Design of Experimental Apparatus for the Measurement of Overall Heat Transfer Rates for Internal Natural Convection in Gas-Vapour Mixtures

The Australian Research Council is funding a project investigating natural convection in cavities filled with unsaturated humid air. Part of the project requires that experiments be performed measuring the overall energy transfer rates between surfaces in the cavities. A major aim of the investigation is to quantify the contribution of moisture migration to the energy transfer between vertical walls.
This paper describes the design of an apparatus in which steady-state, thermal and compositional buoyancy driven flows, with a wide range of Grashof numbers and buoyancy ratios, can be created, and the resulting energy transfer rates measured.
The major design problem is the generation of uniform, time-invariant, unsaturated vapour pressures at arbitrarily oriented, no-slip surfaces, which are independent of the cavity flow field and the mass transfer rate. Various options are explored and the final choice, which requires at the heated wall a stirred salt solution retained behind a micro-porous hydrophobic membrane, and at the cold wall a continuously forming ice layer, is described in more detail.