柴油燃烧颗粒碰撞凝并过程

以柴油机后燃期颗粒的初期碰撞凝并过程为研究对象,在KF模型基础上,通过结合矩方法和颗粒尺寸分布模型,建立了柴油机颗粒碰撞凝并数学模型.探讨了柴油机燃烧过程中,自由分子区中的初级颗粒碰撞频率、凝并速率和剩余表面积的变化规律以及对颗粒尺寸生长的影响作用.结果表明:随着滞留时间增长,初级颗粒的碰撞频率逐渐降低;颗粒粒径分散性的增加能够提高碰撞频率;在同一粒径下,随着凝并特征时间增加,凝并速率明显降低;当粒径小于12.6,nm时,凝并速率对凝并特征时间变化的敏感性较高;初始凝并特征时间越大,表面积减小越慢;颗粒表面积随着滞留时间的增加而减小.     

凝并作用下火灾烟颗粒粒径分布及变化

预测火灾烟颗粒粒径分布特征参数的动态变化可实现探测系统对环境参数响应的动态模拟.顶棚烟气输运中,烟颗粒的布朗凝并是改变粒径分布的主要过程,应用矩方法求解布朗凝并作用下对数正态颗粒粒径分布的烟颗粒非线性凝并方程,得到随燃烧变化的凝并速率下粒径分布特征参数随时间的计算结果.正庚烷试验火烟颗粒分布的计算预测值与实验测量值相近,结果表明,在烟雾浓度快速上升阶段,烟颗粒凝并造成颗粒浓度损失不明显,几何平均粒径变化很小,而燃烧基本结束后,烟颗粒数密度衰减很快.     

燃烧超细颗粒声波团聚的谱分布数值模拟

在简要介绍声波团聚超细颗粒物的动力学机理的基础上,利用区域算法求解超细颗粒声波团聚的动力学方程(GDE),数值模拟了声波团聚前后的超细颗粒的谱分布变化,并同相关实验数据和数值算法进行了比较和分析．区域算法结果和实验数据以及数值解之间吻合较好,并且利用该算法研究了颗粒质量浓度、声波频率和声波强度对超细颗粒团聚效果的影响,结果表明颗粒质量浓度和声强的增加均有利于颗粒的团聚,而声波频率则存在一个最佳值．     

双流体颗粒-壁面碰撞模型在突扩两相湍流模拟中的应用

由于壁面粗糙度的双流体颗粒—壁面碰撞模型中，没有考虑颗粒—壁面碰撞前后颗粒速度相对于平均速度的差别，因此，修正了由此引起的颗粒雷诺应力的变化，并应用其对轴对称突扩流动进行了数值模拟，着重探讨了不同颗粒相边界条件的影响．结果表明，由于考虑了各方向雷诺应力之间的相互转化、雷诺应力从平均运动中得到能量以及壁面对颗粒运动的衰减作用等因素，包括摩擦系数、恢复系数和壁面粗糙度等影响因素的颗粒—壁面碰撞模型和由此得到的简化模型，给出的结果与实验值符合较好，而通常使用的零梯度颗粒壁面边界条件则给出失真的模拟结果。     

燃煤过程亚微米颗粒形成过程的模拟

燃煤中的亚微米颗粒的形成主要经历成核、冷凝和凝并3个过程．通过对颗粒的分布作出一个合理的对数正态分布假设,运用矩方法求解颗粒的通用动力学方程,来研究颗粒的尺寸分布情况．数值计算结果表明,对于不同初始分散性的颗粒,在成核、冷凝和凝并作用下,颗粒的数密度先增大后减小,颗粒的平均体积先减小后增大,分散性先增大后减小,最终趋于一致的均匀程度．对各单个过程的比较结果显示,凝并的作用最大,成核的作用只在颗粒形成的初期,冷凝的作用最小．     

