Study On Influence Mechanism And Optimization Of The Hammer Mill Grinding Performance

Maize is the most productive grain crop in the world.As the most significant energy component,60%of its entire production is used in feed production.Grinding is one of the most crucial steps in feed processing.It influences the production output and effectiveness of subsequent operations such as mixing,granulation,and extrusion,in addition to the nutritional value of the broken particles.However,it has issues such as high energy consumption and low efficiency,with its power consumption accounting for 30%to 50%of the overall power of the production line.Thus,it is imperative to improve grinding effectiveness and reduce energy usage throughout the feed production process.This study aimed to investigate the influence mechanism and optimization of the hammer mill grinding performance.Maize was taken as the research object,a variety of technologies and methodologies,including theoretical analysis,experimental design,statistical analysis,numerical simulation,high-speed cameras,and automatic electronic control were utilized to perform the research systematically.Firstly,the factors affecting the grinding performance of the hammer mill were analyzed and the experimental investigation of the impact breakage characteristics of maize was carried out in this study.After that,the distribution characteristics of the airflow field in the hammer mill and the influence of various factors on them were revealed.Additionally,a DEM model of maize kernel was created,and the contact parameters and the Ab-T10 breakage model parameters of maize kernel were calibrated.Base on the CFD-DEM two-way coupling method,the simulation of the maize grinding process of the hammer mill was established,revealing the grinding mechanism of the hammer mill.Finally,the ruminant arch device and the automatic optimal-search control system of the grinding system was creatively designed.The main research contents and results are as follows:（1）The experimental research on the working performance of the hammer mill was carried out.A laboratory-level grinding system consisting of a hammer mill,hopper,impeller feeder,settling chamber,discharge winch,bag filter,negative pressure fan,and electric control box was designed and established.The study investigated the effects of various variables on the grinding system’s grinding efficiency,power consumption per ton of material,effective power consumption per ton of material,average particle size,and fractal dimension of grinding particles.These variables included moisture content of maize,sieve aperture,rotor speed,hammer-sieve clearance,negative pressure suction air volume,and feeding rate.Regression equations were created between the above test indexes and the experimental variables.The maximum and minimum determination coefficients（R²）of the regression equations were 0.99 and 0.73,respectively.The research findings offered a theoretical framework for investigating the influence mechanism on the working performance of the hammer mill.（2）The experimental research on the impact breakage characteristics of maize was carried out.An impact testing device for a single particle was designed and established,and impact experiments of maize kernels with five different moisture contents were conducted.Five different types of maize kernel samples with varying moisture contents were prepared to study the impact breakage characteristics.The experimental results revealed that the breakage patterns of maize kernels could be divided into unbroken,fracture,and fragmentation,of which the fracture pattern included overall fracture and local fracture.The big particle mass ratio（W_P）was employed to identify the breakage patterns of maize kernels in this study,which was the ratio between the mass of large particle in broken particles to the mass of the maize kernel.According to the experiment results,the regression equations between the W_P values and impact velocity of maize kernels with different moisture content were established,and all of the obtained R² of the equations were greater than 0.92.According to the regression equations,the critical fracture velocities of maize kernels（with moisture contents of 10.86±0.13%,13.87±0.18%,17.24±0.08%,20.23±0.19%and 23.46±0.23%）were determined to be 11.45,15.54,19.84,30.49,and 34.28 m/s,respectively,and the critical fragmentation velocities were 19.55,23.94,28.68,42.07,and46.79 m/s,respectively.The results showed that the critical fracture velocity and critical fragmentation velocity of the maize kernel increased with the increasing the moisture content.The breakage extent of maize kernels was characterized by the average particle size of the broken particles.Using response surface methodology,the model regression equation of the average broken particle size with the impact velocity and moisture content of maize kernels was established,and the obtained R² was 0.96.In addition,the results of the analysis of variance revealed that the impact velocity,moisture content of maize kernels,and their interaction had a highly significant（P<0.01）impact on the breakage extent,with the breakage extent of maize kernels increasing as the impact velocity increased and decreasing as the moisture content of maize kernels increased.The results of this study could serve as a guide for the determination of operational parameters of agricultural material granule processing equipment,as well as offer a theoretical basis and method for the research of the impact breakage characteristics of agricultural material granules.Moreover,the results of scanning electron microscopy（SEM）of maize sections showed that the number of cracks,the total length of cracks,and the width of cracks in the maize kernel increased with the decrease of moisture content.Additionally,with low moisture content,the starch granule gap and the protein base fracture in the maize kernel were more noticeable than with high moisture content,revealing the mechanism of moisture content on the effect breakage properties of maize.（3）The analysis of the distribution characteristics and influencing factors of the air flow field in the grinding chamber were conducted.Using CFD numerical simulation method,this study investigated the effects of screen aperture size,hammer mill speed,hammer-screen clearance,hammer number,and negative pressure suction air volume on the airflow distribution’s influence law on the rotor area,the circulation layer,and the inside and outside of the sieve hole in the grinding chamber.The findings demonstrated that the static pressure had a gradient distribution in the radial direction and was relatively uniform in the axial direction in the rotor region.Notably,increasing the hammer mill speed resulted in a significant improvement in the gradient distribution of the static pressure in the radial direction（P<0.05）.On the other hand,when the volume of negative pressure suction air increased,the static pressure in the radial direction decreased dramatically（P<0.05）.The investigation also discovered that the highest airflow velocity in the circulation layer of the grinding chamber was approximately 83.46%of the tangential velocity of the end of the hammer.Increases in hammer speed and hammer count might significantly enhance the flow velocity of the circulation layer（P<0.05）,whereas increases in the volume of negative pressure suction air might significantly slow it down（P<0.05）.Furthermore,eddy currents were observed in the screen holes,and the eddy currents were more likely to form as the hammer speed and hammer number increased.However,eddy currents were less likely to form in the screen holes with a high screen aperture（φ=6.0 mm）,and the eddy currents created in the screen holes could be destroyed by increasing the negative pressure suction air volume.The study also discovered that as hammer speed and negative pressure suction air volume increased,there was a substantial rise in the static pressure difference between the inside and outside of the screen hole in the grinding chamber（P<0.05）.The static pressure difference between the interior and outside of the screen hole,however,dramatically decreased with an increase in screen aperture size（P<0.01）.