04.06.2010
5th OpenFOAM Workshop, Chalmers, Gothenburg, Sweden, June 21-24, 2010
1. implementation of a new SGS model for LES based on multifractaltheory that accounts for high Schmidt number effects at high Reynolds numbers and moderate mesh resolutions
2. coupling of multifractal model and reaction kinetics with a chemistry-turbulence interaction model
3. implementation and testing of more sophisticated chemistry-turbulenceinteraction models for liquid-liquid reactions (like presumedPDF withILDM, FDF)
04.06.2010
5th OpenFOAM Workshop, Chalmers, Gothenburg, Sweden, June 21-24, 2010