Previous work has shown that for saturated air/water vapour mixtures, water vapour transfer in cavities can cause very large increases in net energy transfer due to its effect on natural convection. Given that many materials used in building envelopes adsorb water vapour, then in many situations a water vapour pressure gradient will exist across a cavity, leading to water vapour transfer across the cavity. In these situations the mixture is generally not saturated, but some effect of the water vapour transfer on natural convection can be expected.
This paper lists the dimensionless groups that are expected to influence natural convection in unsaturated air/water vapour mixtures. It then describes experiments used to determine energy transfer rates in cavities bounded by horizontal isothermal surfaces, the lower and hotter of whích was covered with a saturated. aqueous salt solution generating a vapour pressure less than that of saturated water. The upper and cooler metal surface only exerted a vapour pressure when conditions were such that condensation occurred on it.
Results from the experiments show increases of up to three times in convective energy transfer when condensation occurred, as compared to cases where no mass transfer occurred, even when only modest vapour pressures were generated by the heated salt solution. The magnitude of the results supports the postulate of natural convection enhanced by mass transfer occurring in such cavities, and in many real situations will need to be included in building thermal load calculations.