Stabilization of lifted tribrachial flames in a laminar nonpremixed jet

The stabilization mechanism of lifted flames in a laminar nonpremixed jet has been analyzed and experimentally investigated. An analysis on the flame response by the perturbation from the tribrachial location shows that the lifted flame is unstable for Sc < 1, confirming the experimental findings of the nonexistence of stationary lifted flames for methane and ethane jets. Appreciable upstream mass diffusion from the nozzle exit for Sc < 0.5 explains the flame behaviour for hydrogen fuel such that a nozzle-attached flame exists even up to jet velocities comparable to the speed of sound. Flow-field measurements using laser Doppler velocimetry reveal that the flow velocity at the flame stabilization location for lifted flames is larger than the laminar burning velocity of the stoichiometric premixture, which confirms the increasing effect on flame speed by the flow redirection for tribrachial flames. Also, it is demonstrated that the region between the nozzle exit and the anchoring point of the lifted flames can be treated as a cold jet. (C) 1997 by The Combustion Institute.