The report "A Validated All-Pressure Fluid Drop Model and Lewis Number Effects for a Binary Mixture" presents one in a series of theoretical and computational studies of the subcritical and subpercritical behaviors of a drop of fluid and, in particular, a drop of heptane surrounded by nitrogen The study is based on a fluid-drop model in which, among other things, the differences between subcritical and supercritical behaviors are identified with length scales. It is shown that in the subcritical regime and for a large rate of evaporation from the drop, there exists a mass0fraction "Film layer" immediately below the drop surface and the solution of the model equations has a convective-diffusive character. In the supercritical regime, there is no material surface to follow and this introduces an indeterminancy in the boundary conditions. To resolve the indeterminancy, one must follow an arbitrary boundary, which, in this case, is that of the initial fluid drop. The solution has then a purely diffusive character, and from this solution, one calculates the location of the highest density gradient, which location is identified with the optically observable boundary. It is also shown that the classical calculation of the Lewis number gives qualitatively erroneous results at supercritical conditions, but that an effective Lewis number previously defined gives qualitatively correct estimates of the length scales for heat and mass transfer at all pressures.

This work was done by Josette Bellan and Kenneth Harstad of Caltech for NASA's Jet Propulsion Laboratory. To obtain a copy of the report, access the Technical Support Package (TSP) free on-line at  under the Physical Sciences category.


This Brief includes a Technical Support Package (TSP).
Validated Model of a Fluid Drop for All Pressures

(reference NPO-20702) is currently available for download from the TSP library.

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This article first appeared in the September, 2001 issue of NASA Tech Briefs Magazine.

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