Multicomponent vapor transport model for viscous, transitional, and molecular flow

Abstract

A multicomponent vacuum flow and vapor transport model for a long outgassing tube has been developed and verified experimentally. The model includes advective and diffusive vapor transport processes for a wide range of gas flow, from viscous to molecular. Existing single component approaches to the separation of vacuum flow into molecular and viscous fractions have been extended to include multicomponent transport with partial advection and interdiffusion. A set of nonlinear partial differential equations describing vacuum flow and transport in a tube are derived and solved with an iterative numerical scheme. Comparison of model simulations with laboratory measurements of vapor transport in a long vacuum sampling tube showed that the model accurately represents the process of interdiffusion in all flow regimes. Advection was accurately simulated by the model for all vapor components in fully viscous flow. Partial advection and tube limited diffusion in the transition flow range were also well simulated by the model for an argon–air mixture, while benzene–air and 1,1,1-trichloroethane–air mixtures showed slightly less than predicted partial advection. Reduced advection with these latter compounds may be due to the many energy storage modes available to the multiatomic molecules, which result in less momentum transfer than would be predicted by methods that assume fully elastic collisions between molecules.