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The use of mixing-sensitive chemical reactions to characterize mixing in the liquid phase of fibre suspensions Mmbaga, Joseph Philemon

Abstract

Local energy dissipation rates in the liquid phase of fibre suspensions have been determined in a medium (Save = 10 - 200W/kg) and high-intensity mixer (Save= 1000 - 5000W/kg) by using mixing-sensitive chemical reactions and the Engulfment model of micromixing. The presence of fibre was found to significantly reduce the amount of energy dissipated at the smallest scales, with the reduction mainly dependent on fibre concentration (Cv) and fibre aspect ratio (L/d). Local energy dissipation in the presence of fibres (s) was related to energy dissipation without fibres (SQ) through an exponential function, s = So exp(-aCv) where a is the damping factor that was found to be 52 ± 6 for FBK and 63 + 7 for polyethylene fibres. For spherical particles, the ratio of particle diameter (dp) to the length scale of energy containing eddies (le) was found useful in delienating particle influence on turbulence. Particles with dp/le < 0.1 did not influence local energy dissipation for mass concentration up to Cm = 0.1, whereas, particles with dp/le > 0.1 reduced the local dissipation rates at all concentrations tested. The presence of gas was found to reduce the overall power consumption as well as the local energy dissipation. This was attributed to the diminished ability to transfer momentum from the rotor to suspension as a result of lower density and viscosity around the rotor tip caused by accumulation of gas. Flow visualization with the aid of high-speed video revealed three different types of flow pattern which had direct impact on the overall power consumption. The flow field was also well predicted using computational fluid dynamics (CFD) with the k-e turbulence model. Profiles of dimensionless energy dissipation from measured product distribution were found to reflect the flow pattern.The reduction in energy dissipation rates in low and medium consistency (MC) suspensions has been accounted by a fibre-fibre friction model based on statistical geometry and fluid mechanics. Also, a unified approach based on the apparent suspension viscosity has been shown to correctly predict the change in local energy dissipation rates.

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