Gradient Trajectory Analysis of the Burning Rate in Turbulent Premixed Jet Flames

Abstract

ABSTRACT An analysis of the combustion process along gradient trajectories is presented for a set of Direct Numerical Simulations of turbulent premixed jet flames at different Reynolds numbers and approximately constant Karlovitz numbers. The variation of the jet Reynolds number, ranging from 5600 to 22400, is achieved by increasing the width of the jet and keeping the bulk velocity constant, which also implies approximately constant values of the turbulence intensities across the flames. The flames considered are nominally in the thin-reaction zone regime of the Borghi-Peters diagram of turbulent combustion. The thickening of the inner reaction layer and its enhancement with increasing Reynolds number, observed in previous works on the same database, is linked here to the presence of extremal points in the temperature field (local maxima in the low temperature side and minima in the high temperature side) in the vicinity or inside the inner reaction layer. Therefore, an enhancement of the turbulent flame speed is linked to the interruption of the flamelet structure by turbulence and not by the thickening of the entire flamelet itself. In addition to the expected large thickening of the formaldehyde layer, the layers of species which are characteristic of the inner reaction zone, such as OH and O, show a significant thickening and these species are observed several flame thicknesses ahead of the turbulent flame surface. Finally, it is shown that, in the regions where the inner reactive layer is thickened, the local fuel consumption rate is reduced in comparison to a laminar planar flame, but the total burning rate integrated over the entire flame structure is larger due to the increased volume of the reactive layer. These observations highlight the importance of assessing the balance between two competing phenomena related to high Karlovitz numbers. The first is the effect of turbulence strain and stirring on the chemistry, which usually decreases the local burning rate. The second is the effect on the species fields, which might experience modifications in their topology, such as the introduction of local extremal points, and significant thickening of the volumes between their isosurfaces.