Stabilization mechanism of lifted jet diffusion flames

Abstract

Flame lift and stabilization are studied using numerical simulations of diffusion flames resulting from amethane jet injected into an air background. The numerical model solves the time-dependent, axisymmetric, multidimensional Navier-Stokes equations coupled to submodels for chemical reaction and heat release, soot formation and radiation transport. Simulations are conducted for an undiluted methane jet and for two nitrogen-diluted jets (CH 4 :N 2 /3:1 and CH 4 :N 2 /1:1). The jet exit velocities range from 20 to 50 m/s through a 1-cm-diameter nozzle, coflowing into a 30-cm/s air stream. The flame liftoff height increases linearly with jet exit velocity and the stabilization height increases as the nitrogen dilution of the jet increases. The computations show that the flame is stabilized on a vortical structure in the inner shear layer, which is on the stoichiometric surface at a height where the local axial velocity is approximately equal to the turbulent burning velocity. There is no appreciable chemical heat release in the region below the stabilization point, although a stoichiometric surface exists in that region. The flame base moves upward with the vortical structure to which it is attached, and then quickly jumps down to attach to a new, lower vortex, resulting in an oscillating (1–2 cm) flame liftoff height. The results corroborate parts of both the premixedness and extinction stabilization theories, and suggest that the liftoff mechanism is a result of complex fluid-chemical interactions, parts of which are incorporated in the simplified theories.