Study On Leakage And Diffusion Of Supercritical And Dense Phase CO₂ In Pipeline

There is an unavoidable risk of leakage when CO₂ is transported from source to storage.At present,little research has been carried out on the industrial scale experiment of leakage diffusion rule of high-pressure CO₂ pipeline,and no numerical model has been formed which is more suitable for leakage characteristics.Based on the assumption of non-homogeneity and diffusion source,the whole leakage process is divided into jet section and diffusion section through industrial scale CO₂ emission experiment combined with computational fluid dynamics simulation technology.The release data obtained in the jet section continues to leak as a new diffusion source.According to the numerical model constructed by stages,the leakage characteristics of high pressure CO₂ pipeline after rupture are explored and analyzed.The main contents and results of this paper are as follows:1)The industrial scale CO₂ pipeline leak experimental system has been built and improved,and the problem that the original system did not pay enough attention to the risk factors outside the pipeline has been solved,which enables the experimental device to measure the parameters in the leak area completely,especially the concentration data.After improvement,the evaluation of the far-field concentration range is more accurate..2)Leakage experiments of supercritical phase and dense phase CO₂ at different leakage diameters are carried out on an industrial scale.The leaking process mainly includes expansion,stabilization and attenuation.Dry ice is produced near the leaking outlet but does not accumulate.The relationship between jet intensity,attenuation speed of jet cloud and distance of leakage port is analyzed,and the change rules of temperature and concentration in leakage area under different leakage aperture and leakage phase state are obtained,and the dangerous ranges of high concentration and low temperature are summarized.3)The jet process simulation is carried out.Based on inhomogeneity assumption,dry ice and liquid phase are treated as condensation,gas and liquid thermodynamic properties are described by P-R equation of state,phase transition is realized by loading mass and energy source terms,and a two-phase flow model of CO₂ leakage jet process is built.The results show that the high pressure CO₂ leakage is an underexpansion jet and the velocity field reflects the cylindrical shock wave including Mach disc.The position and size of Mach disc with different apertures and phases are obtained,which conforms to the jet morphology observed in the experiment.The underexpansion structure is divided into initial region,supersonic region and weak sound velocity region.It is found that the condensate phase is dominant near the leakage port,and there is a high pressure gradient on both sides.Negative pressure zone is formed near the leakage port.Negative pressure values of different apertures and phases are determined.The relationship between the axial velocity and the distance of the leak port is analyzed,the maximum speed is determined,and the relationship between the plane size of the diffusion zone and the diameter of the leak port is determined based on the release rate.The relationship between the temperature of the leak port and the distance of the leak port under different leak sizes and phases is analyzed,and the lowest temperature value is obtained.It is found that the freezing point of dry ice has been reached,which is in accordance with the process of dry ice generation observed in the experiment.The jet model built in this paper conforms to the actual leakage process and can accurately describe the jet morphology in industrial practice when the leakage pressure and temperature are known..4)Diffusion process simulation is performed.Based on the diffusion source assumption,the intensity of the diffusion source is calculated according to the release rate,release temperature and release diameter obtained by the jet process,and it is also assumed that CO₂has all been converted into gas phase to build a numerical model of CO₂ leakage diffusion process source strength.The results show that（1）the concentration:the axial concentration is divided into initial rising section,stable rising section and falling section,which accords with the trend and amplitude of concentration change in the experiment.The radial concentration increases in reverse direction.The relationship between this phenomenon and the distance of diffusion source and the diameter of leakage hole is determined.The influence of gravity and wind on concentration distribution is also analyzed.The curves of high concentration hazardous distance under different phase states and diameters were fitted.（2）Temperature:By analyzing the morphological characteristics of the low temperature zone with different phases,it is found that the influence of low temperature is less than that of high concentration,but the low temperature lasts longer near the diffusion source,which is in accordance with the low temperature effect observed in the experiment,and the influence of the change of leakage aperture on the low temperature zone is established.The diffusion model and experiment built in this paper have good applicability and can provide a reasonable forecast for the division of hazard range in industrial field.applicability and can provide reasonable prediction for the division of industrial hazard range.