Transitional flow of non-newtonian fluids in open channels of different shapes

Abstract

Open channels are widely used in the mining industries where homogeneous non-Newtonian slurries have to be transported around plants (Sanders et al., 2002). As water becomes scarcer and more costly due to legislative limitations, higher concentrations of slurries have to be transported. Very little work had been done to predict the laminar-turbulent transition flow of non-Newtonian fluids in open-channels. The effect of open channel on flow of non-Newtonian fluids in the transition region is not well understood. A systematic study on the effect of open channel shape on transitional flow for different non-Newtonian fluids has as far as can be ascertained not been conducted to date. This work investigated the effect of the channel cross-sectional shape on transitional flow of non-Newtonian fluids. There are a number of analytical and empirical methods available for the prediction of transitional flow in open channels. However, there are no conclusive guidelines in the literature that would predict the transitional flow for different shapes. A large experimental database for non-Newtonian flow produced by the Flow Process Research Centre at the Cape Peninsula University of Technology in rectangular, trapezoidal, semi-circular and triangular channels at slopes varying from 1° to 5° was used to achieve the objective. The test fluids consisted of bentonite and kaolin clay suspensions, and solutions of carboxymethyl cellulose (CMC) of various concentrations. The shear stress - shear rate behaviour of each test fluid was measured using in-line tube viscometry. To evaluate predictive models of transitional flow in various channel shapes, a comparison of critical actual velocities with models velocities was conducted for power law, Bingham plastic and yield-shear thinning fluids. After comparison of various models in different flume shapes, Haldenwang‟s critical Reynolds number for rectangular channels was deemed to be the best predictive model. To improve Haldenwang‟s critical Reynolds number, new correlations based on Haldenwang‟s (2003) method were developed for each shape studied and their corresponding critical velocities were compared. By combining all the transition data for the four shapes a new correlation “combined model” was developed for onset of transition and onset of full turbulence which can adequately accommodate the four different channel shapes for all fluids tested.