| The positive siphon pipe is a common way to carry water.In the project,the siphon pipeline is designed according to the basic theory of pressure tube flow.However,in a positive siphon with a long horizontal distance and a large vacuum,the fluid medium is not a single liquid phase but a gas-liquid two-phase flow.The volume fraction increases with the increase of the installation height.The influence of gas on the hydraulic characteristics of the pipe can not be neglected.The hydraulic characeristics of the gas-liquid two-phase flow in the pipeline under the negative pressure condition are different from the conventional positive pressure pipeline.It's very important to know it's hydraulic characteristics for the design and operation of long distance siphon water transmission project with large vacuum degree.Therefore,this paper combines physical model tests,numerical simulations and theoretical analysis methods to systematically study the gas-liquid two-phase flow pattern,two-phase flow hydraulic characteristics and two-phase flow energy loss mechanism in a large vacuum siphon.Through a series of experimental studies on the siphon flow,revealed the reason for the change from a single liquid phase flow to a gas-liquid two-phase flow in the siphon pipeline.Based on the test results,drew the gas-liquid two-phase flow pattern in the negative pressure running pipeline,obtained the criterion of bubble form flow,bubble flow,transition flow and air mass flow in the siphon pipeline,also obtained the influence law of the limit of each flow pattern in the convection pattern of the installation height,slope and diameter convection.For the bubble flow,the effects of factors such as pipe diameter,installation height,slope and upstream and downstream water level difference on quantity of flow,pressure drop,gas content and head loss were studied.According to study,it is more beneficial to the siphon pipeline with the siphon unsteady state.The siphon pipeline flow is independent of the exhausted slope.The exhausted frequency is negatively correlated with the pipe diameter,is positively correlated with the slope,and the exhausted frequency with positive slope is smaller than the reverse one.Established the turbulent motion equation of gas-liquid two-phase fluid in siphon pipeline,and combined with the gas phase concentration distribution function and the Prandtl mixing length theory,the wall shear stress calculation formula of water flow after aeration is deduced,which reveals that the energy loss mechanism of the two-phase flow is different from the single one.By establishing the relationship of drag reduction function,the influence of bubbles on the drag reduction rate after the precipitation from the water flow is analyzed.The results show that the flow resistance of the gas-liquid two-phase flow in the siphon pipeline is greater than the single-phase water flow.And with the aeration concentration increase,the flow resistance gradually increases.The formula for calculating the resistance coefficient of gas-liquid two-phase flow in siphon is proposed,and the influence law of Reynolds number on flow resistance coefficient is analyzed.Based on the test results,it is found that the siphon resistance coefficient considering the influence of the gas phase is smaller than the single liquid phase resistance coefficient without considering the gas influen ce when ?<5%,and the result is oppsite when ?>5%.According to Newton's second law,carried out the stress analysis of the movement process of single and multiple bubbles in the siphon pipeline,then derived the calculation formula of the drag coefficient of the two arranged vertically bubbles.Combined with the differential equation of gas-liquid two-phase motion,the similarity criterion of the simulating test of the siphon is derived by the equation analysis method.It is concluded that the vertical pipeline should adopt the metamorphic model when the hydraulic model test is carried out,and the height direction scale is 1.In order to ensure that pp=pm,?1m=?1p,and the horizontal pipe is made into a normal model to suitable the same lateral width of the vertical pipe.A single bubble and multiple bubble motion equations are established.Based on the standard k-? model and VOF method,the single and multiple bubble motion behaviors in the siphon are numerically simulated.The motion trajectories,the bubble velocity and bubble deformation process of individual bubbles of different sizes under dynamic water and negative pressure conditions are obtained.then analyzed the effect of the bubble on the surrounding flow field in the siphon.Revealed the process of moving two bubbles in the vertical tube and the down tube is simulated,and the process of bubble fusion.And investigated the influence of bubble diameter and quantity of flow on the critical spacing of the bubble.The research results can further study Sports characteristics of the multiple bubbles and group bubbles. |
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