Study on Thermodynamics and Heat Transfer Characteristics of Roller Mill Roller

李聪

武文斌;张文浩

硕士

河南工业大学

磨粉机;;磨辊;;能量;;摩擦生成热;;有限元分析;;温度;;热力学

磨辊的铸造均匀性和辊体等温性是影响磨粉机磨辊性能的重要因素,磨辊辊面的温度能够较大的影响面粉的品质及口感,所以越来越受到行业的关注。磨辊的吸热段主要以热传导为主,遵从傅里叶导热定律。散热段以有限空间的对流换热和辐射换热为主。启动性能的好坏主要取决于磨粉机磨辊的装配质量,而等温性能主要取决于工作过程中辊面的温度均匀性。国内外众多学者也对辊体温度场做过大量的研究,但都只限于轧钢辊,对辊式磨粉机磨辊研究甚少。针对以上问题,本文以布勒MDDK1000/250型磨粉机中直径250mm、长1000mm磨辊为研究对象,基于热力学和传热学基础理论对磨辊的传热特性进行了分析研究,结合ANSYS Workbench有限元分析软件对磨辊温度场进行了分析,最后通过相应的实验对理论和仿真加以验证。明确了辊式磨粉机磨辊温升的影响要素、机理及传热规律,对降低温升预测辊体温度分布提高制粉质量提供了重要的理论依据。本文主要研究及结论如下:1.基于热力学基本原理研究了物料在研磨过程中1B工艺快辊辊体温度的生成及影响要素;研究了1B工艺轧区中小麦对辊面的滑动摩擦力,得出小麦对快慢辊辊面的摩擦生成热功率的比例等于快慢辊辊面与物料之间的滑动摩擦系数之比,这一结论同样适用于1M工艺中快慢辊辊面的摩擦生成热功率的分配。基于理论力学和材料力学的基本理论研究了1B工艺中快慢辊辊间压力及其影响要素,得到当其他量不变时,快慢辊辊间压力随磨辊转速的增大而增大,随喂料流量的增大而增大,随轧距的增大而减小,随快慢辊速比的增大而减小的结论。2.基于热力学基本原理研究并明确了1M工艺中快慢辊辊面的摩擦生成热功率与电动机输出功率之间的量化关系。基于传热学基础理论对磨粉机正常工作中1M快辊的导热热阻、磨辊辊面与外围空气的强迫对流换热系数、磨辊左轴头处齿轮与外围空气的自然对流换热系数、磨辊右轴头处带轮与外围空气的自然对流换热系数及辊面的对外辐射功率进行了分析和计算。得到快辊的导热热阻表达式、磨辊辊面与外围空气的强迫对流换热系数为29.7687W/m ~2℃、磨辊左轴头处齿轮与外围空气的自然对流换热系数为4.31W/m ~2℃、磨辊右轴头处带轮与外围空气的自然对流换热系数为3.61W/m ~2℃、快辊辊面的对外辐射功率为294.04W。3.对磨辊建立了传热数学模型和三维建模,并对磨辊温度场进行了有限元分析与求解,得到磨辊在开机正常工作后370.58min达到稳态,达到稳态时最大温度为78.353℃。如果改变辊体内空腔直径,模拟仿真辊体内空腔直径对辊体温度场的影响,得到辊体内空腔直径改变时,辊体温度场变化较小,可知若想降低辊体温度,增大辊体内空腔直径的途径较不可行。4.在河南省鹤壁市益民面粉厂、江苏省泰兴市曲霞面粉厂、江苏省宇宸面粉厂等对布勒MDDK 1000/250、MDDP 1000/250型磨粉机1M快辊进行了辊面测温,转速及磨粉机电流测量,得到实验数据并绘制了辊面温度、快辊转速及电流与时间的变化曲线、得到快辊辊面稳态温度为75℃。对辊面温度、快辊转速及电流与时间的变化曲线进行了理论分析,明确了快辊辊面温差与磨粉机电流的量化关系并给出了表达式。5.将磨辊温度测量结果与仿真结果比较,得出辊面温度绝对误差为3.353℃,相对误差为4.47%,实验结果与仿真结果基本一致。

