Study On Simulation Of Flow And The Influence On Fractal Growth Of Flocs In Floccution Reactor

In water treatment, flocculation is one of the most important and general units, which has a significant impact on the cost of water treatment ,the subsequent operating conditions and the water quality of outflow to a great extent. As a result, it has become a key field of research and development in water industry. They are unseparated that flocs and flow under the hydraulic condition in flocculation tank. The flocs are entrained by the flow and motion with the flow showing good accompany, so that the flocs can be regarded as a part of the flowing medium and the flocculation process could be considered as hydrodynamic problems when researching it.In this thesis, combining the test and numerical simulation, poly aluminium chloride (PAC) was taken as flocculant and suspended particles of Kaolin and humic acid were regarded as the removal target, in order to study the effects on the flocculation by changing the sizes of the jar through jar tests and discuss the issues of popularizing the conclusions of jar tests to production practices, adopting the hydromechanical similar rules. It is also studied that reactor structure have influence on flocculation, and the simulation of three-dimensional fow field in reactor is carried out for explaining and analyzing the complex phenomenon deeply in the flocculation. Some shortcut, reliable and effective way can be provided for improving the flocculent efficiency and optimizing the flocculent process.The jar test is obtaining much controversy though it is widely used in research and production in water treatment industry. Experimental study should be used in flocculation model test that is little studied domestic and abroad. It investigates their effects on the flocculation and analyzes the cause by conducting jar tests, taking residual turbidity for index, by changing the sizes of the flocculation reactors that are geometric similar. The results show that, that the flow structure is changed when scaling up the flocculating reactor, and there is no similarity in the flocculation with the same velocity gradient.The main force causing the particle collision is inertial centrifugal force and shear stress produced by vortex, especially the vortex shear force. The flocculation is depended on reactor structure which is one of the key factors working on the flocculation, owing that the hydraulic condition in the factor influences the scale and quantity of vortex. The coagulation tests were made by changing the shape and height-width ratio (H/D) of the flocculating reactor, the width of baffles (B) and the height of impellers (C) located in the vessel. The results of the coagulation tests demonstrate that theflocculation efficiency is the best at the condition of circular reactor, H=1.0D, B=0.20D, C=0.33H. The reasonable reactor structure can be obtained under different condition, taking the residual turbidity, fractal dimension of flocs, average particle size for index, and studying the effects of the different hydraulic condition impacting the flocculation. The results of the coagulation tests demonstrate that there is some guiding significance for improving flocculent efficiency by setting reasonable reactor structure,such as the shape and height-width ratio of the flocculating reactor, the width of baffles and the height of impellers located in the reactor. The number of particle collision could be increased and flocculation reaction could be accelerated if the vortex can be controlled effectively during production, eliminating the large scale vortex and increasing the proportion of the small scale vortex.The CFD (Computational Fluid Dynamics) software-Fluent is used to simulated the three-dimensional flow field of flocculating reactors. The numerical results are applied to analyse and explain experimental phenomena to investigate influence of water flow structure under different geometric structures of reactor. The results, basically identical with the results of tests, indicate that the geometric structures of reactor, such as the shape and the height-width ratio of the flocculating reactor, the width of baffles and the height of impellers located in the vessel, can improve the turbulent behavior of water in the reactor effectively and promote the flocculation reaction.