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Show 1. The pre-encounter zone. The fuel and air jets both entrain and mix with the ambient gas (combustion products), but not with each other. Thus molecules containing fuel and air material arriving directly from the fuel and air ports do not meet in this zone ("material" is here meant in the sense of the atoms, irrespective of the molecules that they are bound into at any given point, due to reaction). 2. The two-jet mixing zone. The fuel and air jets merge and mix with each other and with ambient gas that continues to be entrained. Fuel and air material arriving directly from the fuel and air ports meet and, by the end of the zone, are thoroughly intermingled on a molecular scale. If the main combustion process (that associated with substantial heat release) is mixing-controlled, then this zone constitutes the main combustion zone or flame. 3. The dilution zone. The fuel and air material coming directly from the fuel and air ports became fully mixed in Zone 2, so the feature of Zone 3 is the further dilution of that mixture due to the continuing entrainment of ambient gas. In the present context, the ambient gas itself consists of fully mixed fuel and air material in the chemical form of combustion products. Thus the element mass fractions of fuel and air material (atoms) throughout Zone 3 and the ambient gas are uniform and in the ratio of the fuel and air feed rates, w/wa = mjm2, with w/+ wa= \. When P\2 < 40°, the total distance from the burner wall to the end of Zone 2 is (Grandmaison, et al. 1996) around twice the distance, xc, to the point of confluence. Thus, if the combustion is effectively mixing-controlled, the flame length, measured from the burner wall, is approximately Lf= 13 dnWn exp(0.0125/?2 2^2 2), (13) where fi\2 is expressed in degrees. 8.2. Multijet effects The CGRI burner, of course, does not present its air jets in isolation from each other but in a circular bundle of N jets, with N= 1 in the present examples. The jet trajectories start coincident with the air-port axes. However, the jets are drawn toward each other in the field induced by entrainment, so, given enough space for development (which a furnace setting may or may not allow), the trajectories veer inward and converge upon the burner axis. In effect, the jets neck in to merge in a kind of turbulent vena contracta, whereafter the flow proceeds essentially as a single, well-developed round jet. Yimer, Grandmaison and Becker (1995) studied this transition in the case N = 6 with parallel port axes ( & = 0). The distance from burner exit to the vena was around x = 3Dt,s, similar to the distance to the point of virtual origin of the ultimate single jet, where Db,s = (4w6 2 I np(DGxh)m is the effective source diameter for the ultimate jet, mh is the total mass flux from the burner, and Gxb is the total axial momentum flux. W e are not aware of any work on cases with significantly diverging port axes (^ = 10° in the present C G R I burners). In the real burner, moreover, each fuel jet is coupled with two neighboring air jets, and while the fuel jets veer inward toward the air jets, the air jets themselves bend inward at first, as described above.The fuel jets m a y or m a y not reach confluence with the air jets before the air jets merge and reach their vena. Quantitative information on these phenomena is lacking. 16 |