The starting performance and isothermal performance of the mill are an important indicator to measure the performance of the grinding roller.The temperature of the roller surface can greatly affect the quality and taste of the flour,so it has attracted more and more attention from the industry.The heat absorption section of the grinding roller is mainly based on heat conduction and obeys the Fourier heat conduction law.The heat dissipation section is mainly composed of convective heat transfer and radiative heat transfer in a limited space.The performance of the start-up performance mainly depends on the assembly quality of the grinding roller of the mill,and the isothermal performance mainly depends on the temperature uniformity of the roll surface during the working process.Many scholars at home and abroad have also done a lot of research on the temperature field of the roller body,but they are all limited to the rolling roller.There is little research on the roller of the roller mill.In view of the above problems,this paper takes the Buhler MDDK1000/250 mill with a diameter of 250mm and a length of 1000mm as the research object.Based on thermodynamics and heat transfer,the heat transfer characteristics of the grinding roller are analyzed and combined with the corresponding experiments.The temperature field of the grinding roller was analyzed in combination with ANSYS Workbench finite element analysis software.The influence mechanism and heat transfer law of the temperature rise of the roller mill are clarified,which provides an important theoretical basis for reducing the temperature rise and predicting the temperature distribution of the roller to improve the quality of the milling.The main research and conclusions of this paper are as follows:1.Based on the basic principles of thermodynamics,the formation and influence factors of the 1B process fast roll body temperature during the grinding process were studied.The sliding friction of the wheat on the roll surface in the 1B process rolling zone was studied,and the friction of the fast and slow roll surface was obtained.The ratio of the generated thermal power is equal to the ratio of the sliding friction coefficient between the fast and slow roller surface and the material.This conclusion is also applicable to the distribution of the frictional heat generated by the friction roller surface in the 1M process.Based on the basic theory of theoretical mechanics and material mechanics,the pressure between the fast and slow rolls in the 1B process and its influencing factors are studied.When the other quantities are constant,the pressure between the fast and slow rolls increases with the increase of the rotating speed of the grinding rolls.The increase of the material flow rate increases,decreases with the increase of the rolling distance,and decreases with the increase of the fast and slow roll speed ratio..2.Based on the basic principles of thermodynamics,the quantitative relationship between the friction generating heat power of the fast and slow roller surface and the motor output power in the 1M process is studied and clarified.Based on the basic theory of heat transfer,the thermal conductivity of 1M fast roller in the normal operation of the mill,the forced convection heat transfer coefficient of the roller surface and the peripheral air,the natural convective heat transfer between the gear and the peripheral air at the left axis of the grinding roller The coefficient,the natural convective heat transfer coefficient of the pulley and the peripheral air at the right axis of the grinding roller and the external radiated power of the roller surface were analyzed and calculated.The heat transfer resistance expression of the fast roll,the forced convection heat transfer coefficient of the roll surface and the peripheral air is29.7687W/m ~2℃,the natural convective heat transfer coefficient of the gear and the peripheral air at the left axis of the grinding roll is4.31W/m ~2℃,and the right axis of the grinding roll are obtained is3.61W/m ~2℃.The natural convective heat transfer coefficient of the wheel and the peripheral air is 294.04W.3.The mathematical model and three-dimensional modeling of heat transfer were established for the grinding roller,and the finite element analysis and solution were carried out on the temperature field of the grinding roller.The steady state reached the steady state at370.58min after the normal operation of the grinding roller.It is 78.353°C.If the diameter of the cavity in the roller is changed,the influence of the cavity diameter on the temperature field of the roller body is simulated.When the diameter of the cavity in the roller is changed,the temperature field of the roller body changes little.It is known that if the temperature of the roller body is to be reduced,the temperature is increased.The path of the cavity diameter in the roll is less feasible.4.In the Henan Hebi City Yimin Flour Mill,Jiangsu Taixing Quxia Flour Mill,Jiangsu Yuyu Flour Mill,etc.Roller MDDK 1000/250,MDDP 1000/250 mill 1M fast roll was rolled The surface temperature measurement,the rotation speed and the current measurement of the mill were obtained.The experimental data were obtained and the curve of the roll surface temperature,the fast roll speed and the current and time were plotted,and the steady temperature of the fast roll surface was 75°C.The variation curves of roll surface temperature,fast roll speed and current and time were theoretically analyzed,and the quantitative relationship between the roll surface temperature difference and the mill current was clarified and the expression was given.5.Comparing the rolling roller temperature measurement results with the simulation results,the absolute error of the roller surface temperature is 3.353°C,and the relative error is4.47%.The experimental results are basically consistent with the simulation results.