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

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 (R-M) or momentum-controlled flames with large R-M. The present study investigated coupling influences of buoyancy, shear force, and inertia force on tilt angles of turbulent flames with continuous variation of R-M. A global flame tilt angle formula employing buoyancy and momentum was derived and simplified. Experiments of turbulent jet propane flames with medium R-M of 0.1-10 were conducted in a wind tunnel. Based on the results, at lower R-M, 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 R-M, the zone attached to the leeward side of the nozzle became small and eventually disappeared. Three dominated regimes are identified combining R-M and source Froude numbers (Fr): crossflow-dominated (R-M < 0.01, Fr < 0.1), transitional (0.01 < R-M < 10, 0.1 < Fr < 10(3)), and low Mach number jet-dominated regime (R-M > 10, 10 3 < Fr < 10 5). In a specific dominating mode, the theoretical tilt angle decreases with increasing R-M. 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 R-M. Correspondingly, the flame tilt angle (theta in degrees) generally decreases as theta = 74.4 R-M(-0.006), theta = 61.2 R-M(-0.050), and theta = 73.2 R-M(-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. (C) 2016 by The Combustion Institute. Published by Elsevier Inc.