Experimental and theoretical study of two-phase flow through compressible porous media with application to sheet rewetting in wet pressing of paper

This study examines the flow of water and air through a paper sheet and a metal porous plate (or felt by analogy) during expansion in the press nip. A platen-type Dynamic Compression Tester (DCT) is used to simulate the press. The sheet thickness, measured by three eddy current proximity sensors, determines its moisture content under the saturation condition. Results indicate that the sheet is considerably drier at the midnip than the exit from the nip, attributed to either rewetting in the nip or an erroneous thickness measurement.; Independent and simultaneous measurements of the sample thickness were taken to verify the accuracy of the DCT measurements. Experiments were also conducted to study the effect of medium on the proximity sensors and to refine the measurement technique. The results showed that when air is the medium, the DCT measurements are very accurate.; A high speed movie camera filmed the pressing of sheets in the DCT. During expansion discrete water films exist at the sheet-plate interface. As the two separate the films are elongated and ruptured, redistributing the water between the two media. A dye was used to trace the flow of water from the porous plate back to the sheet. The results clearly established rewetting in the nip. The nature of rewet and the parameters affecting it were explored. The results show the magnitude of rewet depends on the amount of water removed in the nip, pore size and structure of the porous plate, sheet freeness, nip pressure, viscosity.; A mathematical model was developed describing rewetting during the expansion phase in the DCT. The model is based on the hypothesis that during expansion, vacuum is created in the sheet-porous plate interfacial region which causes a hydraulic pressure gradient. This driving force draws the water out of the porous plate and into the interface. At high speeds the interfacial water is redistributed between the two media by film splitting. At low speeds capillary action draws the water from the larger pores of the plate into the smaller pores of the sheet. The model predictions agree with the experimental data very closely. (Abstract shortened with permission of author.)...