Abstract
A flame downwash phenomenon was investigated for horizontal jet fire with crossflow perpendicular to the fuel discharge direction. Such a jet fire configuration involves complex flow and air-fuel mixing interactions due to relatively different directions of the fuel jet, buoyant flow, and cross-flow, for which a systematic data does not exist yet. Experiments were conducted with various nozzle diameters (8 ∼ 20 mm) with propane fuel and varying fuel flow rate. The crossflow air speed, generated by a wind tunnel, varied from 1.76 to 4.56 m/s. Results showed that the length of flame downwash first increased and then decreased with the increase in the fuel supply flow rate. The flame downwash length became smaller with the increase in the crossflow air speed at relatively small fuel flow rate and became larger with the increase in the crossflow air speed at relatively large fuel flow rate. A dimensional analysis was performed on flame downwash behavior of the horizontal jet fires based on the analysis of controlling mechanisms, from which three characteristic parameters were identified. First, the dimensionless mass flow rate (Sm˙f/ρ∞)/[Uc×(Uc2/G)2] represents the fuel flow rate normalized by the characteristic air entrainment, which influences the total flame length. Second, the momentum flux ratio of jet to crossflow R represents the relative drag force of negative pressure zone in the leeward side, which determines the amount of fuel or total flame length trapped along the leeward side to produce the downwash flame. Third, the competition of the jet momentum to flame buoyancy flux (m˙fUj/ρ∞)3/4/(m˙fG/ρ∞)1/2 represents the characteristic length in the jet discharged direction where the flame motion turns from horizontal (jet momentum controlled) to vertical (buoyancy controlled) direction. The measured length of flame downwash considering the nozzle diameter, fuel flow rate, and crossflow air speed was satisfactorily correlated by the proposed mechanisms/parameters.
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