Study On Flow Field In The Agitator Tank Of Draft Tube

Stirred tanks are widely used in chemical processing. Mixing process is difficult to measure and quantify, and scaling up is not reliable due to lack of fundamental knowledge of mixing mechanisms. This would lead to alter not only flow pattern alteration, but also a significant influence on mixing, heat and mass transfer. The understanding of spatio-temporal characteristics of the fluid flow in stirred tanks is of great practical importance.It can guide the fluid to flow in and flow out and can improve mixing efficiency by increasing draft tube in the stirred tank. On the one hand, it can enhance stirred intensity, and increase the cut force of impeller to the liquid, and establish the full circulation flow pattern, and cause the fluid to draft tube, and enhance the mixing efficiency. Moreover, it can reduce the feasibility of short-circuited because it can limit the circulation way. The characteristic is obvious in the high viscous fluid and solid-fluid suspension system.The Time-averaged velocity of stirred tank was studied by using Dantec Flowmap 1500 digital particle Image Velocimetry in this paper. The number of image was confirmed by sampling average, the influence of impeller rotational speed, static fluid altitude and impeller diameter were studied on time-averaged velocity, and the flow characteristic was analysed in the experimental process. The conclusions as follows:(1) The flow velocity used PIV was instantaneous and the values might vary with sampling time. In order to remove unwanted disturbance of pulsation on the measurement result, multiple images are needed to screen out. It showed that the time-averaged velocity furiously fluctuated when the number of images less than 230 images, however it became stable when the number of images exceeded 230 in the experimental process. Though fluctuating became milder as the number of images increase, it was limited and the costs were very high. So 300 images were collected in the process of experiment, and then time-averaged velocity was obtained by averaging 300 images.(2) It showed that the flow field was not symmetrical in stirred tank. The reason is that this equipment is not an axial symmetry.(3) The time-averaged velocity was hardly influenced by impeller rotational speed, but it was obvious influenced by the static fluid altitude and impeller diameter.(4) The fluid bumped into wall in the movement process in stirred tank, which leaded to form eddy. The energy was consumed and the mixing efficiency was reduced by this way. Therefore, the part of fluids can come into draft tube in the process of movement, so many holes were discharged in the designed process, in order to promote the mixing of material.(5) The fluid velocity is very low in the district of the hole on draft tube in stirred tank, and there is an eddy at the bottom of stirred tank. In order to eliminate this district, the baffles can be fixed on stirred tank in the designed process.The results of the present work help to give deep understanding to the mixing mechanisms of the mechanically agitated tank, and can be used to as guidance for design and scale-up of the stirred tank.