Study On Hydraulic Transition Process Of Continuous Pressure Bending Pipe

The long-distance water transfer project in the filling process of water and gas transient flow and pressure fluctuations show that the hydraulic transition process of continuous pressurized bends is more complex,and the gas movement in the pipe is not easy to determine;comprehensive bend pipe turbulence structure can be seen,three-dimensional compressible multiphase flow mathematical model is an important means to study the pipeline gas density,gas content and turbulence structure.In this paper,we take the pipeline with large elevation fluctuation through the river(road)as the research object,and use the combination of generalized model test and numerical simulation to study the formation and transformation of the flow pattern of pipeline system in the process of water filling,analyze the fluctuation change of pressure on both sides inside and outside of different kinds of bends,clarify the causes of the change of gas content of pipeline under different water filling conditions,and focus on exploring the hydraulic characteristics of pipeline system based on the change of single flow pattern.The system in the formation of cavity state,the bubble movement caused by the water-gas coupling in the tube and the evolution of the turbulent vortex structure in the bend tube.The research results not only enrich the research content of the hydraulic transition process of the pipeline system,but also have important significance for accelerating the implementation of long-distance water transmission and transfer project construction in China.The conclusions of this study are as follows:(1)The pipeline through the river is no-pressure flow at the early stage of water filling,when the liquid phase fluid in the pipeline reflux occurs at the bend,the pipeline system enters the semi-pressure flow state,when the pipeline filling flow rate is greater than the critical full pipe flow rate,the pipeline system is accompanied by gas discharge to the full pipe pressure flow state,when the pipeline filling flow rate is less than the critical full pipe flow rate,the pipeline system exists stagnant gas,the pipeline is constant for the semi-pressure flow state;pipeline system A large number of flow patterns are formed in the semi-pressure flow state,and the popping flow mainly exists in the pre-filling period,and the segmental plug flow and plug flow are easily transformed into each other in the pre-filling period,among which the number of plug flow formation is once and twice under the first filling and second filling conditions respectively,and the bubble flow exists in the whole filling process as the main way of bubble formation.(2)The motion deformation of large scale bubbles is mainly concentrated in the straight pipe section of the pipeline system,and the bends at both ends of the straight pipe have significant influence on the motion deformation of bubbles.In the 90° bend pipeline system,the head of the bubble will be stretched from elastic to irregular and elongated after entering the bend,and the segment plug flow and plug flow can be easily converted to each other,and the bubble in the vertical tube will sway left and right during the motion;while in the 45°bend pipeline system,the segment plug flow and plug flow are relatively stable and not easy to be converted,but the bubble will be easily stretched and deformed in the inclined tube.(3)pipeline system pressure along with the increase in filling flow rate and increase,and in the American Water Works Association given filling flow rate,the bend tube pressure in the pipeline system under the action of water and gas coupling also occurred more obvious sudden change phenomenon,the maximum peak pressure of sudden change can reach 11.7times the steady state of the pipeline system;through the river pipeline due to accident maintenance caused by the second filling and the first operation of the pipeline filling water Compared with the water-filled condition of the first operation of the pipeline,the pressure fluctuation of the bend at the two ends of the straight pipe section at the bottom of the river is more violent under the second filling,and accompanied by the phenomenon of sudden change in pressure;curved bend and three-section bend due to the interference caused by its structure on the movement of pipeline bubbles,resulting in the instantaneous pressure fluctuation in the three-section bend lags behind the phenomenon of curved bend;90 ° bend is easier to obstruct the discharge of pipeline gas than 45 ° bend,which leads to 90 ° The instantaneous pressure fluctuation in the overall bend piping system is greater than that in the45° bend piping system,for example,the instantaneous Euler number peak at the center of the second bend in the 45° and 90° piping system is 10.47 and 13.91 under the state of 0.8m/s for the first filling of the three-stage bend piping system.(4)The flow pattern formed at the initial stage of the first filling and the second filling is different,resulting in significant differences between the two on the discharge of pipeline gas;because the shear stress on the surface of the bubble increases with the increase of the filling flow rate,the discharge rate of pipeline gas increases with the increase of the flow rate;because 45° bends and curved bends are more beneficial to the discharge of bubbles than90° bends and three-stage bends,and different The bend has a significant impact on the initial gas content of the pipeline,resulting in more complex changes in the gas content of the pipeline system.(5)The gas downstream of the bend is traced back to the upstream pipeline system under the action of density difference,while the vortex formed in the upstream of the bend due to water-gas admixture causes the bubbles inside the bend to move along the outside of the bend to the water-gas interface;along with the migration of the water-gas interface of the bend to the upstream pipeline,the hydraulic conditions inside the bend are relatively stable,and the bubbles are retained in the inside of the bend under the action of shear stress and then form a huge cavity;along with the With the further increase of the cavity volume,the bubbles again to the upstream pipeline system,resulting in the turbulent vortex structure of the bend tube is disturbed,the cavity form and movement of the inner side of the bend tube is disturbed again.