Study On Formation Mechanism Of Benign Gas-Solid Flow Field And Sieve Structure Optimization Of Hammer Mill

Grinding is an important link in feed processing,which not only involves the processing cost of feed,but also seriously affects the quality of feed processing.The ring sieve hammer mill is widely used in feed processing because of its many advantages.When the hammer mill is working,the gas-solid flow field in the grinding chamber will form a circulation layer.The circulation layer is an important factor affecting the performance of the hammer mill,but the conclusions about the influence of the circulation layer on the performance of hammer mill are not the same or even contradictory in the studies at home and abroad.Although some researchers have noticed the important role of exploring the flow field and circulation layer characteristics in improving the performance of the hammer mill,and have achieved many research results.However,due to the limitation of research methods,most of these results are macroscopic,qualitative and experimental results,and there are few quantitative and rational conclusions.With the continuous development of relevant theories and the continuous enrichment of flow field characteristics research methods,it is of great significance to further study the flow field and circulation layer characteristics of the hammer mill by drawing on the advantages of the combination and mutual compensation of various theoretical and practical means.In this thesis,the physical and mechanical properties of the most common grinded corn grain were systematically studied.Based on theoretical analysis and high-speed photography,the material grinding process,gas-solid flow field and circulation layer characteristics were explored,and the influence of circulation layer on the performance of the hammer mill was analyzed.The hypothesis of benign gas-solid flow field characteristics of the hammer mill was proposed,and its formation conditions were analyzed.On this basis,combined with the previous research results of the research group and relevant research results at home and abroad.Considering that the sieve is the key component of the hammer mill,which directly affects the flow field characteristics,and the cost is low,the shape is easy to change.It was proposed to change the characteristics of the flow field and circulation layer by changing the shape of the sieve,so as to form a benign gas-solid flow field.The influence of different sieve shapes on the flow field characteristics,circulation layer and corn grain grinding process was analyzed theoretically and simulated.The formation mechanism of benign gas-solid flow field was revealed.The grinding performance test was carried out,and the key structural parameters of sieve were optimized to improve the performance of the hammer mill.The specific research contents and results were as follows :(1)The physical and mechanical properties of corn grain were studied.The basic physical parameters of corn grain,such as size,density,bulk density,moisture content,static friction coefficient,elastic recovery coefficient and repose angle,were measured.The compression and shear mechanical properties of corn grain were measured.It will provide basic data for establishing discrete element model of corn grain and setting simulation parameters of grinding process.(2)The theoretical analysis and high-speed photography of the material grinding process were carried out,the mechanical model and kinematic equation of material grinding were established,and the key working parameters affecting the performance of the hammer mill were obtained.Based on the high-speed camera,the gas-solid flow field in the grinding chamber,the formation and movement of the circulation layer were photographed,and the characteristics of the circulation layer were described in detail.The formation mechanism of the circulation layer and its influence on the performance of the hammer mill were systematically analyzed.The results showed that the circulation layer made circular motion close to the inside of the sieve,and the characteristics were stable,under the action of high-speed annular air flow,gradient distribution of velocity and pressure layer.To a certain extent,it reduced the grinding efficiency and sieving efficiency of material,and reduced the performance of the hammer mill.On this basis,to change the characteristics of the circulation layer and improve the performance of the hammer mill,a hypothesis of benign gas-solid flow field characteristics conducive to grinding and sieving of material was proposed,and the formation conditions were analyzed.(3)Considering that changing other parts of the hammer mill increases the difficulty of design and manufacture,and the shape change of the sieve is easier.Based on the formation conditions of benign gas-solid flow field,an airfoil-arc sieve was designed.The key structural parameters and value range of the airfoil-arc sieve were determined,and the influence of the airfoil-arc sieve on the flow field,circulation layer characteristics and material grinding process were analyzed.The results showed that the installation of airfoil-arc sieve in the mill could make the gap between hammer and sieve change continuously,and the airflow intensity and direction change continuously,resulting in more flutter and vortex.The disturbance ability of material in the circulation layer was enhanced,and then the characteristics of the circulation layer were changed to form a benign gas-solid flow field conducive to grinding and sieving of material.In addition,the airfoil-arc sieve could also reduce the incident angle of the material impacting the sieve,change the trajectory of the material,and improve the grinding efficiency and sieving efficiency of material.(4)Based on the CFD-DEM coupling method,the corn grain grinding process under the action of different sieves(annular sieve and airfoil-arc sieve)was simulated.The results showed that the installation of airfoil-arc sieve in the hammer mill could significantly improve the flow field characteristics in the grinding chamber.Compared with the annular sieve installed in the hammer mill,the velocity and pressure distribution of the flow field in the grinding chamber was gradient and diversified,and the distribution was scattered and disordered.And there was a strong velocity and pressure layer appear at the gap between hammer and sieve.In addition,the flow field velocity in the grinding chamber and the negative pressure in the central region of the rotor was reduced significantly,and the pressure at the outlet was increased,which formed a benign gas-solid flow field conducive to grinding and sieving of material.Under the action of benign gas-solid flow field,the characteristics of the circulation layer were changed.The particles in the circulation layer no longer move circularly close to the inside of the sieve,but present diversified distribution patterns at the end of hammer.The position of the material overlapped with the trajectory of the hammer end,which improved the probability of material being hit,thereby improving the grinding efficiency and sieving efficiency of material,and improving the performance of the hammer mill.Based on the above research results,the formation mechanism of benign gas-solid flow field of the hammer mill was analyzed.(5)The key structural parameters such as camber,attack angle,small arc radius and the equal number of the airfoil-arc sieve were taken as the experimental factors,and the productivity,electricity consumption per ton and feed temperature rise were taken as the performance evaluation indexes.The Box-Behnken response surface method was used to carry out the orthogonal experiment on the influence of different structural parameters of airfoil-arc sieve on the performance of the hammer mill.The regression model between the performance index and the experimental factors was established.The influence of various factors and their interaction on the performance of the hammer mill was analyzed,and the key structural parameters affecting the visibility were determined.The multi-objective optimization of the airfoil-arc sieve structure was carried out to obtain the Pareto optimal solution set based on genetic algorithm.The optimal combination of structural parameters of airfoil-arc sieve was determined as follows: camber 0.15,attack angle 11.5°,small arc radius 85 mm,and equal number 4.The sample sieve was processed based on the optimization results,and the experimental verification was carried out.The experimental results showed that compared with the circular sieve,the productivity of the hammer mill using the airfoil-arc sieve increased by 6.7%,the electricity consumption per ton decreased by 13.4%,and the feed temperature rise decreased by 23.5%.The optimized airfoil-arc sieve significantly improved the comprehensive performance of the hammer mill.According to the national standard,the optimized hammer mill energy consumption index was better than the first level energy consumption standard of 10.6%,and the energy consumption level was excellent.The research results of this thesis can provide a theoretical basis and implementation method for improving the performance of hammer mill by changing the shape of sieve,and also provide a reference for the study of benign gas-solid flow field in other fields.