考虑壁面粗糙度的双流体颗粒-壁面碰撞模型

提出了考虑壁面粗糙度的双流体颗粒－壁面碰撞模型，将轨道模型中颗粒碰壁模型考虑壁面粗糙度和双流体模型中用概率密度函数积分法处理颗粒与光滑壁面碰撞模型的优点结合起来，引入壁面粗糙度对碰壁颗粒湍流影响的机理。数值模拟结果表明，由于考虑了各方面雷诺应力之间的相互转化，雷诺应力从平均运动中得到能量，以及壁面对运动的衰减作用等因素，包括摩擦系数、恢复系统、壁面粗糙度等物理参数的颗粒－壁面碰撞模型作为边界条件时，得到的结果与实验符合得更好。     

燃煤锅炉烟气中小颗粒的电凝并脱除

电凝并是提高超细粉尘颗粒脱除效率的有效方法之一。使用燃煤锅炉烟气中的粉尘在脉冲高压电场中进行电凝并，得到凝并效率和单位体积烟气能耗、电极数和凝并时间之间的关系。电极数增加对粒径小于3μm的细小颗粒凝并效果影响明显；凝并效率随凝并时间增加而增大，但是当时间超过一定值时，凝并效率趋于稳定，试验得出最佳凝时间为2s；凝并效率随单位体积烟气能耗量变化而变化，能耗为7.7Wh／m^3时．凝并效率最高。     

有直接数值模拟对移动颗粒层除尘的研究

利用气固两相流数值模拟计算模型，分别采用了不同粒径的移动颗粒层过滤除尘器，对不同粒径粉尘颗粒的碰撞次数进行统计，对移动床除尘中过滤介质尺寸与粉尘粒径尺寸之间的相互选择性进行了初步研究。模拟计算了在同一风速下碰撞次数与粉尘粒径以及移动层颗粒粒径之间的关系。计算统计的结果与实验结果对比发现，二者存在定性上的一致。研究结果表明在移动床过滤除尘器不同粒径的过滤层对不同粒径尘粒具有明显的选择性。     

稠密气固流动中颗粒聚集现象的数值模拟

基于确定性轨道和真实碰撞，对二维稠密气固两相流动进行了数值模拟，真实展现了颗粒聚集现象从无到有的发展过程。在此基础上，对颗粒聚集现象影响因素的作用规律进行了研究。结果表明，颗粒间的相互碰撞是造成颗粒聚集的重要原因，即使在初始颗粒体积分数低至0.24％的情况下，仍然有显著的颗粒碰撞发生和聚集出现。颗粒弹性恢复系数越小、颗粒初始体积分数越大，则颗粒聚集现象越明显。这与已有的一些实验和数值计算结果是一致的。     

基于旋流与声波的颗粒复合凝并建模与运动轨迹仿真

为提高微细颗粒间碰撞凝并的概率,提出一种切圆式旋流凝并装置.采用离散颗粒模型(DPM),对切圆式旋流凝并装置内流场和颗粒运动轨迹进行了仿真,分析了旋流速度、颗粒粒径和添加声波与否对颗粒运动轨迹的影响.结果表明:切圆式旋流具有很好的混合与旋流效果,为微细颗粒的凝并创造了有利条件;旋流速度越大,颗粒受旋流的影响越大,发生碰撞凝并的概率也越大;粒径大的微细颗粒受到的离心力大,易与种子颗粒发生掺混运动,粒径小的微细颗粒受到的离心力小,倾向于与粒径相近的微细颗粒发生掺混运动;声波可以增强不同粒径颗粒间的相对运动,有效地提高了微细颗粒与种子颗粒间碰撞凝并的概率.     

Probe measurements and numerical model predictions of evolving size distributions in premixed flames

