AIM 90-0360 Unsteady and Quasi-Steady Vaporization of Spherical Droplet Clouds

Group phenomena for droplets in a spray have often been analyzed on the basis of the quasi-steady assumption even though this assumption has not been completely justified. In this paper the applicability of the quasi-steady assumption to the vaporization of spherical droplet clouds is first analyzed qualitatively by considering the magnitudes of appropriate characteristic scales. Then the governing equations for the unsteady vaporization of a droplet cloud are formulated and solved by numerical methods. The numerical results indicate that cloud vaporization is inherently unsteady and consistent with the analysis of characteristic scales. The unsteady cloud vaporization nearly follows a "D-law" rather than the well "D2-law" which governs the vaporization and conibustion of liquid fuel droplets. Further it ' is found saturation conditions within the cloud cannot be uniquely determined as shown by the quasisteady theory. However, the quasi-steady theory still provides a good prediction of cloud lifetime if the following two conditions are satisfied (1) PO 2 I, and (2) the existence of a thin vaporization layer or wave at the cloud edge. --i