Study On Capacitance Tomography System For Two-Phase Flow Cross-Section Detection

Electrical capacitance tomography(ECT)is a non-destructive detection technique,which can reflect the distribution of media in the pipeline by the capacitance values between the electrodes arranged around the pipeline.The capacitance values of the sensor output enter the computer through signal acquisition unit,and then the cross-section information of the measured field is visualized by image reconstruction algorithm.Compared with the traditional detection technology,this technology has the characteristics of non-intrusion,non-radiation,low cost,good safety performance and fast response.Capacitance tomography technology has great application prospects in the field of two-phase flow detection,such as petroleum exploitation and transportation,coal chemical products and distillation process of condenser tube.Therefore,it is of great significance to carry out research on capacitance tomography system.Capacitance tomography system is composed of capacitance sensor,signal acquisition unit and imaging unit.In this master thesis,the research work is carried out from these three aspects.Firstly,the research background and significance of electrical capacitance tomography,as well as the mathematical basis and working principle are elaborated.The sensor model is established by COMSOL Multiphysics.The wall thickness,the number of electrodes and the coverage of electrodes of the sensor are optimized.By comparing and analyzing the capacitance values of electrodes and its variation,the characteristics of the sensitive field and the image error,it is determined that the wall thickness of the sensor is 3mm,the number of electrodes is 8,and the electrode coverage is 80%.Then the overall design scheme of the signal acquisition unit is introduced.The capacitance detection module based on capacitance digital conversion chip AD7746 and the electrodes state transition module are elaborated in detail.The capacitance between the specified electrodes is measured by the electrodes state transition host computer.Finally,aiming at the problem of non-uniform sensitivity field in the field,the original sensitivity field is filtered and normalized to make the sensitivity field more uniform than the original sensitivity field.Based on the sensitivity field after processing,the classical image reconstruction algorithm LBP and Landweber iteration algorithm are improved.The imaging results show that the improved Landweber iteration algorithm has higher accuracy.The experimental platform of capacitance tomography system is built to carry out the experiments of empty tube,stability and two-phase flow imaging.The experimental results verify the feasibility of the system.The deviation of the system is at the level of fF and the stability of the system is high.The improved Landweber iteration algorithm is used to image the gas-water two-phase flow.The cross-section distribution and liquid content of the two-phase flow can be detected.