Multicomponent droplet vaporization

This chapter complements Refs. 21 and 22 in reviewing the progresses made on the transient, convective, multicomponent droplet vaporization, with particular emphasis on the internal transport processes and their influences on the bulk vaporization characteristics. The interest and importance in stressing these particular features of droplet vaporization arise from the fact that most of the practical fuels used are blends of many chemical compounds with widely different chemical and physical properties. The approximation of such a complex mixture by a single compound, as is frequently assumed, not only may result in grossly inaccurate estimates of the quantitative vaporization characteristics but also may not account for such potentially important phenomena as soot formation when the droplet becomes more concentrated with high-boiling point compounds towards the end of its lifetime. Furthermore, multi-... 

In the next section some of the important time scales are identified and transient droplet heating effects during the spherically symmetric, single-component droplet vaporization are reviewed. Spherically symmetric, multicomponent droplet vaporization and droplet vaporization with nonradial convection are discussed in later sections. 

General Discussion. It was shown in the previous section that the bulk vaporization characteristics of a single-component droplet do not depend too sensitively on the detailed description of the internal heat transfer mechanisms. However, for multicomponent droplet vaporization qualitatively different behavior is expected for different internal transport mechanisms. This is because the vaporization characteristics (for example, the vaporization rate, the flame temperature and location, and the... 

Experimental Observations. Most of the experimental observations on multicomponent droplet vaporization use two-component droplets 49, 62, 63,-64), The observations on the temporal behavior of the droplet temperature (49,64), size (62,63), and composition (64) all indicate that the vaporization processes are controlled by the volatihty differentials rather than by hquid-phase mass diffusion. Since the fuels used in these experiments are quite nonviscous, the above results then indicate that internal circulation of suflBcient strength has been generated by the prevailing forced and/or natural convection. 

General Discussion. We have shown that for the vaporization of practical, multicomponent droplets, qualitatively different vaporization behavior results when extreme internal transport rates are assumed. Since diffusive transport is always present during the transient, it is the... 

The General Discussion of the previous section is equally applicable here, except now proper multicomponent descriptions of the gas-phase transport and the interfacial phase change should be used (50,51, 52). By assuming the gas-phase reactions are again confined to a flame-sheet where the reactants are consumed in a species-weighted stoichiometric proportion, explicit expressions can be derived (50) for y, Tf, H, and the fractional mass evaporation rate of the i species, as functions of the temperature and vapor concentration at the droplet surface. 

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