Question 1
(a) Suggest a flow model for (i) shear-thinning, (ii) shear-thickening fluids and (iii) Newtonian fluids and explain your answer using appropriate diagrams of shear stress against shear rate and apparent viscosity against shear rate. Explain any limitations of the model you have chosen if it is applied to shear-thinning fluids. Your answer should refer to the following terms and define each of them: shear stress; shear rate; apparent viscosity; zero-shear viscosity; consistency.
(b) A jet of water of 22.5 cm diameter impinges normally on a flat plate moving at 0.6 m s/1 in the same direction as the jet. If the discharge is 0.14 m3s1 find the force and the work done per second on the plate.
Question 2
(a) Refer to a standard Rushton turbine design to explain what factors influence the amount of power input required for fluid agitation and mixing and why the scale-up criterion of 'same torque per unit volume' is equivalent to performing scale-up at constant tip speed in the fully turbulent region of mixing
Tests on a small scale tank 0.3 m in diameter show that a blending process between two miscible liquids (both aqueous with properties the same as water) is complete after 1 minute when using an impeller speed of 250 revolutions per minute. It is decided to scale up the process to a tank of 2.5 m diameter using the criterion of constant tip speed.
(i) What speed should be chosen for the larger impeller ?
(ii) What power will be required ?
(iii) What will be the blend time in the large tank ?
State any assumptions you make. Assume a standard Rushton-turbine configuration, and a Power Number of 6.