Particle size distributions (PSDs), measured with a dilution probe and a Differential Mobility Analyzer (DMA), and numerical predictions of these PSDs, based on a model that includes only coagulation or alternatively inception and coagulation, are compared to investigate particle growth processes and possible sampling artifacts in the post-flame region of a C/O = 0.65 premixed laminar ethylene-air flame. Inputs to the numerical model are the PSD measured early in the flame (the initial condition for the aerosol population) and the temperature profile measured along the flame’s axial centerline. The measured PSDs are initially unimodal, with a modal mobility diameter of 2.2 nm, and become bimodal later in the post-flame region. The smaller mode is best predicted with a size-dependent coagulation model, which allows some fraction of the smallest particles to escape collisions without resulting in coalescence or coagulation through the size-dependent coagulation efficiency (γ_SD). Instead, when γ = 1 and the coagulation rate is equal to the collision rate for all particles regardless of their size, the coagulation model significantly under predicts the number concentration of both modes and over predicts the size of the largest particles in the distribution compared to the measured size distributions at various heights above the burner. The coagulation (γ_SD) model alone is unable to reproduce well the larger particle mode (mode II). Combining persistent nucleation with size-dependent coagulation brings the predicted PSDs to within experimental error of the measurements, which seems to suggest that surface growth processes are relatively insignificant in these flames. Shifting measured PSDs a few mm closer to the burner surface, generally adopted to correct for probe perturbations, does not produce a better matching between the experimental and the numerical results.     

汽车排气微粒演变特性的参数分析

通过理论分析的方法对汽车排气尾流中的微粒演变特性进行了研究．建立了描述汽车排气尾流中微粒演变过程的核化模型和凝并模型．在此基础上,对排气微粒在尾流中的核化与凝并过程以及影响因素进行了分析．研究结果表明：核化增加了尾流中超细微粒的数量;燃料硫含量、排气稀释比、尾流温度以及相对湿度等对微粒的核化有较大的影响;凝并将显著改变尾流中的微粒粒径分布;排气微粒的初始数密度和粒径分布对尾流中的微粒凝并具有较大的影响．     

Charge fraction distribution of nucleation mode particles: New insight on the particle formation mechanism

We measured particle size distributions of total and singly charged nanoparticles in premixed flames with different flame stoichiometry and temperature to investigate particle inception.Particle charging in flames occurs by diffusion charging involving ions formed by chemi-ionization reactions in the flame front. It can be described by a Boltzmann charge fraction distribution evaluated at the local flame temperature where the particles interact with the chemi-ions. As the particles coagulate in the post flame zone, their charge fraction is reduced. The charge distribution of the coagulated aerosol again results in a Boltzmann curve, this time evaluated at the local post flame gas temperature where the particles had their last coagulation event. Particle nucleation in the post flame zone, where chemi-ions are drastically reduced, produces uncharged particles.Considering the above charging processes, the charge fraction of the nucleation mode contains information on the location within the flame these particles were formed. The results show that in flames near the particle inception threshold, particles are charged close to the flame front and remain charged even late in the post flame zone. Furthermore, smaller particles undergo less charge neutralization by coagulation as they travel through the post flame zone than larger particles. A different scenario is observed in richer flames; the smaller particles eventually become uncharged, indicating that significant amounts of freshly nucleated particles in these flames are formed in the post flame zone. Whether nucleation preferentially occurs close to the flame front or persists into the post flame zone also depends on flame temperature.     

静电增强湿式除尘器的优化运行

利用颗粒和液滴间的静电吸引力可以大幅度提高传统湿式重力喷淋除尘器的除尘性能.着重分析液滴荷电而颗粒不荷电的湿式除尘器的工作状况,阐述了布朗扩散、拦截、惯性碰撞和静电捕集等4种除尘机理.介绍了典型工况中该除尘器烟尘颗粒度分布,并通过分析捕集速率(沉积核)的方式对该除尘器的运行优化进行了分析.结果表明:含尘气流输运速度越大,液滴喷射速度越小;液滴几何尺度越小,液滴荷质比越高;液气比越大,越有利于可吸入颗粒物的高效脱除.     

