Study On Optimal Hydraulic Design Of Siphon Outlet Channel

Pumping station,which could ensure the safety of people's lives and property and also the smooth development of national economic construction,is widely used in all walks of life,especially in agricultural irrigation and drainage,plumbing work and custom settlements.The outlet channel,whose main function is to recycle kinetic energy,is an important part of a pumping station.If there are bad flow states in it,such as vortex and disconnection,it will not only increase the hydraulic loss,but also affect the normal operation of the pumping station.Siphon outlet channel has the advantages of simple disconnection mode and small hydraulic loss.It is widely used in many large and low-lift pumping stations.Therefore,the optimal hydraulic design of siphon outlet runner is meaningful.A series of numerical simulation studies are carried out on the siphon outlet channel using CFX based on RNG k-? turbulence model.The main contents carried out are as following:(1).The influence of residual circulation at the guide vane outlet on hydraulic performance of outlet channel.(2).The influence of the length of runner pier on the hydraulic performance of of outlet channel under the action of circulation.(3).The influence of several key geometric parameters(including angle of ascending section ?,angle of descending section ?,width of hump section B2,and width of outlet section B3)on the flow passage itself and the pumping system's head and efficiency.Main research results are as follows:1.The residual circulation at guide vane outlet has a larger influence on the hydraulic performance of the outlet channel.With the increase of the flow rate,the flow velocity distribution at the guide vane outlet gets more uniform,and the outflow passage is less affected by the circulation.The hydraulic loss of the passage presents a trend of rising at first and then falling,while the kinetic energy recovery coefficient changes in the opposite direction,and the flow velocity distribution at the outlet section of the passage becomes more uneven.2.Under different flow conditions,the hydraulic performance of the outlet channel varies with the length of the pier.In the condition of small flow rate and design flow rate,the hydraulic loss is positively correlated with the length of the pier.That is the longer the pier,the greater the hydraulic loss,which is different from that of the large flow condition.Under the condition of large flow,the hydraulic loss of flow passage decreases firstly and then increases with the increase of pier length.It can be seen that when the flow is large,a certain length of pier in the flow channel could play a certain function of rectification.3.?The hydraulic loss of the passage decreases firstly and then increases with the increasing of rise angle a.If a is too small,it will increase the horizontal length of the rising section of the flow channel,also can increase the bending angle of the water flow at the inlet bend section that increase the local hydraulic loss of the flow channel.When a is too large,the turning angle at the hump increases,which increases the local resistance of water flow across the hump.Meanwhile,a large area of vortex will also occur in the flow passage.Considering both the hydraulic performance of the outlet channel and the project investment,it is recommended that the value of a should be(30?45)°.?There is an obviously positive correlation between the hydraulic loss and the value of ?,that is,the bigger the ?,the greater the hydraulic loss.A large ? will increase the turning angle of water flow in the hump section.Under the comprehensive influence of water flow circulation and inertia,a large area of disconnection will occur in the lower part of the flow passage descending section,and the vortex area will be enlarged.From the perspective of hydraulic performance,the angle B should be small,but if the angle ? is too small,it will increase the horizontal length of the descending section.Taking both the hydraulic performance of the flow passage and the engineering investment into account,the angle ? is preliminarily recommended to take(25?35)°.?The hydraulic loss of the outlet channel decreases firstly and then increases with the increase of the width of hump section B2.When B2?1.85D0(Do is the diameter of impeller),the comprehensive hydraulic performance of the outlet channel is the best,the overall flow patterns in the outlet is good,the hydraulic loss is minimal,the kinetic energy recovery coefficient is the largest,and the outlet velocity distribution is relatively uniform.It is suggested that the width of hump section B2 should be about 1.85D0.?The outlet width of flow channel B3 has a certain influence on the hydraulic performance and civil engineering investment of the flow channel.If B3 is too large,it will obviously increase the hydraulic loss of the flow channel and weaken the kinetic energy recovery effect of the flow channel.However,when B3/D0>2.5,the hydraulic loss of the flow channel is almost no longer affected by B3.Considering both the civil construction investment and the changes of several hydraulic performance indexes,it is preliminarily suggested that B3 should be taken within(2.3?2.5)D0.