Analysis Of Dense Liquid-solid Two Phase Mixture's Process In Stirred Tank

Stirring operation is widely used in many fields of industry, such as chemical process and mining. Solid suspension in stirred tank is an important and difficult problem of two phase flow. Some scholars have studied on the dilute solid suspension in stirred tank. With the increasing of solid concentration, the collision between particles and turbulent fluctuation intensify the research of dense solid suspension become difficult. The background of this article is Water and ash mixture in cementing truck. This article researches the dense liquid-solid mixture in stirred tank; it analyses the influence of the impellers'type which affects the result of solid suspension.According to the mathematical modeling and numerical simulation, this article analyses the dense liquid-solid mixture in stirred tank. In mathematical modeling, The Eulerian-Eulerian two fluid model is used as the foundation. According to the dynamic of particle, the collision between dense particles and turbulence fluctuation are described using'pseudo-temperature'. The liquid's turbulent movement is expressed with stand k ?εmodel. for the particles'turbulent movement,'Tchen'theory is adopted. In simulation processes, For the dense liquid-solid mixture, Eurlian model is adopted; for the turbulent fluctuation, the stand k ?εmodel id adopted; the stand wall function is chosen to treat the tank wall, the stress caused by particle collision and turbulent fluctuation is solved by particle differential equation; the drag force between phases is solved by syamlal and O'brien equation; the SIMPLE arithmetic is chosen to solve the coupling between velocity and pressure. Momentum equation, continuous equation, turbulent kinetic energy equation and turbulent energy dissipation rate equation are dispersed by first order upwind method; the multiple reference frames is used to simulate the rotating impeller and the stock-still stirred tank.An experiment is used here to justify the validity of model selected. From the simulation the axial concentration profile of simulation is consistent with the data from experiment. That is, the model selected can describe the fluid field in stirred tank and the movement of dense solid particles.Cementing slurry requires the degree of solid-liquid mixture to be uniform suspension. At first, the just off-bottom suspension's rotate speed is analyzed. Further, the degree of uniform suspension is introduced to judge the quality of suspension. In simulation, we output concentration of solid at 20 points to calculate the degree of uniform suspension. The interval of quality of uniform suspension divides the quality to three situations, namely: incomplete suspension, just off bottom suspension and uniform suspension. Comparing the degrees of uniform suspension in different processes, we can judge whose suspension is more uniform.Uniform suspension is the target that requires. We want to arrive to the target with less power consuming. That's why we introduce the judgment of power-agitation power. Agitation power is the function of impeller's type, stirred tank's type, mixture's property, and rotate speed. There are two ways to calculate the agitation power, one is the nomographic method, and the other is the formula of the agitation power. Considering the types we choose in this article, we select the nomographic method to calculate the different impellers'agitation power.In stirred system, the liquid phase is water; the solid phase is the glass beam. We force on the influence of impellers type to solid suspension. The type used including: rushton turbine, pitch blade turbine and high efficiency impeller. In simulation's process, we monitor the area average concentration of different height, when the area average concentration is consistent, the flow field is sufficiency developed, namely increasing the stirred time can't change the flow field. The concentration distribution and velocity distribution in flow field are analyzed to find that the solid phase and the liquid phase have the same velocity distribution, and the concentration distribution is consistent with the change of velocity. No matter which type is used, it can't get to the degree of ideal suspension. Below the impeller, at the bottom and near the tank wall, the solid concentration is usually high; in the center of the vortex, near the shaft and near the top of stirred tank, the concentration is usually low.In order to describe the influence of the impeller, we use the same condition,at a rated speed, comparing the degree of uniform suspension.The high efficiency impeller is the best for suspension, the rushton impeller can arrive to the uniform suspension, but the pitch blade turbine can't satisfy uniform suspension.When using pitch blade turbine and high efficiency impeller, it has different suspension quality with clockwise and anticlockwise Operation. Clockwise operation forms the up-pump and sheer flow; anticlockwise impeller forms the down-pump and the axial flow. The axial flow is suitable for suspension. Namely, anticlockwise impeller can get a better suspension quality.When using the anticlockwise pitch blade turbine, the degree of uniform suspension decrease with the speed increased, and it satisfy the requirement of uniform suspension at a certain speed. For the high efficiency impeller, when the rotate speed decreases, the degree of uniform suspension increases. To some degree, it doesn't satisfy the requirement any longer. Therefore the rotate speed is one of the key factors for suspension.Among the impellers that satisfy the uniform suspension, we calculate the agitation power of them. High efficiency impeller's power is the smallest, the Ruston turbine's is the biggest; although 45 pitch blade turbine can reach the degree of uniform suspension by increasing speed, it is uneconomical.As we analyze above, high efficiency with anticlockwise operation is suitable for the cementing truck. That can satisfy the requirement of uniform suspension with low power consuming.