Analysis Of Heat And Stress Transfer Of Compression With Assisted Vibration For Alfalfa Straw Based On Discrete Element Method

Alfalfa is a forage with high nutritional value.The promotion of alfalfa is hindered by the loose,low density and irregular shape of alfalfa,which is not easy to transport and store.The regular pellet compressed by the loose alfalfa can reduce transportation and storage costs.However,the current biomass compression molding machine has the problem of low quality of molding products and serious wear of the key components.In order to solve these problems,the research group introduced the vibration into the compression molding process.The test verified that vibration compression could improve products quality.To further study the characteristics of vibration on the molded products,this study researched the heat and stress distribution and transfer characteristics within the compressed materials at macroscopic and microscopic scales.A discrete element model of alfalfa straw particles for computer simulation of the vibration compression process was established by measuring the physical characteristics of alfalfa straw and calibration of the discrete element simulation parameters.A vibration compression test system was built to measure the temperature and pressure inside the die,to analyze the transfer of stress and heat during the compression process.Taking the heat and stress transfer effect and the quality of the molded product as the main evaluation indexes,the better working parameters for the test bench ware obtained.The distribution of internal stress and temperature in the compressed material during vibration compression was simulatied by the discrete element method at multiple scales.The transfer characteristics of heat and stress transfer characteristics of alfalfa straw pellet during the vibration compression process and the better range of simulation results were obtained.The research content and results are as follows:1.The intrinsic parameters such as thermal conductivity,specific heat capacity and shear modulus of alfalfa with different moisture content and particle size were tested;the restitution coefficient,static friction factor and rolling friction factor between alfalfa straw and alfalfa straw or alfalfa straw and steel surface were tested.2.A discrete element model of alfalfa particles,which can be applied to the simulation of compression with assisted vibration.The mechanical characteristics parameters required for the discrete element simulation of compression process with assisted vibration of alfalfa straw were calibrated based on the test results of the physical properties of alfalfa straw particles.3.Taking moisture content,vibration frequency and particle size as test factors,and the effect of heat and stress transfer in the molding mold as evaluation indexes,the response surface tests in different working conditions were conducted by biomass compression test bench.It was found that the vibration not only could make the heat and stress transfer efficient in the compression process,but also could promote the heat generation by friction between the pellets.The optimal parameters of alfalfa straw compression were obtained as follows:vibration frequency is 15Hz-20Hz,moisture content of alfalfa pellets is 16%-18%,and the particle size is small.4.Combining simulation results and the alfalfa straw compression test results of the alfalfa straw,the mechanism to obtain the better simulation results was:After the alfalfa straw particles were formed,the position with larger porosity in the pellet had stronger fluidity and the heat transfer was more efficient;the position with higher coordination number had sufficient particle contact and stress transfer was more efficient.The optimal parameters for the compression with assisted vibration of alfalfa straw pellets were obtained from the simulation tests:the vibration frequency is about 17Hz,the moisture content of alfalfa pellets is 16%-20%,and the particle size is small.The simulation test results are close to the physical test results,which proved that the simulation test results are reliable.5.Using moisture content,vibration frequency,particle size,retention time,and external heating temperature as test factors,the alfalfa pellet density,relaxation ratio and shatter resistance of alfalfa pellets as evaluation indexes,the response surface tests with different working conditions were conducted on the biomass compression test bench.It was found that improving the heat and stress transfer during the compression could improve the quality of the molded products.In order to improve the production efficiency,the optimal parameters of compression with assisted vibration for alfalfa straw were obtained:moisture content is 16-18%,vibration frequency is about 20Hz,retention time is150 seconds,and external source temperature is 70?.The test results of this working condition was:the alfalfa pellete density was 649.73kg/m~3,and the relaxation ratio was1.152;the molded pellet was dropped freely from a height of 1m,and there was no mass loss of more than 0.1g.The formed products are of good quality and are easy to transport and store.The findings of this study provide a certain reference basis for designing the structure of the new biomass compression molding machine and the selection of working parameters.