Strona 2_redak - Instytut Agrofizyki im. Bohdana Dobrzańskiego ...

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5.2. Density of consolidated material
The density of granular material is a monotonically increasing non-linear
function of pressure. The function most frequently used for the description of the
relation is a power, exponential or logarithmic function. Gu et al. [59] made a detailed
analysis of empirical relationships used, determining for each the range of
pressure values for which a given function best describes the change in density.
Fig. 5.1. Bulk density values for wheat as a function of moisture content and pressure [161]
In the case of materials of plant origin, another parameter – apart from
pressure – that significantly affects the density of a deposit is the moisture content
of the material. Thompson and Ross [161] made an in-depth study of the density
of wheat grain deposit within the range of pressures from 0 to 170 kPa. They
found that within the grain moisture range from 8% to 12% a half of the change in
the density of the medium was attributable to reorientation of the grains, and the
other half their elastic deformation. Increase in the grain moisture caused an
increase in the contribution of deformation of the grains in the change of the
density of the medium. At grain moisture level of 24% the contribution of
deformation of the grains was about 70% of the change in density. For vertical
pressure values below 100 kPa the authors found a distinct minimum of density at
grain moisture levels within the range of 20-24%. With increasing pressure, the
minimum shifted slightly towards lower values of moisture (fig. 5.1). The authors
described the relation with a non-linear function. A similar course of the relation
was determined – for corn – by Clower et al. [36] and by Loewer et al. [96]. The
curve marked with the symbol 0 kPa in figure 5.1 presents bulk density
decreasing with increasing grain moisture content.