Research Of Gas Holdup Measurement System For Gas-liquid Two-phase Flow In Pipeline Based On Capacitance Sensor

As an important parameter of gas-liquid two-phase flow,gas holdup often affects the performance and safety of industrial equipment.Therefore,the measurement of gas holdup has always been an important research direction of gas-liquid two-phase flow.Capacitive sensor has the characteristics of non-invasive,low cost and simple structure.It is widely used in the gas holdup measurement of gas-liquid two-phase flow in pipeline.In this study,an array capacitance sensor system was designed to measure the gas holdup of gas-liquid two-phase flow in a pipe with an outer diameter of 25mm(inner diameter of 21mm).The visual method was used to verify the measurement effect of the gas holdup measurement system.In this study,the gas holdup measurement system was designed.Comsol Multiphysics was used to simulate and optimize the structure of capacitive sensor,and the number and length of electrode plates of capacitive sensor were determined.The capacitance sensor and hardware circuit in the gas holdup measurement system were designed.FPGA was used to complete the control of sensor system and the communication between upper and lower computers.Using Matlab to build the upper computer for signal acquisition.The collected signal was preprocessd and demodulated in Python.The static measurement method was used to collect the capacitance data of three types of gas-liquid two-phase flow,formed the training set and test set.The data of training set was used to complete the training of BPNN.The static measurement error of the gas holdup measurement system designed in this study is 3.63%.In order to verify the accuracy of the dynamic measurement of the gas holdup measurement system,this study further designed a visual verification platform,and used the gas holdup measurement system to measure the section gas holdup in the pipeline.At the same time,video capture and image processing were performed on the adjacent positions of sensors.Firstly,the contour recognition and feature extraction were carried out for the bubbles in the pipeline,and the Hungarian algorithm was used to track the bubbles in the pipeline.It was observed that the gas-liquid two-phase flow patterns in the pipeline mainly include bubbly flow,plug flow and stratified flow.Under the condition of low input gas holdup,the longitudinal width of bubbles changes little with the increase of input gas holdup,which made the fluctuation amplitude of section gas holdup changed not significantly under the condition of low input gas holdup.Then,a custom algorithm was used to fit the gas-liquid boundary for the stratified flow in the pipeline.It was observed that under the condition of high input gas holdup,the fluctuation of liquid level of stratified flow in the pipeline was particularly intense,which explains the phenomenon of large fluctuation of section gas holdup under the condition of high input gas holdup.Finally,a solution formula of gas holdup based on two-phase flow image was proposed.The average section gas holdup measured by the gas holdup measurement system were validated with the average section gas holdup obtained by the visualization method with a maximum difference of 8.48%,a mean difference of 5.62% and a root mean square difference of 5.51%.The average section gas holdup measured by the gas holdup measurement system were validated with the average volume gas holdup obtained by the visualization method with a maximum difference of 8.66%,a mean difference of 5.01%,and a root mean square difference of6.24%.It can be seen that the gas holdup measurement system has good gas holdup measurement ability.In view of the influence of acquisition point deviation on the effectiveness of the verification method.Firstly,the distribution of the variation coefficient of bubble volume when the bubble moves from one end of the window to the other end of the window was observed,and the average variation coefficient of bubble volume was5.8%.Then,the section gas holdup of the left and right sections of the window was calculated,and the mean difference of the average section gas holdup of the two sections was 1.038% and root mean square difference was 1.36%.It can be seen that under the working condition of this experiment,the variation rate of two-phase flow in the pipeline is small,and the video acquisition position of this experiment is very close to the capacitance data acquisition position.So that the verification method is effective.