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Show residual droplet heating, characteristic of the burning of heavy fuels like hexadecane (Wang, 1983). This residual heating causes the data to deviate slightly from a straight line and to bow downward. For the sake of comparison we have nevertheless fitted a straight line through the data during steady burning and defined its slope as the burning rate constant Kc. Figure 3 shows the corresponding plot for chlorohexane, a typical monochloroalkane. The lightly chlorinated alkane has a rate constant nearly as large as the pure alkane. Figure 4 shows gasification data for the heavily chlorinated C2H2C4 in both oxidizing and inert environments. The two sets of data coincide, implying that C2H2C4 droplets do not burn. Since the average droplet size in sprays is smaller than the sizes presented in this study, and since small droplets are more difficult to ignite than large (Law, 1975), it can be suggested that pure tetrachloroethane cannot burn in the droplet form in conventional spray incinerators. Egual Volatility Mixtures Using C2H2C4 as a representative, heavily chlorinated, non-combustible hydrocarbon, we next explore a blending strategy to induce combustion by the addition of combustibles such as alkanes. In the first step we eliminate volatility differential effects by mixing nonane (C9H20, boiling point = 151°C) with C2H2Cl4 (boiling point = 146°C). Thus the fuel mixture which is vaporized from the droplet surface and participates in the gas-phase reaction should have almost the same composition as that of the liquid mixture. Figure 5 shows D2-t data for a 75% (volume percent) C2H2C4 + 25% C9H20 mixture. It is seen that, after the initial period of droplet heating, there exists a long period of burning at a steady rate. At about 70 ms, however, there is a sudden change in slope from 0.88 mm2/s to 0.56 mm2/s. A separate experiment for the same mixture undergoing pure vaporization yields a vaporizing rate constant Kv = 0.55 mm2/s. We conclude that the instant at which the slope abruptly changes represents the occurrence of droplet extinction. Figure 6 compares representative burning rate profiles of various C2H2CLvC9H20 mixtures. The curves have been translated along the time axis for comparison of their slopes (they are positioned so that the slopes all project through the same initial diameter squared). The 25% C2H2C4 data is linear throughout the observable droplet lifetime, suggesting extinction occurs at a small diameter (near our sizing limit of ,... 20Jlm). In contrast, the 80% data show an early break in slope indicative of extinction at a relatively large diameter. From the data of Figure 6, a burning rate constant can be defined for the period of steady burning. Figure 7 compares the Kc so determined, together with the separately measured Kv, for C2H2C4IC9H20 mixtures of various concentrations. Significantly, even a mixture with a large 75% C2H2Cl4 loading burns almost as fast as pure nonane. Further noting that extinction occurs fairly late in this mixture, it can then be suggested that the addition of even a relatively small amount of alkane (25%) can promote the rapid and complete burning of a heavily chlorinated hydrocarbon. For increasing concentrations of C2H2C4 above 75%, the burning rate drops to the vaporization rate. The drop in the burning rate can be either a heat of combustion effect or a kinetic effect. In the latter case the inhibition of Cion H in the flame becomes an important consideration. In this regard we note that the Cl/H ratio is 0.66, 0.79, 0.96, and 1.19 for the 75%, 80%, 85%, and 90% mixtures respectively. These results show that C2H2C4 droplet combustion is inhibited as the ClIH ratio approaches unity. 3 |