Yim, Samson Sau Shun; (1997) The effect of flow instability on residence time distribution of Newtonian and non-Newtonian liquids in Couette-flow. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The laminar annular flow field formed in the gap of two concentric rotating cylinders with and without a small degree of superimposed axial flow can be used effectively to manipulate the residence time distribution of various species in the gap in order to promote radial mixing with negligible axial dispersion, conditions that are considered important for good heat and mass transfer and reaction processes. This thesis is concerned with an experimental and theoretical study of laminar Couette flow stability in concentric devices. Numerical simulations based on the solution of Navier-Stokes equations are presented for Newtonian and non- Newtonian liquids, showing the effects of operating and geometrical parameters on the transition of laminar to Taylor vortex flow for induced rotational-axial flow in the gap of a pair of rotating cylinders. These simulations indicate that annular rotational flow becomes more stable in the presence of a small degree of axial flow and as the gap width increases. The effect of rotational speed on the breakdown of laminar flow is more complex and for given radius ratio and axial flow rate depends on both the angular speed ratio and the direction of the rotation of the cylinders, counter-rotating flow generally producing a more stable flow than co-rotating. Experimental data are provided on the residence time distribution for flow of several Newtonian water-glycerol solutions and non-Newtonian carboxymethyl cellulose solutions and xanthan gum solutions through the gap of two concentric rotating cylinders operating over a range of conditions. The equipment used consisted of a pair of horizontal cylinders 1 m long with a stationary outer perspex shell and an inner rotating shaft with a variable speed control. The results from these experiments indicate that the residence time distribution of the species in the gap is a complex function of the flow structure in the gap which in turn depends on the operating and geometrical variables. The experimental findings are successfully analysed and discussed using the simulations studies. Recommendations are made for future work.

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