Lifted flame stabilization in developing and developed regions of coflow jets for highly diluted propane

Abstract

Stabilization of lifted flames has been investigated experimentally for highly diluted propane with nitrogen in coflow. For various fuel mole fractions and jet velocities, three distinct flame types are observed: nozzle attached flames, stationary lifted flames, and oscillating lifted flames. When fuel jet velocity is much smaller than coflow velocity, the base of a nozzle attached flame is observed to have a tribrachial structure. Based on the balance mechanism of the propagation speed of a tribrachial flame with flow velocity, jet velocity is scaled with stoichiometric laminar burning velocity. Results show two distinctive lifted flame stabilization modes—stabilization in the developing region and stabilization in the developed region of jets—depending on the initial fuel mole fraction. It has been found that a lifted flame can be stabilized for a fuel velocity even smaller than the stoichiometric laminar burning velocity. This behavior has been attributed to the buoyancy effect, and flow visualization supports it. Oscillating lifted flames are observed with the frequency range of 3–4.5 Hz by the repetitive action of buoyancy due to increased and decreased burning rates during falling and rising periods of flame oscillation.