Measurement Of Gas-solid Flow Parameters Based On Electrostatic Technique And ECT

Gas-solid two-phase flow systems are widely used in various industries such as power plant,chemical industry,pharmacy and metallurgy,etc.Online and accurate measurement of gas-solid two-phase flow parameters is of great significance for the optimized operation of these industrial processes.In this paper,the multi-parameters measurement(flow pattern,local solid velocity,local solid volumetric concentration,local solid mass flowrate and mass flowrate)of gas-solid two-phase flow have been investigated based on electrostatic and capacitance sensing techniques.Firstly,the sensitivities of the electrostatic sensors have been investigated.The three-dimensional simulation models of electrostatic sensors are established by finite element method,and the spatial sensitivity distributions of electrostatic sensors are analyzed.Then the measurement system based on electrostatic sensor array is developed for the measurement of local particle mean velocity of gas-solid two-phase flow.The sampling frequency and sampling number of the system are optimizedly determined by belt conveyor experiment and gravity-fed experiment.Experiments are also performed on a high-pressure dense-phase pneumatic conveying system of pulverized coal to test the system,indicating that the measurement system is capable of achieving local mean velocity measurement of pneumatically conveyed particles with the relative standard deviation less then 5.49%?Secondly,the ECT imaging principle is introduced.The ECT sensor model is established,and its sensitivity is analyzed by finite element simulations.Two image reconstruction algorithms(LBP and Landweber)are compared in numerical and experimental experiments.Results show that the correlation coefficient of the image reconstructed by the LBP is higher than 0.785,while the correlation coefficient of the image reconstructed by the Landweber is better than 0.803.A ECT system based on DSP is designed and developed,mainly including AC-based capacitance measurement circuit design,multiplexing circuit design,data acquisition and control system design and host computer interface design.To test ECT system,static and dynamic experiments are carried out.Results show that the reconstructed images are in good agreement with the test models.Thirdly,the electrostatic effect on AC-based ECT system due to particle charging is investigated theoretically and experimentally.Theoretical and experimental results show that the output of the CN conversion circuit contains a high frequency and modulated capacitance signal and a low frequency electrostatic signal.The capacitance signal represents the change in the capacitance between the independent pair of electrodes within ECT sensor due to variation of gas-solid mixture,and the electrostatic signal of the ECT sensor is produced by particle charge.In the actual measurements,the electrostatic signal is superimposed on output capacitance signal of the AC-based circuit of the ECT system,leading to measurement error of the capacitance measurement and distortion of image reconstruction.As for this problem,an optimized AC-based capacitance measurement circuit by choosing its suitable amplifier and appropriate feedback resistance and capacitance values is presented.The results on a belt conveyor rig illustrate that the saturation of the output signal of the CN conversion circuit is avoided,and further the electrostatic signal superimposed on the output capacitance signal of the C/V circuit can be completely removed by a band-pass filter due to their distinct frequency characteristics.Therefore the electrostatic effect on ECT system can be eliminated by optimizing the AC-based CN conversion circuit of ECT system.Fourthly,an instrumentation system for the measurements of local solid volumetric concentration,local solid velocity,local solid mass flowrate and solid mass flowrate in gas-solid two-phase flow system is developed.It is based on a new type of a Capacitance-Electrostatic sensor(CES).The developed system for the local solid volumetric concentration measurement is verified through analogue simulation experiments and static experiments.Then the system is employed to measure the local solid volumetric concentration,local solid velocity,local solid mass flowrate and solid mass flowrate on a belt conveyor.The experimental results show that the measurement error of the local solid volumetric concentration measurement results are less than 10.43%,the standard deviations of the local solid velocity measurement results are less than 9.56%,and the relative error of the solid mass flowrate is within-19.6%-+14.9%for solid mass flowrate ranging from 0.006 kg/s to 0.103 kg/s,indicating that the system is capable of achieving multi-parameters measurement in gas-solid two-phase flow system.Finally,the developed ECT system and CES system are applied to the measurement of particle distribution,local solid velocity,local solid volumetric concentration,local solid mass flowrate and solid mass flowrate of the flow in the circulating turbulent fluidized bed(C-TFB).The effect of fractal structure on C-TFB is investigated by ECT and CES system.The ECT measurements show that the particle distribution in the C-TFB presents a annular structure when at E-Mod,and the particle distribution at lower bed height become relatively uniform,while at higher bed height also presents a annular structure for the F-Mod C-TFB,indicating that the fractal structure has influence on the uniform distributing the gas-solid flow in the C-TFB,but the functional area is restricted.The CES measurements show that the solid velocity and mass flowrate of the flow in the C-TFB both at E-Mod and F-Mod are increasing with the increasing air intake,while at the same air intake,the particle circulation quantity at F-Mod is higher then that at E-Mod,indicating that the fractal structure can play a certain role in increasing the efficiency of C-TFB under certain conditions.