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Show f 'Wtf**$!t?ii'!- ?* *«^*^^*^'?,<y,*WTf D D (a) x +i, b M^aoEiidE Fig. 10 The two-jet, three-feed mixing problem of Becker and Booth (1975). Section A - A is in the pre-encounter zone-Zone 1-of the two jets generated by feedstreams 1 and 2; in this zone, no mixing occurs between feeds 1 and 2 (fuel and air in the case of combustion in a furnace), but both mix with feed 3. the ambient fluid that they entrain (recirculating combustion products in a furnace). Section B - B is in the early part of Zone 2, the two-jet mixing zone where feeds 1 and 2 mix with each other as well as with feed 3. Section C - C is in the middle of Zone 2 Zone 2 ends at a distance of about L + Lx from the jet source plane (the burner wall of the furnace). Section D - D is in Zone 3, die post-two-jet-mixing or dilution zone where feeds 1 and 2 are completely mixed with each other but continue to mix with feed 3 (c) Time Fig. 11. Local gas composition as a function of time in die two-jet, three-feed mixing problem. The situation, in the realization of Becker and Booth (1975), is sketched m Fig. 10. and the records shown illustrate behaviour at points in some of the marked sections: (a) A point in either jet in Sect A - A which is in Zone 1, the pre-encounter zone of the jets The shaded portion represents the mass fraction of feeds 1 or 2 (fuel or air m combustion) in the mixture, and the clear portion above (the rest of the space within the frame) represents the mass fraction of feed 3 (the entrained ambient fluid, or in furnace combustion, the entrained recirculating combustion products) There is no mixing between feeds 1 and 2 at this stage. (b) A point in Sect. B - B which is in Zone 2, die two-jet mixing zone. In terms of the combustion example, the black-shaded portions represent the mass fraction of fuel, the grey-shaded portions the mass fraction of air, and the clear portions within the frame, die mass fraction of recirculating combustion products The point is very early in die zone Although turbulent mixing is at different times bringing in eddies of fuel/product mixture and eddies of air/product mixture, fuel and air have barely begun to mix molecularly and are highly segregated, practically never present together. (c) A point in Sect C-C which is well into Zone 2. The black curve represents the mass fraction of fuel, die grey curve the mass fraction of air, and recirculating combustion products make up the balance At this point, molecular mixing is well advanced, so fuel and air are typically present together but in varying proportions. |