Particle Image Velocimetry Investigation On Flow Characteristics Of Solid-liquid System In The Stirred Tank At Dense Concentration

Treatment of solid-liquid mixtures in the stirred vessel is widely used in food processing,petrochemical industry and biological pharmacy industry.Studies of flow characteristics of solid-liquid system in the stirred tank not only give us deeper understanding for the operating units such as crystallization,fermentation and ore flotation but also provide detailed data for the assessment of numerical simulations.As for the optical measurements in the solid-liquid stirred tank,most of the published paper focus on a relatively low solids loading,typically less than 1%.However,the solids volume fractions in the crystallization process that was often carried out in the agitated tank were usually more than 20%.In this research,Particle Image Velocimetry(PIV)technique combined with refractive index matching was used to measure the flow characteristics of solid-liquid system in the stirred tank.And the upper limit-with the particles in this experiment-was 20%by volume.In this experiment,mixture of phenyl silicone oil and methyl silicone oil was made as the continuous phase to match the refractive index of borosilicate glass spherical with a diameter of 2 mm.In the fully developed condition,the particle-angle-resolved PIV experiments were carried out to explore the flow characteristics of liquid and particle distributions when solids volume fraction increased from 0%to 15%and impeller angle increased from 0° to 60°.In the start-up process,the instantaneous flow characteristics of liquid and particle distributions with different operating conditions were investigated.In the research of fully developed conditions,a strong phase couple effect was found between the continuous phase and dispersed phase.With the increase of solids loading the maximum attenuation of mean velocity and turbulent kinetic energy are 48%and 70%,respectively.Nevertheless,the liquid velocity is found to have an enhancement in the bottom region with the increase of solids volume fraction.In the research of start-up process,with the increase of impeller speed,the erosion effect that the fluid acts on the particle bed is more significant and the homogeneity and height of the particle cloud also have an increase.The number of suspended particles has an increase when solids volume fraction raises from 5%to 15%.When the impeller speed is constant,the erosion of fluid to granular bed at the upward mode is stronger than that at the downward mode,and the particles get suspended earlier,but the maximum height of the suspended particles of the former mode is lower than that of the latter mode.The critical velocity for the fluid to erode the granular bed is around 0.1-0.25 m/s.