Heuristic study on the interaction between heat exchange and slow relaxation processes during wood moisture content changes

Abstract

Abstract “Non-Fickian” diffusion of wood moisture refers to the unexplained anomalous behaviour of water vapour sorption in wood. The depression of moisture diffusion rates in thin sections of wood tissue during moisture content change at high relative humidity is one of the least-understood aspects of “non-Fickian” diffusion. In a previous work, a rate-limiting effect by two cooperative couplings of moisture and heat transfer, was shown to generate some signature features of “non-Fickian” diffusion. The present work aims to heuristically study the simultaneous action of this thermal rate-limiting effect and intrinsic slow relaxation processes in wood. Using an analytical non-geometrical model, exact solutions are obtained which show that the thermal rate-limiting effect acts directly and equally on diffusion and all relaxation processes, effectively time-stretching the entire dynamic wood moisture response. The magnitude of the time-scaling effect is proportional to the coupling strength between heat and mass transfer, which increases progressively with the relative humidity.