Laminar burning velocity and Markstein lengths of methane-air mixtures

Spherically expanding flames propagating at constant pressure are ennployed to determine the unstretched laminar burning velocity and the effect of flame stretch as quantified by the associated Markstein lengths. Methane-air mixtures at initial temperatures between 300 and 400 K, and pressures between 0.1 and 1.0 MPa are studied at equivalence ratios of 0.8, 1.0, and 1.2. This is accomplished by photographic observation of flames in a spherical vessel. Power law correlations are suggested for the unstretched laminar burning velocity as a function of pressure, temperature, and equivalence ratio. Zeldovich numbers are derived to express the effect of temperature on the mass burning rate and from this, a more general correlation of burning velocity, based on theoretical arguments, is presented for methane-air mixtures. Flame instability is observed for mixtures at high pressure, and the critical radius for the onset of cellularity ir; correlated with Markstein number. Experimental results are compared with two sets of modeled predictions; one model considers the propagation of a spherically expanding flame using a reduced mechanism, and the second considers a one-dimensional flame using a full kinetic scheme. The results are compared with those of other researchers. Comparison also is made with iso-octane-air mixtures, reported elsewhere, to emphasize the contrast in the burning of lighter and heavier hydrocarbon fuels. (C) 2000 by The Combustion Institute.