Compressional dynamics of fibrous porous media I: from the coffee press to industrial machinery

We present an experimental and theoretical investigation of compressional dewatering of dense porous fibre suspensions. Fixed rate, uniaxial compression experiments are performed for a variety of compression rates for comparison to a 1D two-phase model, for both nylon and cellulose pulp fibres suspended in viscous fluid. The constitutive functions representing the solid network’s compressive yield stress and permeability are calibrated using independent experiments. Comparisons are made between the model and experiment of both the compressional load and the solid phase velocity. It is found that reasonable representation of nylon fibre suspensions is achieved by a traditional plasticity model for the solid phase, however a strain-rate-dependent extension to the solid network stress in the model is necessary for agreement with cellulose pulp suspensions. The source of this rate-dependent stress is attributed to the hollow, porous-wall structure of cellulose pulp fibres. The model for pulp suspensions can be compared to results found on a pilot-scale Twin Roll Press, for which reduction of a 2D version of the model yields a similar set of equations as the uniaxial compression tests.