CFD Simulation Of Dynamic Filtration System With Rotating Disks

The permeate flux of membrane module is mainly hindered by the concentration polarization and membrane fouling.Increasing the shear stress on the membrane surface has been found to be a good solution to enhance the permeate flux of membrane filtration. Dynamic filtration systems with rotating disks can yield very high permeate fluxes due to the very high shear stress generated at the membrane.Although these systems have been extensively studied with experiments,only a few CFD(Computational fluid dynamic) researches have been carded out to fully understand the intrinsic situation of these systems. Our research team work together with the team leaded by Luhui Ding of University of Technology of Compiegne to investigate three types of dynamic filtration systems with rotating disks.Full 3 - D CFD simulations have been implemented in this paper on the basis of the experimental data supported by the Ding's team.Firstly,the hydrodynamic situation of the dynamic filtration system with rotating smooth disk is simulated,aiming to investigate the influence of disk rotating speed and inlet flow rate on fluid rotation,shear stress of the membrane,mechanical power consumed by the equipment. It can be found recirculation zones near lateral wall due to the opposite movement of fluid on the rotating disk and stationary membrane surface,the rotating flow pattern is plan to see in the equipment.Comparing to the situation of stationary disk's,the pressure on the membrane is increased by the rotating of disk.The velocity induced factor k will decrease with the increasing of rotating speed of disk when the inlet flow rate is higher than 30 L/h, representing the fluid rotates much slower than the disk.The shear stresses calculated by CFD show the shear stresses have a direct proportional to the radial position and under the same operating condition,the surface-average shear stress of disk is almost twice that of membrane surface.The mechanical efficiency of the disk is almost 59%.The required energy of pump is quite small compared to that of disk.Then,the simulation for dynamic filtration system of disk with vanes has been carried out.The results show the introduction of vanes turn the flow field in unsteady state and an huge recirculation appears inside the equipment which varies with time.By the mixture model, it is proved the mixing effect of vanes can reduce the particle concentration on the membrane surface.The calculation for velocity induced factor k demonstrates that the introduction of vane make rotating velocity of fluid nearly the same with disk's,which makes the shear stress of membrane surface is not dependent on the inlet flow rate.At the same rotating speed,the disk with vanes brings shear stress on the membrane 4 times larger than that of the smooth disk. The energy calculation shows that the introduction of vane does not reduce the mechanical efficiency of the disk.Finally,the multishaft disk(MSD) system has been simulated and the results show though the inner flow field is complicated,the fluid between disks exhibits obvious rotating flow and even the obvious recirculation.The shear stress of overlapping area is larger than the other areas on the disk.By comparing the shear stress with smooth disk system,it is considered that expression for predicting the surface-averaged shear stress for the smooth disk can also be used for MSD with a modified k.