Investigation Of Gas-Liquid Two-Phase Flow With Electrical Capacitance Tomography

With the improvement of environmental protection requirements and the transformation of the energy structure,natural gas demand has increased year by year.The implementation of the coal-to-gas conversion project has triggered a nationwide gas shortage.The efficient extraction and transportation of natural gas is essential to increase supply capacity.Natural gas is usually in the form of gas-liquid two-phase flow during the extraction and transportation.Identifying flow patterns and measuring gas content of gas-liquid two-phase accurately are of great significance for safe production and transportation.Compared with traditional single-phase meter measurement methods,Electrical Capacitance Tomography(ECT)technology has the advantages of no radiation,non-invasion,non-intrusive,fast imaging,and low cost,thus ECT is widely used in multi-phase flow measurement.However,electrical capacitance tomography still faces many challenges in the measurement of gas-water two-phase flow.For example,the high pressure of the fluid during the online measurement of gas-water two-phase flow needs thick wall of the ECT sensor.Besides,the dielectric constant ratio of gas-water is as high as 80,which is much higher than that of oil-gas or gas-solid medium.The main purpose of this research is to explore the influence of the media permittivity on the electrical capacitance tomography,and the effect of the wall thickness of the ECT sensor on the imaging quality.The ECT sensor that can be used for gas-liquid two-phase flow pattern identification and online monitoring of gas volume fraction is designed and applied for dynamic test.The influence of the dielectric constant of the medium in the sensing area on the imaging quality is studied by numerical simulation.The research variable in the simulation is the full-field dielectric constant,which is set to 2,5,10,20,40 and 80 in turn,while the dielectric constant of the empty field is set to 1.The flow patterns involved in the simulation are stratified flow,annular flow,and bubble flow.The algorithm for image reconstruction are linear back-projection(LBP)algorithm and the Landweber iterative algorithm.The reconstructed images were quantitatively evaluated and analyzed by relative image errors,correlation coefficients,and void fraction.The research is conducted under two types of wall thickness.By increasing the value of the dielectric constant of the empty field,that is,reducing the dielectric constant ratio of the two kinds of medium,the distortion of the reconstructed image at the high permittivity ratio can be improved.Through numerical simulation,the differences of sensor performance under different wall thicknesses are studied,including the effects of wall thickness on capacitance measurements,the sensitivity distributions,and the reconstructed images.The relative image error and the correlation coefficient are used for the evaluation of the reconstructed images.In the study of the effect of wall thickness,an 8-electrode ECT sensor with the inner diameter of 50 mm is used.The wall thickness increases from 2.5 mm to 15 mm,and simulation calculations are performed for ECT sensors with six kinds of different thickness.The permittivity ratio is set to 80 and 2.5,respectively,to simulate gas-water and gas-oil phases.The effect of different normalization methods on the image reconstruction are compared.Both the LBP and Landweber iteration are used for image reconstruction.The distribution of the excitation signal and the distribution of the sensitive field in the sensor are analyzed under different wall thicknesses.The capacitances of three flow patterns at different wall thicknesses are also evaluated.In addition,analysis about the void fraction of reconstructed image with the wall thickness is given.The experimental research of gas-liquid two-phase flow includes static experiment and dynamic test.Static experiment is conducted to compare and verify the effect of wall thickness on the reconstructed images.Three ECT sensors with different wall thickness are designed and air-oil and air-water are used as imaging medium in static experiment.Based on the analysis of capacitance measurements,a quantitative evaluation of the sensor performance is made with signal-to-noise ratio and void fraction.Finally,combined with the numerical simulation and static experiment,an ECT sensor is designed for online imaging of gas-water two-phase flow and gas volume fraction measurement.The gas-liquid two-phase flow pattern identified by ECT has a good agreement with the result of T-D flow pattern map.The online monitoring of the void fraction can be achieved by the pixel summation method and the average value of the capacitance,and the trend of void fraction measured by the two methods is basically the same.