旋风静电除尘器流场分布测试研究

利用五孔探针系统对直筒式旋风静电除尘器模型的三维流场进行了测定与分析.结果表明:随着入口风速、供电电压增加,旋风静电除尘器内气流的切向和径向速度分别增加,各截面轴向速度沿半径的分布基本相似;与旋风除尘器相比,旋风静电除尘器内气流的切向速度和径向速度的变化更为平缓,入口风速为8.5 m/s时,在0.3～0.5倍简体直径时,切向速度达最大.     

旋风静电除尘器流场分布测试研究

利用五孔探针系统对直筒式旋风静电除尘器模型的三维流场进行了测定与分析。结果表明：随着入口风速、供电电压增加，旋风静电除尘器内气流的切向和径向速度分别增加，各截面轴向速度沿半径的分布基本相似；与旋风除尘器相比，旋风静电除尘器内气流的切向速度和径向速度的变化更为平缓，入口风速为8．5m／s时，在0．3～0．5倍简体直径时，切向速度达最大。     

旋风除尘器内部流场的数值研究

通过建立旋风除尘器模型，模拟了排气管插入深度对旋风除尘器的压降、气流的切向速度和轴向速度的影响。研究表明，增加排气管的插入深度会增大旋风除尘器的压力损失，但对其内部压力分布的影响却比较小；增加排气管的插入深度对气体切向速度和轴向速度的影响也比较小，在不同高度上，内外旋流交界处切向速度的最大值都有所增加，这会产生更大的离心力，除尘效率也会因此而提高。     

球柱形旋风除尘器分离性能数值模拟与实验

针对传统柱锥形旋风除尘器存在细颗粒分离效率低的问题,提出了一种新型球柱形旋风除尘器。采用数值模拟和实验研究手段,分析了柱段高度对球柱形旋风除尘器分离特性的影响。模拟结果表明:当球柱形旋风除尘器柱段高度不为零时,随着柱段高度的增加,静压力逐渐变小;球柱形旋风除尘器内流体的切向速度均呈"M"型分布;流体轴向速度随着半径的减小,其绝对值先增大后减小,在中心轴线处又开始增大;流体径向速度均关于中心轴线对称。实验结果表明,当球柱形旋风除尘器柱段高度为150 mm时,总分离效率最高,可达到92.01%。研究结果可为旋风除尘器中细小颗粒分离应用提供指导,对提高5μm以下颗粒分离效率具有重要意义。     

基于大涡模拟的旋风分离器内流场数值模拟研究

为了研究旋风分离器内流场的运动状况，借助Fluent软件，采用大涡模拟，基于曲线坐标系的SIMPLEC算法，对切向入口的Stairmand旋风分离器内流场进行了三维数值模拟研究。数值结果表明，大涡模拟适合于三维强旋流的流场模拟，分离器内部的流动空间可分为内、外2个流动区域，在不同的流动区域中，气体压力、速度场的分布有较大的差异。而且压力分布与速度分布不是绝对的轴对称分布。数值模拟结果对其设计优化研究有一定的应用价值。     

内置涡核破碎翼旋风分离器内颗粒受力数值分析

内置涡核破碎翼旋风分离器具有维持较高效率同时降低阻力的特点。研究其内部颗粒受力情况对揭示其工作机理有重要意义。利用计算流体力学方法(CFD),采用雷诺应力模型(RSM)模拟流场,在此基础上,应用拉格朗日离散化模型(DPM)对传统Lapple型旋风分离器加设涡核破碎翼前后的内部颗粒进行追踪,并对其进行受力分析。结果表明:内置涡核破碎翼旋风分离器在颗粒分离时间及受力在数值上均与传统分离器有所差别;涡核破碎翼叶片较短时,径向合力主要表现为向外,离心力占主导作用,利于颗粒收集;叶片较长时,颗粒受到气流扰动作用加剧,径向上的运动随机性增加从而不利于分离。     

基于颗粒能量耗散模型的多相网格质点法的旋风分离器模拟

提出一种配合网格质点法的能量耗散模型,考虑颗粒之间碰撞引起的能量耗散效应以描述颗粒的团聚行为。采用该模型模拟的旋风分离器颗粒流发现:较其他模型具有较高的预测精度,模拟结果与实验吻合较好,验证了耗散模型可行。旋风分离器的中心形成负压区域,而在靠近壁面处形成高压区域。气相场呈现以中心为轴同转向不同流向的双螺旋结构,大部分颗粒被夹带到壁面附近,聚集在一起,形成聚团并产生分离,模拟结果发现旋风分离器在模拟条件下的分离粒径为1×10~(-5)m。     

