木屑压缩成型颗粒圆周表面形貌分形参数研究

Study on fractal parameters of circular surface morphology of sawdust compression molding particles

精确地表达生物质成型颗粒圆周表面粗糙形貌是生物质成型机关键部件摩擦磨损机理分析及磨损预测研究的关键。选择原料为混合木屑的生物质成型颗粒,在直径6 mm、颗粒度1~3 mm、含水率11%、密度1.2 g/cm^(3)的条件下,测出其圆周表面形貌及轮廓数据,利用成型颗粒圆周表面粗糙形貌所具有的统计自相似性和标度不变特性,采用盒子计数法、结构函数法、变差法3种方法分别计算出成型颗粒圆周表面的分形维数D和分形特征参数G,再根据W-M分形函数,建立生物质成型颗粒圆周表面粗糙形貌的分形函数模型,进行了数值模拟。结果表明:木屑压缩成型颗粒圆周表面粗糙度均值为1.45μm时,D的均值约为1.6,G值约为2.24×10^(-5)m。分形模型对木屑压缩成型颗粒圆周表面的粗糙形貌模拟准确合理,所测得的分形参数准确;在测定分形维数D的过程中,盒子计数法的计算精度相对更高。本研究为重构准确可信的生物质成型颗粒圆周表面粗糙形貌数字化模型,实现成型颗粒圆周表面与成型模具接触状态仿真分析奠定基础。

The rough surface morphology of biomass formed particle is the basis for the accurate analysis of the interaction between the biomass formed particles’ surfaces and the forming mold.Thus,an accurate description of the rough morphology of the biomass formed particle circular surface is the key basis for analyzing the friction and wear mechanism,as well as the wear prediction of the key components of biomass briquetting machine.Since fractal theory has good applicability to describe natural phenomena with scaling rate characteristics,while the rough morphology of the biomass formed particle circular surface has the characteristics of statistical self-similarity and scale-invariant.Thus,the fractal theory can be used to describe the surface morphology of the biomass formed particle circular surface.In this study,the biomass molded particles of blended sawdust with 6 mm diameter,1-3 mm granularity,11% moisture content and 1.2 g/cm^(3) density were selected to extract profile data from outer surface of molding particle with a surface roughness tester.Three different methods,the Box-Counting Method,Structure Function Method and Variation Method were also used to calculate the fractal parameters of outer surface of the molded particle:fractal dimension D and fractal feature G.Furthermore,a fractal function model of biomass molded particle outer surface roughness was developed according to the W-M function followed by numerical simulation.It was revealed that,when the average roughness of outer surfaces of the biomass molded particles was 1.45 μm,the average fractal dimension D was about 1.6 and the average G was approximately 2.24×10^(-5)m.The fractal model has a good simulation of the rough surface morphology of biomass molded particle so that the fractal parameters of D and G were corrected and the overall fractal model could well explained and was universally applicable to the rough surface morphology of biomass molded particle.The Box-Counting Method had a higher calculation accuracy and hence was the most reliable one with regard of surface roughness of biomass molded particle among the three methods.More importantly,this study provided a foundation for reconstructing the accurate and credible digital model of the rough morphology of the biomass formed particle circular surface,and achieving the simulation of the contact state between the circular surface of the biomass formed particle and the forming mold.The theory basis could be used to predict and analyze the mechanical properties,friction and wear mechanism of biomass molding process.