Experiment And Theoretical Investigation On Dust Reduction By Synergies Effect Of Spray And High-frequency Standing Wave Field

Coal dust is one of the outstanding problems that threaten the safety of production and the health of employees in mining areas for a long time.The prevention and control of the respiratory dust in coal mines is a worldwide problem.It is urgent to seek new technologies and mechanisms for the reduction of respiratory dust.Considering the high-concentration dust production in mines and the high-humidity conditions of spray dust reduction,a new technology scheme for coal dust reduction by the implementation of high-frequency standing wave sound field and spray were proposed in mine roadways and driving face.A quantitative theoretical description and analysis of the physical mechanism contained in the process of acoustic wave and spray synergistic dust reduction were carried out by theoretical analysis,numerical simulation and experimental research methods,which provides a theoretical basis for the implementation of dust reduction technology by the synergies effect of acoustic wave and spray in coal mines.The main research contents and conclusions are as follows:(1)Based on the successful application of sound wave agglomeration theory in smoke dust control,artificial rainfall and other fields,it is proposed to strengthen dust reduction by synergies effect of spray and high-frequency standing wave field in the mine.In order to explore the mechanism of this method and the possibility of engineering application,model experiments were used to analyze the effects of two types of the dust reduction schemes the synergies effect of acoustic wave and spray.The results show that the dust reduction efficiency can be significantly improved by adopting dust reduction scheme with synergies effect of"ultra-fine droplets+sound wave+high pressure spray",and the dust reduction efficiency of the respirable coal dust can be increased by more than 10%(compared with the single high pressure spray dust reduction method),while the synergies effect of"sound wave+high-pressure spray"has no obvious effect.On this basis,the influences of nozzle diameter,spray water flowrate,ultrasonic atomizer water flowrate,ultrasonic power and air velocity on dust reduction efficiency were studied by the dust reduction scheme of"ultra-fine droplets+acoustic+high-pressure spray".(2)Based on the numerical simulation method of Lagrangian method,the estimation formula of the dust collection efficiency of a single droplet was obtained.The results show that(1)for the respirable dust in the range of d_p=1～10μm,increasing the spray velocity of the droplets can significantly improve the dust collection efficiency of the droplets;(2)The existing models for dust collection efficiency of the droplets(based on potential flow model)overestimates the dust collection efficiency of droplets.In this paper,under the unified background flow pattern of droplets,new estimation formulas of droplet dust collection efficiency due to three mechanisms of Brownian diffusion,interception and inertial collision were presented respectively,which solves the problem of inconsistent in solving the dust collection efficiency in the existing theory.(3)The formula proposed in this paper can be used to estimate the dust collection efficiency of droplets,which can be applied to spray dust reduction in actual spray dust reduction or other similar application projects.(3)Based on particle acoustic dynamics theory and CFD-EDM method,the motion and agglomeration behavior of ultrafine droplets and dust in the acoustic/fluid coupling field were numerically solved.The results show that under the same acoustic agglomeration conditions,a larger particle leads to a larger the acoustic radiation force;The collision probability between particles increases greatly with the increase of acoustic energy density;With the increase of acoustic frequency,the particle aggregation effect is enhanced;The reduction of wind speed is conducive to the migration of particles to acoustic pressure nodes,and these factors are important parameters affecting the acoustic agglomeration effect.The reduction of wind speed is conducive to enhancing the aggregation effect of particles.Through theoretical analysis,an estimation formula of the effective length of the acoustic agglomeration chamber(or the implementation acoustic space)is proposed,which can be used to guide the design of practical acoustic and spray dust reduction projects.(4)Considering the multi-mode logarithmic normal distribution characteristics of underground dust,the particle mass balance equation equation was solved by quadrature method of moments,the collection efficiency of polydisperse particles and the spatial-temporal evolution law of particle size spectrum are obtained by the established dust collection efficiency model of droplets.The results show that(1)for the case of monodisperse particles,with increase of the spray velocity of droplets,the dust collection efficiency of particles in the range of all particle sizes is significantly improved;With the increase of the air flow,the collection efficiency of particles reduces;At the same of spray flowrate,using small droplets can obtain more hight reduction efficiency.(2)For the case of polydisperse particles,the collection efficiency of particles passing through the droplets field increases exponentially with time.For small particles or medium scale particles(d_p<0.4μm)passing through the droplet field,the geometric mean diameter and geometric standard deviation of particles remain unchanged;When alarge particles group(0.4μm<d_p<10μm)passes through the droplet field,both its geometric mean diameter and geometric standard deviation become smaller;When large particles or small particles pass through the droplet field,the evolution process of particle size distribution has a typical"self holding"characteristic,that is,it always meets the initial logarithmic normal distribution characteristics.In this study,a scientific understanding of the mechanism of high-frequency standing wave sound field and spray synergetic dust reduction had been formed,a new model for estimating the dust collection efficiency of droplets had been proposed,and the theoretical defects of the existing models for the dust collection efficiency of droplets had been corrected.In addition,the movement equation of dust and droplet due to sound/flow field was established,and the hysical mechanism of dust reduction p by the synergies effect of sound wave and spray was revealed.Finally,the prediction model of dust reduction efficiency of high-pressure spray on multi-mode logarithmic distribution dust was constructed,which enriches the current theory of spray dust reduction.The result provides a new idea for dust control in the mine.