Images of the quenching of a flame by a vortex—To quantify regimes of turbulent combustion

Abstract

A laminar toroidal vortex is interacted with a laminar premixed flame in order to isolate and to visualize some of the fundamental physics of turbulent combustion. Localized quenching of the flame was observed using planar laser-induced fluorescence imaging of superequilibrium OH molecules in the counterflow flamefront region near the vortex leading edge. A quenching limit curve was measured as a function of vortex size and strength. In the second part of the study, the measurements are combined with concepts proposed by Poinsot, Veynante, and Candel in order to infer the thin flame limit, namely, the onset of distributed reactions, on a classical premixed turbulent combustion regime diagram. The measured thin flame limit indicates when laminar flamelet theories become invalid, since quenching allows hot products and reactants to coexist. Results are compared with the Klimov-Williams criterion. Vortex core diameters were as small as the flame thickness in some cases. The main conclusion is that small vortices are less effective at quenching a flame than was previously believed; therefore the inferred regime within which thin flame theories are valid extends to a turbulence intensity that is more than an order of magnitude larger than that which was previously predicted. Results also indicate that micromixing models, which assume that the smallest eddies exert the largest strain on a flame, are not realistic. Measured trends are in agreement with direct numerical simulations of Poinsot et al., but absolute values differ. The measured vortex Karlovitz number that is required to quench a flame is not constant but decreases by a factor of four as vortex size increases from one to five flame thicknesses. Thin-film pyrometry was used to quantify the radiative heat losses; quenching occurs when the products cool to approximately 1300 K, which is in agreement with stretched laminar flame calculations that include detailed chemistry. The quenching Karlovitz number for propane-air flames differs from that of methane-air flames, indicating the importance of detailed chemistry and transport properties. Flame curvature was observed to cause enhancement (or reduction) of the local reaction rate, depending on the Lewis number, in a manner that is consistent with stretched flame theory. microscale (U'rms/A)/(SL/,~) to equal unity. This prediction has been denoted the Klimov-Williams criterion [2]; the Taylor microscale (A) is assumed to be the appropriate

Direct numerical simulation study of statistically stationary propagation of a reaction wave in homogeneous turbulence

“…This quantitative difference can be attributed to the lack of thermal expansion effects in the present computations. In the case of premixed combustion, small-scale eddies are known to be inefficient in wrinkling reaction-zone surface [48,49], in particular, because they rapidly disappear due to dilatation and an increase in viscosity within the flame preheat zone. However, because these two phenomena vanish in a constant-density flow, reaction waves do not destroy the smallest-scale turbulent eddies and, therefore, do not reduce the highest local rates of stretch of reaction zones under conditions of the present DNS.…”

Lewis number effects in distributed flames

“…The distributed flames of [6][7][8] are confined by periodic lateral boundary conditions, which means there is a pool of hot fluid that is mixed with the fuel by fullydeveloped turbulence. The Bunsen flames of [9][10][11][12] and the single vortex-flame interactions of [13,14] that extinguish at high Karlovitz number, all experience large-scale global mean stretch, which is the likely explanation for the difference in behavior.…”

Quantitative scalar dissipation rate measurements in vortex-perturbed counterflow diffusion flames

“…First, we provide an experimental technique for the quantitative measurement of v stoich and use it to estimate this parameter for quasi-steady and vortex-perturbed flames approaching extinction. As far as diagnostics is concerned, the study of flame-vortex interaction has yielded two-dimensional images of various observables in the toroidal mixing layers [6,12]. A technique based on Raman spectroscopy, very similar to the one to be discussed here, was used in Ref.…”