Mechanism of Heat and Mass Transfer in Moist Porous Materials

Abstract

A mathematical simulatin carried out to study the physical mechanisms of moisture diffusion into hygroscopic fabric during humidity transients. On the basis of a mathematical model developed to describe the coupled heat and moisture transfer in wool fabric, the moisture–sorption mechanisms are investigated for fabrics made from fibers with different degrees of hygroscopicity. Theoretical predictions on the moisture uptake and temperature changes under humidity transients are compared with those measured previously in a sorption–cell experiment for fabrics made from wool, cotton, acrylic fiber, and polypropylene fiber. It is concluded that hte physical mechanism of moisture diffusion into highly hygroscopic fibers such as wool and cotton can be described by a two stage moisture diffusion process: a fast Fickian diffusion with a concentration–dependent diffusion coeffecient and slow diffusion with a time–dependent diffusion coefficient. For weakly hygroscopic fibers such as polypropylene fiber, the moisture sorption process can be described by a single Fickian diffusion with a constant diffusion coefficient. Heat transfer in moist textiles takes place by conduction, infra–red radiation, and distillation. Until the fiber is saturated, the evaporation process is determined mainly by the fiber’s sorption properties. Conductivity increases with water content and also depends on a fiber’s sorption properties. Key words: Moisturediffusion; hygrospic fabrics; humidity transients