催化裂化沉降器顶部旋风分离器升气管外壁结焦的原因分析

本文在催化裂化沉降器升气管外壁颗粒沉积实验的基础上,结合气相流场数值模拟的结果,分析了升气管外近壁气相流场的分布特点和颗粒所受径向力的分布特点.结果表明对应工业旋风分离器结焦区域为边界层增厚区,非结焦区域对应边界层分离区.对应工业旋风分离器结焦区域,气体、颗粒径向速度均朝内;对应非结焦区域,气体、颗粒径向速度均朝外.在此基础上分析了结焦的原因.     

双进气道旋风分离器内不同粒径颗粒流动特性

采用改进的RNG k-ε湍流模型和欧拉多相流模型,对一种单入口双进气道旋风分离器内的气固多相紊流过程进行数值模拟。计算得到旋风分离器内不同粒径颗粒速度和浓度分布规律,结果表明:大粒径颗粒比小粒径颗粒轴向速度分布更平坦,切向速度峰值位置和外准自由涡区也越向壁面靠近;与普通单入口旋风分离器相比,相同处理量时,此种旋风分离器内速度和不同粒径颗粒浓度分布轴对称性更好,大粒径颗粒切向速度峰值位置外移更明显,筒体段颗粒有更向壁面浓集的趋势,锥体段不同轴向位置处中心旋流区双进气道的颗粒浓度低于单进气道的。小粒径颗粒捕集能力增强,有助于提高分离器分离效率,减少不稳定流动导致结焦的颗粒源供给,从流动角度保证了抗结焦和长周期稳定操作。     

轴对称进口回转通道对旋风分离器分离特性影响的研究

针对单进口旋风分离器内气流轴不对称问题,将单进口旋风分离器入口方式改造为轴对称型回转通道形式（即采用双进口与单进口等长通道长度的新型切向进气方式）,从旋风分离器的进风量、入口含尘量及阻力损失等方面,分析了轴对称型进口回转通道对旋风分离器分离特性的影响。分级分离效率的试验与计算结果表明,新型旋风分离器的分离性能不仅优于单进口,而且稍优于Ｓｔａｉｒｍａｎｄ高效旋风分离器     

径向入口结构的旋风分离器内三维流场的数值研究

采用雷诺应力模型(RSM)对径向入口结构的旋风分离器内气相流场进行了数值模拟。不同于传统的切向入口结构,径向入口设计使得新型旋风分离器在保证较高分离效率的同时更能适应高压下的作业,且降低了工程焊接难度。通过模拟结果与实验值的对比发现,RSM模型能很好地预测新型旋风分离器内部气相流场,且模拟结果表明:旋风分离器内部流场呈现非轴对称性,主要表现为沿轴向气流的旋转中心与旋风分离器的几何中心不重合,且在分离空间内各轴截面出现具有周期性的摆动涡核。分离空间内切向速度场以0.8倍升气管直径为边界,呈现自由涡与强制涡结合的兰金涡形式,随着入口角度和升气管直径比(dr=dr/D)的减小,切向速度增大,内外旋流区也随之变化。此外,升气管内切向速度呈"U"形分布,由于速度分布中心不断发生变化,亦存在摆动涡核且摆动频率较分离空间的大。     

旋风分离器短路流的估算问题及其数值计算方法的讨论

旋风分离器升气管下口短路流量的大小是评价分离器性能的指标之一,现有的短路流计算方法对升气管下口短路流出现的高度范围的选取并不明确,采用不同的高度范围计算结果可能完全不同,故而分离器设计、开发及应用前后往往需要经历反复的实验验证过程,既费时又费力。通过对双进口及普通单进口分离器径向速度场和轴向速度场的分析,自定义了一种基于数值计算平台的短路流计算方法,要点为根据分离器升气管下口位置横截面上径向流动向心与离心趋势的拐点划分短路流量计算的范围,基本为与升气管同心的环状面,利用CFD软件可以很容易地计算出此环面内下行的流量,即通常意义的短路流量。该计算方法的优势在于能够较准确地定义短路流计算的范围,并且对于新型多进口分离器和普通单进口分离器都适用,计算过程简单、结果可靠。     

