| Long distance water transmission pipeline may generate severe vibration due to the intense pressure fluctuations of gas-water two-phase flow, which could lead to the leakage in the pipe joints and reduce the structural stability, and even worse it may cause pipe burst. In order to improve the security of long distance water transmission pipeline and study the vibration characteristics of pipeline under different flow regimes, this thesis is going to talk about the amplitude-frequency characteristics of pipe vibration for both gravity flow and pressurized flow under different operating conditions, which is based on the basic theory of gas-liquid two-phase flow regimes and the coupling effect between two-phase flow and the pipes.A model of pipe vibration of gas-liquid two-phase flow was established by the combination of theories and experiments, and it was applied to analyze the properties of pipe structure and the effect of gas-liquid two-phase flow on the instinct properties of pipe vibration. Plexiglass pipes were used to build the test apparatus of long distance water transmission pipeline to build long-distance pipeline. The water velocity was controlled at 0.7~1.9 m/s by the electric control valve and the air flow rate was controlled at 0~3.0 m3/h by outlet pressure of the air compressor. In the test apparatus, the steady working conditions and the transition process could be simulated for both gravity flow and pressure flow.. Tri-axial acceleration sensors were applied to detect the vibration of the horizontal pipe, the upward inclined pipe of 45 °, the downward inclined pipe of 45 ° and the other parts near the valve under steady working conditions, so that the axial and radial acceleration spectrum for pipes could be obtained. The electric control valve and air operated reed valve were employed to carry out the experiments of transition process, which explained the effect of valve closing and valve accident on the amplitude-frequency characteristics of pipe vibration.The theoretical and experimental results show that the natural frequency of the pipe system will increase as the pipe diameter increases or the elastic modulus of pipe increases or the pipe length decreases or the pipe density decreases. The increase of void fraction in pipe could lead to the increase of the natural frequency in pipe systems and enhance the intensity of pipe vibration. Under the same water velocity, the axial vibration becomes more intense as the void fraction increases, and presents a rapid increase in bubbly-slug flow, and it will reach the climax in slug flow. The pipe resonance may occur due to the wide range of vibration frequency in slug flow in the horizontal pipe, which should be avoided in the actual operation.The elbow in upward inclined pipe shows intense vibration because of the connecting coupling effect. The vibration of downward inclined pipe has a close relationship with the development of the air mass, so that partly-filled pipe-flow should be avoided in this section. The local disturbance produced by the two-phase flowing through the valve has a greater impact on the radial vibration than the axial vibration in the upstream of the valve and will increase the vibration frequency. The experiments of closing valve quickly show that the presence of gas in pipe will reduce the water hammer, and as the void fraction goes up, the duration time water hammer and pipe vibration will become longer. All the conclusions above could be beneficial to the design, reconstruction and operation of long distance pipeline. |
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