Local measurements of fluid and particle velocities in a stirred suspension

An experimental approach is proposed for the measurement of the local behaviour of continuous and dispersed phases in a stirred suspension at low particle concentrations (0.5 vol.%). The basic principle involves the simultaneous measurement of the local velocity and particle size with a phase Doppler velocimeter and the separation of the data obtained from large particles (representing the dispersed phase) and very small particles (representing the motion of the continuous liquid phase). This technique was applied to the investigation of a fully baffled cylindrical vessel, with a flat bottom, stirred by an industrial axial propeller. The diameters of the vessel and the propeller and the vertical clearance were 0.3 m, 0.14 m and 0.1 m respectively. The liquid was water and spherical glass particles were used as the dispersed phase. In the absence of particles, the circulation pattern created by the propeller in the middle plane between two baffles revealed a major circulation loop in the lower part of the vessel and a minor contra-rotative loop in the wall surface corner. The calculated pumping coefficient, circulation flow number, non-dimensional time of renewal and circulation time were 0.62, 0.89, 11.76 and 8.15 respectively. The tangential velocities remained at values of less than 15% of the impeller tip velocity in the plane of the measurements. Axial, radial and tangential mean and root-mean-square velocities for the carrier liquid phase and particles were measured in the two-phase flow, using particles with a mean diameter of 253 mu m. These measurements showed that the particles lagged behind the liquid phase in the upward parts of the flow field, but were ahead in some downward parts. The root-mean-square axial velocities obtained for the particles were always greater than these obtained for the continuous phase. Non-homogeneities in the suspension were observed by the local mean diameter field. The effects of the stirring rate and size of the particles were also checked in the upward part of the flow in the vessel. (C) 1997 Elsevier Science S.A.