Analysis And Application Of Diffuser Angle In Outlet Conduit Of Large Low Head Pumping Station

The hydraulic performance of the outlet conduit plays an important role in the large low head pumping station,and the quality of the hydraulic design not only affects the pump system efficiency,but also affects the safety and stability of the pumping station.The diffusion angle has a great influence on the flow pattern and hydraulic performance of the conduit,and is one of the key parameters of hydraulic design for the three-dimensional shape of the outlet conduit in the pumping station.Equivalent diffusion angle,which is converted by flow area,is demanded to design the outlet conduit in the current " Design Code for Pumping Station ",in order to facilitate the engineering technical personnel to design and calculate.In fact,by this method,the outlet conduit design is simplified from three-dimensional to two-dimensional,so that the actual flow in the outlet conduit could not be reflected and it will fail to meet the high standard and requirement of the modern large low head pumping station.With the continuous development of modern science and technology,especially the continuous improvement of CFD theory and technology,the understanding of three-dimensional flow field in the outlet conduit has been deepened continuously,and the hydraulic design of the outlet conduit could be designed from the three-dimensional perspective so that the problem of equivalent diffusion angle could be avoided.The problem of equivalent diffusion angle applied in the hydraulic design of outlet conduit in large low head pumping station was analyzed.The diffusion flow in the outlet conduit was studied in-depth by the method of numerical simulation,and it is proposed that the equivalent diffusion angle is replaced with the plane diffusion angle and facade diffusion angle to optimize the hydraulic design for the three-dimensional shape of outlet conduit.The research results were applied to engineering design and achieved satisfactory results.The main research results of this paper are as follows:(1)The problem is analyzed about the equivalent diffusion angle of outlet conduit should be between 8° and 12° in the "Design Code for Pumping Station"(GB50265-2010).The diffusion angle is a key parameter of hydraulic design for the three-dimensional shape of outlet conduit,it has great influence on the flow field and hydraulic performance of the conduit.The equivalent diffusion angle is calculated based on the flow area conversion,when the two different shape outlet conduits which are "flat and wide" type and "high and narrow" type have the same areas at the outlet section,they will have the same equivalent diffusion angle,so there exists theoretical contradiction.(2)The three-dimensional diffusion flow in the outlet conduit is decomposed into the diffusive flow in the planar direction and facade direction.For the "flat and wide" type outlet conduit,the diffusive flow is mainly in the planar direction;while the diffusive flow is mainly in the facade direction for the "high and narrow" type outlet conduit.If the outlet conduit is designed based on the equivalent diffusion angle,it may result in that the diffusion flow in the outlet conduits are obviously different while the equivalent diffusion angles are the same.(3)According to the statistical results,the range of facade diffusion angles are between 5 °and 10 ° and the range of the plane diffusion angles are between 15 ° and 23 ° for the outlet conduits of large low head pumping stations which were built.It is indicated that the flow in the outlet conduit is less diffused in the facade direction but more diffuses in the planar direction.The reasons are that the diffusion in facade direction is limited by factors such as hydraulic layout of pumping house,amplitude of water level in outlet pool,gate size and civil engineering work amount,the diffusion in planar direction is limited by few factors.(4)The three-dimensional flow fields of outlet conduits with different planar diffusion angles and facade difusion angle were calculated by the method of numerical simulation.The results show that the planar diffusion angle and facade diffusion angle are separately suitable to value as about 18° and 8° for the outlet conduit of large low head pumping system.Oversize planar diffusion angle will lead to vortex in the conduit,too small planar diffusion angle will lead to oversize conduit length and the civil engineering work amount will increase.(5)The research results are applied to the optimum hydraulic design for the outlet conduits of large scale front shaft tubular pump system and inclined 25 ° axial-flow pump system,respectively.The model test results of pump system show that both the pump systems achieve very good excellent hydraulic performance,the pump system efficiency at the optimum operating point for the front shaft tubular pump system reaches 83.7%,and the pump system efficiency at the optimum operating point for the inclined 25 ° axial-flow pump system reaches 81.0%.