Shrinkage constant rate period

Constant rate period. Liquid flows to the surface to replace that lost to evaporation. This is the stage where most of the shrinkage and warping occurs, as a result of the c pillaiy pressvire differentials within the film. 

To maintain a flow of liquid to the surface at a constant rate, the green body must shrink. A solution to this equation gives the shrinkage at various radii in the spherical green body as a fiinction of time during the constant rate period. The boundary conditions for this equation are... 

Experimentally, drying curves (Fig. 1) show two distinct regions a constant rate period (CRP), which is followed by a falling rate period (FRP). In the CRP, the evaporation rate is constant and shrinkage of the article occurs. At the end of the CRP, shrinkage stops and the FRP begins. 

Because it is the pressure gradient that causes differential strain and cracking, it is evident that fast evaporation and low permeability are detrimental. Cracks usually appear at the end of the constant rate period when the shrinkage stops and the meniscus recedes into the pore. At this point, the radius of the meniscus is minimal and depends on the pore radius tp and contact angle 6 ... 

These ideas can now be applied to the drying of lumber boards, which is assumed to be stress-free at the beginning. At the beginning of drying (constant drying-rate period), sap throughout the entire board remains free. No shrinkage occurs hence, stress buildup is absent. 

As opposed to Scherer, recent models are non-isothermal and not restricted to essentially one-dimensional problems (such as an infinite plate, cylinder or sphere). Some of them consider the drying gel as a biphasic medium, that is, a solid matrix that remains saturated, so that the second drying period cannot be described. Others can also simulate shrinkage and stress after the liquid/gas phase boundary has receded into the gel, as already explored by Scherer (1987b) for a flat plate by assuming a constant evaporation rate until the gel is completely dry. 

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