Numerical Simulation Of Transient Flow Characteristics In Pipeline With Trapped Air Mass

The existence of trapped air masses makes the flow in the pipeline more complicated,which will affect the transportation capacity of the pipeline and bring hidden danger to the safety of the pipeline.Based on the National Natural Science Foundation Project(52079107),this paper uses the numerical simulation method and FLUENT to calculate the transient flow characteristics in the vertical horizontal bend with trapped air mass.The influence of driving pressure,volume of air mass and air mass release on transient flow is studied.The transient flow process and its response characteristics with impact pressure are analyzed.The main conclusions are as follows:The transient flow is divided into two cases:no air mass release and air mass release,and the transient flow process is described respectively.In the case of no air mass release,due to the expansion and compression of the air mass,the water body moves forward and backward alternately.In the case of air mass release,the flow state can be divided into three cases due to the different exhaust volume.One is that when the air mass release is small,the transient flow process is similar to that without air mass release;One is to discharge a large amount of gas,and the pipeline system contains a small amount of gas for compression and expansion;The other is that the gas is completely discharged,and the stable flow is soon restored.In the case of no air mass release,the effects of driving pressure and air mass volume on transient flow characteristics are analyzed.The larger the driving pressure is,the larger the system pressure is and the smaller the fluctuation period is.The maximum peak value of the system pressure is 1.4-2.3 times of the driving pressure.There are two forms of air mass volume change:the length of gas bearing end and the change of pipe diameter,The effects of the two forms on transient flow characteristics are as follows:The longer the length of the gas bearing end of the pipeline,the smaller the system pressure,the greater the pressure fluctuation period,the smaller the pressure reduction range,and the average pressure attenuation of every 100mm increase of the gas content length is no more than 5%;The pipe diameter has little effect on the fluctuation of system pressure.After the analysis and calculation of air mass release,the pressure fluctuation curves can be divided into three types:A,B and C curves when the driving pressure is 7 m head and the length of air terminal is 300 mm:A,B,and C curve.Type A curve is when the exhaust volume is small(s/S?0.0031),the system pressure is mainly the pressure of compressed and retained air mass.One type of curve B is to discharge a large amount of gas(0.0044<s/S ?0.0278).The pressure in the pipeline system is composed of water hammer pressure and pressure of compressed air mass.The other type of C curve is that the gas is completely discharged(0.0316<s/S ?0.1264),and the water hammer pressure is dominant in the system pressure.In the design of the exhaust valve,if A small amount of exhaust is needed,the A-type curve can be selected to correspond to the size of the vent hole;if A large amount of exhaust is needed,the B-type curve can be selected to correspond to the size of the vent hole;if all exhaust is discharged,the C-type curve can be selected to correspond to the size of the vent hole.