| Cryogenic two-phase flow is widespread in aerospace,energy and chemical industry,it is of great significance to accurately obtain the two-phase distribution state of cryogenic fluid in the pipe.The difference of temperature,density,viscosity and dielectric constant between cryogenic fluid and normal temperature fluid makes the measurement of two-phase flow a new challenge.Capacitance method,as a common cryogenic liquid level measurement method,is widely used.As an extension of capacitive sensor technology,electrical capacitance tomography(ECT)has been widely studied and applied in petroleum,fluidized bed,medicine and other fields.But the research of ECT in the measurement of cryogenic two-phase flow is in the development stage.In order to realize industrial application,it also faces many difficulties,such as cryogenic ECT is more vulnerable to measurement noise,the test of low-temperature on the material and layout of sensor electrode,etc.The feasibility of cryogenic ECT is studied theoretically and experimentally in this paper,results obtained the inversion imaging of liquid nitrogen/nitrogen vapor,verified the feasibility and accuracy of ECT for cryogenic fluid,and paid special attention to the performance of the sensor and optimizes it accordingly.The main work and conclusions are as follows:(1)Theoretically verified the accuracy of classical ECT inversion algorithms applied to inversion imaging of cryogenic fluid phase distribution.Simulation research carried out based on eight-electrode sensor found that ECT can identify the basic position of gas-liquid phase at cryogenic,and the classical inversion algorithm can reflect the approximate phase distribution in high real-time.Comparing three representative cryogenic working fluid pairs(liquid hydrogen/vapor hydrogen,liquid nitrogen/vapor nitrogen and liquid methane/vapor methane),it is found that ECT can be well identified when the ratio of bubble/droplet to pipe diameter is greater than 0.2.(2)Experiments verified the accuracy of ECT classical algorithm and sensor in liquid nitrogen/nitrogen vapor inversion imaging.A liquid nitrogen/nitrogen vapor static ECT test-bed was designed and built.It is found that the inversion image can basically reflect the actual phase distribution.Among them,the maximum difference between the actual and inverted liquid volume fraction is no more than 7%.The three factors affecting the imaging,including the contraction of the sensor pole,the boiling of the liquid phase and the noise intensity,are analyzed to prove that there is room for improving the quality of the inversion results.According to the deficiency of static experiment,a vacuum insulated cryogenic two-phase flow visualization experiment platform is improved and built for preliminary experiment.(3)The feasibility of electrical capacitance volume tomography(ECVT)of cryogenic fluid is numerically studied.The feasibility of ECVT for the measurement of two-phase flow of cryogenic fluid is studied.It is found that the staggered sensor poles,the axial protection poles are beneficial to the imaging,there is an optimal value for the number of pole layers,and the influence of pole shape on the imaging results can be ignored.It is verified that the influence of noise on ECVT under different flow patterns is in an acceptable range,and ECVT is expected to be applied to industrial measurement of cryogenic fluids.(4)A2D/3D simulation and experimental platform of ECT based on MATLAB and COMSOL Multiphysics is designed.It is used to collect,demonstrate and numerically study the influence of sensor structure,sensitivity and algorithm on imaging quality. |
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