Tilt angle of turbulent jet diffusion flame in crossflow and a global correlation with momentum flux ratio

Abstract

The flame tilt angle is an important phenomenological parameter of the jet fire in crossflow. Previous work focused on buoyant-controlled flames with small jet-to-crossflow momentum flux ratio (RM) or momentum-controlled flames with large RM. The present study investigated coupling influences of buoyancy, shear force, and inertia force on tilt angles of turbulent flames with continuous variation of RM. A global flame tilt angle formula employing buoyancy and momentum was derived and simplified. Experiments of turbulent jet propane flames with medium RM of 0.1–10 were conducted in a wind tunnel. Based on the results, at lower RM, flames presented a three-zone structure: the flame attached to the leeward side of the nozzle, the long symmetric tail of the flame, and the junction that connects these two zones. With increasing RM, the zone attached to the leeward side of the nozzle became small and eventually disappeared. Three dominated regimes are identified combining RM and source Froude numbers (Fr): crossflow-dominated (RM < 0.01, Fr < 0.1), transitional (0.01 < RM < 10, 0.1 < Fr < 103), and low Mach number jet-dominated regime (RM > 10, 103 < Fr < 105). In a specific dominating mode, the theoretical tilt angle decreases with increasing RM. When the domination mode changes from crossflow to transition and jet flow, the mode bandwidth of the experiments becomes narrower in the log coordinate of RM. Correspondingly, the flame tilt angle (θ in degrees) generally decreases as θ=74.4RM−0.006, θ=61.2RM−0.050, and θ=73.2RM−0.115. For the crossflow-dominated regime, most of the jet fluid is entrained into the down-wash area, causing a large tilt angle. For the transitional regime, the vortex in the leeward side of the nozzle moves upward and becomes small, causing a medium angle. For the jet-dominated regime, the flow near the leeward sides of both the nozzle and the jet points upward, causing a small angle and accompanying the backward-rolling jet-type vortices with coherent structure.