排气管尺寸对旋风分离器流场影响的数值模拟

用雷诺应力湍流模型（RSM）模拟研究旋风分离器排气管尺寸对旋风分离器流场的影响.结果表明：单入口旋风分离器的非轴对称性在环区更明显;在排气管壁存在滞流区,排气管尺寸减小,该滞流区变薄;在分离区,De/D≥0.4时,旋风分离器的中心位置存在向下旋流,该旋流造成一定返混,对提高旋风分离器效率不利;随着De/D的减小,内旋流切向速度提高,中心处的向下旋流速度减小,总压降大幅提高;当De/D=0.3时,中心处向下旋流消失,提高了分离效率.     

旋风分离器内流场的非轴对称性特点

采用Fluent6.1软件提供的代数应力模型(ASM)计算了旋风分离器内的气相流场,并在此基础上讨论了流场的非轴对称特点.在旋风分离器的三维速度中,切向速度在环行空间具有明显的非轴对称分布,向下进入分离空间后很快发展为轴对称分布;轴向速度和径向速度在环行空间存在一定的非轴对称性,而到分离空间后基本上是轴对称分布了.旋风分离器内流场的这种非轴对称特点是由于切向入口的非轴对称结构产生的.     

升气管插入深度对旋风分离器内部流场影响的数值模拟

针对旋风分离器内部复杂的三维强湍流,在仿真过程中,采用雷诺应力模型(RSM),利用贴体网格技术,模拟得到不同升气管插入深度时的旋风分离器内的切向速度分布、轴向速度分布和进出口压降。结果表明,当升气管插入深度减小时,会使总压力损失有所降低,但有一部分气流将从入口直接进入升气管形成短路,使旋风分离器的分离性能下降;对于特定的旋风分离器,升气管插入深度存在最优值,可保证较高的分离效率和较低的压降。     

旋风分离器入口结构改进及其对“短路流量”的影响

常规切向进口旋风分离器的气流进入旋风分离器后必定要经过排气芯管外壁和筒体内壁之间,因此不可避免会使得相当一部分气流没有经过分离空间而直接从排气芯管底部排出(短路流量),这也是影响旋风分离器分离效率的重要因素之一。在前人工作的基础上,对旋风分离器的进口结构进行了改进:使得旋风分离器的入口具有一定截面角,并借助数值计算技术,分别对传统的和具有一定入口截面角旋风分离器内的三维流场进行了数值模拟,计算了芯管底部的"短路流量",结果表明:进口具有一定截面角可以明显减小芯管底部的"短路流量",这对改善旋风分离器的分离效率具有重要的实际意义。     

导流板对旋风分离器内非轴对称流动的影响

采用雷诺应力模型(Reynolds Stress Model,RSM)对环形空间设有导流板的旋风分离器内的气相流场进行模拟研究,分别考察了导流板径向、周向位置和导流板半径、数目对旋风分离器内气相流动的影响。结果表明:导流板半径和导流板数目对气相流动的影响最大;导流板数目为2时,可有效抑制非轴对称流动,但导流板数目进一步增多会使流场的轴对称性变差;导流板半径越大,流场的轴对称性越好;外旋流区的切向速度越大,越有利于提高分离器的性能。     

Modeling the gas flow in a cyclone separator at different temperature and pressure

The gas flow field in a cyclone separator, operated within a temperature range of 293 K–1373 K and a pressure range of 0.1–6.5 MPa, has been simulated using a modified Reynolds-stress model (RSM) on commercial software platform FLUENT 6.1. The computational results show that the temperature and pressure significantly influence the gas velocity vectors, especially on their tangential component, in the cyclone. The tangential velocity decreases with an increase in temperature and increases with an increase in pressure. This tendency of the decrease or increase, however, reduces gradually when the temperature is above 1000 K or the pressure goes beyond 1.0 MPa. The temperature and pressure have a relatively weak influence on the axial velocity profiles. The outer downward flow rate increases with a temperature increase, whereas it decreases with a pressure increase. The centripetal radial velocity is strong in the region of 0–0.25D below the vortex finder entrance, which is named as a short-cut flow zone in this study. Based on the simulation results, a set of correlations was developed to calculate the combined effects of temperature and pressure on the tangential velocity, the downward flow rate in the cyclone and the centripetal radial velocity in the short-cut flow region underneath the vortex finder.     

