Combustion and Flame, Vol.162, No.12, 4622-4642, 2015
Analysis of dynamic models for large eddy simulations of turbulent premixed combustion
Dynamic procedures to automatically determine flame wrinkling factors from known resolved fields in large eddy simulations of turbulent premixed combustion are investigated from a priori tests processing a DNS database of a turbulent swirled flame. These flame wrinkling factors measure the ratio of total to resolved flame surfaces in the filtering volume and enter directly or indirectly into various flamelet combustion models through the sub-grid scale turbulent flame speed. They are usually modeled by algebraic expressions derived assuming equilibrium between turbulence motions and flame dynamics, a situation generally not reached during early stages of flame developments. Dynamic models then appear as a promising alternative to flame wrinkling factor or flame surface density balance equations to handle out-of-equilibrium situations. Attention is paid to three key requirements: (i) the correct prediction of propagating laminar flame fronts; (ii) the replacement of the averaging volume introduced to determine resolved and test-filtered flame wrinkling factors by a Gaussian operator easier to implement on unstructured meshes and/or massively parallel machines; (iii) the use of a local model parameter, evolving both in space and time. The two first requirements suggest basing the procedure on flame surface conservation instead of on chemical reaction rates. The saturated form of the Charlette et al. efficiency function , Xi(Delta) = (Delta/delta(l))(beta), where Delta is the filter width and delta(l) the flame thickness, is found to be very well suited to dynamic determination of the model parameter beta, easy to implement and very robust in practice, as confirmed by preliminary a posteriori tests. (C) 2015 The Combustion Institute. Published by Elsevier Inc. All rights reserved.