颗粒在脉动气流场中的运动与分离规律

发布时间：2018-03-21 20:43

本文选题：脉动气流分选　切入点：分选机理　出处：《中国矿业大学》2015年硕士论文　论文类型：学位论文

【摘要】：脉动气流分选技术作为一种新型气流分选技术,利用颗粒在非稳定气流中的运动规律,弥补了传统气流分选按密度分选的不足,目前已在电子废弃物和工业废催化剂回收利用,非金属矿分选等领域得到了应用,为矿物分选、二次资源回收利用提供了新的思路和方法。但由于非稳定气流中,气流和颗粒运动复杂,对脉动气流的机理研究尚有不足,限制了脉动气流分选技术的进一步发展。论文分析了已有脉动气流分选机理的不足。认为脉动气流利用颗粒到达沉降末速前的加速度差进行分选,缺乏理论和实验支持,并与实际不符。而以单颗粒动力学方程为基础的虚拟质量力理论,无法解释高密度颗粒按密度分选的行为,并且对阻力系数的修正需要进一步研究。为了完善分选机理,本文提出了基于颗粒群的脉动气流分选理论,认为分选柱内的浓相区域是实现颗粒按密度分选的主要区域,对脉动气流波形的作用原理也进行了讨论。论文通过大涡模拟模拟了颗粒在脉动气流场中的绕流问题,通过脉动气流与稳定气流下阻力系数的比较,发现脉动气流下阻力系数的变化与雷诺数区间和气流加减速有关。同时对球体绕流、涡旋结构的变化情况进行了研究,分析了阻力系数变化的原因。研究结果表明,阻力系数变化不是颗粒按密度分选的主要原因,减少颗粒处于低雷诺数段时间,控制气流加减速段比例有助于提高分选效果。为了进一步验证基于颗粒群的分选理论,论文以脉动气固流化床为载体,研究了颗粒在浓相环境下的运动规律。通过研究脉动气流操作参数对床层和气泡特性的影响,发现了气流脉动频率对床层稳定性影响较大,但床层膨胀率变化绝对值不大;气流脉动频率的增加有助于气泡生成频率和尺寸增加。通过观察外加颗粒的运动规律,发现气泡对于颗粒下落起加速作用,虚拟质量力能够促进颗粒按密度分离,并随气流脉动频率和颗粒体积浓度增加而变得显著。论文最后对-6+3 mm的难选细粒煤进行了实际分选实验,Ep值达到0.17,最佳分选效率为48.08%,可燃体回收率达到68.22%。利用前述分选机理解释了脉动气流振幅、脉动频率、入料量对分选效果的影响,提出了临界脉动频率和临界颗粒体积浓度的概念。本文结合数值模拟和实验技术对颗粒在脉动气流场中的运动和分离规律进行了研究,完善了脉动气流分选理论,为实际分选过程中,操作参数的控制,设备结构和分选工艺的设计提供了理论基础,对于其它涉及脉动气固两相流的研究,具有一定的参考价值。
[Abstract]:As a new type of gas flow separation technology, pulsating flow separation technology makes use of the movement law of particles in unstable airflow, which makes up for the shortage of traditional airflow separation according to density. At present, it has been recovered and used in electronic wastes and industrial waste catalysts. The application of non-metallic ore separation and other fields has provided a new way of thinking and method for mineral separation and secondary resource recovery. However, due to the complex movement of air and particles in unstable airflow, the study on mechanism of pulsating flow is still insufficient. The further development of pulsating flow separation technology is limited. The paper analyzes the shortcomings of the existing mechanism of pulsating airflow sorting. It is considered that pulsating airflow is separated by the acceleration difference before the particle reaches the final velocity of settlement, which lacks theoretical and experimental support. The virtual mass force theory based on single particle dynamics equation can not explain the separation behavior of high density particles according to density, and the modification of resistance coefficient needs further study. In this paper, the theory of pulsating airflow separation based on particle group is put forward. It is considered that the dense phase region in the column is the main region to realize the separation of particles according to the density. The working principle of pulsating flow waveform is also discussed. The particle flow around the pulsating flow field is simulated by large eddy simulation, and the drag coefficient under pulsating and steady airflow is compared. It is found that the variation of drag coefficient under pulsating flow is related to the Reynolds number range and the acceleration and deceleration of airflow. At the same time, the variation of the vortex structure and the flow around the sphere are studied, and the reasons for the variation of the drag coefficient are analyzed. The change of drag coefficient is not the main reason for particle separation by density, and the reduction of particle size at low Reynolds number, and the control of the proportion of airflow acceleration and deceleration are helpful to improve the separation effect. In order to further verify the separation theory based on particle group, In this paper, the moving law of particles in dense phase environment is studied with pulsating gas-solid fluidized bed as carrier. By studying the influence of operating parameters of pulsating airflow on bed and bubble characteristics, it is found that the frequency of pulsating gas flow has a great influence on the stability of bed. However, the absolute value of the bed expansion rate is small, and the increase of the gas flow pulsation frequency contributes to the increase of the bubble formation frequency and the size. By observing the motion law of the added particles, it is found that the bubble accelerates the falling of the particles. Virtual mass forces facilitate the separation of particles by density, At the end of the paper, the actual separation experiment of 6 ~ 3 mm fine coal was carried out, and the optimum separation efficiency was 48.08, and the recovery rate of combustible body reached 68.22. Finally, the actual separation experiment was carried out for the refractory fine coal with 6 ~ (-3) mm. The optimum separation efficiency was 48.08 and the recovery rate of combustible body was 68.22. The aforementioned sorting mechanism explains the amplitude of the pulsating airflow, The effects of pulsating frequency and feed quantity on the separation efficiency are discussed. The concepts of critical pulsation frequency and critical particle volume concentration are proposed. The motion and separation of particles in pulsating airflow field are studied by numerical simulation and experimental techniques. The theory of pulsating airflow separation is perfected, which provides a theoretical basis for the control of operational parameters, the design of equipment structure and separation process in the actual separation process. It is of certain reference value for the study of pulsating gas-solid two-phase flow.
【学位授予单位】：中国矿业大学
【学位级别】：硕士
【学位授予年份】：2015
【分类号】：TD94

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