Modelling And Flow Field Measurement Of Sedimentation Tanks

In recent years, the demand of water quality is stricter with the development of the high living standard. However, the water pollution problem is especially serious in China with the development of economics. So the treatment ability of water treatment plant should be higher. To solve this prolem, the Computational Fluid Dynamics (CFD) was employed to investigate sedimentation unit treatment processes. In order to have a more efficient sedimentation tank, operation analysis was conducted and some design parameters were optimized.The design of sedimentation tanks is based on ideal model, the dimensions of sedimentation tanks was determined based on hydraulic rate or hydraulic retention time. The assumption of uniform flow always departs from real case. There are many factors that affect the performance of a settling tank, such as inlet and outlet structures, tank geometry, wind movement, rainfall, concentration or density gradient, and temperature. The transport and mixing characteristics of SS was mainly affected by flow, so the removal efficiency of SS by sedimentation was affected by these factors. In order to investigate the operation status, flow field analysis was conducted under different conditions. The main work and results of this dissertation were shown as followings: The k-εtwo-layer turbulence model combining mass transfer model and settling model was used to establish a mathematical model describing of flow, mixing and settling characteristics in rectangular primary settling tanks. The Semi-Implicit Method for the Pressure-Linked Equations (SIMPLE) method was used on pressure correction.The hybrid finite analytic method with staggered grid was used to solve this model. The simulation results obtained using the mathematical model were compared with the experimental data and simulation results available in literatures, and the results of comparison indicate that the profiles of the velocity field at Re=10 900 and the recirculation length increasing with Reynolds number range from 2 500 to 25 000 in case of incoming flow coming from the bottom are in line with the experimental results and simulation results of Imam; the Flow-Through Curve (FTC) obtained using the mathematical model are in good agreement with the curves based on experimental data and simulation results of Imam, and the accuracy of predicted FCT is higher than Imam's results. Under the low Suspended Solids (SS<150~200mg/L) concentration, the influence of solid phase to the fluid phase could be ignored, the mathematical model could be used to predict the remove efficiency, and the results are in good lines with other predicted values available in literatures. The flow field changes when the incoming flow coming from the middle part of the inlet, there are two vortices in the inlet region in this case, and the predicted results were verified by comparing with Adams and Rodi's work. It is therefore concluded that the mathematical model and HFAM approach can be used to simulate the turbulent flow, mass transfer and settling process in rectangular primary settling tanks with low SS concentration wastewater.Based on a series of numerical calculations, the features of flow field with incoming flow coming from the bottom of rectangular primary settling tanks were summarized and analyzed. The flow field can be divided into three regions: (Ⅰ) recirculation region behind the reaction baffle; (Ⅱ) the main flow region; (Ⅲ) a small vortex, with the opposite circle of region I, near the corner of the outflow weir. It is found that the shape and size of regionⅠis easily influenced by Reynolds number, and its'size increase with Reynolds number, regionⅢis also increased with Reynolds number. However, the increasing scope is very small, the area ratio of vortexⅢoccupy the whole settling tanks is also very small. RegionⅡis influenced by Reynolds number, its'size decrease with increase of Reynolds number. The shear stress between regionⅠand regionⅡis fairly intense, as a results, the floc that formed in the former unit process are easily to be broken up, so the hydraulic conditions should be controlled in a reasonable range. The pressure field is mainly affected by the reaction baffle and weir, there exist two low pressure regions: (1) one behind the reaction baffle, a small low pressure region; (2) the other at the top of the outlet weir, contaminations are suffered to the sucking effect in this region, so contaminations are easily carried out in case of low settling tanks with high flow rate.In order to investigate the flow field under different flow conditions and provide the validation for the numerical simulation results, 2D Laser Doppler Velocimeter (LDV) was employed to conduct a series of experiments. The features of flow field were investigated under various flow rate Q and incoming flow height Hin. The recirculation length and the length at which the flow reaches to uniform flow were measured. The experimental results showed that: (1) there existed a big recirculation zone behind the reaction baffle, and the flow velocity was small in the recirculation region; (2) the length of recirculation increased with the increasing of the flow rate; (3) the length of recirculation increased with the increasing of the reaction baffle height; (4) comparing with the flow rate, the variation of the reaction baffle height can affect the flow field more significantly. Based on the validation of the established mathematical model describing of flow, mixing and settling characteristics in rectangular primary settling tanks under low Suspended Solids (SS<150~200mg/L) concentration, the mathematical model was used to optimize some main design parameters. The relative submergence height and the length-to-height ratio were optimized under low SS. Sedimentation tanks can run at high removal efficiency with the relative submergence height at the range of 0.20~0.50, and the length-to-height ratio is no less than 8. The increasing of removal efficiency is not obvious when the length-to-height ratio is greater than 12, for saving place and the convenience of layout the settling tank in water treatment plant, the design of length-to-height is no more than 12.