颗粒物料在半封闭式回转鼓内的混合特性

采用DEM离散单元法,对不同转速与倾角下半封闭式回转鼓内颗粒物料的混合过程进行了模拟。通过“颗粒接触数”定义的分离指数S,分析了不同转速和倾角对回转鼓内颗粒物料径向与轴向混合特性的影响。结果表明：转速与倾角对回转鼓内颗粒物料径向与轴向混合特性有显著的影响;倾角不变,转速分别为15r/min、30r/min、45r/min时,颗粒物料的径向与轴向混合速度随转速的增加而增加,当转速超过30r/min后,增加转速对径向与轴向混合速度的影响越来越小;转速不变,倾角分别为0°、17°、34°时,增大倾角能有效的增加轴向混合速度,但对径向混合速度没有促进作用,当倾角超过17°后,轴向混合速度的增幅随着倾角的增加而逐渐变小,而径向混合速度随着倾角的增大而减小,但增加转速可以减小径向混合速度下降幅度。     

Gas-solid mass transfer in a rotating drum

Gas-solid volumetric mass transfer coefficients (k_SA) were measured in a rotary drum by evaporation of n-decane into dry air from the surface of porous solids. The effect of drum rotational speed, N (0.09 to 2.0 min^?1), solids volume fraction, η (0.043 to 0.25) and the presence of baffles on k_SA were investigated. In the presence of baffles, k_SA was independent of ηand higher than in the case of a rolling bed where no baffles were present. For the rolling bed case, k_SA increased with increasing η. Mass transfer in the rolling bed was modeled based on the particle motion.     

回转干馏炉内颗粒间传热特性的数值模拟

将离散元方法与颗粒热传导模型相结合,研究了页岩灰颗粒与油页岩颗粒在回转干馏炉内的混合传热过程,采用混合指数、颗粒平均温度和温度标准偏差作为评价混合传热效果的指标,分析了填充率、炉体转速、油页岩粒径及抄板形式对颗粒间混合传热特性的影响规律。结果表明,炉体转速和油页岩粒径是影响颗粒混合传热效果的主要因素,而填充率和抄板形式对混合传热效果的影响相对较小。当炉内未设抄板时,随着填充率和油页岩粒径的增大,颗粒间分层现象使混合传热效果变差,而随炉体转速的提高传热效果呈现出先增强后减弱的趋势;设抄板时,抄板形式对炉内颗粒间的混合起到不同程度的扰动作用,从而使传热效果得到显著改善。     

Solid‐liquid mass transfer in a rotary drum

The solid‐liquid mass transfer coefficient for slurries agitated in rotating drums with baffles was measured using the dissolution of β‐naphthol as a tracer in silica sand slurries. The effects of drum rotation rate (0.33 to 15 min^?1), slurry holdup (4.4% to 17%) and solids volume fraction (0 to 0.62) on the solid‐liquid mass transfer were investigated with a slurry particle diameter of 4.6 × 10^?4 m. Mass transfer coefficients were corrected for the effect of particle shrinkage due to dissolution of the tracer. The mass transfer coefficient increased with increasing rotational speed of the drum from 0.005 to 0.045 mm.^5‐1. Two types of slurry motion were observed; well mixed slurry flowing over the baffles and segregation of solids on the baffles. The mass transfer coefficients from the present study, along with literature data, correlated with the Froude number to the 0.36 power, for Froude numbers from 10^?6 to 10^?1.     

基于离散单元法的丝状颗粒传热数学模型

丝状颗粒作为一类长径比较大的非球形颗粒,其传热特性及相关技术广泛应用于工农业生产的诸多领域。但目前颗粒在运动过程中传热问题的研究还很不充分,特别是对于丝状颗粒,更是缺乏有效的数学模型进行描述。从颗粒传热机理出发,提出了一种基于离散单元法的丝状颗粒传热模型,模型中综合考虑了颗粒碰撞（接触）传热、颗粒的内部导热以及颗粒与气体间的对流换热。利用该模型,对固定床中堆积丝状颗粒的热量迁移过程进行了数值模拟,着重比较了各种传热方式对传热过程的影响。研究表明,对流换热对整体传热量的贡献较大。此外,还获得了不同工况下颗粒温度随时间的变化规律。     

Experimentally validated computations of heat transfer in granular materials in rotary calciners

Heat transport through flowing particulate materials is an essential component of modern technologies such as heterogeneous catalytic reactors, high performance cryogenic insulation, construction material, and powder metallurgy. In catalyst manufacturing, heat transfer through granular media occurs in the drying and calcination stages. In this paper, we describe the use of experiments and discrete element methods to examine flow, mixing, and mass and heat transport in rotary calciners. Alumina powder (200 μm) and cylindrical silica pellets (2 mm diameter and 3 mm long), which are common support materials for catalysts, are used in our experiments. A parametric study was conducted by varying the material properties of granular material, and rotational speed of the calciner. We use the discrete element model to simulate flow, mixing, and heat transport in granular flow systems in rotary calciners. Simulations and experiments show that the rotation speed has minimal impact on heat transfer. As expected, the material with higher thermal conductivity (alumina) warms up faster in experiments and simulations. Various baffle configurations (rectangular and L-shaped flights) in the calciner and their effect on the flow and heat transfer of granular material are simulated. The average wall-particles heat transfer coefficient of the granular system is also estimated from the experimental findings.     