旋风分离器内高速旋转流场的数值计算方法选择

数值计算方法已成为研究旋风分离器内部流场的重要途径。利用计算流体力学方法对旋风分离器内部气相流场进行了数值模拟研究。分析了不同湍流模型、离散化方式和压力插值方式对旋风分离器内部计算流场的影响,并将所得数值结果与已知的实验数据进行了对比,以期得到最适合的数值模型。结果表明:对于旋风分离器内部复杂流场的流动,运用雷诺应力模型(Low-Re Stress-Omega)能够较好地预测出强制涡中心涡流的运动情况,同时采用SIMPLEC算法、二阶迎风离散化方式和PRESTO压力梯度插补格式能够获得最好的预报结果。     

直筒型导叶直流式三相旋流器排气管上部内流场的实验研究

使用五孔球探针对一种用于分离锅炉净化烟气中含尘液滴的直筒型导叶直流式三相旋流器排气管上部的气相流场进行了实验测定。得到了这一区域的速度和静压分布。发现了排气管上部筒体中心存在强烈的内旋流;外层气体不断汇入内旋流,内旋流呈轴向加速运动;绝大部分气体不经过排气管外环形空间下行直接由排气管排出。主体段切向速度为Rankine组合涡。     

直筒型导叶直流式三相旋流器排气管上部内流场的实验研究

使用五孔球探针对一种用于分离锅炉净化烟气中含尘液滴的直筒型导叶直流式三相旋流器排气管上部的气相流场进行了实验测定。得到了这一区域的速度和静压分布。发现了排气管上部筒体中心存在强烈的内旋流；外层气体不断汇入内旋流，内旋流呈轴向加速运动；绝大部分气体不经过排气管外环形空间下行直接由排气管排出。主体段切向速度为Rankine组合涡。     

水力旋流器速度场的PDPA测试研究

应用相位多普勒粒子分析仪(PDPA)对液液水力旋流器模型的内部流场进行了测试,分析了旋流器内部速度场的特点。通过进口流量的变化,测试了该模型的不同截面上的速度场分布情况。测试结果表明:随着流量的增大,旋流器轴截面上的速度都增大,旋流变大,有利于连续相和分散相的分离;旋流器中连续相和分散相速度分布趋势相同,但在管芯处,两者速度滑移明显。     

卡鲁塞尔氧化沟反应器两相流PIV图像处理研究及实验分析

在单相数字粒子图像测速度(PIV)技术的基础上,研究了PIV两相流动的测试及图像处理方法.利用数学形态学的原理以及互相关法,对两相粒子图像进行标定、识别、区分和流场分析,编辑了相应的软件.并采用此技术对卡鲁塞尔氧化沟模型直道及弯道段二维液固两相流速进行测量,克服了传统点测量方法无法获取全场流动同步信息的缺陷,较好地反映了不同位置处固液流动分布的基本特性.结果表明,沟内径向流速远小于轴向,流速大小相差约一个数量级;同一断面上轴向流动分布是决定水力特性的主要因素;径向流速和水体动能是决定污泥沉积位置的主要因数,固相总体流速小于液相.     

柴油机微粒捕集器复合再生性能及其场协同分析

基于柴油机微粒捕集器瞬态复合再生机理,建立了CFD三维仿真模型,并利用漩涡破碎燃烧模型和场协同数学模型对柴油机微粒捕集器瞬态复合再生过程的速度矢量和温度梯度进行了数值模拟和场协同分析,结果表明,再生时间为175 s时的微粒捕集器过滤体后端壁面峰值温度达到1 184 K,且其过滤体内速度矢量和温度梯度的协同度最好,最优燃烧再生区域所占最大比值为0.58,表明此时微粒捕集器过滤体内再生燃烧速率最大,其再生效率最高。     

Unsteady absorption of sulfur dioxide by an atmospheric water droplet with internal circulation

Unsteady absorption characteristics of sulfur dioxide by an atmospheric water droplet in motion are predicted numerically and analyzed theoretically to recognize the physical mass transport processes inside an aerosol droplet, which is frequently encountered in the atmosphere. Considering the absorption of sulfur dioxide by a droplet in cloud or fog with various velocities, three different Reynolds numbers, viz., Re_g=0.643, 1.287, and 12.87 are studied and compared with each other. The results indicate that for the Reynolds number of 0.643, sulfur dioxide always penetrates toward the droplet centerline throughout the entire absorption period. This is due to the mass transfer dominated by diffusion along the radial direction. In contrast, when the Reynolds number is 12.87, the strength of the vortex motion inside the droplet is strong enough. It results in that, most of the time the concentration contours parallel the streamlines and the lowest SO₂ concentration is located at the vortex center. As a consequence, the diffusion distance is reduced by a factor of three and the absorption time for the droplet reaching the saturated state is shortened in a significant way. With regard to an intermediate Reynolds number such as 1.287, a two-stage mass transfer process can be clearly identified. In the first stage, it is dominated by one-dimensional diffusion, in which over 50% sulfur dioxide is absorbed before the saturated state is reached. In the second stage, the vortex motion mainly controls the mass transfer. However, the contour core is inconsistent with the vortex center. This is because the characteristic time of mass diffusion is in a comparable state with that of droplet internal circulation. The present study elucidates that the strength of a droplet's internal motion plays a vital role in determining SO₂ absorption process.     

