Pressure distribution on the surface of Rushton turbine blades - Experimental measurement and prediction by CFD

The distribution of pressure over the leading and trailing faces of blades on a Rushton turbine was investigated using both experimental and computational methods. Pressures on mechanical impeller blades are of interest for several reasons, including calculation of power input to the tank, assessment of the mechanical design of the impeller, and predicting the gas entrainment rate for impellers in gas-liquid systems. Experimental measurements of pressure on the surfaces of the impeller blades in a rotating Rushton turbine have been made using a hollow blade fitted with pressure tappings and connected to an external pressure cell. Pressures on the trailing face show a pattern indicative of roll vortex formation and detachment. Pressures are used to calculate the power number, which compares fairly well with power according to a torque meter. Fluid flow in the laboratory tank was simulated using computational fluid dynamics (CFD), using a Multiple Frames of Reference method to account for impeller motion. Since the region of interest represents a small fraction of total tank volume, a second simulation was carried out in which better grid resolution was obtained by restricting the computational domain to a zone surrounding a single impeller blade. Pressures on the impeller blades are predicted by the CFD simulations and both methods show reasonable agreement compared with the experimental: measurements, with some improvement using the second method. The CFD results were also used to calculate power, and both CFD methods show good agreement with measurements from a torque meter. The study shows that CFD can provide a very useful tool for the analysis of impeller blade design and process issues related to pressure in the impeller region.