Visualization of the Process of Water Extraction from Transparent Model Samples in Convective and Acoustic Drying

Abstract

Visualization of a field being dried in a model saturated sample has been carried out. It has been shown that the process of moisture extraction under the acoustic action is faster compared to the convective action. Criteria dependences of the dimensionless relaxation time on the Biot criterion have been obtained. It has been found that the acoustic action decreases the coefficient in this dependence without changing the exponent. The speeding-up of the drying of materials in a high-intensity acoustic field is associated with the processes proceeding both on the surface of the samples and in their bulk [1]. At the stage with a decreasing rate of drying, which is realized in practice, the processes in the bulk of the material are determining. To elucidate the physical mechanism of the increase in the rate of moisture extraction in a high-intensity acoustic field, it seems to be useful to visualize the processes proceeding in the bulk of the material being dried. One way of solving this problem is the drying of transparent model samples. The internal picture is observed in two regimes of drying — convective and acoustic ones, which permits gaining an insight into the mechanism of the influence of the acoustic field. Figure 1 schematically represents the experimental facility in which transparent model samples were placed (AA and BB denote the cross sections of the sample and the model facility). As these samples, we used two glass plates 1 with a fine-mesh net 2 clamped between them by means of a plastic profile 3. The space between the plates was filled with distilled water through a medical needle 5 introduced into the internal cavity. The temperature was controlled by a thermocouple 4. Then the thus prepared sample was set in the drier channel with a rectangular cross section 6 so that the open edge of the net was on the leeward side of the flow. As a sound source, a Hartmann generator 8 was used. The operating conditions of the facility were determined by the brake pressure of the working gas in the settling chamber of the nozzle P and the position of pistons 9 and 10. The intensity level of the acoustic field was measured by a pressure transducer 7. The light source 11 illuminated the model sample through the optical window 12. The video camera 13 was mounted on one side of the facility and registered the dynamics of the moisture redistribution in the net in the drying process. For the transparent samples to be able to model capillary-porous phenomena, the characteristic sizes of the meshes were selected so that water could rise, under the action of the capillary forces, in the meshes of a vertically positioned model sample. A brass woven net with "window" sizes of 0.2 mm and a wire diameter of 0.1 mm proved to be suitable for investigations. Note that these sizes somewhat exceed the characteristic sizes of large pores, for example, in wood. The net thickness is 0.250 mm, and its length and width are, respectively, 51 and 18 mm. The experiments were performed in two drying regimes — in an acoustic field of intensity 171 dB and frequency 520 Hz and by a convective method. The working-gas brake pressures in the settling chamber were approximately equal for these regimes. After three-minute acoustic-convective interactions the sample was weighed on an analytical balance to determine the quantity of extracted water. The kinetic curves of drying for the two regimes (in the acoustic field and without it) are given in Fig. 2. On the Y-axis, the quantity of extracted water referred to the initial weight of water has been plotted. One can see a considerable increase in the drying rate and the total amount of extracted water under the acoustic action.