Thorpe, Graham R (2007) Heat and moisture transfer in porous hygroscopic media: two contrasting analyses. In: 5th International Conference on Heat Transfer, Fluid Dynamics and Thermodynamics, 1-4 July 2007, Sun City, South Africa.

Abstract

The heat and mass transfer phenomena that occur in beds of ventilated hygroscopic porous media are coupled. This results in two transfer waves traversing bulks of porous medium that initially have a uniform temperature and moisture content, and that are subsequently ventilated with air that is not in thermodynamic equilibrium with the solids. The more rapidly travelling wave is known as a temperature wave and the slower wave is known as the moisture wave. Between the two waves the bed of porous medium is at its dwell state. In an Eulerian formulation of the differential equations that govern heat and mass transfer in beds of hygroscopic porous media a solution is obtained using computational fluid dynamics software. This approach has the advantage of enabling a wide range of geometries and operating conditions to be investigated. A second analysis relies on the observation that thermodynamic states of transfer waves lie on straight lines in - space. The Lagrangian analysis is used to investigate the effect on heat and mass transfer of the differential of the integral heat of wetting of the solids with respect to temperature. The numerical and analytical solutions are compared, and it is shown that the numerical solution captures dispersive effects arising from the finite intra-particle resistance to moisture transfer and thermal conductivity. It is observed that accounting for the differential of the integral heat of wetting of the solids with respect to temperature in the analysis has a significant effect on the predicted velocity of the temperature wave. However, this may be an artifice arising from the fact that the sorption isotherm was obtained by extrapolation to low moisture contents. It may be that sorption isotherms need to be constrained to ensure that the ratio of the differential heat of sorption to the latent heat of vaporisation of water is independent of temperature. It is shown that such isotherms do indeed lessen the effects of the integral heat of wetting on the velocity of the temperature wave. However, the velocity of the moisture transfer wave is found to be strongly dependent on the form of the isotherm.

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