Flame/stretch interactions in laminar and turbulent premixed flames

The flame/stretch interactions of laminar and turbulent premixed flames are considered both experimentally and computationally. Potentially strong effects of flame/stretch interactions due to preferential-diffusion phenomena within practical turbulent premixed flames were suggested by experiments and numerical simulations of spherical outwardly propagating laminar premixed flames. These considerations were limited to conditions where ignition disturbances, pressure variations, intrinsic unsteadiness of propagating spherical flames, and radiative heat losses were small. Flame reactants consisting of H 2 /O 2 /N 2 and several light hydrocarbon/air mixtures were studied for various fuel-equivalence ratios and pressures of 0.5-4.0 atm at normal temperature (298±3K). The measurements and predictions yielded several interesting results, as follows: Flame response to stretch was linear using a local conditions hypothesis to define characteristic flame length and time scales, yielding constant Markstein numbers for given flame conditions; effects of stretch were surprisingly strong with up to 100 percent variations of laminar burning velocities resulting from rather modest stretch rates well below extinction conditions (i.e., Karlovitz numbers less than 0.5); there was a progressive tendency for greater ranges of unstable preferential-diffusion conditions (negative Markstein numbers) as pressures were increased for all reactant mixtures studied; and several contemporary detailed treatments of multicomponent transport and chemical reaction mechanisms yielded reasonably good predictions of laminar burning velocities and their sensitivity to flame stretch due to preferential-diffusion effects. The predictions suggest that the strong sensitivity of the present flames to stretch is mainly caused by preferential diffusion of light radicals and stable species relative to typical stable reaction products and heat, with increased preferential-diffusion instability at elevated pressures resulting from reduced radical concentrations in the reaction zone due to increased radical recombination rates. The potential practical importance of flame/stretch interactions was examined by considering the properties of strongly turbulent premixed flames. These measurements involved premixed H 2 /O 2 /N 2 and C 3 H 8 /air flames propagating in the thin wrinkled flamelet regime within isotropic turbulence. Test conditions included unstable, near-neutral, and stable flames with respect to effects of preferential-diffusion. The experiments yielded several interesting observations, as follows: 1) Rates of turbulent flame propagation progressively decreased as flame stability with respect to preferential-diffusion effects increased even through unstretched laminar burning velocities and turbulence properties were the same; 2) Distortion of the flame surfaces by turbulence as the flames grew caused their fractal dimensions to progressively increase from a value of 2.0, appropriate for a smooth surface, to asymptotic values in the range 2.3-2.4, irrespective of preferential-diffusion stability conditions; 3) Other parameters characterizing the extent of distortion of the flame surfaces showed no tendency to approach asymptotic values for available observation times, however, raising questions about the existence of steady turbulent flame propagation properties for the present test conditions; and 4) The extent of flame surface distortion progressively increased at a given flame diameter, but decreased at a given time of propagation, as preferential-diffusion stability was progressively increased even though unstretched laminar burning velocities and turbulence properties were the same. These flame/stretch interactions in turbulent flames can be explained by noting that stable (unstable) preferential-diffusion conditions tend to retard (enhance) distortion of the flame surface by turbulence for outwardly propagating spherical turbulent premixed flames in much the same way that preferential-diffusion effects interact with small disturbances to yield either stable (unstable) flames for nonturbulent conditions.