T.P. Elson

CAVERN SIZES IN AGITATED FLUIDS WITH A YIELD STRESS

Abstract Highly viscous, non-Newtonian Xanthan gum solutions and two transparent model fluids with similar Theological properties have been studied under aerated (up to 1 vvm) and unaerated conditions in a 0.29m diameter agitated vessel. Rushton disc turbines of size 1/3 and 1/2 of the tank diameter have been used alone and also in conjunction with 6-bladed, 45°-pitch axial flow turbines of the same size at speeds up lo 24 rev/s, enabling specific power inputs of up to 15 W/kg to be imparted. Flow patterns were studied by flow visualisation and hot film anemometry. When the fluids have a yield stress, the fluid divides into a turbulent well-mixed cavern which increases in size with increasing speed with the remainder stagnant. A model for the size of the cavern fits the experimental data well for both aerated and unaerated mixing. Large diameter combinations produce good mixing at about 1 to 2 W/kg which is about 1/3 to 1/4 of that required with small diameter combinations. Single disc turbine impellers a...

Flow dynamics in a draft-tube bubble column using various liquids

Abstract Gas hold-up and liquid circulation rates in a draft-tube bubble column (0.22 m diameter, 85 l capacity) fitted with various diameter draft-tubes are measured for water and aqueous solutions of ethanol, glycerol and carboxymethylcellulose (CMC), respectively. The data are analyzed in terms of a development of the simple energy balance incorporating flow contraction coefficients to quantify deviations from ideal flow. Ethanol and glycerol enhance the entrainment of gas bubbles into the downcomer compared with pure water and inhibit liquid circulation. In CMC solutions, however, large coalesced bubbles are easily disengaged at the top of the column with little entrainment into the downcomer and promote the highest liquid circulation rate. Bubble column geometry, i.e. draft-tube diameter, affects both bubble entrainment and the flow contraction coefficients, and interacts with the fluid properties to affect both gas hold-up and liquid circulation.

THE GROWTH OF CAVERNS FORMED AROUND ROTATING IMPELLERS DURING THE MIXING OF A YIELD STRESS FLUID

The growth of caverns, formed around rotating impellers in a yield stress fluid during mixing in a stirred vessel, has been studied by observing impeller speeds at which fluid motion was first obse...

Drag reduction in aqueous cationic soap solutions

Abstract The pipe flow drag-reducing properties of mixtures of alkyltrimethylammonium halides with 1-naphthol in aqueous solution have been investigated. The effects of solution concentration, soap-naphthol ratio, soap molecular weight and solution temperature upon drag reduction and swirl decay time are reported. The critical wall shear stresses above which the drag-reducing properties cease correlate well with swirl decay time. At low soap concentrations greater than equimolar proportions of 1-naphthol with the soap are required for maximum drag reduction. The drag-reducing properties of these solutions are greatest at and around the Krafft point of the pure soap. A phenomenon similar to onset for polymer solution drag reduction is reported for these soap solutions.

X-RAY FLOW VISUALISATION OF FLOW PATTERNS DURING THE MIXING OF YIELD STRESS, NEWTONIAN AND DILATANT FLUIDS

Abstract This paper describes the use of an X-ray flow visualisation technique to visualise flow patterns in opaque fluids. Flow patterns were observed in a pseudoplastic fluid possessing a yield stress, in a dilatant clay suspension and in a Newtonian fluid for comparison. Impeller Reynolds numbers were in the range 4.5 ≤ Re≤ 74. The flow patterns observed in the fluid possessing a yield stress indicate that the predominant motion in the mixed cavern surrounding the impeller is circular in nature. Comparison of the flow patterns observed in the dilatant suspension, power law flow behaviour index n ≅ 3, with those in the Newtonian fluid al the same Reynolds number, shows that bulk motion throughout the vessel is greater but extensional flows seem reduced. The observed flow patterns support the prediction that when mixing a power law fluid with n > 2 an increase in impeller speed will lead to a reduction in impeller Reynolds number.

The interaction of yield stress and viscoelasticity on the Weissenberg effect

Abstract The interaction of fluid yield stresses and viscoelasticity upon the Weisenberg effect has been investigated in several ways. Firstly, a simple theoretical model for the Weissenberg effect has been developed (assuming the Weissenberg hypothesis) and used to predict surface profiles for solutions without yield stresses and for solutions of similar rheologies but with yield stresses superimposed. Secondly, the surface profiles for pairs of polymer solutions were observed and compared. Each comparison consisted of two solutions of similar experimentally determined rheologies except that one solution exhibited a yield stress whereas the other did not. Thirdly, the model for the Weissenberg effect was used to predict the surface profiles for these pairs of solutions using their experimentally determined rheologies. The results of each of these analyses were in agreement, i.e., the Weissenberg effect is suppressed when yield stresses and stagnant fluid regions are present. In many processes, for example fluid mixing, it is important to know whether a fluid is viscoelastic or not and the Weissenberg effect is an indication of this viscoelasticity. The effects of stirred tank scale-up criteria upon the presence of a noticeable Weissenberg effect around a rotating impeller shaft are discussed.

Velocity profiles of concentric flow between coaxial, rotating cylinders with a stationary lower boundary

Abstract A mathematical model for the effect of a stationary lower boundary upon the concentric flow between coaxial cylinders, with the inner cylinder rotating and the outer fixed, is proposed and tested. The model is based on simple principles and agrees well with velocity profiles measured using a tracer technique and reported here.