燃烧油滴的曳力和尾涡造成的湍流变动的数值研究（英文）

燃烧油滴/颗粒的曳力和尾涡造成的湍流变动反映了燃烧油滴/颗粒和气体湍流的相互作用.关于燃烧对颗粒曳力的影响,文献中曾报导,燃烧的煤粒增大了曳力,但是燃烧的焦炭粒减小了曳力.至于燃烧油滴的曳力,文献中报导了增大或减小曳力的相反结果.在液雾燃烧数值模拟中往往采用无燃烧的固体颗粒的曳力规律.关于颗粒尾涡导致的湍流变动,众所周知,大颗粒的尾涡效应会增强湍流.但是燃烧油滴的尾涡造成的湍流变动至今不清楚.本文给出了气体绕过燃烧的单油滴和油滴群的数值模拟研究.结果指出,单个燃烧油滴的曳力比等温流动中固体颗粒的曳力小得多,而且燃烧油滴群的曳力比单个燃烧油滴的更小.与固体颗粒尾涡增强湍流效应相反,本文发现燃烧的单个油滴的尾涡效应是降低湍流.同时,随着气体相对速度的增大,燃烧油滴群的尾涡效应一开始是降低湍流,但是后来却是增强湍流的.     

固宝床层中四种不同粒径柱状活性炭流动阻力特性

采用活性炭脱硫制酸,床层阻力是活性炭床层在工程技术设计中一个重要的特征数据,通过测量4种不同粒径活性炭的阻力特性,为工程设计提供了依据.实验表明在层流区时,平均阻力系数随Re数的增大而减小,层流向紊流过渡区时,平均阻力系数随Re数的增大而增大.入口效应仅在低Re数、床层总阻力较小时对床层平均阻力系数影响较大.在层流区时小颗粒直径的活性炭(1mm)床层平均阻力系数随床层高度增大而增大,大颗粒直径活性炭(3mm,6mm,10mm)床层平均阻力系数随床层高度的增大而减小.过渡区中大颗粒直径活性炭的平均阻力系数随床层高度增大而增大.     

单油滴蒸发燃烧周围流场及火焰结构的数值模拟研究

采用一步总包有限反应速率模型模拟了不同相对速度、颗粒尺寸、环境温度条件下单个油滴燃烧。对于全包围燃烧模态,火焰半径的模拟结果与周力行的理论解析解进行对比发现,周的理论解在周向角小于90°范围内和数值模拟结果接近,大于90°,理论解不能精确预测火焰锋面半径;油滴表面局部蒸发率较尾部火焰燃烧模态下的蒸发率小;燃烧状态下油滴颗粒的阻力系数要比冷态条件下的阻力系数小,不能简单套用Wallis-Kliachko公式计算。     

燃煤电站爆米花灰空气动力学参数的实验测定与计算

燃煤电站爆米花灰颗粒可能严重威胁SCR(选择性催化还原)系统催化剂的性能,建立CFD(计算流体力学)模型,优化爆米花灰分离捕集方案是预防爆米花灰危害催化剂的必要手段。为模拟爆米花灰颗粒的流动特性,实验测定、计算了不同粒度大小共90个颗粒的尺寸、密度、阻力系数和球度等空气动力学参数,并拟合得到了阻力系数比与球度的β-Ψ近似关系曲线。实验和计算结果表明,爆米花灰颗粒表观密度随粒度的增大而增大,且与颗粒内部孔隙率大小关系密切,平均表观密度约1 182 kg/m~3。颗粒平均阻力系数0.765,平均球度0.66;随着颗粒粒度的减小,颗粒的球化程度逐渐增大,平均阻力系数有所减小。     

高温鼓泡流化床的流化行为

床层温度在20－1000℃范围内，以4种粒径的煤灰为实验物料，研究了不同表观气速下最小流化速度，床层平均空隙率，压力波动标准偏差和主频的变化规律，最小流化速度随床层温度升高而减小；相同床温下，平均空隙率随表观气速升高而增大，不同床温下，压力波动偏差随流化数增加而增大。相同流化数时，B类颗粒的压力波动标准偏差受床层温度变化影响较小，而D类粒子随床层温度升高压力波动标准偏差减小，胡着流化数增加，压力波动主频减小。     

一次风速度对煤颗粒群着火特性影响的实验研究

利用Hencken型平面携带流反应器,研究了一次风速度对两种高挥发分褐煤和一种贫煤射流着火特性的影响.实验结果表明,低雷诺数(Re=878)条件下,贫煤煤粉气流先后发生外层单颗粒着火燃烧和内层颗粒群着火燃烧,煤粉颗粒着火和燃烧轨迹十分规则.但在高雷诺数(Re=4,392)条件下,贫煤煤粉气流更难形成群燃火焰,呈现出暗红色火焰.随着一次风速度的增加,尽管煤粉颗粒的停留时间减小,但湍流强度的增加使颗粒加热速率以及挥发分析出的强化作用占主导,使得煤粉气流的着火距离减小.此外,群燃火焰在挥发分聚集到一定程度后产生,是颗粒群燃烧的特有现象,而煤种挥发分含量的增加和有效聚集有利于群燃火焰的出现.     

