【作者】 李德付；

【导师】 解茂昭； 尹洪超；

【作者基本信息】 大连理工大学 ， 动力机械及工程， 2013， 博士

【摘要】 回转窑是一种具有燃烧、换热、化学反应以及物料输送等主要功能的生产设备。具有容积大、能耗高、物料长滞留、燃料适应性强等特性。但回转窑与其它热工设备相比热效率低。如何促进物料与空气的混合,强化气固换热,减少热损失是亟待解决的问题。为此,必须对回转窑内物料、煤粉和空气的流动和传热特性开展深入研究,用以指导回转窑技术和结构改进,从而实现提高运转效率和降低能耗的目的。同时,由于问题的复杂性与典型性,该项研究对稠密气固两相流学科领域也具有重要理论意义。本文以一台Φ4.3×64m水泥回转窑为工程背景,对包括燃烧器和篦冷机在内的完整的回转窑系统进行了多维数值模拟研究。性能优良的燃烧器能够保证为水泥回转窑提供最佳的热工条件。本文首先对一台四通道旋流燃烧器内的气固两相流进行了模拟研究,应用欧拉-拉格朗日计算方法,对窑内煤粉-空气两相湍流流场进行模拟,系统地研究了各风道不同送风量及旋流叶片角度对燃烧器出口流场中的速度分布的影响,详尽的分析了不同工况下冷态射流湍流场的流态变化规律。计算结果表明,燃烧器的中心回流区对实现快速点火和稳定燃烧具有重要作用,内旋流风和高速外风使得煤粉和空气充分混合,并卷吸了高温二次风,促进了煤粉燃烧。提高外风风速会促进煤粉颗粒的运动,但风速需要控制在适宜的范围内。其次,基于欧拉-欧拉法,建立了回转窑内物料与气体的两相流动模型,分别对其二维横截面与三维窑体内的流动和传热特性进行数值研究。把颗粒流体的动理学理论引入该项研究,将物料颗粒的随机运动比拟为分子热运动,引入颗粒温度作为描述颗粒随机运动动能的变量,建立颗粒的本构方程,从而封闭固相动量方程。考虑了物料反应、辐射传热及窑壁间接式传热等诸多因素。讨论横截面内物料、壁面和自由空间气体之间的换热机理,应用热量源项在二维模型中考虑轴向温度梯度的影响,分析物料、壁面和自由空间气体三相间的换热机制。通过计算得到了颗粒物料在回转窑内随窑壁回转的复杂运动特性及其空间分布规律,获得了回转窑内温度分布曲线,及不同区域气体、物料和窑壁之间的换热量。计算结果与文献试验值相吻合,说明该计算模型具有较好的预测能力,可以为改进与优化回转窑的工艺操作流程提供指导。最后,针对熟料在篦冷机内的输运与冷却换热过程,应用欧拉-欧拉两相流计算方法及多孔介质模型,进行了二维数值模拟,主要分析了熟料的输运、熟料与气体的换热特性。计算结果表明,颗粒增大会使熟料床的厚度明显增厚,直径为20mm的熟料与空气换热最好,空气温度升高最大。更多还原

【Abstract】 Rotary kiln is a critical equipment for cement production in which complex processes,e.g. material transportation, combustion, thermal exchange and chemical reaction occur simultaneously. The kiln has features like large volume, high energy consumption, long processing time, excellent fuel adaptability and etc. However, the rotary kiln is facing the shortage that its thermal efficiency is much lower than other thermal equipment. Therefore, the optimization of the mixing material and air, enhancing thermal exchange between phases and reducing heat loss is an urgent problem to be solved. To fully understand the mechanism of the multiphase flow and heat transfer in the kiln and, accordingly, improve its operating efficiency and reduce energy consumption, it is necessary to conduct deep investigation on the working processes in the kiln. Moreover, due to the complexity and typicalness of the problem, the research on kiln simulation has important theoretical significance to the discipline of dense gas-particle two-phase flow. In this thesis, taking a Φ4.3×64m cement rotary kiln as the engineering background, the complete kiln system, including a burner and a grate cooler, is numerically investigated based on multi-dimensional models.A burner with excellent performance could offer the best thermal condition for cement production. Hence, a research on a4-channel swirling burner is presented firstly, in which an Eulerian-Lagrangian approach is applied to simulate the turbulent field in the burner. Velocity distribution in the burner outlet area is analyzed at various inlet air flow conditions and various angles of the swirling blade, also, the characteristics of the turbulent jet flow field are studied under different cold conditions. The result shows that the central recirculation zone is important for a quick ignition and stable combustion; both the internal swirling flow and the high speed external flow contribute to an adequate mixing ofair and coal particles, and entrain the hot secondary air flow to enhance combustion. Increasing the high speed external flow can also promote the movement of the coal particles, however, it should be controlled in a reasonable range.Moreover, based on the Eulerian-Eulerian approach, a particle-air two-phase flow model is built for the rotary kiln to investigate the flow and heat transfer characteristics in a two-dimensional cross section and three-dimensional space, respectively. The kinetic theory for "particle fluid" is introduced in which an analogy between the random motion of particles and the thermal motion of molecules is adopted. A concept of granular temperature is used as a random parameter for describing particle kinetic energy, and the constitutive equation for the particle phase is established, which closes the solid momentum equation. A detailed discussion is presented for the heat exchange among the material, the wall and the air flow in free space where a heat source term is applied to calculating the temperature gradient in the axial direction. The mechanism of heat exchange between phases is discussed. Temperature distribution is obtained by numerical calculation to determine the heat quantity exchanged among phases, and between the material and the kiln wall. The result shows satisfactory agreement with experimental data, indicating that the model has a good prediction ability, and can provide useful guidance for the improvement and optimization of the operating process in the rotary kiln.Finally, focusing on the processes of the material transportation and heat exchange in the grate cooler and employing the Eulerian-Eulerian two-phase method and a porous medium model,2-D numerical simulation was carried out, to analyze heat exchange characteristics between various medium in the cooler. The calculation results show that the thickness of material bed increases significant with particle size increasing. Specifically, when the particle diameter reaches20mm, the best heat exchange efficiency and the highest temperature raise can be obtained.更多还原