Effect of modelling assumptions on the effective water diffusivity in wheat
2004
Effective diffusivity of water in spring wheat cv Broom was estimated with three models employing thin layer drying data. Firstly, an isothermal model with a prescribed constant surface moisture content was utilised to determine the effect of choice of grain geometry (spherical or axisymmetric ellipsoid) on the estimated diffusivity. Air temperature and initial moisture contents ranges represented values used in wheat drying practice. The ratio of diffusivities for ellipsoids to those for spheres was found to approximate to wheat sphericity squared, and the results agreed with previous findings in hard wheat. In both, diffusivities followed an Arrhenius behaviour with temperature, with a constant activation energy around 30 kJ mol-1 and a pre-exponential factor that increases linearly with initial moisture content. Another objective was to improve prediction of shape of drying curve, especially at the beginning, where simulated moisture fell faster. Predictions with the ellipsoidal model, coupling heat and mass transfer with convective boundary conditions were not improved. More accuracy was achieved using a time-varying, prescribed surface moisture content with an isothermal model. A further improvement resulted from relating diffusivity with mean grain moisture. Thus the work clarified the relationship between diffusivity estimated from drying data using three models and has shown how values obtained from simpler geometry can be corrected to be used in two- or three- dimensional representations.
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