用激光衍射法研究柴油机高压喷雾(Ⅱ)——环境温度、压力和反弹对粒度分布的影响

本文用基于激光衍射原理的Malvern2604C 型粒子分析仪,研究了定容弹中PT喷油器的高压喷雾粒度分布特性受环境温度、压力的变化以及碰壁反弹的影响。结果表明:在800K 环境温度下,在距喷孔40mm 处仍有未蒸发的燃油滴存在,且随温度的升高,SMD 有增大的趋势,而环境压力的增加,则使SMD 减小。射流碰壁后,反弹回来的油滴变得更细、更均匀。     

湍流射流火焰抬举高度的实验研究

湍流射流燃烧作为工业燃烧室中普遍存在的燃烧方式,研究湍流射流火焰不仅能促进实际燃烧室的设计改造,更能增强对湍流燃烧理论的理解。在轴对称伴流射流燃烧器实验平台上,研究了湍流自由射流火焰抬举高度随射流速度的变化及氮气稀释和伴流速度对火焰抬举高度的影响。实验结果表明湍流自由射流燃烧火焰抬举高度随射流速度呈线性增长;随氮气稀释摩尔分数的增加其抬举高度的线性斜率增大,射流火焰吹出喷嘴的雷诺数降低,火焰更易发生抬举;同时,氮气稀释摩尔分数的增加也导致射流火焰发生吹熄时雷诺数减小,射流火焰在射流速度完全进入湍流之前发生吹熄;伴流速度小于0.3 m/s时对火焰抬举高度的影响不大,当伴流速度大于0.3 m/s时抬举高度随伴流速度的增加呈线性增长,当射流速度大于20 m/s时,伴流速度的影响降低;对比伴流与稀释对抬举高度的影响可知射流速度大于30 m/s时对伴流的敏感性大于稀释,而在射流速度小于30 m/s时对稀释更敏感。     

环境温度对掺水乳化柴油喷雾燃烧特性的影响

柴油机中燃用掺水乳化柴油有提高燃烧热效率的潜力,并同时降低碳烟和NOx排放.利用定容燃烧弹台架,对比试验了纯柴油和掺水乳化柴油的燃烧特性,并重点研究了环境温度对掺水乳化柴油喷雾和燃烧特性的影响.结果表明:相比于纯柴油,掺水乳化柴油能明显降低燃烧过程中碳烟的生成量;掺水乳化柴油中水的蒸发和微爆作用随环境温度的升高逐渐增强,能有效促进喷雾雾化和油、气混合过程;随着环境温度的升高,掺水乳化柴油的滞燃期及火焰升举长度均减小,燃烧火焰亮度增大.高环境温度工况下,碳烟前期生成速率和后期氧化速率均增大,使得不同环境温度工况下掺水乳化柴油最终碳烟的排放量相差较小.     

不同湍流强度下液雾燃烧的直接数值模拟

采用欧拉方法求解气相场,采用拉格朗日方法跟踪液滴颗粒,对液雾燃烧进行了直接数值模拟研究.分析了不同湍流强度下液雾燃烧的液滴蒸发特性和燃烧特性.发现在低混合分数区域,液滴蒸发速率约等于零.当混合分数大于夹层内平均混合分数的时候,液滴蒸发速率随混合分数线性增加.液雾燃烧的着火时间与相同条件下气体混合物的着火时间相比会更长.湍流在混合分数空间中抑制了液体燃料的蒸发,但是能够加强燃料和氧化剂的混合,从而进一步促进化学反应的进行.     

Investigation of catalyst particle hydrodynamic and heat transfer in three phase flow circulating fluidized bed

Vaporization of gas oil droplets has significant effects on the gas-solid flow hydrodynamic and heat transfer characteristic. A three dimensional CFD model of the riser section of a CFB have been developed considering three phase flow hydrodynamic, heat transfer and evaporation of the feed droplets. Several experiments were performed in order to obtain the data needed to evaluate the model using a pilot scale CFB unit. The Eulerian approach was used to model both gas and catalyst particle phases comprising of continuity, momentum, heat transfer and species equations as well as an equation for solid phase granular temperature. The flow field and evaporating liquid droplet characteristics were modeled using the Lagrangian approach. The catalyst particle velocity and volume fraction were measured using a fiber optic probe. The comparison between model predictions of catalyst particle velocity and volume fraction with the experimental data indicated that they were in good agreements and the Syamlal-O'Brien was the most accurate drag equation. The CFD model was capable of predicting the main characteristic of the complex gas-solids flow hydrodynamic and heat transfer, including the cluster formation of the catalyst particles near the reactor wall. In addition, the simulation results showed droplet vaporization caused reduction of catalyst particle residence time. Moreover, the higher ratios of the feed to catalyst flow rates led to the lower values of the catalyst temperature profile minimum.     

