余 龙¹, 庞冬冬¹, 陈双琪¹, 佘敏敏¹, 李勇铜¹, 沈文朋², 孟亚慧³

(1. 兰州理工大学 石油化工学院, 甘肃 兰州 730050; 2. 天华化工机械及自动化研究设计院有限公司, 甘肃 兰州 730060;3. 甘肃省特种设备检验检测研究院, 甘肃 兰州 730050)

引用格式:

余龙, 庞冬冬, 陈双琪, 等. 立式粉体干燥器内二元湿颗粒气-固逆流过程的数值模拟[ J]. 中国粉体技术, 2024,30(2): 82-95.

YU L, PANG D D, CHEN S Q, et al. Numerical study on the gas-solid countercurrent process of binary wet particles in a vertical powder dryer[J]. China Powder Science and Technology, 2024, 30(2): 82-95.

摘要: 【目的】为了研究二元湿颗粒在立式粉体干燥器干燥单元气-固逆流过程中的流动特性, 分析不同条件下二元湿颗粒的空间分布情况, 实现二元湿颗粒在气-固逆流过程中的优化设计。 【方法】采用计算流体力学( computational fluid dynamics,CFD)与离散元(discrete element method,DEM)耦合液桥力模块的分析方法,分别探讨颗粒含水质量分数、粒径比和质量比对二元湿颗粒流动特性的影响及液桥力变化规律。 【结果】在气-固逆流过程中,颗粒质量分数在径向和轴向分布上呈现“边壁大中心小”“上大下小”的规律;颗粒速度随着轴向高度的增加而增大,从边壁区域到中心区域的颗粒速度呈现减小趋势;随着颗粒含水质量分数和粒径比的增加,颗粒的运动由接触力和液桥力共同控制逐渐变成液桥力占主导,干燥单元顶部容易堵塞;随着质量比的增加,接触力对颗粒运动的控制进一步加强,在颗粒含水质量分数较小的情况下占主导作用。 【结论】较大的质量比可以缓解干燥单元顶部堵塞,同时要选取适当的颗粒含水质量分数,才能削弱颗粒结块对流动特性研究的干扰。

关键词: 气-固逆流; 含水量; 二元湿颗粒; 液桥力; 数值模拟

Abstract

Objective When flowing into the drying unit of the vertical powder dryer, the binary wet particles tend to agglomerate, resulting in blockage at the top of the drying unit. According to the principle of solid flow, the drying unit operates in a similar mannersimilarly to a gas-solid countercurrent downer. However, limited research studies on binary wet particles in the gas-solid countercurrent dryers. The spatial distribution of binary wet particles is analyzed under various working conditions. The related methods and results are conductive to the process of a vertical powder dryer.

Methods In this paper, firstly, the analysis model was established, coupling liquid bridge force module of computational fluid dynamics (CFD) and discrete element method (DEM). Secondly, the reliability of the above model was verified through experiments. Finally, this study quantitatively described the effects of moisture content, particle size ratios and mass ratios of coarse and fine particles on the flow characteristics of binary wet particles. Furthermore, the study studied mechanism of the changes in the liquid bridge force between coarse and fine particles.

Results and Discussion According to the model established above, it is found by simulation that high moisture content leads to particle agglomeration, as well as the presence of gas vortices and local high-speed areas in the drying unit. When the particle size ratios are greater than or equal to 2, there is a significant difference in particle size between the two, resulting in a high filling rate of fine particles within the coarse particle skeleton, and making it more likely for particle agglomeration to exceed the bed size, leading to blockage at the top of the drying unit and a sharp decrease in the particulate concentration of wet particles within the unit. Under the influence of the upward flow of gas on the surface, the particles that are moving downward gradually shift from the central region to the side wall region, leading to an accumulation of particles near the side wall, causing an increase in the non-uniformity of the radial distribution of the particles. Moreover, with an increase in moisture content and the particle size ratio of coarse and fine particles, both the number and mean residence time of particles also increase, leading to an increase in the liquid bridge force, while the contact force decreases relatively. However, as the mass ratio of coarse and fine particles increases,the number and mean residence time of particles decrease, resulting in a stronger influence of contact force on particle flow characteristics and a relatively weaker influence of liquid bridge force, especially when the moisture content is low. At approximately 0. 8 s, the maximum number of wet particles is 1 249, the maximum collision frequency is 61 270 Hz, and the maximum number of liquid bridges is 1 678.

Conclusion In this paper, the flow characteristics of binary wet particles and the variation of liquid bridge force are reported. The results reveal that the particulate concentration and velocity are relatively higher in the side wall region and decrease towards the center region. The particulate concentration shows higher levels at the top, and gradually decreasing along the downer, while the particle velocity increases with the axial height. Moreover, in a dry particle system, the flow structure is classified as dilute phase flow, and the number of contacts is very small, sometimes even approaching zero. In a wet particle system, when the particle size ratios are 2 or greater, a higher particle size ratio increases the likelihood of blockage at the top of the drying unit. However, increasing the mass ratios can help alleviate the blockage. Thus, this study is beneficial for achieving better heat transfer efficiency in gas-solid countercurrent dryers for binary wet particles, and supplements the deficiency of gas-solid countercurrent systems.

Keywords: countercurrent gas-solid flow; moisture content; binary wet particle; liquid bridge force

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