Transient droplet heating during

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 liquid phase equations determine the droplet temperature T r, t) and also the volumetric fraction 4>- r, t) during the transient droplet heating. Then and are used to solve the gas phase equations. Note that the boundary and initial conditions, (40.19)-(40.22) and also (40.26) show how the liquid-phase equations are coupled with the gas-phase equations, and therefore, an iterative method has to be employed to solve the equations. 

In the lumped parameter model, the transient temperature of a single droplet during flight in a high speed atomization gas is calculated using the modified Newton s law of cooling, 1561 considering the frictional heat produced by the violent gas-droplet interactions due... 

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. 

The transient nature of the cavitation event precludes conventional examination of the conditions generated during bubble collapse. Chemical reactions themselves, however, can be used to probe reaction conditions through the use of comparative-rate chemical thermometry. 4 These kinetic studies revealed that there were in fact two sonochemical reaction sites the first (and dominant site) is the bubble s interior gas phase while the second is an initially liquid phase. The latter corresponds either to heating of a shell of liquid around the collapsing bubble or to droplets of liquid ejected into the hot-spot by surface wave distortions of the collapsing bubble. 

---