OCR Text |
Show 11 At points very early in this zone, at first encounter, molecular diffusion has scant effect and so, while fuel material coming directly from the fuel port may sometimes be present, and air material directly from the air port at others, they are virtually never present together They are segregated, as demonstrated in the record shown in Fig. 11(b). In this state wxw2 = wxw2 +w'xw2 = 0, so w[w2 = -W.W2. (39) As molecular mixing progresses, such fuel and air are increasingly seen together, and the covariance w[w'2 grows from negative to positive. At the downstream limit of the two-jet mixing zone, fuel and air have become completely mixed with each other across the flow, and are present in the proportions set by the mass fluxes of the feedstreams. Then wx I w2 = wx I w2 - w[ I w2 - w[ lw2 = riiXo I rii2o, 40) giving w[w, 2 = w'xw'2, (41) where w't = ^w't 2 , i = 1, 2 or 3. Zone 3: The dilution zone. In the post-two-jet-mixing region, the now single turbulent jet, in which fuel and air are fully mixed, continues to entrain, and mix with, the ambient combustion products. In terms of a furnace operation, the mixture of fuel and air arriving directly from the fuel and air ports continues to mix with, and be diluted by, recirculating combustion products. The basic characteristic of this zone is dilution. 4.2.3. The two-jet mixing index From what has been presented above, the correlation function (or coefficient) RX2^w[w'2lw[w'2 (42) is a highly significant index of the progress of two-jet, or fuel/air, mixing Mathematically, -1 < RX2 < 1. At the end of the two-jet mixing zone, where fuel and air have become fully mixed at the molecular level, the upper limit, Ri2 = 1 is reached. At the beginning, where fuel and air are virtually segregated and turbulent eddies of both visit the same point in space at different times, Rn is evidently strongly negative. H o w close it may get to Rn= -1 is not clear a priori, but some features of behaviour at high segregation levels are nevertheless evident. Let b\ be the intermittency function for the presence of material of feed 1 (fuel) arriving directly from the feed port, and b\ that for material of feed 2 (air): 8i={\ whenw, > 0, 0 whenw, = 0}, (43) where / = 1 or 2 W e have w! = -w. when^=0, |