Development Of A Void Fraction Measurement System Based On Capacitance Array Method

At present,two-phase flow is widely used in the fields of nuclear engineering,food safety,petroleum metallurgy and medical production.However,because the two-phase medium in the two-phase flow is random and variable,especially in the packed channel,the distribution of the flow channel is extremely complex,and it is more difficult to obtain the distribution parameters of the flow field.Therefore,it is more meaningful to use relevant technical means to detect the two-phase flow in the unpacked channel and the packed channel.Capacitance Tomography Technology uses the difference of dielectric constant between two phases in space,and determines the change of phase distribution on the cross section of the tube by measuring the change of capacitance value with the help of sensor.This method has many advantages,such as non-invasive,non-contact,non-radiation,not affected by light and wide application range.In this paper,the sensor structure,signal processing circuit and how to apply in the packed channel are studied in the capacitance tomography system,and a system for measuring the cavity share of the two-phase flow in the unpacked channel and the packed channel is developed.The main research work of this paper is as follows:1.This paper optimizes the structural parameters of the non-filled capacitance sensor,analyzes the influence of the change of structural parameters on the performance of the sensor through the relevant sensor performance indexes and the capacitance normalization model,and summarizes the relevant laws.A set of optimized parameters are obtained as the manufacturing parameters of the sensor,and it is found that when the filler material and shield are not added,the series normalization model should be selected to process the data,on the contrary,the parallel normalization model should be selected.2.The filling channel is introduced into the capacitance sensor,which simplifies the difficulty of the capacitance tomography system to measure and process the two-phase parameters of the filling channel.At the same time,the influence of the filling ball parameters on the sensor performance is analyzed,and the relevant law is summarized.It is found that the larger the porosity and the smaller the filling ball size,the better the sensor performance is improved.The limit of the sensor measuring range is about 3 times the height of the plate;The material with high dielectric constant is selected to fill the ball,which has little influence on the sensor performance.It is also found that the sensitivity distribution of the field is more uniform for the shunt normalization model.3.By comparing four kinds of capacitor detection circuits,the AC excitation capacitor/voltage conversion circuit with better performance is selected.Combined with relevant literature,the signal processing circuit is designed and debugging.Through reasonable design of timing sequence,reducing reference signal and selecting specific continuous data,the problems of coupling capacitance,channel inconsistency,stability and grounding error are alleviated or even eliminated.4.The error calibration of the measuring precision of the designed system is carried out by using the static experiment of 8 different media distribution and the pressure drop method of double position.The results show that the errors in the static experiment and the dynamic experiment are within the range.At the same time,through static experiments,it is found that when the medium in the sensor tube is mainly water,and the distribution of water is mainly near the sensor electrode,the parallel model should be preferred to process the data,and the series model should be preferred when the media distribution is opposite.In this case,the measurement accuracy of the system is higher and the error is smaller.5.The designed system was used to measure the void share in the unfilled channel and the filled channel with spherical diameter of 0.8,1.2 and 2cm.The influence of gas-liquid flow rate change on the void share was briefly analyzed,and it was found that the change of the void share in the unfilled channel and the filled channel was basically consistent with the law of the change of the void share in the filled channel with the gas-liquid flow rate.This indicates that the designed system can be used for the measurement of the cavity fraction of the two phase flow in the unpacked channel and the packed channel.