OCR Text |
Show number will be decreased by a factor of two or three, representing the shorter diffusion length inside the droplet due to circulation cellsts . The most extreme internal circulation results in nearly equal species and thermal diffusion and represents a distillation-limited droplet . The solid lines in Fig. 1 show the droplet concentration profile for Le=l, at times 60 msec and 120 msec. In this case, the species are replenished at the surface as soon as they are vaporized, and the maximum benefit from blending strategies, i.e., transport of combustible hydrocarbons into the gas phase, is realized. However, blending strategies cannot be optimized by just considering mixing within the droplet. Later results show that the volatility and ignition characteristics of both components are important to insure early ignition and late extinction of the droplet. A volume integration of the data in Fig. 1 shows that for the low Lewis number case, at 60 msec, only 11% of the remaining droplet mass is nonane, and at 120 msec, only 3% is nonane. Since in hazardous waste incineration we are concerned with droplets burning to completion, this rapid consumption of nonane could signal the onset of premature extinction, because nonane is required to maintain combustion. These Le=1 results contradict preliminary experimental data" which show that droplet concentration remains constant throughout the droplet lifetime indicating little if any internal circulation, which can be well- represented by a Lewis number of approximately tenu . • 24.0] 0.8 ~20.0 c () 0 ~, ... 0.6 () ~16 . 0 ~ (ij U. .,g ~ c 12.0 «:I 0.4 .g ~ () Q) «:I II) Q) 8.0 ~ a: 0- 0.2 '0 ~ co 4.0 (:J Q) J: Nonane Tet rachloroethane 0.0 0.0 +:-:=-====;==-...;;;;:::::;==-"T"---......--- - ......------, 0.0 10.0 20.0 30.0 40.0 50.0 60.0 Radial Distance Figure 2. Concentration and heat release profiles around droplet at time 120 msec., liquid Lewis number equal to one. Results from ·the Le=1 case yield an interesting conclusion about the controlling vaporization process in a distillation-limited droplet. Figure 2 shows that in the distillation limit (Le=l)' the combustion characteristics of each component play an important role in the rate of depletion of both components. The nonane and TECA, blended originally in a 20/80 mass ratio (35/65 volume ratio)' have identical volatilities, which in a distillation-limited processes might lead to the expectation that the components will vaporize from the droplet at equal rates. However, in a hot oxidizing environment, the nonane ignites and burns creating a preferential fuel-vapor concentration gradient, as shown in Fig. 2, which causes the nonane to vaporize from the droplet much faster than the TECA. H we examine the relative strength of the nonane and TECA chemical reactions, also shown in Fig. 2, we see that in this model the TECA barely ignites, even after passing through the nonane flame, while the nonane produces an intense flame relatively close to the droplet. From these. results we see that in a distillation-limited process, the characteristics of the flame surrounding the droplet influence the vapor concentration profiles of each component, thereby controlling the vaporization process within the droplet. 5 |