Propagation Characteristics Of Ultrasound In Non-uniform Gas-Solid Two-Phase Medium

Gas-solid two-phase media is widely used in industrial production processes and other fields,and the monitoring of solid-phase parameters is of great research significance.In recent years,ultrasonic wave has been widely used for measurement of solid-phase parameters of gas-solid two-phase flow because of its advantages of non-contact,strong penetrating force and good directivity.Based on the research background of pulverized coal particle size monitoring in coal-fired power plants and the combination of theory and experiment,this paper mainly studies the propagation characteristics of ultrasonic wave in non-uniform gas-solid two-phase medium,and provides a theoretical basis for the development of particle size online measurement system,which has a good application prospect.The main research results are as follows:(1)In the gas-solid two-phase medium,the viscous attenuation,scattering attenuation and thermal attenuation during ultrasonic propagation are studied according to the extended coupled phase model,the ECAH model and the Gregor-Rumpf model respectively.When the particle size is within the 50?m~500?m range,ultrasonic waves with frequencies of 20 kHz,40kHz,60 kHz,and 80 kHz all show that the total acoustic attenuation coefficient decreases as the particle size increases.(2)The ultrasonic measuring system consists of an ultrasonic transducer,drive device and a signal acquisition and processing system.The ultrasonic transducer is the core part of the whole measuring system.The piezoelectric transducer with a frequency of 20 kHz has a cross-section diameter of 10 cm,and the piezoelectric transducer with a frequency of 40 kHz,60kHz and 80 kHz has a cross-sectional diameter of 8cm.The geometric model is established by using COMSOL multiphysics coupling software to simulate the ultrasonic transducer.The vibration mode and frequency domain characteristics show that the piezoelectric transducers of four frequencies tend to be stable when the circular radius of the air domain is less than 15 cm,which provides an accurate range for the placement position of solid particles in the experiment.(3)The glass beads with similar properties to the pulverized coal are used as experimental research objects.The layered hypothetical model is proposed which is considered that the non-uniform gas-solid two-phase medium can be spatially divided into multiple air layers and local accumulation layers of solid particles.The propagation in a uniform gas-solid two-phase medium can be considered as a combination of the propagation of the air layer and the propagation of the local aggregate layer of the solid phase particles.The acoustic attenuation characteristics of the particle accumulation layer conform to the existing gas-solids two-phase medium propagation model.According to the above stratified model,the propagation characteristics of three typical non-uniform gas-solid two-phase media in single,double and three-dispersion distributions are studied.The result shows that when the distribution spacing is less than 10 mm,the acoustic pressure attenuation rate is approximate to satisfy the exponential attenuation law.The ultrasonic wave with a frequency of 80 kHz further studies the influence of the extended distribution distance on the ultrasonic propagation characteristics when the solid particles are double dispersed distribution,when the solid phase distribution spacing is within the wavelength range of 5~10 times,the acoustic attenuation is changed from exponential decay to square attenuation.When the solid phase distribution spacing is greater than 10 times wavelength,the acoustic attenuation satisfies the square attenuation law.This is because the new equilibrium state is re-established between the ultrasonic wave and the non-uniform gas-solid two-phase flow to realize the energy distribution of freedom.