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A-PRIORI DIRECT NUMERICAL SIMULATION ASSESSMENT OF THE EFFECTS OF LEWIS NUMBER ON THE PERFORMANCES OF SUB-GRID SCALAR FLUX MODELS FOR LARGE EDDY SIMULATION OF PREMIXED FLAMES

DOI: 10.1615/ICHMT.2014.IntSympConvHeatMassTransf.480
pages 633-645

Nilanjan Chakraborty
School of Mechanical and Systems Engineering, Newcastle University, Newcastle-Upon-Tyne, NE17RU, United Kingdom

Markus Klein
Institut fur Mathematik und Rechneranwendung Universität der Bundeswehr München W.-Heisenberg-Weg 39, Neubiberg, Germany

Аннотация

The performances of the models for the sub-grid scalar flux of reactive scalars for Large Eddy Simulations (LES) of premixed turbulent combustion have been assessed based on a Direct Numerical Simulation (DNS) database of freely propagating turbulent premixed flames with global Lewis number Le (=thermal diffusivity/mass diffusivity) ranging from 0.34 to 1.2. It has been found that the sub-grid scalar flux components exhibit counter-gradient transport for all cases considered here. However, the extent of counter-gradient transport decreases with decreasing values of filter width Δ and increasing value of Le. The sub-grid scalar flux shows counter-gradient transport when the velocity jump across the flame due to heat release overcomes the effects of turbulent velocity fluctuation. The effects of heat release and flame normal acceleration strengthen with decreasing Le and this trend is particularly strong for flames with Le << 1 (e.g. Le = 0.34 ) due to thermo-diffusive instabilities, which in turn increases the extent of counter-gradient transport. The performances of several models have been assessed with respect to the sub-grid scalar flux components extracted from explicitly filtered DNS data. It has been found that the standard gradient hypothesis does not adequately capture the quantitative behaviour of the sub-grid scalar flux components for all filter widths in all cases considered here. The models which account for local flame normal acceleration perform better than the standard gradient hypothesis model especially for filter widths where the flame remains completely unresolved. In general the performance of the models, which account for the alignment of local resolved velocity and scalar gradients, remains relatively better than the other alternative models irrespective of Le.

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