局部磨损对α型旋风分离器内流场及分离性能的影响

以α型旋风分离器为研究对象,基于欧拉-拉格朗日方法,采用雷诺应力模型（RSM）、颗粒离散相模型（DPM）、E/CRC磨损方程对分离器内流场与磨损特性进行数值模拟。通过分析速度矢量、切向速度、颗粒运动轨迹等参数的分布规律,研究了局部磨损对设备内流场及分离性能的影响。结果表明,α型旋风分离器入口正对壁面磨损最为严重,最大磨损率约为1.4×10^-5kg/(m²·s)。磨损引起壁面几何结构的改变,导致气流方向发生偏转,不利于主流的稳定与固体颗粒的分离。随局部磨损的加剧,排气管下口短路流急剧增大,从而导致排气管下口以下区域流体流量减少,外涡切向速度降低;细颗粒的逃逸现象更加明显,粗颗粒运动轨迹趋于重合,更易形成高浓度灰环加剧壁面磨损。与未磨损时相比,局部磨损厚度50mm时,3μm粒径颗粒的分离效率由74.38%降低至54.97%,分割粒径d₅₀由0.73μm增大至2.36μm;设备压降降低了约15.41%。     

气液旋流分离器进口宽高比优化的数值模拟

利用FLUENT提供的RSM和DPM模型对不同入口高度和宽度的气液旋流分离器进行了数值模拟. 结果显示,当增大宽度或高度时切向速度与分离效率减小,但压降降低;当宽度大于环形空间的间隙时,部分进气流量直接作用于排气管上,影响内部流场;减小入口宽度或高度时引起的压降无明显差别,但减小宽度可提高分离效率而高度则相反. 入口高度(a)与分离器筒体直径(D)的比值a/D和宽度(b)与分离器筒体直径(D)的比值b/D约为0.2时,压降基本相同,但分离效率相差约3.6%. a/D约为0.38时,分离效率约为95.6%,压降约为340 Pa;而b/D约为0.25时效率为96.3%,压降约为320 Pa,入口宽度对分离器性能的影响比入口高度更显著.     

Test of flow field on the annular meridian plane in a tubular membrane separator with rotary tangential flow

Enhancement of membrane microfiltration by rotary tangential flow is a new technique, which is based on the hydrocyclone mechanism. It improved the structure of the general membrane separator and the form of the liquid suspension flowing into the separator, so as to increase membrane fluxes and decrease membrane fouling. In our research, a tubular membrane separator with rotary tangential flow was designed for the first time. The flow field characteristics of polypropylene tubular membrane microfiltration in this tubular separator were studied systematically by means of the Particle Image Velocimetry (PIV) test. Streamlines and velocity distributions of the meridian plane of the separator under different operating parameters were obtained. The velocity distribution characteristics of rotary circular tangential flow were analyzed quantitatively with the following conclusions being obtained:

• (1) In the non‐vortex area, no matter how the operating parameters (flux, entry pressure) change, the velocity near the rotary tangential flow entrance is higher than the velocity far from the entrance at the same radial coordinates. In the vortex area, generally the flow velocity of the inner vortex is lower than that of the outer vortex. At the vortex center, the velocity is the lowest, the radial velocity being generally equal to zero. In the vortex zone, the radial velocity is less than the axial velocity.
• (2) Under test conditions, the radial velocity and the axial velocity of the vortexes' borders are 1–2 times the average axial velocity in the annular gap of the membrane module. The maximum radial velocity and axial velocity of Taylor vortexes are 2–5 times the average axial velocity in the annular gap of the membrane module.
• (3) In the vortexes that formed on the meridian plane, it was found that mass transfer occurred between the inner and outer parts of the fluid. Much fluid moved from the outer vortexes into the inner ones, which was able to prevent particles blocking the membrane tube.

