Investigation On The Mechanism Of Air-Entraiment In Pipes And Experimental Study On Fluid Transportation With Siphon Pipe Under Negative Pressure

Fluid transportation with siphon under negative pressure is an effective method.If compared with free open channel flow and pressurized pipe transportation,although it has many successful applications during the rehabliation projects,but its application was limited because it hasn't been adopted by the planning and design institute due to lack of theorical support,so there is an urgent demand to carry out the deep theoretical study and experimental investigation to derive a scientifically rational explanation.In the pressurized pipe transportation system,the influence of pipe roughness on the hydraulic friction factor is a crtical topic,however,for the fluid transportation with siphon under the negative pressure,the air-phase problems,such as air entrainment due to intake vortex,entrapped air and cavitation inside the pipes because of the vaccum and negative pressure in the pipes,will have significant impact on the pipe's flow capacity.The problem that the flow capacity was influenced by the air entraiment and entrapped air,which has been disclosed for the siphon transportation,is also universally valid for the normal pressurized pipe system with a less submerged intake and an undulating layout.The vaccum is a critical condition to ensure that fluid can be siphoned under the siphon transportation.Thus how to avoid the vaccum broken due to air admission is a must-have theory and key technology in the application and promotion of the large scale siphon.In 90s of the 20^th century,a siphon head invented by Mr.Weng Youbin from Quanzhou Dayu Vaccum Flow Co.,Ltd.,made the application of siphon for big and long pipes possible,so the 3^rd water transportation,flow under negative pressure,appeared.However its internal mechanism was unclear to the public so far,so with a focus on the fluid transportation theory and the key technology of?Weng's siphon?,the problems of air entraiment due to intake vortex,flow resistance increase due to air entrapped in pipes,and the pipe friction factor under different inflow conditions have been systematically investigated in this dissertation with both methods of theoretical analysis and experimental study.The main research achievements and innovation points include:1.After a complete literature review about the previous studies on the air-water flow in the pipes and the ingoes and removal of air in a pipeline system,the balance and control of amount of air in a pipe system has been systematically proposed.Based this theory,the mechanism that how the problem of air in pipes has solved with Weng's siphon has been analyzed and addressed firstly by the author,and the study shows that the necessary minimum submergence has been greatly reduced with the multi-holes design feature of the siphon head at the pipe inlet,so the air entrainment at the pipe inlet due to intake vortex was avoided.2.Through the study on the possible location prone to air accumulation?air pocket?inside the pipes and based on the current full pipe flow theory,a new formula to express the change of pipe resistance with air in pipes has been proposed in this dissertation for the first time.And a theoretical study on the resistance factor with the influence of air in pipes both on the friction loss and minor loss has been carried out.Later a siphon test rig with a full transparent undulating pipe system has been built to carry out the experiments on the flow under different inlet flow conditions and w/o air in pipes.Through this preliminarily comparison experiment,the analysis shows that a lower friction factor has been achieved for the siphon inlet flow condition than the normal inlet flow.In this preliminary experiment,that the resistance increased with air in pipes was validated,the flow behavior,movement of air pockets and the hydraulic jump has been observed through the transparent pipe when there is big amout of air in the downward sloping pipes.4.A further experiment on the influence of the negative pressure on the pipe friction factor has been carried out on a more simplied pipe layout,the friction factor for the hydraulic smooth pipe under the negative pressure has been obtained up to a Reynolds number of 6*10⁵ through the experiment,the result shows there is an obvious difference from the previous measurements such as that from Nikuradse,so a friction factor with 25%³0%lower than the normal condtion has been firstly measured by the author for a siphon pipe under the negative pressure and a high Reynolds Number?corresponds to high Froude Number?.Moreover the trend analysis on the friction factor against the Reynolds Number shows:the flow transition from laminar to turbulent is different from that of the normal pressurized flow because there is an important impact of the dissolved air separated out from the water on the flow under a low Reynolds Number?corresponds to a low Froude Number?.5.Cavitation is one of the main air resources in the pipes.In this dissertation,detailed literature review,analysis and experiment and summary on the cavitation research for pipes and siphons have been performed.Some high siphon tests have been performed on the phenomenon of the dissolved air separated out from the water,cavitation bubbles with pipes of different diameters and layouts.The author carefully observed and analyzed the development and the characteristic of bubbles at the siphon hump,and found that big air pocket merged from the small bubbles is the rootcause for siphon broken and flow blockage.Based on this observation,design and install a buddle of stabilizing tubes inside the siphon hump has been proposed to inprove the high siphon stability.It was found that the rootcause of the maximum siphon height cannot exceed one atomosphere height is the large quantanty of nuclei inside the natural water itself through the cavitation test.Therefore optimizing the design of siphon tube can only increase the maximum siphon height a little bit,but in principles it cannot break through the limitation of one atomosphere pressure.If change to other liquid medium free of gas?with relatively high tensile strength?,then siphon can exceed such limitation and therefore make the best use of both advantage of flow under negative pressure and gratity flow.6.The successful stories of Weng's siphon application in water transportation have been studied in depth;through the performance comparison before and after the modernization,not only the superior hydraulic performance for the application of Weng's siphon has been disclosed,but also the importance of controlling the resistance increase due to air for the long distance transportation has been addressed.Through the detailed study about the engineering data,the author derived the information of the pipe friction factor of the big-scale siphon pipe and minor loss coefficient of the prototype siphon head.In this dissertation,a special case of the powerhouse vibration of a pumpedstoraged power plant has been investigated and reported based on the detailed study on the field measurements of the typical pressure fluctuations,and the internal hydraulic resonance have been discussed and evaluated,and it was derived finally that the strong rotor-stator interaction including the phase resonace occurred at the runner upstream and the resonance with the cavitating vortex rope occurred downstream the runner and inside the draft tube was the main rootcause.After a detailed analysis,necessary actions recommendated for a successful troubleshooting has been proposed by the author.From 2013 to 2018,this troubleshooting was successfully diagonosed and solved by the National Grid and GE Hydro.After the comparison of the runner design before and after the modernization it is found that the clearance between the runner and guide vanes has been significantly increased,which resulted in a much lower pressure fluctuation in the vaneless space,and finally the previous vibration and noise has been successfully removed.