Study On Numerical Simulation Of Local Three-Dimensional Flow Fields With Entrapped Air Pockets In Pressured Pipelines

With the rapid development of economy, the water of internal or peripheral city cannot meet the increasing water demand. At this time, people have to look for water in further place and establish long distance water delivery systems between source of water and city. For the complex terrain, undulating ground and long distance pipeline, it's likely to accumulate air in the hump place, the closed end and valve place and form big air pocket in this kind of system. If there is no enough exhaust valves or the installation position is wrong, it will cause the air in pipeline cannot be exhausted promptly. In such cases, if someone suddenly starts pump or turns on the valve in pressure pipeline system, it will cause current rush to air pockets and appear abnormal pressure near the air pockets. The gas-water hammer pressure produced by this special transient flow phenomenon will far outweigh the general water hammer pressure and this is one of the major factors in pipe burst. When there are several air pockets in the pipeline, water hammer wave superposition will cause higher water hammer pressure which makes pipe explosion accident to occur. Therefore, the study on gas-liquid two phase transient flow in the pressure pipeline has important theoretical significance and practical value.Based on gas-liquid two phase flow theory and computational fluid dynamics principle, this paper has established the physical model of entrapped air pockets in pressured pipelines. This paper has got the local flow change rule by using VOF model to simulate the field.Through simulating dozens of conditions which have different diameters and different length, the author thinks that when pipe length is 10 m, water hammer pressure remains stable. By using interface track technology of VOF model, this paper gets the changing rule of the local flow field. The results show that air pockets are break down and burst in process of compression and expansion, leading to abnormal pressure occurred. When the pipe contains several air pockets, they will influent each other, which makes the local flow field shake seriously. This paper has proofed the gas changeable process equation, through the simulation analysis and calculation, the gas changeable process can be divided into four sub processes, respectively is: quickly compressed process, slowly compressed process, quickly expanded process, slowly expanded process. The pressure of air-mass in the first two sub processes is rising and reaches to the peak value at the end of slowly compressed process, then falls until another quickly compressed process begins. Besides, this paper applied the method above to practical project, through the local flow field simulation on the case of water filling stage and the case of power failure accident in the long distance water delivery systems, has got the local flow field change rule which can provide suggestions for long distance water delivery project design and operation management.