The effect of coflow velocity on a lifted methane-air jet diffusion flame

The effect of coflow velocity on flame liftoff is studied using numerical simulations of methane-air diffusion flames. The numerical model solves the time-dependent, axisymmetric (2-D), Navier-Stokes equations coupled to submodels for chemical reaction and heat release, soot formation, and radiation transport. The computations predict a flame structure similar to that observed experimentally. The flame stabilization point is located on the stoichiometric surface in the inner shear layer, Animations of the simulations show that the radial and axial location of the stabilization point varies in time by 1-2 cm, as the flow field is distorted by passing vortices in the inner shear layer. Flame liftoff heights compare well with those observed experimentally. The computations show that liftoff height increases with jet exit velocity and with the air coflow velocity. For higher coflow velocities, the inner-shear-layer vortices begin to form farther downstream, and the jet spreads more slowly. The scalar dissipation rate along the stoichiometric contour decreases sharply at the liftoff height. All of the liftoff data for a wide range of jet (20-50 m/s) and coflow (10-1500 cm/s) velocities collapse onto one curve when liftoff height is plotted against an effective velocity. These data indicate that the momentum of the coflowing stream acts in combination with the jet momentum to push the base of the flame farther away. This suggests that the momentum at the flame base is a strong factor in determining flame liftoff height.