The basic aim of this project is to predict the heat and mass transfer rates given the temperature and humidity of the cavity walls. A complete attainment of this aim is still some way off, however, since convective transport is highly nonlinear and under many commonly occurring sets of boundary conditions the fluid will be turbulent. More specifically, then, the first aim of this project is to develop a consistent mathematical model of the transport processes and identify the key dimensionless parameters and possible regimes of behaviour of the system.
The second aim is to focus on some of these regimes, to model the behaviour within them, and to determine their approximate boundaries in the parameter space. The purpose of these simplified models is to provide the functional form for correlations of results from experiments and numerical solutions. This is necessary as the large number of governing dimensionless parameters (nine in the current model) makes an exhaustive set of either experiments or numerical solutions prohibitively expensive.
As well as the fundamental significance of natural convection in the sciences of heat and mass transfer and fluid mechanics, and the practical and economic importance of understanding and predicting vapour transmission rates in the technological applications listed above, the findings of this study have environmental implications in the context of the thermal design of building envelopes, particularly in warm humid climates. Environmental implications are possible on the individual, regional and global levels. For example, if there is a high vapour transmission rate through a building wall cavity, this may well lead to moisture content related deterioration of other parts of the structure (TenWolde 1989; White 1989) while the attendant large energy transfer rate could significantly affect the comfort of the occupant individuals who, in turn, may go out and buy a large air-conditioning unit. This, of course, increases demand for and dependence on electricity which is supplied either by the damming of natural watercourses (regional environmental degradation) or the carbon dioxide-producing combustion of fossil fuels, both of which have been associated with global warming. On the other hand, a better understanding of the physics of vapour transport could lead to the design of more energy efficient housing in the tropics. As such advances are some way in off in the future, this project is more about uncovering the basic mechanisms of vapour transport and describing them in a simple but hopefully illuminating manner.
Considering the building wall cavity problem in more detail, the reason for horizontal vapour transfer can be explained as follows. Many common wall materials, such as brick, concrete, wood and plaster are porous and can both hold and transmit large amounts of moisture (Close, Suehrcke & Masatto 1995). The outside of an external wall may be exposed to sun and rain. The vapour pressure exerted by a porous material increases with both temperature and moisture content, so that the air in the cavity in contact with the surface of the outer leaf may well be more humid than that on the inner side. Given this information, the task is to investigate how quickly the vapour crosses the cavity and how much thermal energy it takes with it.
This project is a fundamental study, and the conclusions to be presented are of sufficient generality to be relevant to all the applications listed. The building wall cavity problem, however, will be kept constantly in mind as an example and as a guide for selecting, when necessary, which parts of the parameter space to investigate.