Wind Tunnel Simulation of Neutral Atmospheric Surface Layers by the Counter-Jet Technique

A new technique to generate thick turbulent boundary layers In relatively short distances, which is capable of modifying and controlling rapidly the mean and unsteady profiles of the simulated layers, is presented. The increased thickness is achieved in the "I.IT. Environmental Wind Tunnel" by providing large momentum defects at the wall through upstream oriented, spanwise discrete wall jets, with changeable jet velocities and controllable jet angles. Various mean velocity profiles of the boundary layer (which can be represented by a wide range of power law exponents) are obtained at the same streamwise position using different settings of the counter-jet parameters and different types of artificial surface roughness. The transverse uniformity of these layers is also documented. Selected measurements of the flow field in the vicinity of a "building" model tested in three surface layers are compared in order to examine the sensitivity of measured effects to changes in the surface layer characteristics. Viewing the flow field with the aid of the modular flow concept, changes in the wake of the "building," in the flow above its roof and in the shear layer spreading downstream from its top are recorded through profiles of mean velocity and turbulence intensity. The effect of the wind direction with respect to the model is also investigated.     

On the influence of the urban roughness sublayer on turbulence and dispersion

The concept of the urban roughness sublayer is discussed and this lowest atmospheric layer over a rough surface is shown to have a non-negligible vertical extension over typical urban surfaces. The existing knowledge on the turbulence and flow structure within an urban roughness sublayer is reviewed, focusing on the height dependence of turbulent fluxes and a scaling approach for turbulence statistics, such as velocity variances, in the above-roof part of the roughness sublayer. Finally, the implication of this turbulence and flow structure upon dispersion characteristics is investigated. The most prominent difference of explicitly taking into account the roughness sublayer in a dispersion simulation (as compared to assuming a `constant flux layer') is a clearly enhanced ground level concentration far downwind from the source. For the example of a tracer release experiment over a (sub) urban surface (Copenhagen) it is shown that introducing the roughness sublayer clearly improves the model performance.     

旋风分离器排气管最佳插入深度的实验与分析

采用实验方法考察物料特性、入口气速、排气管直径、入口截面积4种因素对排气管最佳插入深度的影响。实验结果表明,排气管的最佳插入深度主要受粉料中位粒径和粒径分布的影响,中位粒径越小,包含的小粒径颗粒越多,最佳插入深度越大,而与其他3项参数无关。另外,插入深度在最佳位置附近变化时,效率波动不大,远离最佳位置后,效率大幅下降,并且在大排气管直径,大入口条件下,效率对插入深度更为敏感。短路流和内旋流二次分离的协同作用是形成以上现象的主要原因。     

Influence of wall roughness on the dispersion of a passive scalar in a turbulent boundary layer

Many towns and cities consist of similarly sized buildings in relatively regular arrangements with smaller scale roughness elements such as roofs, chimneys and balconies. The objective of this study is to investigate how small scale roughness elements modify the influence of the large scale organized roughness on the dispersion of a passive scalar in a turbulent boundary layer. Wind tunnel experiments were performed using a passive tracer released from a line source and concentration profiles were measured with a Flame Ionisation Detector. The measurements are compared with numerical solutions of the advection–diffusion equation.The results show that decreasing the cavity aspect ratio increases the turbulent vertical mass fluxes, and that the small scale roughness enhances these fluxes, but only in the skimming flow regime. Numerical simulations showed that outside the roughness sub-layer (RSL) the changes in surface roughness could be accounted for by a simple variation of the friction velocity, but inside the RSL the spatial variability of the flow imposed by the roughness elements has much more influence. A simple model for a spatially averaged dispersion coefficient in the RSL has been developed and is shown to agree satisfactorily with the concentrations measured in these experiments.     

Wind tunnel study of air entrainment into two-dimensional dense gas plumes at the Chemical Hazards Research Center

Experiments in a neutrally stable wind tunnel boundary layer were made for two-dimensional (quasi-line) sources of carbon dioxide dispersing over two types of uniformly spaced (billboard) surface roughness elements. Velocity and concentration measurements were made with each surface roughness over a wide range of source Richardson number by varying carbon dioxide release rate and wind speed. Concentration measurements were made with a FID gas analyzer using an ethane tracer in the source gas, and velocity measurements were made with independent LDV and HWA systems. For each surface roughness, this paper describes the wind tunnel boundary layer and presents alongwind and vertical concentration profiles in the gas plume. Vertical velocity and concentration profiles were measured at selected downwind distances, and the profiles were integrated to confirm the consistency of the measurements with the mass of carbon dioxide released. The data are intended for development of improved vertical turbulent entrainment correlations for use in dense gas dispersion models applied to hazardous chemical consequence analyses.