| The unbalanced distribution of water resources has become an important factor restricting the development of China's economy,long-distance water conveyance engineering and transregional water transfer engineering have become the main means to solve these problems,and these projects will inevitably cause the high and low fluctuation of pipelines due to topography during construction.When the hydraulic components are regulated by the control system in engineering,once the transient flow phenomenon occurs,it will cause safety accident and economic loss.In the traditional water hammer simulation method,one-dimensional quasi steady friction model is often used to calculate,and this method can not accurately describe the extreme value and attenuation process of pressure wave.With the construction of many complex water conservancy projects,this method can not meet the requirements of water hammer protection calculation in engineering,so it is necessary to introduce unsteady friction into the calculation method and improve the accuracy of calculation results.With the deep study of water hammer,the traditional method of characteristic is not suitable for solving the shock phenomenon in pipeline,and by using the method of Lax-Wendroff scheme combined with Upwind scheme,Beam-Warming scheme and Ghost Cell method,a method suitable for calculating shock wave in pipeline is deduced,the accuracy and stability of the calculation are also analyzed.By using computational fluid dynamics to calculate the phenomenon of water hammer in pipelines,the characteristics of turbulence and the dynamic change of shock wave under transient flow are analyzed,and the methods to study water hammer problem are added.At the same time,the reverse problem method and signal analysis are used to study the pipeline leakage,extract the characteristics of the leakage signal,and use it for the leak detection,the location calculation of the leakage point,and then compare the other common detection methods,a method suitable for pipeline leakage detection is selected.In this paper,the analysis of transient flow phenomena and the study of pipeline leakage detection methods are very important to ensure the safety of the project and improve the economic efficiency of the project.The main research work and research results of this paper are as follows:(1)Based on the theory of transient flow,the mathematical model and boundary conditions in the pipeline are established,the transient flow phenomenon in the pipeline is further studied on a test rig,the water hammer pressure of the pipeline is calculated by introducing IAB friction model and MIAB friction model,and the results are compared with the quasi steady friction and test data.At the same time,the applicability of unsteady friction to practical engineering is discussed preliminarily.The results show that the period of hammer wave in the attenuation process is in agreement with the experimental value,which shows that the wave speed value of the theoretical calculation is in accord with the actual value.In the extreme value and attenuation trend of the pressure in the pipeline,the unsteady friction has a more accurate simulation effect on the attenuation of the pressure in the pipeline.When the unsteady friction is applied to the practical engineering,the nondimensional parameters of the pump after the pump stoppage,the water level in the surge tank maintains good consistency with the test value,and the calculation of the water hammer protection with the quasi steady friction model can meet the engineering needs,but the result is less accurate than the experimental value.(2)The water hammer equation is discretized by Lax-Wendroff scheme,the upstream and downstream boundary is processed by Beam-Warming scheme,Upwind scheme and Ghost Cell method,and the calculated results are compared with the experimental data.The results show that the above method can be used to simulate the pressure oscillation of the pipeline when water hammer occurs,the difference between the oscillation period of water hammer wave and the test is small.Among the three methods for dealing with boundary conditions,the Upwind scheme and the Ghost Cell method are stable,and the results are not sensitive to the Courant number.The results of Beam-Warming scheme are sensitive to the variation of Courant number and grid number.When the grid number is 1000,the Courant number is more than 0.96,which leads to the divergence of the results.When the grid number is less than 1000,the solution will oscillate in varying degrees.The Upwind scheme and the Ghost Cell method are stable,but the calculation precision is low,and the result of the Beam-Warming scheme is higher than 1000 meshes,so the result will not have the numerical oscillation(3)Using three-dimensional computational fluid dynamics to calculate the water hammer phenomenon in pipelines can obtain more flow field information than the traditional one-dimensional method,by simplifying the boundary conditions in the calculation,taking into account the the compressibility of water,and comparing with the experimental data,the results show that CFD has high accuracy when calculating the pressure of water hammer.The attenuation trend of pressure oscillation is basically consistent with the test value.The theoretical formula calculates that the velocity distribution of the pipe section is basically consistent with the CFD.Based on the sensitivity analysis of radial distance,axial distance and minimum mesh thickness in CFD computational grid,the optimum mesh parameters are obtained,axial sectional coefficients Fd=0.92,radial sectional coefficients F,=0.029,first layer mesh thickness Ft=5x10-4mm.In the region near the wall boundary layer,the viscous force dominates the action,thus resulting in a larger velocity gradient,which increases the wall shear.Under the combined effect of the radial viscous force of the pipeline and turbulent transport,the velocity gradient does not increase indefinitely,thus limiting the continuous development of the wall shear force.When the valve is closed,a shock wave is generated at the valve and begins to propagate through the pipeline.The first shock wave has the greatest energy,causing the flow line to break and flow in the opposite direction near the wall of the pipeline,which is consistent with the change law of velocity distribution in the pipeline(4)By using transient reflexions method,negative pressure wave method to calculate the leakage condition in pipeline,the influence of leakage position,leakage,initial constant flow pressure and pipe friction on the accuracy of the test result is analyzed,and the characteristics of each method are compared.The results show that the transient hydraulic reflection method has a good effect on the small leakage,This method can also show the abnormality of water hammer waveform when the leakage of pipeline is less than 5%,and the larger the leakage,the more obvious the abnormality of the waveform.Using this method for pipeline leak detection,the accuracy of the calculation results depends on the accuracy of the water hammer waveform calculation,such as the friction of the pipeline which affects the waveform attenuation,but the calculation accuracy requires the valve to generate the excitation signal close quickly,the valve closing speed has a significant impact on the test results,valve closing time greater than 2 seconds this method is invalid.When the transient flow pressure wave method locates the position of the leak,the time difference of the pressure wave reflection can be used to locate the leak point,when the leak point is located upstream of the pipeline,the error of the location is not more than 5%,the leakage point is located downstream of the pipeline,the error of the positioning is not more than 10%,the positioning accuracy is higher.For a given flow,the negative pressure wave method can be used to detect the presence of a leak by analyzing the pressure signal,that is,there is pressure difference in the pipeline,the greater and faster the need to cause transient flow changes,the greater and steeper the pressure wave,the better leakage detection results.Negative pressure wave method is suitable for large leakage,sudden leakage detection,and slow leakage small leakage detection effect is not obvious.After analysis with HHT,the pressure signal in the constant flow is mainly low frequency,the frequency component of pipeline leakage pressure is distributed in the whole frequency band,not concentrated in a certain frequency band;the amplitude begins to decrease with the frequency increase,which is due to the pressure drop in the pipeline caused by the leakage,and the low frequency movement in the pipeline decreases,the high frequency movement increases. |
PDF Full Text Request