Study on water hammer phase transition characteristics of dense/liquid phase CO2 pipeline

Under water hammer conditions, dense/liquid phase CO2 pipelines are prone to enter the phase equilibrium zone due to significant pressure fluctuations, further disrupting flow stability. At present, the research on the sudden changes in physical properties and pressure wave transmission characteristics caused by CO2 phase transition during water hammer transient process is not clear. This article uses experiments to screen the state equation suitable for CO2 phase characteristic calculation. Based on this, the characteristic line method is used to solve the one-dimensional pipeline water hammer flow process. When the water hammer pressure is lower than the saturation pressure, the corresponding gas phase fraction is solved using the isentropic principle. The results indicate that a phase transition occurs under water hammer conditions when the operating temperature is 280–300 K and the operating pressure is 0.59 %–4.55 % higher than the saturation pressure. When a phase transition occurs, for the valve front, 1.33 % gas generation will increase the pressure wave velocity by 1.67 %, and for the valve rear, 0.86 % gas generation will increase the pressure wave velocity by 0.61 %. This study provides a basis for the safe and stable operation of CO2 pipelines.