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Show relative to that of the fuel, G 2 » G , . Since momentum flux is the dynamic measure of jet source strength, it follows that the air jet is strong and the fuel jet weak. The gas surrounding these jet flows is of combustion products, imagined to be uniform in temperature and composition and virtually at rest far away. In actual burner operation, the ambient gas is of recirculating, fairly slow-moving, combustion products that have lost some enthalpy to furnace heat transfer. In mixing theory, this is effectively a three-feed mixing process, the feeds being the air, the fuel, and the recirculating products. The strong air jet develops like an isolated free jet, oblivious to the weak fuel jet. The fuel jet, however, moves through the field of ambient gas flowing radially inward upon the air jet, driven by entrainment, and itself entrains gas from that field. The momentum of the material entrained by the fuel jet is thus directed toward the air jet, and this gathering of transverse momentum causes the fuel jet to veer toward the air jet by which it is ultimately, in effect, entrained. Grandmaison, et al. (1996) analyze this problem through standard jet physics. They predict the jet trajectories and the mass entrainment by each jet along its trajectory up to the point of confluence. Confluence is defined by the encounter of the fuel jet trajectory with the statistical boundary of the air jet, point c in Fig. 14. The distance d\2 between the jet ports is the characteristic length scale of the system. The controlling operating parameters are the angle fi\2 between the port axes and the fuel/air momentum flux ratio y/\2 = GJG2. Predictions for f3\2 = 0-40° and various values of y/\2, of special interest in the present context, are: 1. The location (x, y, z) = (xc, yc, 0) of the point of confluence at y/\2 = 0.005-0.08: xc = 6.5dl2 y/\l exp(0.0125/?;>!2 2), (8) yc = 0.194 xc, (9) where and (x,y, z) = (x, 0, 0) on the air-port axis, which also describes the air jet trajectory. 2. The apogee (maximum transverse excursion) of the fuel-jet trajectory at y/\2 = 0.02: ym = rf12(l+0.65 tan #2exp(3.94 tan # 2)), (10) located at approximately x = 0.7 xc. 3. The local mass flux in either jet before they meet, at y/\2 = 0.005-0.08: m = 032mjX/ DJS, (11) where j = 1 or 2 and 0/*- ^m)lnpjjj = Djmjpj lpm (12) is the effective jet-source diameter. This the same expression as for an isolated free jet moving in the x direction, which is what it should be for the air jet but it is interesting that it is also a good approximation for the fuel jet at >L?I2 < 40°. Grandmaison, et al. (1996) go on to follow the course of mixing beyond the meeting of the jets, drawing upon experimental results of Becker & Booth (1975). They identify and characterize three zones, Fig. 14: 15 |