Investigation of Flame Stabilization, Lift-off, and Blowout Behavior

Flame stabilization and lift-off represent phenomena of great practical and fundamental interest. Lifted laminar flames have often been used to gain a fundamental understanding on the stabilization of turbulent lifted flames. Recent studies indicate that the leading edge of a laminar lifted flame has a tribrachial (or triple) structure, and the stabilization mechanism has been explained based on the balance of the propagation speed of tribrichial flame and the local flow velocity. Our combined experimental-numerical research is focusing on the stabilization and lift-off behavior of partially premixed flames. The experimental effort is being directed by Professor Ishwar K. Puri.

Two scenarios are being considered. The first scenario is depicted in the attached figure, and pertains to the lift-off and subsequent (downstream) stabilization of a partially premixed (triple) flame from its burner-stabilized position. After this triple flame lifts off from the burner, it propagates downstream approximately along a locally unity equivalence ratio (f = 1) isocontour. This appears to be a characteristic property of triple flames. The base of the lifted flame expands with an increase in the lift-off height and the flame curvature of the base decreases as the lift-off height increases. The flame establishes at a characteristic stabilization position due to the combined effects of flame stretch, flow divergence, and preferential diffusion of various species. The axial velocity reaches a minimum near the triple point with a value that corresponds roughly to the laminar burning velocity of unstretched adiabatic stoichiometric methane-air flames. In the second scenario, we are investigating the stabilization behavior a propagating partially premixed flame, which is ignited in a nonuniform mixture in an axisymmetric jet flow downstream of the burner exit. The fundamental issues dealing with flame stabilization and lift-off, as well as global properties-such as lift-off height of partially premixed flames as a function of equivalence ratio, velocity, Lewis number, nitrogen dilution, etc.-are being investigated.