Gas-liquid-pulp fiber slurry three-phase flows are encountered in a number of stages in paper making, including flotation deinking, delignification and bleaching. In most of these stages the objective is to promote mass transfer between the gas and liquid-pulp mixture, often accompanied by chemical reaction. Efficient operation of these devices evidently depends on the degree of mixing they support, and hence on their flow regimes. On-line, non-intrusive monitoring of flow regimes in these systems can thus be valuable for diagnostics and control. The behavior of gas-liquid pulp slurry mixture is different than gas-liquid two-phase flows, primarily due to the flocculation phenomena, whereby at consistencies (weight fraction of dry pulp in the water-pulp mixture) as low as 0.5% three-dimensional networks of pulp form, with their size and strength increasing as consistency is increased (Bennington, 1989; Lindsay et al., 1995; Heindel and Garner, 1999).

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