Numerical Simulation Of Flows Within The Outside Helical Flow Modules And Their Separation Performance

Microfiltration(MF), also called micropores filtration, is a pressure-driven membrane process and finds wide applications in chemical, medical, food industry, and so on. As a new rising technology, it has its great market prospects and commercial values. The key problems for MF are concentration polarization and membrane fouling. MF assembly with outside helical flow is a new kind of equipment which can enhance permeate flux by means of the operating principle of a hydrocyclone. And up to now many researches of it has been mainly done experimentally. In this paper, the characteristics and details of the laminar flow fields in the annular gap of the modules were theoretically studied by numerical simulation method first time. The mechanism of the microfiltration with outside helical flow was analyzed. Its results were compared with the former experimental results and those of outside axial flow MF and would provide the theoretical basis for optimization design. The main contributions of this work are included as follows:(1) Numerical tests of the outside axial and helical flows were carried out respectively. The velocities and pressure profiles and shear distributions have been obtained and they were compared due to the different flow field. It is concluded that when no particles are deposited, the helical flow would lower the pressure on membrane surface to some little extent but not markedly. But for realmicrofiltration, the distribution of shear stress on the membrane of the module with outside helical flow would have benefit to make smaller number of particles deposit on the membrane.(2) By means of orthogonal test and regression method, four new models, trans-membrane pressure model, flux model, trans-module pressure drop model, and efficiency model, were established, which roundly describe the performance of outside helical MF by depicting the flux, energy loss, and handling capacity simultaneously.(3) Under the ideal conditions of no particle deposition, the complex effects of flow parameters and geometry on performance of microfiltration with outside helical flow were discussed. And it is suggested that when designing, several factors should be taken into account comprehensively. It is noted that increasing MF capability will, at the same time, increase the energy loss of the module.(4) The conditions of no particle deposition or making deposited particles move along the membrane surface were determined with the help of the analysis of forces acting on particles, from which, it may be considered that in the helical flow these conditions would be easier to reach than in the axial flow. When these conditions be satisfied in the whole region, there would be no or only a dynamic cake on the membrane surface. In addition, to ensure a long time steady microfiltration, sometimes over-high flux should be avoided.(5) Qualitative analysis of the formation of vortex and its effect on MF had been done. It shows that when the velocity are big enough, eddies would appear on longitudinal section. These vortexes would have much influence on particles' sedimentation or movement. The MF would, therefore, be enhanced. At the same time, there is an optimal tangential inlet velocity in order to avoid the negative effect on the pressure on the membrane for microfiltration.In sum, in the case of laminar flow, the outside helical flow would have a little influence on MF of purity water. For the filtration of suspension, on the other hand, it does enhance the MF flux. So it should be pointed out from the above analysis that to make the module operate better, it is the better choice to ensure the flow inthe annular gap of it to be a turbulent flow.