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UBC Theses and Dissertations

Insights into the jamming mechanisms in pressure screening Villalba Chehab, Miguel E.


The phenomenon of particles jamming in constrictions is a common occurrence in everyday situations. It is also a critical problem in particle separation processes such as pressure screening. The majority of the screening literature has focused on strategies to improve the efficiency of the screening operation but, fundamentally, less is known about the jamming mechanisms. In this thesis, we present four studies which provide insights into the jamming mechanisms in pressure screening. In the first study, we elucidated how jams form locally under idealized conditions. Jams may occur by single-particle bridging or flocculation of the suspension. We demonstrated an intermittency for both bridging and flocculation events which follow a Poisson process. Bridging is primarily related to the number of fibres passing through the slot. Flocculation is related to the time interval whose mean decreases inversely with the square root of concentration. In the second study, we explore a hysteresis which has been observed between the jamming and cleaning cycles in the screen. We speculated that the hysteresis stems from the elastoviscoplastic rheology of the pulp network. Using a model fluid with those characteristics, two different flow states were achieved under the same applied pressure—a signature of hysteresis. Complete jamming of the aperture was shown to occur between suction and flow-reversal. In the third study, we analysed imaging data of jamming near the screen apertures. We demonstrated that variations in the distribution of area fluctuations, deviating from the Gaussian state, result from heightened intermittency—an indication of the onset of jamming. In the fourth study, we performed pilot-scale screening trials where the pressure differential was measured. We confirmed that changes in the fluctuations of pressure indicate the onset of jamming. The minimum rotor speed to maintain operation was found to be related to the ratio of fibre length to slot width. Based on our findings, we suggest fine-tuning of the frequency and duration of pressure pulses to align with the mechanical behaviour of the fibre network. Further studies should focus on understanding the role of changes in the global flow state which may aid in the hosting conditions for jams.

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