THE problem pursued in this project is how fast a vapour will cross a vertical space filled with a nonreacting gas. A simple example is water vapour transmission across an air-gap in a building wall. There are also several technological applications such as partial pressure distillation and solar desalination (Hu & El Wakil 1974), and crystal growth by physical vapour transport (PVT) (Jhaveri, Markham & Rosenberger 1981) and chemical vapour deposition (CVD) (Klosse & Ullersma 1973). The enclosures considered consist of a simply connected space for the gas-vapour mixture bounded by more or less solid walls which may be sources or sinks for the vapour or impermeable to it. The gas is typically completely confined.
Locally, there is a net migration of vapour due to diffusion, down gradients in its mass fraction with respect to the mixture, and advection with any bulk flow of the fluid. There is always a bulk flow in such problems due to the interfacial mass fluxes at the vapour source and sink. Additionally, the density of the mixture will vary, depending on the relative composition, and so the resulting buoyancy forces lead to natural circulation. In all practically occurring cases, such a system will be far from isothermal, either because the vapour source is a source because it is heated or because of the energy absorbed and released by the vapour on evaporation/sublimation/desorption and condensation/incorporation/sorption. These temperature variations must, in general, be taken into account when calculating the buoyancy forces for the convective flow. The energy transfer is often a quantity of interest in itself, for example in assessing the thermal performance of air-spaces as part of a building envelope. Like the vapour, energy is transported through the fluid mixture by advection and diffusion (conduction). Since vapours typically have significantly higher specific heat capacities than gases (e.g. that of water vapour is about double than of air under normal conditions), the thermal energy transported with the diffusive flux of the vapour--the `interdiffusion flux'--can be important.
In modelling vapour transport through gas-filled enclosures, then, attention must be paid to the strong coupling between the concentration, temperature and velocity fields.