Study On3-D Calculation Of Turbulent Flow And Optimum Design Of Front-positioned Pit Tubular Device

Today, with the growing tension of world resources shortage, our country is carrying out the implementation g of strategic water resources allocation. The east and middle routes of the inter-basin South-to-North Water Transfer Project have been started. With the rapid growth of China’s overall national economy, the enhancement of comprehensive national strength, the acceleration of urbanization, regional economic development and the improvement of urban flood control system standards, a large number of low lift medium-sized pumping stations have been built or will be planned in coastal city flood control and water logging engineering, urban water supply security protection project, modern agricultural irrigation and drainage project, and water environment and ecological water project. Therefore, to carry out the optimum design and research of low lift hydraulic pump device has important theoretical and practical significance to ensure the safety and reliability of pump station operation and to save investment and reduce energy consumption.Low-lift pump devices have many types, and pit tubular pump device is a relatively new one. Although it has made some preliminary results in recent years, there are many key technical issues needed to be solved, such as the lack of hydraulic optimum design experience, single research method rather than multi-program comparison, and the traditional specific installation test. Although numerical simulation and optimization research have been taken into practice, it is still lack of systematic expression and necessary characteristics experimental verification. Therefore, reasonable choice of pit tubular pump device parameters, optimum design of hydraulic profiles and reasonable match of inlet and outlet conduit need further research.With the help of numerical computation and plane potential flow theory, the numerical computations of3-D turbulent flow and optimum hydraulic design for the inlet and outlet conduits and the whole pump devices are completed respectively, improvements of the hydraulic performance of the pit tubular pump system are studied deeply in this paper. The main works are as follows:(1) According to plane potential flow theory, combined with part of domestic pit tubular pump stations’pit shapes, potential flow analysis is applied to the comparative analysis of the front-positioned pump system inlet conduit’s outline, thus it also provides a theoretical basis for optimal design.(2) According to the hydraulic optimum objectives of the inlet conduit,2initial groups of11pit inlet conduit programs are developed, and the CFD software FLUENT is used to carry out the numerical simulation of3-D turbulent flow as well as the calculation of flow field and hydraulic loss. On this basis, two optimal inlet conduit programs are compared carefully in depth.(3) According to the hydraulic optimum objectives of the outlet conduit, the numerical simulation of3-D turbulent flow is carried out in one outlet conduit program. Selecting high specific speed model pump, combined with the two inlet conduit optimum programs, two front-positioned pit tubular pump devices are composed. Numerical simulation of turbulent flow and comparative analysis are carried out on them under each flow condition.(4) On the basis of the numerical simulation, combined with the existing pump station design experience, systematic design procedure and mathematical expression of inlet and outlet conduits are summarized.The conclusions obtained from the research of the front-positioned pit tubular pump systems in this paper are as follows:(1) Obtained by potential flow analysis of uniform stream around the pit, the streamline shape is:"front:straight, middle:convex, rear:straight ".The inlet conduit’s outline designed to be close to the flow line has the minimum impediment to the flow, therefore a reasonable outline of pit inlet conduit should be designed as linear, and its appearance is:"two sides narrow, wide in middle."(2) Pit head outline should be symmetrical convex circular (or elliptical) arc, and reducing the curvature value of the arc can reduce the low-speed zone. However, due to the short distance to inlet conduit importing section and low flow rate, the pit head’s outline also can be designed as the semi-circular or semi-regular ellipse.(3) The outline of pit’s tail has a great influence on inlet conduit’s outline, because it decides the contraction of flow lines here. When the tail has concave or convex shape, the flow lines shrink rapidly, while the tail has an ellipse shape, the flow lines shrink slowly. Therefore the tail line should be designed as concave or convex symmetrical circle (or ellipse) arc, in order to reduce the low-speed zone. Because there is larger flow rate here, it should not use integrated circular or elliptical arc, to avoid increasing the flow loss of pump inlet.(4) According to the passage design requirements, the outlet conduit designed in this paper turns out to get small hydraulic loss. It is also proved that the flow velocity is uniform and there is no flow separation, local vortex and other undesirable flow patterns. The methodology used in the outlet conduit design is reasonably practical.(5) By the potential flow analysis and optimal design, the two front-positioned pit tubular pump devices have good overall performance and high efficiency, as well as wide high efficiency area. Thus it can be concluded that pit tubular pump device has good hydraulic performance at low lift and can be provided as an option for low lift pumping stations.