outlet

The water outlet of Kenting is near the houbi lake. The sea water presents different blue colors, with distinct layers and beautiful scenery.

outlet

This entry is compiled and applied by "popular science China" Science Encyclopedia

to examine

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There are many forms of water outlet, the common ones are one character type, eight character type and gate character type. The location, form and outlet velocity of the outlet of the drainage pipe should be determined according to the drainage water quality, downstream water consumption, water flow and water level variation range, dilution and self purification capacity, water flow direction, wave situation, terrain change, weather and other factors, and the consent of the local health department and shipping management department should be obtained. The connection between the outlet and the water bank should take anti scour measures In general, mortar block stone is used for wall protection and bedding. In the area affected by frost heaving, the outlet should be built with frost heaving resistant materials, and its foundation must be set below the freezing line.

hydraulic characteristics

introduce

With the rapid development of China's national economy, the power grid capacity is increasing all over the country, and the problem of uneven power consumption is becoming increasingly prominent. Solving the peak shaving capacity of power grid has become one of the urgent problems in current power production. Developing large-scale pumped storage power station is one of the ways to alleviate this problem, The water inlet / outlet of the upper reservoir is the water inlet when generating electricity and the water outlet when pumping water; the water inlet / outlet of the lower reservoir is the water outlet when generating electricity and the water inlet when pumping water. According to the form of its water inlet / outlet, it can be divided into two types: side type and shaft type. According to its structure, shaft type can be divided into open type and cover plate type, There are few cover plate shaft type water inlets / outlets, so many engineers and technicians are not familiar with the hydraulic characteristics of cover plate shaft type water inlets / outlets, and there are few engineering examples for reference. Only Xilongchi < sub. < / sub has been built. The existing relevant papers are either for power generation conditions or for pumping conditions, and the test size and method are described, Based on the shape optimization project of the upper reservoir inlet / outlet of Mashan pumped storage power station in Wuxi City, the water flow pattern of this kind of water conservancy project is analyzed systematically, This paper makes a preliminary study on the hydraulic characteristics of the cover plate shaft inlet / outlet. Because the operation design parameters of each project have certain differences, the specific test and calculation results are only meaningful for specific projects. Therefore, the author only studies the hydraulic characteristics of the inlet / outlet

research method

There are two methods to analyze the hydraulic characteristics of the intake / outlet: one is the model test, the other is the numerical simulation. In this study, combined with the model test and numerical simulation, the hydraulic characteristics of the shaft type intake / outlet are analyzed

(1) Three dimensional model test. In this study, the test is based on the cover plate shaft inlet / outlet of the upper reservoir of Mashan pumped storage power station. The model is designed according to the gravity similarity criterion (Froude criterion), and the normal model is adopted. In the test of vortex and flow circulation, the Reynolds number Re and Weber number we of the prototype are large; in the model test, the Reynolds number Re and Weber number we of the prototype are large, In order to minimize the effects of viscous force and surface tension, it is necessary to increase the Reynolds number Re and Weber number we of the model. The common method in model test is to increase the model flow to 2.0-3.0 times of the design flow to increase the Reynolds number Re and Weber number we of the model so as to observe the vortex motion

(2) Two dimensional sliced flow model test. The three-dimensional hydraulic physical model test is insufficient in visibility. In order to make up for this defect, the two-dimensional sliced flow model test can be supplemented. The flow pattern characteristics of the inlet / outlet can be understood more intuitively, so as to optimize the shape of the inlet / outlet and judge the optimized effect, In order to simulate the characteristics of axisymmetric flow diffusion, the two-dimensional sliced flow model is designed according to the gravity similarity criterion, and a larger model scale can be selected, The flow is calibrated according to the weight method, and the error is within 8%. Air is added into the suction of the pump to observe the flow state. There are many deficiencies in the binary slice flow, so it is mainly used to observe the main flow direction under the pumping condition and make qualitative description

