Solid Suspension Characteristics Of Coaxial Mixers In Solid-liquid Stirred Tanks

The coaxial mixer is a novel kind of mixer with wide adaptability.Owing to their flexibility and high efficiency,coaxial mixers are widely used in industrial production.Few researches about coaxial mixers were reported since their invention,and most reported researches were focused on coaxial mixers' performance in single phase system.Critical impeller speed and power consumption of coaxial mixers needed for just off-bottom solid suspension in solid-liquid system with viscous fluid were experimentally studied.First of all,a new method to determine the just off-bottom suspension condition based on the pressure measurement technique was invented.Experimental results showed that the new method is applicable to solid-liquid as well as gas-liquid-solid systems.Based on the proposed method to determine the just off-bottom suspension condition,solid suspension performance of 3 combinations of coaxial mixers(Anchor + RT/PBTU/PropellerU),3 single impeller mixers with 2 groups of diameter(RT/PBTU/PropellerU;D=0.35T/D=0.53T)were investigated and compared.Results showed that coaxial mixers were more efficient than traditional single impeller mixers concerning power consumption,especially when the outer anchor speed was relatively high.Among the 3 combinations of coaxial mixers,Anchor+PropellerU consumes the least energy to attain the just off-bottom suspension condition.In addition,solid suspension performance of coaxial mixers operating under co-rotation,single-inner-impeller and counter-rotation modes were compared.It was found that operating the coaxial mixer under co-rotation mode can achieve better solid suspension performance than the others 2 modes.What's more,considering the influence of fluid viscosity,particle diameter,solid volume fraction and density difference between solid and liquid phases,correlations for the prediction of coaxial mixers' critical impeller speed for solid suspension were proposed.To obtain more detailed information about solid distribution and flow field,CFD study was also carried out.Gidaspow drag model and modified Brucato drag model were compared.Results showed that the modified drag model predicts better results as compared to Gidaspow model.Relationship between average solid volume fraction in the plane which is 1mm above the tank bottom(?vp1)and the inner impeller speed(Ni)was numerically investigated.It was found that average solid volume fraction in the plane decreases slowly with the increase of impeller speed at first,then decrease drastically with further increase of impeller speed,and finally becomes constant and does not change with the increase of impeller speed.According to the curve of?vp1-Ni,predicted critical impeller speed was obtained and showed good agreement with experimental data.In addition,solid flow fields of different impeller combinations,solid fraction distribution under different rotation modes and impeller speeds were obtained through CFD simulation.Effects of liquid viscosity and solid volume fraction on solid distribution through the tank were also studied.