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Modelling radio frequency/vacuum drying of wood Koumoutsakos, Anastasios

Abstract

The purpose of this study was to investigate the electromagnetic energy dissipation coupled with heat and moisture mechanisms in wood during radio frequency/vacuum (RF/V) drying. This involved both numerical and experimental studies. Specifically, the objectives were to establish a continuous RF/V drying model, to examine first if a one-dimensional model is sufficient to capture the basic mechanisms of drying. Subsequently, this was extended to a two-dimensional model in order to study anisotropic effects. Experimental data required by the model were two parameters, namely, the bound moisture diffusion coefficient and the intrinsic permeability. In order to evaluate the predictability of the model developed, experimental information from a series of RF/V runs using a single specimen under different power densities and ambient pressures were performed. These data were used to evaluate the capabilities of the model. The mathematical model was derived by averaging the conservation equations over a representative differential volume. The final form of the model was developed after a detailed discussion of the controlling resistances and transport mechanisms during RF/V drying. Such a model may readily exclude liquid capillary transport, which is an important factor in convective drying. Comparison of the model prediction with the experimental data obtained revealed a good agreement for a variety of drying conditions, particularly with those predicting the total average moisture evolution and the total drying time. Extension to a two-dimensional model was suggested to overcome its shortcomings in predicting accurately the temperature evolution and in describing in detail the internal heat and mass transfer phenomena during wood RF/V drying. Experiments were carried out to determine the permeability and the bound water diffusion coefficients of western hemlock and western red cedar for both heartwood and sapwood in the longitudinal, tangential and radial directions, respectively. The permeability data, though exhibited high variability, were found within the range reported in the literature. Diffusion coefficients were determined as a function of moisture content and temperature for all directions and wood types. The trends of the curves obtained are in agreement to theoretical ones. The fiber saturation point was determined for all the above cases. The sorption isotherms were calculated and the parameters of the Hailwood-Horrobin equation were calculated using a non-linear regression technique. All the results are discussed within the framework of developing a two-dimensional mathematical model for the RF/V drying of timbers. Finally, a small representative number of experiments was carried out by using "50 Ohm amplifier" technology. The improvement of the uniformity of the electric field was obvious through the temperature and final moisture measurements. The predictions of the two-dimensional form of the model were in good agreement for all cases examined. The agreement is considerably better compared to that found for the one-dimensional model. The value of using evolution strategies for the overall optimization of the process is also presented.

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