Coupled diffusion of temperature and moisture

The interdependence of heat and moisture in solids has been discussed in [1–3]. It was demonstrated [3] that the effective diffusion constants measured in typical experiments [4] are equal to the diffusion coefficients of the material only in the case of very weak coupling. The processes of heat and moisture transfer can be coupled depending on the specified environmental conditions. An initially uniform dry solid can be suddenly immersed in a body of water of the same temperature which is kept constant. The solid will absorb moisture at a rate which is initially proportional to the square root of time [2, 3, 5]. The slope of the moisture content versus square root of time curve can then be used to compute the effective diffusion coefficient. It depends [3] on the diffusion coefficients for both heat and moisture and on the coupling parameters associated with the heat and moisture flow rate. In addition to this reinterpretation of experimental results of classical tests, there is the related diffusion of heat in the same test. For conditions in which moisture diffuses into the solid, heat has also diffused into the solid at a rate porportional to the square root of time initially resulting in an increase in temperature of the solid [2]. Ultimately, as the diffusion of moisture slows down, the flow of heat is reversed and the temperature of the solid decreases to its initial value. These are similar reciprocal effects for the case in which a slab is suddenly subjected to a change in surface temperature with no change in the surface moisture concentration [2, 3]. Experimental results on coupling of heat and moisture in textile materials can be found in [5].