Studies Of Transient Shrinkage Of Bio-porous Material And Its Effect On The Heat And Mass Transfer

Volume decreasing and tissue shrinking of bio-porous materials often occur accompanied with the water removal during the convective drying under the hot air. The convective mass transfer coefficient between the surface of the material and the hot air and the water diffusion coefficient will decrease with the decrease of the volume and tissue shrinkage. And these will affect water further diffusing from the internal to the external and evaporating from the surface of the material to the hot air. The transient tissue shrinkage of bio-porous material under convective drying and its effect on the heat and mass transfer were explored in this dissertation.Firstly, a transient shrinkage recursion model was proposed for bio-porous material on the base of the continuous media theory and the relationship between the shrinking volume and average moisture content of the particles. The validity of the proposed model was also tested by mathematical reasoning and numerical calculation. The problem of the transient tissue shrinkage was studied based on this model. As suggested in the result, the external of the bio-porous material firstly appeared contraction due to the water loss, rather than synchronous contraction from the outside to the inside with the wet sub-external migration and the humidity's decrease during the drying process. There was no shrinkage of the parts without loss water. Furthermore, the external shrinked more greatly than the internal due to more water loss in the external.Secondly, a model of tissue shrinkage-water effective diffusion coefficient was also proposed for the relation between the local water effective diffusion coefficient and the change of the local diffusion area based on the transient tissue shrinkage of the bio-porous material. The changing mechanism of the local water effective diffusion coefficient of the bio-porous material was investigated based on the model proposed. The results showed that the local water effective diffusion coefficient decreased with the shrinkage of the area of the water diffusion. And the local water effective diffusion coefficient didn't change if the area of the local water diffusion remained constant. Furthermore, the local water effective diffusion coefficients were equal due to the uniform of the porosity in the bio-porous media.Finally, the effects of the characteristic dimension and the shape of the bio-porous material, and the effects of the temperature, the relative humidity and the velocity of the hot air on the characteristics of the drying were explored based on the shrinkage of the volume and the decrease of the convective mass transfer area between the material and the hot air. The results showed that the whole drying process should be divided into the first drying process controlled by the external drying condition and the second drying process controlled by the internal water diffusion of the material, rather than the constant drying rate process and the falling drying rate process described in the classical drying theory. Furthermore, the characteristic parameter judging whether the drying process is in the first drying phase or in the second drying phase was the external element's water activity rather than the critical average moisture content.