(3) Numerical simulation. According to the introduction of the literature, the number of grids for the three-dimensional model of Xilongchi shaft is more than 8 × 10. If the model is a little more complex, such as adding details such as vortex beam, the number of grids will increase more. The scale of the three-dimensional model that can be borne by the personal computer is (2 ~ 3) × 10 grids, and the model with more than 10 grids needs to be run on the parallel computer, The period of model debugging and calculation is also long

There is a lack of sufficient data to prove that the accuracy of 3D model can replace the physical model, and there is no report about the successful simulation of vortex phenomenon in outlet flow by 3D model, The grid number of the model can be controlled below 10. No matter the difficulty of modeling and the computational efficiency, the 3D model is much higher than the 2D model. Although the description of the 2D axisymmetric multi model is coarser than the 3D model, considering the efficiency, accuracy and other factors, the 2D axisymmetric rngk - ε model is recommended to simulate the water flow under the pumping condition, The three-dimensional simulation is carried out when time and funds permit

feature analysis

2.1 main body shape

A large number of experiments and numerical simulations were carried out to control the reverse velocity area of the outlet. The model test of two-way flow was carried out, and some results were obtained. This study focuses on the upper structure. The main forms of the upper structure are vortex beam, horizontal vortex beam, stepped vortex beam and vortex plate

2.2 several basic characteristics of water flow under pumping condition

(1) Under the pumping condition, after the flow turns, there is an obvious main flow area, which is very similar to the jet. The model test and axisymmetric numerical simulation of the horizontal vortex beam are carried out under the pumping condition. When the height from the bottom plate to the vortex beam is 5 m, the test and numerical simulation results of the horizontal vortex beam structure (the axisymmetric plane model used in the calculation process, the axisymmetric plane model used in the calculation process, the axisymmetric plane model used in the calculation process, the axisymmetric plane model used in the calculation process, the axisymmetric plane, The space occupied by the diversion pier is not considered; the corrected velocity is the calculated velocity multiplied by the cross-section area ratio). L is the distance from the bottom plate, h is the water depth. It can be seen from the test and calculation results that, generally, there is an obvious backflow area near the bottom plate, the velocity gradient between the main flow area and the backflow area is large, and the velocity distribution in the main flow area is wedge-shaped, The three-dimensional simulation results and the two-dimensional axisymmetric numerical simulation results show that the main flow area is obvious. In this study, the average velocity at the inlet is higher, which is 4.02m/s, and the main flow area is more obvious

According to the principle that the anti vortex beam does not directly contact the main stream, the height of the anti vortex beam is adjusted, and then the slice test and model test are carried out according to the adjusted hydraulic numerical optimization results. The slice test shows that the tracer bubble no longer rises through the anti vortex beam gap, and the model test shows that the outflow will form surface flow, but there will be no surge phenomenon, The water surface is relatively stable. It is preliminarily considered that the stepped anti vortex beam destroys the large-scale vortices formed in the outflow process, so as to suppress the possible surge. The test results show that the stepped anti vortex beam structure is beneficial to maintain the stability of water flow along the mainstream direction, and the principle that the mainstream does not directly collide with the anti vortex beam is reasonable, The height of vortex beam determined by axisymmetric numerical simulation can be used as design reference

According to the hydraulic tests of four different engineering structures, it is found that the head loss without vortex beam structure is the smallest, while the vortex plate structure is the most unfavorable. The vortex plate structure used diffuses the forced compressed water around the horizontal, while the other methods used are equivalent to a certain relaxation, gradually adjusting the flow direction, so the energy loss is relatively small, The head loss coefficients of various shapes are measured at the beginning section of the diffusion end of the pipeline and far away from the reservoir

(2) In general, the diffuser can be used near the shaft inlet / outlet to increase the cross-section area and reduce the flow velocity. Some engineers and technicians increase the angle of the diffuser in order to shorten the length of the diffuser. This method is not desirable, It is generally believed that the uniformity can be achieved only after passing 10-20 times of the pipe diameter distance. However, in practice, the distance of secondary flow is far greater than this value. When the diffusion angle of the shaft diffusion section is too large and the water flow appears separation, the phenomenon of bias flow is very easy to appear, which causes the side effect of the increase of local flow velocity

Chinese PinYin : Chu Shui Kou