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.
Peer Reviewed
http://deepblue.lib.umich.edu/bitstream/2027.42/30696/1/0000341.pdf