Flow topologies in different regimes of premixed turbulent combustion: A direct numerical simulation analysis

Abstract

The distributions of flow topologies within the flames representing the corrugated flamelets, thin reaction zones, and broken reaction zone regimes of premixed turbulent combustion are investigated using direct numerical simulation data of statistically planar turbulent H 2 -air flames with an equivalence ratio φ = 0.7. It was found that the diminishing influence of dilatation rate with increasing Karlovitz number has significant influences on the statistical behaviors of the first, second, and third invariants (i.e., P , Q, and R) of the velocity gradient tensor. These differences are reflected in the distributions of the flow topologies within the flames considered in this analysis. This has important consequences for those topologies that make dominant contributions to the scalar-turbulence interaction and vortexstretching terms in the scalar dissipation rate and enstrophy transport equations, respectively. Detailed physical explanations are provided for the observed regime dependences of the flow topologies and their implications on the scalar dissipation rate and enstrophy transport.

Effects of Reaction Progress Variable Definition on the Flame Surface Density Transport Statistics and Closure for Different Combustion Regimes

“…The distributions of RPV for the flames considered here are presented elsewhere Wacks et al, 2016) and thus are not shown here. In statistically planar flames, ̃ is a unique function of the mean direction of flame propagation and thus all the terms are presented here as functions of ̃ for different definitions of RPV.…”

“…A three-dimensional DNS Wacks et al, 2016) database of H 2 -air flames with an equivalence ratio of 0.7, employing a detailed chemical mechanism (Burke et al, 2012) with 9 species and 19 chemical reactions, is considered here. The unburned gas temperature 0 is taken to be 300K, which leads to an unstrained laminar burning velocity = 135.6 cm/s and heat release parameter = ( − 0 ) 0 ⁄ = 5.71 (where is the adiabatic flame temperature) under atmospheric pressure.…”

“…The unburned gas temperature 0 is taken to be 300K, which leads to an unstrained laminar burning velocity = 135.6 cm/s and heat release parameter = ( − 0 ) 0 ⁄ = 5.71 (where is the adiabatic flame temperature) under atmospheric pressure. The numerical implementation pertaining to this database has been discussed elsewhere Wacks et al, 2016) in detail and thus will not be repeated here. Turbulent inflow and outflow boundaries are taken in the direction of mean flame propagation and transverse boundaries are taken to be periodic.…”

Scalar dissipation rate transport conditional on flow topologies in different regimes of premixed turbulent combustion

“…The dilatation rate is predominantlypositive within the flame and thus S5 and S6, which are typical of = (−∇ • ⃗ ) > 0, are rarely obtained there. The relative strength of dilatation rate and likelihood of obtaining high positive values of ∇ • ⃗ are significantly smaller in case C than in cases A and B because energetic turbulent eddies penetrate into the reaction zone and disturb the chemical reaction, which affects the magnitude of ∇ • ⃗[19]. However, previous analyses indicated that the magnitude of heat release rate does not necessarily decrease with increasing Karlovitz number but the heat release rate becomes less significant in comparison to turbulent transport contributions[28,29].…”

“…1 [12,13] (S2 topology is schematically shown for later discussion). Relatively limited effort has been directed to the analysis of flow topology distributions in turbulent combustion [14][15][16][17][18][19][20][21]. This analysis considers a detailed chemistry…”