Transient droplet heating during vaporization

Transient Droplet Heating during Spherically Symmetric Single-Component Droplet Vaporization... 

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. 

The d -law assumes a constant Tg. However, in many practical situations the temperature of the droplet when introduced into the evaporator is far below this final, equilibrium value. Hence an initial transient heating period exists during which y, and Tf all increase whereas H decreases. Furthermore it can be estimated also that the sensible heat required to heat the droplet is of the same order as the latent heat of vaporization. Hence droplet transient heating effects on the bulk vaporization characteristics are expected to be significant. Two such models, representing extreme rates of internal heating, will be discussed. 

Another experiment of interest is that by El Wakil et al. (49), in which the center and peripheral temperatures were measured for a vaporizing droplet subjected to mild forced convection. The measurements show that there are essentially no differences between these two temperatures, not even during the initial transient heating period. Visual observations also revealed the existence of fairly rapid internal circulations. These imply that the assumption of a uniform droplet temperature may be quite realistic for droplet vaporization with some external convective motion. 

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