CFD Simulation Of Characteristic And Scale-up Effect In Mixing Process

Mixing is one of the most important and most used operations in industrial production process. The measures of mixing enhancement are needed to be adopted in order to get the best result in momentum transfer, energy transfer, mass transfer and reaction process. Currently, the large-scale productivity has been the developing trend in chemical production, but the scale-up effect in devices generated after scaled up would make the large scale production in chemical industry be in danger. Therefore, it is of vital significance in optimizing and scaling up the mixing equipment to take the study on the mixing process in mixing equipment and find out the mixing characteristic, as well as scale up the equipment and make clear the scale-up effect.First, FLUENT software and planar laser induced fluorescence technique are used in the study of the ejector, the conclusions are as followed:(1) The Realizable k-ε turbulent model is used to study the hydromechanical characteristics and the mixing process in ejectors, and the velocity, pressure as well as the concentration fields are obtained. It is shown that there is a turbulent core region in the mixing section and the front of the diffuser, where the values of velocity, turbulent kinetic energy and turbulent dissipation rate are high, and also the mixing of the materials mainly occurs in this area.(2) The effects of ratio of jet flow velocity to suction flow velocity and the suction flow feeding angle on the velocity distribution in ejectors are studied to discuss the flushing action to the wall of ejectors. It is shown that the velocity distribution shape changes from convex to concave and the velocity core deflect to the opposite side of the suction feeding inlet when the velocity ratio changes from4to0.25. The more the suction flow, the more serious the deflection is, and at this time, the flushing action to the wall is the strongest. And also, when the suction flow feeding angle changes from0°to45°, the deflection of the velocity core changes from opposition side to the side of the suction flow feeding inlet. When the angle is24°, there is no clear deflection in the ejector. In that case, the flushing action is the weakest.(3) PLIF experimental technique is adopted to measure the concentration field in ejectors, and the impact of the operating parameters on the mixing effect is also studied. The results show that the distance required for mixing completely decreases with the increase of the suction flow rate at a certain jet flow rate, and increases with the decrease of the jet flow rate at a certain suction flow rate, and the deflection of the concentration core occurs. When the ratio of suction flow velocity to jet flow velocity is fixed, the distance needed decreases with the increase of the value of the two velocities.(4) Numerical simulation method is used to scale up the ejector. It is shown that there exists scale-up effect in ejectors. Scale-up effect is the phenomenon that parameters could not be repeatable. In ejectors, the scale-up effect has mainly been seen in the decrease of turbulent dissipation rate and vorticity, as well as the distance needed to mix completely. The velocity and turbulent kinetic energy are not changed apparently after scaled up. For the scale-up effect of ejectors without diffuser angle, turbulent dissipation rate and vorticity decrease after scaled up, and the mixing distance is longer than that before scaled up.Secondly, the Standard k-ε turbulent model is adopted to study the fluid flow and mixing process in SK static mixer. The results show that the velocity distribution in the mixer has a relationship with the shape of mixing elements, and the distributions of velocity and turbulent kinetic energy have period rules. SK static mixer has a good mixing effect to both water-salt water system and water-toluene system. Meanwhile, the fluid flow and mixing process in some other types of static mixers such as SV, SX and HEV static mixers are also studied. It is shown that the SX static mixer has the best mixing effect, and the HEV static mixer has the worst mixing effect. In the study of the scale-up effect in the SK static mixer, the scale-up effect has mainly been seen in the decrease of turbulent dissipation rate and vorticity, as well as the distance needed to mix completely.At last, the multiple reference frame (MRF) method is applied to simulate the hydromechanical characteristics and the mixing process in the dynamic mixer with two feed inlets. The results show that the value of the velocity is relatively high near the leading edge of mixer blade. Because of the feed and the effect of the blade, there are several flow circulations in the mixer, and the values of turbulent kinetic energy and turbulent intensity are lower near the wall and in the bottom of the mixer and higher in the center region of the blade. As well, the concentration distribution in the dynamic mixer below the two inlets is uniform for both water-salt water and water-toluene mixing components. It is shown that dynamic mixer has a good mixing effect for the two sets of mixing components.