柱状生物质颗粒与钢球颗粒在滚筒中的混合特性

采用离散单元法DEM（discrete element method）对圆柱形生物质颗粒和钢球颗粒在滚筒中的混合进行了数值模拟,分析了滚筒转速和颗粒数量比对混合质量的影响。结果表明：在本文设定的工况下,颗粒的混合模式为阶梯模式,并且颗粒在混合时可以分成3个区域,即左面的单层钢球颗粒区、中间的钢球颗粒和生物质颗粒混合区、右面的生物质颗粒堆积区。左右两边的颗粒混合效果较差,中间的颗粒混合效果较好。当滚筒转速相同时,钢球颗粒和生物质颗粒数量比为3000∶200时的颗粒混合效果比钢球颗粒和生物质颗粒数量比为3000∶100时的好,即当钢球颗粒数量远大于生物质颗粒数量时,增加生物质颗粒的数量可以提高混合效果。在钢球颗粒和生物质颗粒数量比相同的情况下,当滚筒转速在5~25r/min的范围内,滚筒转速越高,颗粒的混合质量越好,并且颗粒混合达到稳定的时间就越短。     

Flow of slightly settling slurries in a horizontal rotary drum

An experimental study was conducted to examine the flow of slightly settling slurries in a horizontal rotary drum. The effects of the slurry flow rate, feed solids concentration, particle settling velocity and drum rotational speed on the hold-up solids concentration were investigated. At low drum speeds, the hold-up solids concentration was much higher than that of the feed. However, at higher drum speeds, the hold-up solids concentration approached that in the feed. At a given drum speed, the hold-up solids concentration was found to vary linearly with the feed solids concentration. A semi-empirical correlation was developed to predict the hold-up solids concentration using the drum Froude number, the single particle drag coefficient and a dimensionless slurry feed rate.     

Simulation of the granular flow of cylindrical particles in the rotating drum

This study aims at unveiling the effect of particle shape on granular flow behavior. Discrete element method is used to simulate cylindrical particles with different aspect ratios in the rotating drum operating in the rolling regime. The results demonstrate that the cylindrical particles exhibit similar general flow patterns as the spherical particles. As the aspect ratio of the cylindrical particles increases, the active‐passive interfaces become steeper, and the number fraction, solid residence time, and collision force in the active region decreases. The mechanism underlying the difference is the preferential orientation, with particles of greater aspect ratios increasingly orientating their longitudinal axes perpendicular to the drum length. Also, particle alignment in the active region is more uniform than that in the passive region. The results obtained in this work provide new insights regarding the impact of particle shape on granular flow in the rotating drum. © 2018 American Institute of Chemical Engineers AIChE J, 64: 3835–3848, 2018     

移动床中颗粒接触传热的数学模型

密相颗粒流动过程中的传热行为研究还不充分.在综合考虑颗粒接触传热的多个过程的基础上提出了颗粒接触传热模型,并与DEM模型结合模拟了移动床中颗粒与加热面间的传热过程.比较各传热过程的关系,可知通过接触面传递的热量对整体有效传热量的影响最大;有效传热系数主要受与加热面接触的颗粒数目、接触时间、颗粒 加热面间接触面的大小等因素的影响;当颗粒的平均粒径相同时,不同的颗粒粒径分布对颗粒系统的传热影响不是很大;当颗粒的粒径增大,有效传热系数降低;颗粒与加热面间的传热量在空间上的分布是不均匀的.     

间接加热式列管回转干燥机传热系数的测试方法

在对列管回转干燥机内传热过程进行深入分析的基础上,设计了简便、可靠的实验系统,并运用非稳态传热理论导出列管壁面与物料颗粒间的传热系数与被测量参数的关系式,从而建立了一套用于间接加热式列管回转干燥机的加热管壁面与物料颗粒间传热系数的测试方法。实验装置的传热系数测量结果显示,列管壁面与颗粒间的瞬时传热系数随筒体转动呈现周期性的变化规律,随着列管位置的升高,传热系数呈逐渐降低的趋势。通过对测试结果的误差分析表明,对传热系数测量精度影响最大的因素是紫铜管表面积,其余依次为壁面升温速率、加热电流、列管壁面温度和物料温度的测量。对常见的6种运行工况的传热系数测试结果进行了可靠性分析,结果表明,所产生的最大相对误差小于3.5%,壁面升温速率的测量误差是总误差的主要构成部分。     

