Study On Steady-state Vibration And Transient Process In Water Conveyance Pipeline Based On Gas-liquid Two-phase Flow

The water conveyance project is the lifeline project of the city.The safe operation of the water conveyance pipeline directly affects the normal water consumption of residents and enterprises.Therefore,it is of great significance to ensure the safety of the water conveyance pipeline to maintain social stability and promote economic development.Due to the air entrainment by pumps and other mechanical equipment,the air release from water,and the poor exhaust of the air valve,the flow in pipe is actually gas-liquid two-phase flow.The potential safety hazard of the water pipeline is pipe vibration caused by the gas-liquid two-phase flow under the steady flow.In the transient process,transient flow that may cause pipe burst is the potential safety hazard.Therefore,this thesis studies the pipe vibration caused by gas-liquid two-phase flow under the steady flow and the transients of gas-liquid two-phase in the transient process.Moreover,the method of air content identification in the pipeline is adopted to guide the maintainance for the exhaust valves.A total of 144 cases(combination of different air contents and water velocities)were selected to study the flow regimes under different pipeline layouts.By analyzing the forces on bubble,in the horizontal pipe,higher pressure can make the bubbles move faster;higher water temperature will make the average bubble diameter larger and slower moving speed.The vibration of the horizontal pipe under the action of gas-liquid two-phase flow usually increases with the increase of flow velocity,but it is more affected by the air fraction.The existence of air will shift the vibration frequency of the horizontal pipe to the lower frequency and lead to more instense vibration.The amplitude of axial vibration acceleration increases with the increase of air content,and the axial vibration in the elbow is more severe.Pipe vibration caused by the butterfly valve at low air fractions is quite similar to the ball valve,and at high air fractions,the pipe vibration caused by butterfly valve is significantly more intense.Through the analysis of the natural frequency of simple supported pipe system,it is found that the natural frequency of the system increases with the increase of pipe diameter.The pipe with high modulus of elasticity can increase the natural frequency of the pipe system,while the increase of material density can reduce the natural frequency of the pipe system.Through the numerical simulation of the buried pipe vibration,it is found that when the pipe constraints are the same and the inner wall of the pipe is under the same load,the larger the elastic modulus of the pipe,the smaller the pipeline vibration.With the same pipe material,larger pipediameter can cause smaller pipe vibration.Within the normal depth range of water pipeline,the increase of buried depth can not reduce the amplitude of vibration.When the bottom of the pipeline is weakly constrained,the vibration of the pipeline causes the displacement of the bottom of the pipe to be larger and affects the soil underneath.As a result,the vibration of the pipe roof is weakened and the displacement of the pipe roof is reduced.On the basis of the experimental facility in this paper,two one-dimensional gas-liquid two-phase transient models based on the discrete vaporous cavity model(DVCM)and the discrete gas cavity model(DGCM)are established.The experimental and numerical results show that DVCM can accurately simulate gas-liquid two-phase transient flow at low air fractions.However,DGCM is more suitable for the safety design of the pipeline where gas-liquid two-phase transient flow may occur.Based on the numerical simulation of DVCM,the results show that the existence of gas will cause the viscoelastic effect(VE)on pressure damping greatly weakened.The relative importance of unsteady friction(UF)and VE on transient pressure damping tends to increase with the increase of air fraction.In viscoelastic pipelines,the air fraction(?)has a much greater influence on the transient frequency shifting than VE.Therefore,When the transient model is applied in viscoelastic pipelines,it is necessary to not only calibrate the wave speed and steady friction,but also to check the viscoelastic parameters when ?(27)1%;When 1% ?? ?2.37%,only the wave speed and steady-state friction should be checked.By analyzing the time-frequency characteristics of the pressure signal in the downward sloping pipe,it is found that the pressure fluctuation of the bubbly flow is relatively stable.As the air content increases,the pressure signal fluctuates more and more violently.Generally,the fluctuation amplitudes of pressure signals of different flow regimes could be ranked in a descending order: stratified flow,blow-back flow,plug flow,bubbly flow.In addition,the frequency-domian distributions of pressure signals of different flow regimes are mainly concentrated within 10 Hz.The pressure signals of bubbly flow have no obvious dominant frequency,while the pressure signals of plug flow,blow-back flow and stratified flow have obvious dominant frequences,and it appears to be gradually noticeable(close to 0 Hz).In this paper,two support vector machines(SVM-1 and SVM-2)are constructed to identify the air content in the downward sloping pipe.SVM-1 is used to separate the bubbly flow from the other three flow regimes.SVM-2 is used to distinguish plug flow from blow-back flow and stratified flow.The results of numerical experiments show that power spectral density(PSD)is most suitable feature for SVM-1,while short-time zero-crossing rate(SZR)is the most suitable feature for SVM-2.For the preprocessing of the pressure signals,simple smoothingis better than wavelet filtering.Moreover,higher can bring higher identification accuracy.It is verified by experimental data that SVM-1 and SVM-2 can achieve the best identification accuracies of 94.3% and 93.9% respectively when the sampling rate is 1 k Hz and the sampling time is 8 s.