Research On The Dynamic Characteristics Of The Coal Particle In The Furnace Under Acoustic Action

The acoustic influence combustion technology in boiler furnace is an important technique to improve combustion and heat transfer efficiency using acoustic theory and technology.However,there is lack of knowledge of nonlinear propagation characteristics of high-intensity sound,propagation characteristics of acosutic wave in boiler flue gas containing solid particles,oscillating flow characteristics around coal particle in the acoustic field and heat and mass transfer chracteristics around coal particles under the action of acoustic field.This technology is embarrassed without enough acoustic knowledge.Therefore,it is of great significance for academic research and engineering application to study the dynamic characteristics of coal particles under the action of acoustic field.In this paper,the nonlinear wave equation of acoustic wave is derived,and the nonlinear propagation characteristic of acoustic wave in the power plant boiler is studied.Fisrtly,the mechanism of distortion and harmonic generation occurring in propagation process of high-intensity sound with pressure level of 160 dB was analyzed based on the hypothesis of simple wave theory,while the formation distance of shock waves was calculated for high-intensity sounds of different frequencies.Secondly,the strict solution of burges equation for viscous fluid in the furnace was calculated,and the propagation characteristics of high-intensity sound were analyzed under the circumstance of nonlinear effect and dissipation effect.Finally,the propagation characteristics of high-intensity sound in the relaxation medium were discussed after solveing relevant nonlinear wave equations by numerical methods.Numerical simulation were conducted on the attenuation mechanism and acoustic velocity of audio waves in the furnace of power plant boiler,based on which formulas of sound attenuation coefficient and acoustic velocity in the granular medium containing solid particles were built,respectively,so as to analyze the effects of following factors on the attenuation coefficient,such as the acoustic frequency,particle concentration,particle size and flue gas temperature,etc.Moreover,attenuation characteristics of acoustic wave and acoustic velocity in the furnace of fluidized bed boiler containing solid particles of higher volumetric fractions were also studied based on multi-body multiple scattering theory,and subsequently corresponding formulas were corrected.The entrainment characteristics of the coal particles in the acoustic field and the secondary effects of acoustic streaming were analyzed.On the basis of those,two-dimensional unsteady mass and momentum conservations for laminar flow in spherical coordinates were developed with actual parameters of power plant boiler to study flow field distribution characteristics around a pulverized coal particle in acoustic field with sonic frequency.Displacement amplitude of incident acoustic wave is greater than characteristic length of particlea,and aocusitc Reynolds number is less than 20.Velocity field,axial pressure gradient,shear stress and flow separation angle around particle were analyzed at different acoustic Reynolds numbers and Strouhal numbers.Base on two-dimensional,unsteady and laminar equations for mass,momentum and energy conservation and transport,heat and mass transfer characteristics around single coal particles entrained by flue gas under the action of acoustic field were studied.The temperature field,local Nusselt,surface-averaged Nusselt and space-and time-averaged Nusselt numbers around the coal particles at the sound pressure level varying in 145-167 dB and at the range of audio frequency were analyzed,respectively.Base on those equations,the heat and mass transfer characteristics around the pulverized coal particle under the action of a high intensity acoustic field with superposed steady component were also studied in this paper.The local Nusselt,space-averaged Nusselt and space-and time-averaged Nusselt numbers around the coal particles,as a function of Sound Pressure Level,frequency and the ratio of the acoustic velocity to the steady slip velocity between the particles and the main flow,have been discussed,at the sound pressure level varying in 150-170 dB and at the range of audio frequency.Experiments were also conducted to study the heat transfer,in particular the heat exchange at the surface,considering the effect of the acoustic wave around a stationary copper sphere.The results of the theorical study were verified by the the experimental investigation.The results of this paper provide a theoretical foundation for the application of acoustic enhancement combustion in power plant boiler.