Research On Mechanism And Performance Of Swirling Flow Pneumatic Conveying System For Coal Particle

Green coal logistics system,linking coal mining,processing,transportation,utilization,and recycle,is the central hub of intensive,safe,effective,green,and modern coal industry system building.The closed transportation technique of coal is the key of green coal logistics and the way to combat environmental probloms.To realize short distance green coal logistics,the coal pneumatic conveying is proposed in this disseration,which have the characteristics of land-saving,eco-friendly,operationally simple,flexible,and secure.Therefore,the main goals of this dissertation are to reveal the theoretical foudation of swirling flow pneumatic conveying of coal particle,the dynamic and kinematic characteristic of coal particle in the gas flow,and the performance of pneumatic conveing system.Based on the gas-solid flow theory,the coupled computational fluid dynamics and discrete element method(CFD-DEM)simulation and coal particles pneumatic conveying experiment are addressed for investigation,which are meaningful for coal particle pneumatic coveyng investigation and industrial application.Based on the quantified description of coal particle size and morphology characteristics,the mechanism of coal particle swirling flow pneumatic conveying is studied.The morphology characteristics of coal particle are censused on the basisi of three demimensional scraned coal particle geometrical models.Then the differential equation of particle motion,the motion equations and the contact dynamics equations of particle collision process are deduced on account of the coal particle morphology characteristics.Further,the governing equations,including the motion equations of coal particle group,the mass conservation and momentum conservation equations of gas flow,are presented.The aforementioned governing equations theoretically support coal particle swirling flow pneumatic conveying investigation.The applicable swirling generator are researched and developed on the basis of exisiting swirling generation technique and theory.CFD-DEM model is developed to compare the flow profiles and the particle motion in swirling flows generated by spiral wall swirling generators.The swirling intensity and velocity profile of the particles downstream of the swirling generator are investigated in detail.The results indicate that the peak region of the axial velocity is rotated quasi-periodically in the downstream direction of swirling flow.The dispersion of the particles increases with the inlet velocity and the swirling intensity.Stronger swirling intensity is suitable for the initiation of pneumatic IV conveying.However,medium swirling intensity has the highest efficiency for stable conveying.The internal spiral pipe swirling generator is a favorable option for swirling pneumatic conveying systems for coal particles.Baesd on the results of numerical simulation,side-inlet-vanes swirling generator is developed and experimentally measured.A decay model of swirling intensity is fitted by regarding of non-dimensional measure distance,Reynolds number,and tangential flow ratio.Based on the independently developed critical velocity measurement experiment device of coal particle,effect of particle size and swirling intensity on coal particle critical velocity are studied.The critical velocity is defined as the pickup velocity based on analysis of definitions and physcial meanings of common critical velocities for pneumatic conveying.Then,the initial test condition is determined on account of exiting pickup velocity models.The experiment results show that the main effect factors of coal paritlce incipient motion are the flow drag disturbance,the impaction disturbance from other picked coal particles,and the support boundary unstability disturbance.In the axial flow field,coal particles tend to be partly picked up layer-by layer,and the downstream of windward side is the priority picking area.Meanwhile,in the swirling flow field,coal particles are prone to gradually pick curl up in the windward side and transport in form of entirety movement.The pick up velocity of coal particle increases first and then decreases with swirling intensity,and monotonously increases with particle size.Take into accouting of swirling intensity and non-dimensional particle size,the pick up velocity of coal particle with size of 5-15 mm is fitted on the basis of Kalman pick up velocity model.The trend of particle breakage and pipe erosion in the interaction process between coal and wall are numerically studied on account of the reconstructed coal particle DEM model.On the strength of parallel bonding method and overlapped method,effect of particle shape and swirling intensity on coal particle breakage and pipe erosion are respectively discussed.Results show that the downward fluctuation of the particle mechanism energy is coupled with the particle and wall collision.However,the particle breakage shows a positive correlation with the energy difference.The higher the sphericity,the higher the particle agglomerate integrality ratio.The Exp3p2 exponential function can fit the variation of the numerical simulations with less than 2% fitting errors.Meanwhile,the integrality ratio of coal agglomerates continuously increases with the swirling intensity.The elbow direction affects the magnitude,distribution,and position of the maximum of the erosion rate.The mean erosion rates generally vary with the particle sphericity in a fluctuating pattern,and the fitting curves presents a nearly inclined “S” pattern in all calculated elbow directions.The mean erosion rate evidently decreases with the swirling intensity.On the strength of conveying performance index analysis,the coal particle pneumatic conveying performance is experimentally studied from the aspect of transportation energy efficiency and stability.Influence of swirling intensity on conveying performance is experimentally studied in terms of energy storage and energy consume between the measures points,which are defined by static pressure and static pressure loss,respectively.Moreover,effect of swirling intensity on coal particle conveying stability is discussed by regarding the relative standard deviation of static pressure fluctuation.In the 0.2 tangential flow ratio condition and the conveying velocity just under 40 m/s,the static pressure value in swirling flow field greater than axial flow field that with same mean conveying velocity.The absolute static pressure loss linearly increases with mean conveying velocity.Meanwhile,the slope of absolute static pressure loss variation increases with angential flow ratio.The relative static pressure loss decreases with the mean conveying velocity.However,the decline,caused by mean conveying velocity increasing,slow down in condition of high tangential flow ratio.There are four main factors that influence the conveying stability,including the disturbance of hopper storage variation,the disturbance of air source,the disturbance of rotary feeder,and the disturbance of noise,and the frequency ranges of the aforementioned four factors are less than 0.1 Hz,0.1-1 Hz,1-10 Hz,and more than 10 Hz in this work,respectively.The middle two facters specifically highlight than the other factors.In the low air velocity with medium swirling intensity and high air velocity with weak swirling intensity,the swirling flow field shows significant advantage of conveying stability.Meanwhile,at frontend of conveying pipe,the swirling flow field is of better conveying stability.However,at the end of conveying pipe,the reverse happens.