Laminar burning velocities and Markstein numbers of hydrocarbonair flames

Abstract

Effects of positive flame stretch on the laminar burning velocities of hydrocarbon/air mixtures were studied experimentally using outwardly propagating spherical flames. The test conditions included propane, methane, ethane, and ethylene-air flames at various fuel-equivalence ratios and normal temperature and pressure. Karlovitz numbers generally were less than 0.3 so that the flames were remote from quenching conditions. Within this range, the ratio of the unstretched (plane flames) to stretched laminar burning velocities varied linearly with Karlovitz numbers, yielding Markstein numbers that were independent of Karlovitz numbers for a particular reactant mixture. In addition, Markstein numbers varied in a roughly linear manner with fuel-equivalence ratios over the range of the measurements, which were somewhat removed from flammability limits where behavior might differ. Effects of stretch were substantial: Markstein numbers varied from -2.5 to 7.2, yielding corresponding laminar burning velocity variations of 0.4-2,7 times the value for an unstretched (plane) flame over the test range. The ranges of fuel-equivalence ratios for unstable preferential-diffusion conditions (negative Markstein numbers) were as follows: propane, greater than 1.44; methane, less than 0.74; ethane, greater than 1.68; and ethylene, greater than 1.95. Fuel-equivalence ratios for maximum flame temperatures and laminar burning velocities are near unity for the present flames; therefore, neutral preferential-diffusion conditions are shifted toward fuel-equivalence ratios on the unstable side of unity, in qualitative agreement with recent approximate theories treating the effects of stretch on laminar premixed flames.

Active control for statistically stationary turbulent premixed flame simulations

“…In addition, the relative insensitivity of the φ = 0.75 flame is apparent. If we associate a curvature Markstein number, ᏹ κ , with the slope of the correlation for each case in Figure 11, then the data matches the trend reported in [39], including the change of sign of the Markstein number near φ = 0.75. The magnitude of the Markstein number is sensitive to the definitions of flame thickness, burning speed, flame isopleth, etc.…”

“…We now apply the control methodology described above to a series of methane flames modeled in significantly greater detail, using the GRI-Mech 3.0 chemistry mechanism (53 species, 325 reactions) and a mixtureaveraged diffusive transport model. Three flames are chosen to highlight variations observed in a methane flame's response to flowfield flame surface curvature (see, for example, Tseng et al [39]). The three cases have stoichiometries, φ = 0.55, 0.75, 1.0.…”

Curvature and Velocity of Methane-Air Bunsen Flame Tips

“…The experimental task can be simplified if the experiment is designed in such a way that only one of the two contributions to the flame stretch discussed above is present or there is a relation between the two. This goal has been achieved in previous works by using spherical flames propagating inward or outward [8], or flames in the stagnation point flow configuration. In the first case the front curvature alone determines the stretch, whereas in the second case the flame is flat and the only contribution comes from the strain rate [9].…”

Morphology and burning rates of expanding spherical flames in H2/O2/inert mixtures up to 60 atmospheres

“…Groff [4] first measured spherical flame speeds at high pressures, up to 5 atm, by directly imaging the propagating flame front and also through pressure measurements. Lately, Taylor and coworkers [5][6][7] and Faeth and coworkers [8][9][10][11][12][13] determined val-0 s u ues for various fuel compositions and pressures through systematic subtraction of stretch effects using similar imaging techniques. The values so de-0 s u termined, with different stretch-compensated relations, are in reasonably good agreement with each other, as well as with computational calculations and the data from counterflow experiments.…”