Fusion Measurement Method For Gas-solid Flow Parameters Based On Electrostatic And Capacitance Sensors

Gas-solid two-phase flow exists widely in many fields such as energy, chemical, metallurgy, pharmaceutical and foods industries. It is of great significance to achieve the accurate, online and continuous measurement of solid particle velocity, concentration and mass flow rate for the research of flow mechanism, energy saving, the safety and optimized control of system operation. Electric methods for the gas-solid flow measurement, mainly including electrostatic and capacitance sensing techniques, have been increasingly attractive due to their advantages of non-invasion, simple structure, low-cost, high reliability and non-radiation. In this paper, the flow parameters (particle velocity, concentration and mass flow rate) measurement based on electrostatic and capacitance sensing techniques have been studied to provide a basis for the development of reliable measurement instrumentation system.On the basis of the ring-shaped linear electrostatic sensor array (LESA), an electrostatic sensor matrix (ESM) is firstly proposed and its sensitivity and spatial filtering characteristics of are investigated. Each arc-shaped LESA of the ESM has a local sensing space and the central frequency of its output signal is proportional to the particle velocity. Further, a spatial filtering method based on the ESM is proposed for local particle mean velocity measurement in a gas-solid two-phase pipe flow. And then, a local particle mean velocimeter based on the ESM is designed and established. Performance of the velocimeter is evaluated on a gravity-fed particle flow rig. Experimental results demonstrate that the relative standard deviation of the measured velocities by the velocimeter is less than 11%.An integrated electrostatic sensor in combination with cross-correlation technique is designed and used to explore the velocity characteristics of anthracite and petroleum coke particles on a dense phase pneumatic conveyor. The ring-shaped electrodes in the integrated sensor are capable of measuring the "mean" velocity of solid particles over the whole cross-section of the pipeline, while the arc-shaped electrodes are employed to determine the local velocity of those particles near them. The experimental results demonstrate that the flow characteristics depend on the physical properties of solid particles and carrying gas, as well as the operation conditions of the conveying system. The increase of the flow rate of supplement gas or differential pressure between the feeding and receiving hoppers will lead to higher flow velocity with a lower particle concentration. And higher conveying system pressure is favorable for the suspension of particles. The anthracite particles mainly present a stratified flow in the experimental conditions, and petroleum coke particles are much easier to be suspended in the gas flow than the anthracite particles due to the less density. The local velocities from the arc-shaped electrode pairs mounted on the top of the pipeline are usually higher than those on the bottom, and the velocity from the ring-shaped electrodes falls between them for a stratified flow. The velocity from the ring-shaped electrode pair, indicating the mean velocity of particles over the whole cross-section, is usually higher than the local velocities from the arc-shaped electrode pairs for a suspension flow.Auto-correlation method combined with LESA is firstly used for the particle velocity measurement in gas-solid flow. The particle velocity is proportional to the transit time in the auto-correlation function of the output signal from LESA. Experiments are carried out on a gravity-fed rig and a dense-phase pneumatic conveyor to verify the method. With an appropriate sampling frequency, the mean particle velocity can be measured accurately by power spectrum and Auto-correlation analyses. However, the Auto-correlation analysis has a simple and reliable processing procedure and is more suitable for the online measurement. Moreover, the application of LESA is dependent on the flow pattern and only suitable for suspension flow. It is difficult to be used for the velocity measurement of stratified flow.The influence of particle electrification on capacitance method for the particle concentration measurement is theoretically and experimentally investigated based on a capacitance sensor with helical surface-plate electrodes and the AC-based capacitance measurement circuit. The results indicate that the capacitance signal and the electrostatic induction signal have different frequency band ranges. The influence of the particle electrification on capacitance measurements can be eliminated by rationally designing the capacitance measurement circuit, and a modified measurement circuit was further proposed to eliminate the influence by removing the electrostatic signal superimposed on the capacitance signal. The optimization of the capacitance sensor with helical-surface electrodes are further carried out on 10mm and 50mm pipes. With a fixed axial length of the electrode, the sensitivity in the center of the pipe is first larger and then becomes gradually less than those near the pipe wall with the increase of the electrode width. Therefore, there is an optimal width of the electrode to obtain a homogeneous sensitivity for the capacitance sensor and it can be easily determined by trial and error. After calibration, the optimized capacitance sensor can be used for the accurate particle concentration measurement.On the basis of in-depth research on the electrostatic sensing technique for the particle velocity measurement and the capacitance sensor for the concentration measurement, an integrated instrumentation system is developed for the particle velocity, concentration and mass flow rate measurement based on electrostatic and capacitance sensors. Experiments have been carried out on the V-belt pulley rig and the gravity-fed rig, respectively. Experimental results demonstrate that the measurement error of the solid mass flow rate is -3%～8% within the range of 0.13 kg/s～0.9kg/s., which proves that the system is capable of the online and continuous measurement of particle flow parameters. Moreover, the instrumentation system is designed based on DSP and can be directly applied in industrial environment to provide particle flow parameters to system control unit.