微通道内气体-近壁颗粒流动传热数值模拟研究

数值模拟了微通道受限空间内气体-近壁颗粒流动与传热过程,所建模型考虑微尺度气体的可压缩与变物性特征,且在通道和颗粒壁面采用速度滑移和温度跳跃边界条件以考虑滑移区气体动量/能量非连续效应。在此基础上,计算分析了克努森数(Kn)和颗粒偏移比对颗粒表面拖曳力系数(C_D)以及传热努塞尔数(Nu)的影响规律。研究结果表明:受气体稀薄效应影响,颗粒表面拖曳力系数呈减小趋势,换热过程也相应削弱;随颗粒与壁面距离减小,颗粒表面拖曳力系数相应减小,而颗粒与其周围气体的传热过程由于近壁效应呈增强趋势。     

A coupled level set and volume-of-fluid simulation for heat transfer of the double droplet impact on a spherical liquid film

A coupled level set and volume-of-fluid method is applied to investigate the double droplet impact on a spherical liquid film. The method focuses on the analysis of surface curvature, droplet diameter, impact velocity, double droplets vertical spacing, the thickness of the liquid film of two liquid droplets after the impact on a spherical liquid film, and the influence of flow and heat transfer characteristics. The results indicate that the average wall heat flux density of the double liquid droplet impact on a spherical liquid film is greater than that of a flat liquid film. Average wall heat transfer coefficient increases with the increase in the liquid film’s spherical curvature. When the liquid film thickness is smaller, the average wall heat flux density of the liquid film is significantly reduced by the secondary droplets generated from the liquid film. When the liquid film thickness is larger, the influence of liquid film thickness on the average wall heat flux density gradually decreases. The average wall heat flux density increases with the increase in impact velocity and the droplet diameter; it also decreases with the increase in double droplets vertical spacing.     

Effects of ambient pressure,gas temperature and combustion reaction on droplet evaporation

The effects of ambient pressure, initial gas temperature and combustion reaction on the evaporation of a single fuel droplet and multiple fuel droplets are investigated by means of three-dimensional numerical simulation. The ambient pressure, initial gas temperature and droplets’ mass loading ratio, ML, are varied in the ranges of 0.1–2.0 MPa, 1000–2000 K and 0.027–0.36, respectively, under the condition with or without combustion reaction. The results show that both for the conditions with and without combustion reaction, droplet lifetime increases with increasing the ambient pressure at low initial gas temperature of 1000 K, but decreases at high initial gas temperatures of 1500 K and 2000 K, although the droplet lifetime becomes shorter due to combustion reaction. The increase of ML and the inhomogeneity of droplet distribution due to turbulence generally make the droplet lifetime longer, since the high droplets’ mass loading ratio at local locations causes the decrease of gas temperature and the increase of the evaporated fuel mass fraction towards the vapor surface mass fraction.     

Subcritical and supercritical droplet evaporation within a zero-gravity environment: Low Weber number relative motion

A validated comprehensive axisymmetric numerical model, which includes the high pressure transient effects, variable thermo-physical properties and inert species solubility in the liquid phase, has been employed to study the evaporation of moving n-heptane droplets within a zero-gravity nitrogen environment, for a wide range of ambient pressures and initial freestream velocities. At the high ambient temperature considered (1000 K), the evaporation constant increases with the ambient pressure. At low ambient pressure, the evaporation constant becomes almost a constant during the end of the lifetime. At high ambient pressures, the transient behavior is present throughout the droplet lifetime. The final penetration distance of a moving droplet decreases exponentially with increasing ambient pressure. The average evaporation constant increases with ambient pressure. The variation is almost linear for reduced ambient pressures smaller than approximately 2. For higher values, depending on the initial freestream velocity, the average evaporation constant either becomes a constant (at low initial freestream velocities) or it non-linearly increases (at high initial freestream velocities) with the ambient pressure. Droplet lifetime decreases with increasing ambient pressure and/or increasing initial freestream velocity.     

