UBC Theses and Dissertations
Understanding of no-load power in low consistency refining Rajabi Nasab, Nina
Low Consistency (LC) refining is the primary means of improving the strength and smoothness of paper by imparting energy to fibres through repeated fibre-bar interactions. The useful part of the energy modifies the morphology of the fibres and the remaining, no-load power, mainly overcomes the hydraulic, pumping and mechanical losses in the refiner. This thesis is aimed to explore the no-load power in LC refining both experimentally and computationally. The contribution of this thesis comes in three parts. Firstly, the effect of consistency, operational and plate design parameters on noload power was experimentally determined on two pilot scale LC refiners with different plate diameters. The obtained data were used to provide a statistical model for prediction of no-load power. To study the effect of diameter and groove depth, the no-load power consumption of some mills corresponding to their operating conditions, and the specifications of the relevant refiner discs were collected. Based on this model, no-load power is described in terms of two main components, hydraulic and pumping powers, and an empirical equation is proposed. Secondly, we numerically examined the two-dimensional flow of a Newtonian fluid in the gap formed between two opposing cavities which represent the cross-sectional flow in LC refiner. A large number of unsteady simulations were conducted to characterize the effect of gap size on the flow field over the range of velocities. Then, we examined material transport between the cavities by introducing a passive scalar to represent the motion of tracer particles. Over the range of parameters studied, we identify two characteristic flow fields, defined as either steady or unsteady. We also find that particles are transported to the region near the leading edges of the bars only under the conditions of unsteady flow. Thirdly, we extended the numerical study by characterizing the effect of cavity depth on the flow field over the range of velocities. We find that the aspect ratio of the cavity dictates three characteristic flow fields based on the number of vortices formed within cavity and we propose criteria for cavity aspect ratio in terms of the refiner application.
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