四甲基硅烷燃烧法制备气相白炭黑的研究

介绍了以四甲基硅烷为原料,采用气相燃烧的方法制备超细白炭黑的新工艺。探讨了火焰构型、四甲基硅烷给气速率、甲烷给气速率、氧气给气速率、载气(氮气)通气速率等反应条件对产物的理化特性的影响。结果表明,前驱体浓度、初级颗粒在火焰中停留时间和火焰温度是影响白炭黑颗粒粒径大小的主要因素,前驱体浓度大、火焰中停留时间长、火焰温度高、产物粒子粒径越大。通过调节适量载气,可以很好地控制白炭黑粒径大小,使其粒径分布更均匀,成功制取粒径为9.46nm的超细白炭黑。并指出二氧化硅纳米颗粒在扩散火焰中经历化学反应、成核、凝并及团聚等几个主要过程。     

汽油机颗粒物数量排放及粒径的分布特性

采用 DMS500快速颗粒取样分析仪对一台气道喷射国Ⅲ汽油机进行了颗粒物粒径分布特性的试验研究.结果表明,汽油机排气中颗粒物以核态为主,仅部分工况存在积聚态.怠速时呈现包括核态和积聚态的双峰分布;低转速时表现为核态的单峰分布;中等转速时粒径分布范围扩大,既存在核态颗粒物,也存在积聚态颗粒物.相同转速下,随负荷的增加核态颗粒物数密度峰值先降低后增加;相同负荷下,随转速的升高核态颗粒物数密度峰值和峰值粒径均降低,积聚态颗粒物数密度峰值有增加的趋势.催化器对核态颗粒物的净化效果较好,对积聚态颗粒物的净化效果较差,对低转速下的粒径分布有明显影响     

Modeling of particulate carbon and species formation in coflowing diffusion flames of ethylene

A model of species and particulate formation in laminar diffusion flames is presented. The kinetic model is based on the chemistry of fuel oxidation and pyrolysis, the formation of aromatics and their growth into particle nuclei, particle growth by surface reactions, coagulation, and finally particle oxidation. A sectional model is used for the particle phase. The sectional method divides the particle mass range into classes of species each with a rate equation for surface growth, coagulation, and oxidation. An inception model links the gas-phase mechanism with the smallest particle section. Predictions are compared with experimental data in two laminar coflowing diffusion flames of ethylene for which experimental profiles of stable species, aromatic compounds, high-molecular-mass precursor species, and soot are available. The predictions show good agreement with data for total particulates, defined as the sum of soot plus nano-organic carbon particles. The model has a continuous size distribution and is able to address nanoparticles which comprise a significant part of the total particle loading. A conclusion from the sensitivity analysis is that the inception process, the molecular growth process by aromatic addition on particle nuclei, and surface addition of C₂H₂ all play important roles which need to be studied in greater detail to predict the right size distribution and volume fraction of particulates formed in flames.     

燃煤过程中微细颗粒生成机理及其模拟研究的进展

燃煤过程中产生的飞灰颗粒呈现出双峰状的尺寸分布,其中的微细颗粒因其表面富含有毒痕量元素且不易捕集,对人类健康造成极大危害,故受到广泛关注.研究表明,微细颗粒是通过煤中无机难熔矿物的还原蒸发、成核、凝结以及凝并等过程产生并长大的.针对煤中的主要无机矿物元素,系统地论述了微细颗粒形成的各个过程以及各过程子模型的建立,在深入理解微细颗粒生成机理的基础上提出了进一步的研究方向.     

Numerical study of the effects of internal and external forces on the nanoparticle mixing in a ferrofluid flow

Micromixers are an essential part of microfluidic devices. Numerous studies have previously discussed improving the mixing efficiency in micromixers. In the present study, the scale analysis of different forces has been done to predict their influence on the behavior of the particles. Results show that the order of the magnitude of gravitational force is lower than other forces in this study and it is not effective. Then, in the absence of the magnetic field, the effects of different forces on the mixing of magnetic nanoparticles have been studied using the one‐way coupling method. According to the numerical results, it is observed that the Brownian force is essential for the normal behavior of nanoparticles. The thermophoretic force causes the penetration of particles into pure water, but homogeneous mixing does not occur. Finally, two methods have been introduced to improve mixing, including increasing the velocity of ferrofluid and magnetic field implementation. Both approaches make particle distribution more homogeneous. Incidentally, both of the above‐mentioned factors are useful for improving the mixing process. Using them together yields more homogeneous mixing in a shorter length of the mixing channel.