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CFD simulation of mixing dynamics in agitated pulp stock chests Ford, Clara

Abstract

Agitated pulp chests function as low-pass filters to reduce high frequency variability in pulp properties (mass concentration, freeness etc.) ahead of many pulping and papermaking operations. Tests made on both industrial and scale-model chests have shown that their dynamic performance is far from ideal, with a significant extent of non-ideal flow (short circuiting, recirculation and stagnation) possible (Ein Mozzafari et al., 2004). In this work, the flow field of a 1/11th scale-model pulp chest was modeled using commercial CFD software (Fluent) with the pulp suspension treated as a Bingham plastic. A multiple reference frame approach was used with coupling between reference frames made using a velocity transformation. The flow profiles predicted by the simulation agreed qualitatively with the observed profiles in the experiments. The power input predicted by the simulations was slightly higher ( + 12%) than the measured power. The velocity field obtained from the CFD model was used to obtain the system's dynamic response to a frequency-modulated random binary input signal. This data was then used as input to a dynamic model developed by Ein Mozzafari (2002) that treats flow within the chest as following two streams: one that bypasses the mixing zone and one that enters it. For both streams, the fraction of suspension passing through each zone was determined and a time constant and delay time computed. These parameters were then compared with parameters measured experimentally under identical operating conditions and the parameters associated with the mixing zone found to agree to within ±25%. The CFD simulation provides detailed information on the velocity profile within the chest and allows the location(s) of poor mixing regions to be identified. Hence, design methods and trouble shooting of existing stock chests can be significantly improved.

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