正丁醇水溶液液滴的蒸发特性实验研究

自湿润流体是一种具有特殊的表面张力特性的二元流体,了解其蒸发传热特性对于揭示其强化传热机理十分重要。为了探究添加自湿润流体液滴的蒸发特性,采用液滴形状分析仪(DSA100)研究了不同温度(30、40、50、60℃)下铜底板上去离子水、正丁醇水溶液(质量分数为0.5%)液滴的蒸发特性。结果表明:加入少量正丁醇溶液并不影响去离子水液滴的蒸发模式,但在去离子水中加入正丁醇溶液增大了初始液滴的接触半径、减小了初始接触角度,在底板温度较低时增快了液滴蒸发平均速率,加剧了液滴蒸发过程中Marangoni流动的强度,可以达到强化换热的效果;随着底板温度的不断增加,正丁醇水溶液液滴的平衡接触角以及蒸发速率均与温度成正相关,符合液滴的基本蒸发模式。     

Prediction of local heat transfer characteristics of dilute gas‐solid flows through an adiabatic,horizontal pipe

The local heat transfer characteristics of gas‐solid flows through an adiabatic, horizontal pipe are numerically studied using the two‐fluid model of Ansys Fluent 15. First, the model is validated with the experimental results available in the literature for the air temperature and average Nusselt number. Then, the local heat transfer characteristics of gas‐solid flows, such as temperature profiles of gas and solid, gas‐solid Nusselt number, logarithmic mean temperature difference, and effectiveness of gas and solid, are studied by changing different parameters (gas velocities 15‐24 m/s; inlet solid loading ratios 0.1‐1; particle diameters 100‐400 µm). It is observed that increasing the particle diameter and inlet gas velocity increases the gas temperature and decreases the solid temperature, increases the logarithmic mean temperature difference, and decreases the thermal effectiveness of gas and solid. However, increasing the solid loading ratio decreases the gas and solid temperatures, decreases the logarithmic mean temperature difference, and increases the thermal effectiveness of gas and decreases the thermal effectiveness of solid. Moreover, increasing the particle diameter decreases the gas‐solid Nusselt number, whereas increasing the solid loading ratio and inlet gas velocity increase the gas‐solid Nusselt number.     

不同冷气/燃气温比下高温涡轮叶片旋流冷却流固耦合特性研究

采用流固耦合方法对燃气轮机高温涡轮叶片旋流冷却结构进行数值模拟分析。探究了不同冷气/燃气温度比条件下旋流冷却的流动与传热特性、叶片前缘区域固体温度、热应力以及热应变分布。研究表明：在进气腔入口雷诺数固定的条件下，随着温度比升高，冷气密度降低，冷气流速逐渐提升，同时湍动能升高，靶面努塞尔数逐渐升高；当温度比较低时冷气的流速较低、单位时间冷气带走的热量较少，当温度比较高时冷气温度较高、单位质量冷气所能吸收的热量有限，靶面处热流密度先升高后降低。受靶面热流密度分布影响，随着温度比升高，叶片前缘固体的温度、热应力以及热应变先降低后升高。     

Combustion of a coal char particle in a stream of dry gas

The burning rate, surface temperature, drag, and extinction conditions of a single char particle moving in a gas are computed numerically. The effects of the size and velocity of the particle and of the temperature and composition of the gas are examined in the framework of a simple model that includes O₂ and CO₂ heterogeneous reactions and, in some cases, a diffusion-controlled CO oxidation flame in the gas around the particle. In agreement with known results, the burning rate is found to increase with the velocity of the particle when the Reynolds number of the gas flow ceases to be small. The temperature of the particle increases with the temperature and oxygen mass fraction of the gas and is little affected by the size and velocity of the particle, except in the vicinity of extinction. The drag coefficient is a decreasing function of the particle size and velocity in the range of Reynolds numbers that has been analyzed. The presence of CO₂ in the gas may have an important effect on the gasification of small particles.     

Two-phase modeling of evaporation characteristics of ethanol droplet under force-convection environment

Studies on the evaporation phenomenon of a pure ethanol droplet have been mostly confined to the semianalytical modeling in stagnant ambient. Investigation into this aspect in a convective environment by considering the Navier–Stokes equation is also minimal. Hence, in this study we analyze and investigate the evaporation characteristics of a single-component spherical-shaped isolated pure ethanol droplet under force convective air environment by considering both gas- and liquid-phase motions, nonunitary Lewis number in the interface, variable Stefan flow (blowing) effect, and the transient droplet heating. The finite difference method is utilized while solving the governing equations of the spherical polar coordinate system for species, momentum, and energy transfer. The maximum Reynolds number and ambient temperature are kept at 100 and 600 K, respectively. The present work is validated by comparing the normalized surface regression curve of the droplet with the earlier experimental and theoretical results. Using the current simulated data, flow and temperature profiles of both gas and liquid regions are visualized in streamline and isotherm contour plots at various instants of time. It is observed that at a moderate Reynolds number a detached vortex forms at the downstream location of the droplet. However, the detachment length increases with time. The temperature gradients along the droplet surface are observed at the initial stage. Moreover, the heat-up period occupies about 20% of the total lifetime of the droplet. The droplet life and heat-up period decrease with an increase in free-stream velocity. In addition, the saturation temperature increases with ambient temperature.