Effects of body forces on vorticity and enstrophy evolutions in turbulent premixed flames

Abstract

The effects of body forces (alternatively Froude number) on both vorticity and enstrophy evolutions within the flame brush have been analysed using Direct Numerical Simulations (DNS) data of freely propagating statistically planar turbulent premixed flames subjected to different turbulence intensities.The turbulence parameters are taken to represent the thin reaction zone regime of premixed turbulent combustion. The enstrophy has been found to decay significantly from the unburned to the burned gas side of the flame brush for high turbulence intensities and this trend is particularly prominent for the strengthening of the body force promoting unstable stratification. However, local instances of enstrophy generation have been observed and in some cases the decay of enstrophy is arrested across the flame brush for small turbulence intensities. This trend strengthens with the increasing magnitude of the body force promoting stable stratification. The enstrophy generation due to the baroclinic torque is primarily responsible for this local enstrophy generation for small turbulence intensities especially under the body force promoting stable stratification. This baroclinic torque contribution is also found to be responsible for anisotropic behaviour of vorticity components within the flame brush. The vortex stretching and viscous dissipation terms have been found to be the leading order source and sink terms, respectively, in the enstrophy transport for high turbulence intensities especially in the case of body force promoting unstable stratification. However, baroclinic torque, and the sink term due to dilatation rate continue to play significant roles even for high turbulence intensity cases considered here but their relative importance increases with decreasing turbulence intensity especially under the body force promoting stable stratification. The surface-weighted entrainment velocity has been found to be mostly unaffected by the body force in this analysis, and a minor influence can be discerned in the case of small turbulence intensities where an unstable stratification tends to promote high negative values of entrainment velocity only towards the unburned gas side of the flame brush.

Edward E. O'Brien contributions to reactive-flow turbulence

“…This enthusiastic response is another indication of the respect Ted O'Brien has within the international research community of turbulence and reactive flows. These contributions are on diverse topics including combustion instability, 150,151 scalar mixing, [152][153][154][155][156] homogeneous isotropic turbulence, [157][158][159][160] turbulent premixed flames, [161][162][163][164][165][166][167][168][169][170][171] turbulent non-premixed flames, [172][173][174][175] wallbounded turbulence, [176][177][178] turbulent combustion modeling, [179][180][181] FDF/PDF, [182][183][184][185][186][187][188][189][190][191][192] and two-phase turbulent flows. [193][194][195][196]<...…”

Conditioned structure functions in turbulent hydrogen/air flames

“…Substantial influence of combustion-induced thermal expansion on turbulence in flames has been known since the seminal papers by Karlovitz et al 1 and Scurlock and Grover. 2 Over the past two decades, rapid development of Direct Numerical Simulation (DNS) methods and tools allowed researchers [3][4][5][6][7][8][9] to reveal various manifestations of this influence and to document significant changes of basic features of turbulence in premixed flames. Such results reviewed elsewhere [10][11][12][13] call for revisiting the problem of modeling turbulence in reacting flows.…”

Effects of thermal expansion on moderately intense turbulence in premixed flames

“…Since the pioneering work by Karlovitz et al 1 and Libby and Bray, 2 effects of thermal expansion on turbulence (e.g., so-called flame-generated turbulence 1 ) and turbulent scalar transport (e.g., so-called counter-gradient diffusion 2 ) in premixed flames have long been a challenging research subject. [3][4][5][6][7][8][9][10][11][12][13][14][15] Numerical studies reviewed elsewhere [16][17][18][19] indicate that the influence of combustion-induced thermal expansion on turbulence within a premixed flame brush is well (hardly) pronounced in weakly (highly) turbulent flames. Recent Direct Numerical Simulation 11,[20][21][22][23] (DNS) and experimental 24,25 investigations further support this view.…”