The mathematical modeling of liftoff and blowoff of turbulent non-premixed methane jet flames at high strain rates

A model is described for non-premixed turbulent combustion. It is applied to subsonic methane jet flames and, on the basis of presented evidence, is based on combustion in strained premixed laminar flamelets. When the fuel discharges into the air, the shear-generated aerodynamic strain rate is initially sufficiently high to quench both diffusion and premixed flamelets. Farther downstream, the strain rate relaxes and premixed burning ensues. Data are drawn from models of premixed laminar flames and include heat-release rate-temperature profiles, Markstein numbers, and positive and negative stretch rates for flame quenching. A conditional probability density function (PDF) for reactedness is introduced and Reynolds stress, second-order closure is adopted. Predicted liftoff heights and blowoff velocities are in good agreement with available ineasurements, up to quite large diameters. Dimensionless correlations of these are presented as are detailed flow and combustion structures for the base of the flame. The contributions of different mixture fraction bands to the mean heat release are computed. The relative contribution of rich burning decreases with a decrease in pipe diameter and an increase in flow velocity. An increase in flow velocity results in leaner combustion and an increase in flame stretch rate that eventually produces extinction at blowoff. The implications of this for modeling are discussed.

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