| In recent years,strong sound wave combustion supporting and dust removal technology has been applied in utility boilers,and achieved good economic and environmental benefits.This shows that strong sound waves have great potential in strengthening pulverized coal combustion and heat exchange process of heat exchangers.However,this technology has not been fully popularized in coal-fired power station boilers.The fundamental reason is that this technology lacks a clear understanding of the basic physical relationship between the strong acoustic field and the heat transfer,combustion and ash removal process and the mechanism of action.For example,there is a lack of sufficient basic theoretical research on the motion characteristics of coal-fired particles under the action of strong sound waves,the influence of strong sound waves on the heat transfer behavior of coal particles and heat exchanger tubes,and the nonlinear acoustic streaming characteristics formed by the interaction between strong sound waves and coal-fired particles and heat exchanger tubes.Based on the above background,in order to improve the comprehensive thermal efficiency of boilers,this paper carried out research on nonlinear acoustic streaming effect and heat transfer characteristics under the action of strong acoustic waves,aiming to provide basic theories and key technical support for promoting the application of acoustic technology of power station boilers.The main work of this paper is as follows:(1)Based on Navier-Stokes equations,this paper analyzes the driving mechanism of nonlinear acoustic streaming under periodic strong acoustic wave disturbance,and deduces the time averaged second-order nonlinear acoustic streaming governing equation in the form of thermal viscosity in static background field.It is further clarified that the thermal viscous dissipation of acoustic waves in the acoustic boundary layer on the object wall is the main mechanism of forming the classical boundary layer driven acoustic streaming.In order to solve the nonlinear acoustic streaming governing equations,two numerical methods,Reynolds stress method(RSM)and limit slip velocity method(LVM),are proposed.It is shown that the Reynolds stress method can realize the accurate simulation calculation of acoustic streaming inside and outside the acoustic boundary layer for any physical model,but it requires high computational resources.In contrast,the limit slip velocity method is only applicable to the numerical calculation of acoustic streaming outside the acoustic boundary layer for geometrically simple physical models,but has the advantage of low requirements for computational resources.(2)The motion equation of pulverized coal particles under the strong sound wave is deduced theoretically,and the formula of nonlinear entrainment coefficient is given.The effects of acoustic parameters,fluid physical parameters and coal particle physical parameters on the oscillation characteristics and entrainment effect of coal particles were studied.The theoretical calculation shows that the particles at different positions from the acoustic source have different oscillation periods and amplitudes,showing rich oscillation characteristics.The nonlinear entrainment coefficient is negatively correlated with particle location,particle mass and sound frequency,but positively correlated with flue gas temperature.Furthermore,the dynamic numerical model of coal particles in plane standing wave acoustic field is established,and the effects of secondary effects such as acoustic streaming drag force and acoustic radiation force on the motion characteristics of coal particles are comprehensively considered.The calculation shows that there is a critical diameter dcrit=160 μm under the action of standing wave sound field of 150 dB and 1000 Hz.When the coal particle diameter is less than the critical particle diameter,the coal particles are mainly affected by the acoustic streaming drag force,on the contrary,they are mainly affected by the acoustic radiation force and agglomerate towards the sound pressure wave node.Finally,the nonlinear acoustic streaming characteristics around coal particles in standing wave field are numerically calculated by using the Reynolds stress method(RSM),and the general variation of radial streaming velocity distribution and vortex scale around coal particles with Reynolds number Re and Strouhal number Sr are obtained by dimensionless parameter analysis.The results show that with the increase of Sr and Re,the vortex structure scale in the acoustic boundary layer decreases exponentially and and the shear stress on the particle surface increases.(3)The flow field and heat transfer characteristics of nonlinear acoustic streaming around heat exchanger tubes in traveling wave field are numerically calculated by Reynolds stress method(RSM).For a single heat exchange tube,the dimensionless parameter analysis shows that the dimensionless acoustic streaming intensity decreases in a specific exponential function with the increase of Strouhal number Sr,and increases in a linear form with the increase of Reynolds number Re.Further,the nonlinear acoustic streaming characteristics in the structural field of heat exchange tube bundle and tube array are calculated.The effects of acoustic pressure level,acoustic frequency,acoustic incident direction and tube spacing on the characteristics of acoustic streaming between tubes are analyzed.The research shows that there are abundant acoustic streaming vortex characteristics around the heat exchange tube,and there are fluid phenomena such as vortex merging,shrinking and splitting.The acoustic streaming vortex has a strong shear stress on the wall of the heat exchange tube.Finally,the influence of the change of acoustic streaming field on heat transfer is calculated by coupling.The calculation shows that the inner and outer vortices of acoustic streaming formed by low-frequency strong sound and high-frequency strong sound on the wall of heat exchange pipe determine the different heat transfer behavior of heat exchange pipe.(4)The convective heat transfer characteristics of hot copper sphere and hot steel pipe under the action of strong sound wave are experimentally studied,and the action mechanism of sound wave in different frequency bands on convective heat transfer is analyzed.The experimental results show that the strong acoustic streaming effect of low-frequency and high-intensity sound is the main mechanism affecting heat transfer.For a hot steel pipe,when the direction of a sound wave is consistent with the direction of buoyancy,the heat transfer coefficient of the hot steel pipe surface shows a "V" distribution under the action of low-frequency strong sound waves.On the contrary,the heat transfer coefficient of the hot steel pipe surface shows a "Λ" distribution under the action of high-frequency strong sound waves.In addition,it is found that sound waves can not only enhance the convective heat transfer process but also hinder heat transfer.The enhancement or weakening of the heat transfer process by sound waves is related to the structural and acoustic parameters of the heat source.(5)Based on Nyborg perturbation theory,the time-averaged second-order nonlinear acoustic streaming governing equation in the background physical field is theoretically derived,and a numerical model for nonlinear acoustic streaming under the background physical field is established.The influence of background flow field and non-uniform temperature field on nonlinear acoustic streaming characteristics is analyzed.The results show that when the velocity of the background flow field is greater than 0.1 times of particle velocity amplitude of sound field,the nonlinear acoustic streaming is easy to be submerged by the background flow field.In addition,the nonlinear acoustic flow is very sensitive to the temperature gradient field.With the increase of the temperature gradient,the nonlinear acoustic streaming vortex near the high temperature region expands rapidly,and its flow intensity increases exponentially. |
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