（4）The contact parameters of maize and Ab-T10 breakage model parameters were calibrated.The maize kernel model was constructed using the 3D scanning technology with combination of section method.The restitution coefficients of particle to low-carbon-plate（p–w）and particle to particle（p–p）,as well as the dynamical friction coefficients,were calibrated using the inclined surface drop method and the funnel method,respectively.According to the findings of impact studies of maize kernels,the breaking probability and t₁₀ of maize kernels increased with an increase in specific impact energy and reduced with an increase in moisture content of maize kernel.Based on the theoretical formula of breaking probability and t₁₀ in the Ab-T10 breakage model,the data of impact experiments of maize kernels were fitted（adjusted R²>0.86）,and the parameter values in the theoretical formula were determined,calibrating the Ab-T10 breakage model parameters.Through the test verification,the relative errors of the sizes of the three axes and volume between the maize kernel model and the real maize kernel were 3.81%and 0.48%respectively.The maximum relative errors between the simulation values and the measured values of the inclined surface drop test and the funnel test were 4.38%and 6.98%respectively,and the relative error of broken maize particle average size between the simulation value and the measured value is less than 6.54%.These results indicated that the particle model,the calibrated contact parameters and Ab-T10 breakage model parameters could effectively characterize the grinding performance of maize kernels.The findings of this study may provide theoretical importance for the subsequent simulations of the maize grinding process through CFD-DEM two-way coupling method.（5）The grinding mechanism of the hammer mill was studied.High-speed photography was used to record and examine the maize grinding process in the grinding chamber.The ruminant trough may alter particle movement and breaks up particles.Hammers have three possible impacts on maize kernels:centered,eccentric,and parallel.The maize grinding process inside the hammer mill was simulated by CFD-DEM two-way coupling method.The results showed that after hammer-type impact’explosive’grinding,there were three primary trajectory pathways and five branch trajectories for maize.The medium-speed particles hit the sieve and were sieved or rebounded,whereas the low-speed particles essentially stayed in their initial position.As some of the components of the high-speed stream flowed toward the rotor’s center,others collided with the sieve and were sieved or rebounded.The re-emitted high-speed particles and the leftover particles gradually formed a circulation layer.The ruminant trough could damage the circulation layer by changing its trajectory and breaking some particles in the circulation layer.Grinding maize depended primarily on the impact of the hammer and sieve,and sieved particles were including discharged impact powder and circulating powder in the grinding chamber.The particle screening procedure was split into three categories based on the link between the maximum size of the particles on the tangential projection surface of the screen hole and the screen aperture size.The aforementioned conclusions were also theoretically examined.（6）A ruminant arch device was designed and an automatic optimal-search control system of the grinding system was built.Based on the DEM simulation results of the maize grinding process,it was found that the ruminant trough had limited damage to the circulation layer,and particles accumulated in the trough.To address this issue,a new ruminant arch structure was designed and tested using DEM simulation,which showed that it improved the grinding efficiency of the hammer mill by 13.26%and increased the geometric mean particle size of crushed particles by 5.78%compared to the ruminant trough.The automatic optimal-search control system was designed using a Siemens S7-200smart PLC as the control processor and Raspberry Pi Model 3B as the computing processor,which was composed of an electromagnetic relay group,analog output module,analog input module,and MCGS configuration display screen.The system allowed for the automatic optimization of three working parameters of rotor speed,feeding speed,and negative pressure suction volume,and real-time monitoring of the grinding system’s working process.Three operational modes of grinding system were established based on the limitative conditions of working parameters optimization,which were performance,energy-conserving,and balance.The results of the verification experiments showed that the automatic optimal-search control system effectively improved the working performance of the grinding system.The grinding efficiency of the aforementioned three operational modes was increased by 13.40%,5.11%and 9.79%,respectively,compared to the routine operating condition.Additionally,the power consumption per ton of material was reduced by 1.20%,13.65%,and 5.19%,respectively.Overall,the research achieved its purpose of improving the efficiency and saving energy of the hammer mill.The innovations mainly include:Innovation Point 1:The impact breakage characteristics of maize were ascertained,and the influence mechanism of the moisture content on the breakage characteristics of maize was revealed from the microscopic scale by SEM.Innovation Point 2:The distribution characteristics and influence of the velocity of the circulation layer,the vortex in the screen hole,and the pressure difference between the inside and outside of the sieve were systematically revealed.It was proposed that the trajectory of broken particles mainly presented three main trajectories and five branch trajectories,which clearly showed the maize grinding process,circulation layer movement,and powder discharge from the screen hole.Innovation Point 3:The ruminant trough was improved,and a new ruminant arch was designed,which were more destructive to the circulation layer and could improve the grinding efficiency.The automatic optimal-search control system was innovatively established to realize the automatic optimization of the working parameters of the grinding system.