Cao, Yingchen (2016)
Examinations of the shear stress on MBR-membrane plates by a single bubble using CFD.
Technische Universität Darmstadt
Masterarbeit, Erstveröffentlichung

Kurzbeschreibung (Abstract)

Nowadays the use of membrane filtration process is rapidly increasing in industrial fields, especially in the field of water and wastewater treatment. Aeration is one of the most important processes in MBR systems. The cost of aeration is the main operating cost. It can provide the biomass with oxygen and prevent them from settling down. Besides, the shear stress created by the aeration process can scour the solid from the membrane surface to control the growth of cake layer and increase the filtration flux.

At the same time computational fluid dynamics (CFD) has become a leading modeling and analysis tool for fluid simulation because of its advances in computational performance. Recently more and more studies have used CFD techniques to investigate and model the fluid flow through a membrane module and to study the membrane performance.

The aim of this thesis is to investigate the shear stress on the membrane plate induced by a single bubble in the MBR system. A better understanding of the shear stress caused by aeration in MBR, which depend greatly on operating conditions and bubble size, can be achieved by using numerical simulation. This thesis focuses on the operation conditions, bubble sizes and the geometry of the membrane models etc. as well as their influence on shear stress.

The thesis firstly introduces the basics of membrane and CFD as well as the model theories. Then a model for simulation of the membrane systems is established and its independence on the mesh size and model geometry was tested. Experiments for validation are conducted. The results of this simulation and the results from the experiments and from literature are presented and compared. Finally in models with an appropriate geometry, an appropriate mesh size and most importantly reliable results, different simulations are carried out to simulate the bubble formation, to investigate the effect of different parameters on the bubble motion, to study what affects the shear stress and shear force, especially in the escape zone, where the bubble moves out of the gap between the submerged flat sheet membranes.

The novel of this study lies in the flexibility of membranes. The membrane in almost all CFD simulations for membrane in previous studies was set as rigid wall. In the last part of this thesis, the membrane was set to be flexible. A two-way system coupling was applied for the simulation of the membrane movement and the rise of the bubble.

At first the formation of bubbles was studied. The amount of gas injected into the water, the way it is injected (continued aeration and discontinued aeration), the inlet configurations and the inlet velocity of the gas determine the bubble shape and size. Then it was found out that the bubble size has a very powerful impact on the bubble motion and shear stress exerted by rising bubbles. An increase in bubble size leads to an increase in maximal shear stress, which also means an increase in cleaning effect on the membrane surface in the filtration. When the bubble size is larger than the gap width, this cleaning effect is no longer that much significant. As for the bubble rising velocity, it reaches its maximal value at a bubble diameter of 5 mm, above which, the wall effect of the membrane makes the bubble slow down. This effect was more significant for the bubbles, whose diameter is larger than the channel gap of membranes. For these bubbles a slug flow could be observed, and according to their spherical-cup shape, they belong to Taylor bubbles. To identify the most effective multi-phase flow pattern for fouling control, membrane module configurations with different gap width were evaluated with CFD. The shear stress was found out to be highest at the smallest gap distance. Considering the clogging problem for small membrane gaps, it is concluded that the rise of a 5 mm bubble between the membranes with a gap of 6 mm might be the optimal conditions for the aeration during the filtration process. The fluid velocity was also investigated in this study. The averaged shear stress shows a linear dependence on the fluid velocity and the bubble motion can increase the shear stress at low fluid velocity significantly. 2d simulation and 3d simulation were also compared in this study. The numerical results from 3d simulation were more reliable. At last a two-way system coupling simulation was performed to investigate the effect of membrane movement on the shear stress. It was found out that the maximal shear stress produced by membrane movement and bubble rising is more than ten times greater than that only induced by the rise of a single bubble.

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