Effects of pressure gradients on turbulent premixed flames

Abstract

In most practical situations, turbulent premixed flames are ducted and, accordingly, subjected to externally imposed pressure gradients. These pressure gradients may induce strong modifications of the turbulent flame structure because of buoyancy effects between heavy cold fresh and light hot burnt gases. In the present work, the influence of a constant acceleration, inducing large pressure gradients, on a premixed turbulent flame is studied using direct numerical simulations.A favourable pressure gradient, i.e. a pressure decrease from unburnt to burnt gases, is found to decrease the flame wrinkling, the flame brush thickness, and the turbulent flame speed. It also promotes counter-gradient turbulent transport. On the other hand, adverse pressure gradients tend to increase the flame brush thickness and turbulent flame speed, and promote classical gradient turbulent transport. As proposed by Libby (1989), the turbulent flame speed is modified by a buoyancy term linearly dependent on both the imposed pressure gradient and the integral length scale lt.A simple model for the turbulent flux u″c″ is also proposed, validated from simulation data and compared to existing models. It is shown that turbulent premixed flames can exhibit both gradient and counter-gradient transport and a criterion integrating the effects of pressure gradients is derived to differentiate between these regimes. In fact, counter-gradient diffusion may occur in most practical ducted flames.

A direct numerical simulation study of vorticity transformation in weakly turbulent premixed flames

“…In the DNS literature, a number of images of instantaneous vorticity fields can be found, [20][21][22][23][24][25][26][27][28][29][30] with a few recent DNS papers [31][32][33][34] aiming at quantitatively investigating vorticity changes due to premixed turbulent combustion. In particular, Treurniet et al 31 simulated weakly and moderately turbulent premixed flames characterized by various density ratios σ and documented anisotropic generation of vorticity in the cases of σ = 6 or 4, whereas the computed vorticity decayed within turbulent flame brush in the case of a lower density ratio, i.e., σ = 2.…”

Subgrid combustion modeling for premixed turbulent reacting flows

“…t is an eddy viscosity, a c is the turbulent Schmidt number and #,-is a Cartesian coordinate component. The presence of counter-gradient diffusion in turbulent premixed combustion has been well established in theory (Libby and Bray, 1981), experiments (Cho et al, 1988;Li et al, 1996) and in direct numerical simulation (DNS) (Veynante et al 1995. The turbulent flux normalized by the reactant density (p 0 ) and the mean jet velocity (U 0 ) are shown in Figs.…”

Lean premixed opposed jet flames in fractal grid generated multiscale turbulence

“…24). Veynante et al [73] derived the Bray number as a criterion for the transition between GD and CGD in freely propagating flames with subsequent refinements provided [74,75]. Chen and Bilger [76] further analysed the transition from GD to CGD using Bunsen flames.…”

“…However, in the turbulent opposed jet configuration the geometry imposed pressure gradient exerts a significant influence on the transition from GD to CGD. Efforts have been made to account for such effects [70,74], though a comprehensive evaluation of the resulting model formulations present difficulties in the current context. However, the current data, see …”