Copyright © 2004 Society of Chemical Industry     

Stereoscopic PIV studies on the swirling flow structure in a gas cyclone

Understanding the swirling flow in a gas cyclone is of great importance in improving the cyclone design. Once the three-dimensional strong swirling flow is fully understood, cyclone performance such as pressure drop and separation efficiency can be improved by optimizing the cyclone design. The swirling flow was investigated by the stereoscopic particle image velocimetry (Stereo-PIV) in this work. The instantaneous whole-field tangential, axial, and radial velocities were measured simultaneously in the cylindrical and conical separation zone, and in the dust hopper area of the cyclone with gas inlet velocity of .The time-averaged flow pattern in the cylindrical and conical sections of the cyclone showed: a typical Rankine vortex with inner quasi-forced vortex and outer quasi-free vortex which is generated by tangential gas velocity; inner upward flow and outer downward flow of axial gas velocity; and centripetal flow in the region close to the wall due to the presence of radial gas velocity. In the dust hopper, a secondary longitudinal circular flow is formed in the annulus area between the conical body and the cylindrical wall. Experimental results indicate that the separated particles may be re-entrained into the cyclone from the bin to degrade the separation efficiency of the cyclone.     

旋风分离器高温流场的实验测量

引言 对旋风分离器内流场的认识是研究其分离过程的基础.长期以来,人们对旋风分离器内流场的结构、速度分布等进行了大量的测量和分析,认识到旋风分离器内是一个三维强旋旋湍流场.     

柱状旋流分离器零轴速面分布特性模拟分析

采用雷诺应力模型对柱状旋流分离器气相流场中的零轴速包络面分布进行了数值模拟分析,考察了旋流不稳定性、流速以及结构参数变化对其分布的影响。指出由于分离器旋转流动不稳定性的固有存在,零轴速边界在整个分离空间会呈非轴对称的扭曲状态;在研究速度范围内,流场处于自模化区,分离器运动相似,入口速度变化对零轴速边界的分布影响不大;升气管直径与入口面积对升气管入口区域的零轴速边界分布影响较大,随升气管直径的减小和入口面积的增大,零轴速边界逐渐收缩。远离升气管入口区域,升气管直径与入口面积的影响减弱,零轴速边界宽度逐渐增大,并趋于一致;使流场趋于稳定的升气管直径与入口面积,截面零轴速边界会保持较好的圆度。     

分流型芯管对导叶式旋风管内颗粒逃逸的控制

Experimental and computational fluid dynamics was used in this study to predict the escape particles and evaluate the performance of PSC type cyclone tube with slotted vortex finder.The simulation results showed that the PSC type cyclone tube could remove the particles with a diameter greater than 5 μm.The PSC type cyclone tube increased the grade efficiency of particles with a diameter greater than 2 μm as compared with the Shell type cyclone tube.Short circuit flow occurred around the vortex finder slots and there was almost no short circuit flow under the vortex finder inlet.Most small particles escaped from vortex finder slots of the PSC type cyclone tube.The slotted vortex finder could develop “upwards flow” near the vortex finder inlet outside wall and control the escape particles under the vortex finder inlet.The force analysis of particles near the slotted vortex finder slots showed that gas flow carried the particles with a diameter smaller than 3 μm out the separator.     

Effect of a Stick on the Flow Field in a Cyclone and the Pressure Drop Reduction Mechanism

Measurements of the flow field in a cyclone separator with and without a reducing pressure drop stick (Repds) showed that the Repds reduces the peak tangential velocity, the axial velocity gradient, and the radial gradients of static and total pressure and reverses the axial static pressure gradient. These changes reduce the energy consumed by the rotating kinetic energy, the internal friction, the turbulent kinetic energy, and the drag of the negative pressure difference. The results are used to discuss why the separation efficiency remains high while the pressure drop is reduced. The results also show that a 24% "short flow" occurs near the cyclone entrance. Analysis of the changes in the flow field and the pressure drop due to the thin stick shows that the Repds increases the pressure drop in the outer vortex zone and reduces the pressure drop in the inner vortex zone. Therefore the pressure drop reduction with the Repds is due to its wake vortex, which leads to the hypothesis that the pressure drop in turbulent flow can be reduced by adding vortexes.