OCR Text |
Show We conclude that especially for a well-mixed droplet , the vaporization characteristics will be governed by the chemical reactions at the flame surrounding the droplet. Therefore, the processes we are most interested in modeling, namely the gasification rates of each component, are highly dependent on the submodel used to simulate chemical kinetics at the flame. This is an important conclusion for future computational studies of multicomponent droplets. In the last section of this paper, we discuss alternative chemical models and recommend an approach for future studies. The strong effect of fl&m.e chemistry on droplet vaporization adds merit to the theory and experimental evidence" that blending a chlorinated hydrocarbon with a lower volatility fuel delays droplet extinction by preventing rapid vaporization of the fuel. A lower volatility fuel will offset faster fuel chemistry at the flame, and create a more equal vaporization rate for the two components. ! = f Blended Droplet Ignition / Extinction 2000.0.....-----------------, 180Q.0 1800.0 1700.0 Non.n. &. 1800.0 E ~ E :;, ·.Ex, ~ 1600.0 '.00.0 1300.0 1l....J./~====t.tI=. ============-_~ 1200.0 41 r.trlchloro.th.n. 1100.04-----,r---"T""---r----"T---.----; 0.00 0.02 0 .0. 0.08 0.08 0.10 0 .12 TIme. seconds Figure 3. Maximum temperature in gas surrounding droplet as a function of time for pure and blended droplets. Peak in temperature indicates ignition (our criteria)5 . Droplet ignition can be seen by plotting the maximum temperature in the gas surrounding the droplet as a function of time. Figure 3 shows such a plot for pure nonane, pure TECA, and the blended droplet (35% nonane, 65% TECA, by volume) . Since the ambient air is at 1200K, the maximum gas temperature will be 1200K until ignition occurs. Ignition for the pure nonane droplet is easily identified by the rapid rise in gas temperature surrounding the droplet. We conclude that the pure TECA droplet does not ignite, based on the insignificant temperature rise of the surrounding gas. This result supports experimental data" which shows that the gasification rate for a TECA droplet in oxidizing and inert environments is the same - indicating that pure TECA does not ignite. However, Fig. 3 shows that blending 35% nonane by volume provides enough energy to cause ignition around the TECA droplet. The gas temperature around the blended droplet shows a noticeable increase when ignition occurs, but the temperature rise is much lower than for the pure nonane droplet, due to a lower concentration of nonane in the gas phase, and the very low heat of combustion of TECA, as shown in Fig. 2. Figure 3 indicates that the blended droplet is extinguished at approximately 0.11 seconds. Droplet extinction occurs when the gasification rate, fuel concentration, and maximum gas temperature indicate a lack of chemical reaction. Figure .. shows the square of the droplet diameter, D2 , vs. time, where the slope of this curve is equal to the gasification rate (area change per second). In the limit of infinitely-fast kinetics and an infinitesimallythin flame, combustion theory says that for a droplet at its boiling point (all heat transfer to the droplet is 6 |