一种新型受热面传热和流动特性的数值模拟及实验研究

针对余热利用过程中低温热源的含尘量高、不连续及不稳定等特点, 提出了一种新型菱形受热面结构。在传热过程中, 该受热面表现出管束叉排布置的特征, 传热过能力较强, 流动阻力较大, 壳侧对流换热表面传热系数较高。在实施吹灰过程中, 该受热面呈现出管束顺排布置的特征, 易清洗, 吹灰效率高。采用数值模拟和实验方法研究了新型受热面结构的传热和流动特性, 给出了壳侧的Nusselt数和摩擦因子随Reynolds数的变化规律。实验结果和数值分析均表明, 该受热面能够适应现有余热利用过程的基本要求, 在便于清灰和除垢的同时实现高效传热。     

A comprehensive heat transfer model for rotary desorbers

A model that describes the heat transfer processes within a rotary desorber is presented. This model has several advantages over previous models. The method for calculating the wall-to-bed heat transfer rate includes the effects of moisture in the feed solids and allows for evaporation from the bed before the bulk bed temperature reaches the saturation temperature of the moisture. The method for calculating radiative heat transfer in the desorber accounts for the real nature of the participating gas and the amounts of CO₂ and H₂O in the gas phase from both combustion and evaporation. Results from experiments performed using a pilot-scale desorber are discussed, and the model is validated by comparing the predicted bed temperature profiles and evaporation rates to those found experimentally. A preliminary attempt to account for the effects of a non-uniform bed on the evaporation rate from the bed is also presented along with several parametric studies.     

Mathematical model of heat transfer during solid‐state fermentation in well‐mixed rotating drum bioreactors

We present a mathematical model that describes heat and mass transfer during solid‐state fermentation (SSF) of Aspergillus oryzae in a well‐mixed rotating drum bioreactor (RDB). In addition to the substrate bed and the headspace, the model recognises the bioreactor wall as a subsystem, allowing it to identify the role of this subsystem in heat removal from the bed. Model predictions agree well with previously published experimental data obtained in a rotating drum bioreactor of 0.19 m diameter and 0.85 m length, with maximum temperatures up to 15 °C greater than the incubation temperature being reached. The model offers insights into the rate limiting steps in heat removal, and how SSF performance might be improved in the experimental system. Copyright © 2003 Society of Chemical Industry     

一种回转窑余热回收用集热器的实验研究及其结构优化

减少回转窑表面的散热损失对冶金、化工等高耗能行业的节能减排具有重要意义。针对现有窑表面集热器回收效率较低的现状,提出了一种新型集热器,并通过实验测量与数值计算分析了其换热性能。建立了集热器换热量与管长、管径及管子数等结构参数的数学关系,分别以换热面积、进出口压降以及改进的熵产数为优化目标,利用遗传算法分别对其进行了结构设计,结果表明,优化后集热器的辐射换热所需的换热面积减小15%,对流换热所需的换热面积减少20%左右。优化后两种形式的换热面所消耗的泵功明显减少。优化后集热器的传热熵产数与优化前无明显变化,而流动熵产数得到明显降低。     

An investigation of heat transfer from a vertical tube in a spouted bed

An experimental investigation on heat transfer from a vertical tube in a gas—solid spouted bed has been conducted. Values of heat transfer coefficients (h_s) have been found to vary between 95 and 230 W/m²°C. Maximum values of h_s are always at the spout axis and decrease monotonically towards the column wall. The drop is most important at the spout-annulus boundary. Most of the effects from different parameters can be explained by the flow patterns of the gas, although particle convection still contributes to some extent to the heat transfer rate. Equations correlating h_s in the spout and the annulus have been developed.     

强制对流传热的换热表面结垢特性实验研究

An experimental study was conducted to investigate the fouling process of calcium carbonate on the heat transfer surface, during forced convective heat transfer. The dynamic monitoring apparatus of fouling resistance was set up for the present experiments. The fouling behavio（s were examined under different factors including fluid velocity, hardness,alkalinity, solution temperature, and wall temperature. Asymptotic fouling curves varying with time were obtained. The fouling rate and asymptotic fouling resistance increased and the induction periods were shortened with the fluid velocity decreasing, hardness andalkalinity increasing, and solution temperature and heat transfer surface temperature increasing. Thecomponents of fouling that formed on the heat transfer surface included crystallization fouling and particulate fouling. The thermal performance parameter of fouling,ρfhf, varied from 380 to 2600 kg·W·（m^4·K）^-1, increasing with growing velocity and decreasing solution temperature, hardness or alkalinity. Furthermore, the thermal conductivity of fouling, λf, varied from 1.7 to 2.2 W·（m·K）^-1 .