Effect of the Solid Loading on a PFBC Cyclone with Pneumatic Extraction of Solids

This paper presents the effects of solid loading on the performance of a cyclone with pneumatic extraction of solids. The cyclone is a non‐conventional design, especially used for hot‐gas cleaning applications such as pressurized fluidized bed combustors (PFBC). A scaled‐down cold‐flow model was employed for the research. Experiments were conducted at 9–14 m/s inlet gas velocities, inlet solid loadings ranging from 30 to 230 g/kg gas, and bottom gas extraction percentages from 0.3 to 1.5%. Experimental results of pressure drop resistance coefficients and collection efficiency were compared with literature predictions. At PFBC operating conditions, cyclone geometry and solid concentration are the main parameters influencing cyclone pressure drop and collection efficiency. The vortex penetration in dipleg causes lower pressure drop values and higher collection efficiencies than predicted. These parameters can be suitably predicted for PFBC cyclones by introducing a modified penetration length in Muschelknautz's model [1]. For the present cyclone design, a new correlation of pressure drop, including the influence of solid loading, is proposed. A new method for detecting cyclone fouling, not previously addressed, is also presented, based on the evolution of the pressure drop resistance coefficient. An enhanced separation efficiency has been found, related to collection efficiency, which is especially important for particle sizes below 10 μm revealing agglomeration effects.     

并联旋风分离器的旋流稳定性分析

分别选用2台和4台直径300 mm的相同PV型旋风分离器作为分离元件,共用进气管、集气室和排尘室,以中心对称方式组成两种并联分离器,并通过数值模拟比较单分离器与两种并联方案中各分离元件气相流动的特点. 气体介质为常温常压空气,入口气速15~30 m/s. 结果表明,2台或4台并联时各分离元件流量偏差分别不超过0.35%和0.28%,压降最大偏差为0.79%和0.43%,流量分配均匀,灰斗内窜流返混不明显,且4台并联时效果更好. 4台并联时分离元件排尘段的稳定性指数比2台并联或单分离器降低过半,旋流稳定性显著增强. 对称排列的分离元件在公共灰斗中会形成具有自稳定性的对称涡系,对分离元件内旋进涡核的摆动有约束作用,旋流稳定性增强.     

Ungewöhnliche Zyklonabscheider

Unusual cyclone separators. Conventional cyclone separators are seldom suitable for dust cleaning of gases according to present standards. The reason is secondary flow within the body disturbing the process of separation based on elementary cyclone theory. This can be avoided by special design of the cyclone e. g. geometry of the separation chamber, the position of openings, flow guides within the cyclone, dimension and geometry of the bin, bleeding and bypass of the gas, switching of cyclones, and means for dust agglomeration.     

Zur Berechnung der Partikelabscheidung in ZyklonenA model for particle separation in cyclones

In the present paper a new model is developed for cyclone design. This is based upon results of recent experimental and theoretical studies on gas flow and particle collection in cyclones. Particle transport is interpreted as a process, in which a random particle motion due to flow turbulence is superimposed on a mean motion. The particle flux within the cyclone is calculated using a simple flow model. In this model the separation chamber is divided into four characteristic regions. The model covers all important design parameters and requires little computational effort. Calculated grade efficiencies are in good agreement with the experimental results.     

Zur Berechnung der Partikelabscheidung in Zyklonen

In the present paper a new model is developed for cyclone design. This is based upon results of recent experimental and theoretical studies on gas flow and particle collection in cyclones. Particle transport is interpreted as a process, in which a random particle motion due to flow turbulence is superimposed on a mean motion. The particle flux within the cyclone is calculated using a simple flow model. In this model the separation chamber is divided into four characteristic regions. The model covers all important design parameters and requires little computational effort. Calculated grade efficiencies are in good agreement with the experimental results.     

环流式旋风除尘器内流场的数值模拟

采用 CFD 模拟软件 Fluent 6.2 提供的雷诺应力模型(RSM)对环流式旋风除尘器内的流场进行了数值模拟研究.并与热线热膜风速仪实验测试结果进行了比较.模拟结果与实验结果基本吻合.结果表明:环流式旋风除尘器特殊的流路设计,避免了内外旋涡的相互干扰,增强了旋转速度,规整了流形,减小了强湍流对性能的影响,消除了旋风除尘器易产生的短路流和二次返混,提高了除尘效率,降低了设备的压降.通过对影响除尘器性能的局部涡进行分析,为进一步优化结构提供了参考依据.     

Effects of separation space diameter on the performance of a novel reverse flow cyclone

ABSTRACT

A numerical study was carried out to investigate the effect of separation space diameter on the performance of a novel reverse flow tangential inlet cyclone design by using the Eulerian-Lagrangian approach. The design of this cyclone is based on the idea of increasing vortex length and decreasing pressure drop compared with traditional cyclones. This novel cyclone differs from the traditional cyclones with separation space and vortex limiter instead of the conical part. A qualitative numerical study was performed to analyze the effect of separation space diameter on the cyclone performance at different flow rates by evaluating velocity profile, pressure drop, fractional and overall efficiencies. The results show that the collection efficiency of smaller particles increases while pressure drop decreases significantly with the increase in separation space diameter for D₁/D < 0.5.     

环流式旋风除尘器的工业应用

本研究工作在实验室研究的基础上 ,对环流式旋风除尘器进行工业放大 ,并对工业环流式旋风除尘器的分离效果和压降进行了测试。工业应用表明 ,环流式旋风除尘器处理量大 ,操作稳定 ,放大效应小 ;工业环流式旋风除尘器与常规型旋风除尘器相比 ,用于合成氨造气和复合肥车间干燥窑炉除尘的效率提高了 15 % ,压降降低了 4 0 %左右     

差异旋风分离器并联性能测量及流场分析

通过改变旋向和芯管直径,设计了3种差异旋风分离器,并按中心对称方式组成了3种并联方案：相同分离器、旋向差异分离器和芯管差异分离器并联。在冷态实验装置上,测量了单分离器和并联分离器的性能,并利用FLUENT软件分析了并联分离器的流场。结果表明,并联分离器的效率均高于单分离器,且效率-气速曲线未出现“驼峰”;与相同分离器并联相比,旋向交替变化时并联总压降较小,分离效率也更低,但各分离器流量分配均匀,未发现“窜流”现象;当芯管有差异时,并联总压降增大,各分离器进口流量分配不均匀,且进、出口流量平均相差6.0%,公共灰斗中存在“窜流”,旋流稳定性变差,效率降低。为了保证并联分离器的性能,应采用相同分离器对称并联的方式。     

双循环旋风分离器除尘机理的数值模拟

双循环旋风分离器通过将主进口设置在筒体中部,将顶部进气口设置为回流口,消除了进气口附近的二次流,避免了短路流,将大于3μm颗粒的分离效率提高至接近100%,并避免了少量11—15μm颗粒的短路逃逸。为了探索该设备的除尘机理,借助CFD软件,通过数值模拟研究的方式,辅助分析了2种进气口在分离性能上不同,传统旋风分离器不能完全分离3—8μm和11—15μm颗粒的机理,以及消除二次流的方法。计算结果表明:当回流气速低于主进气速时,会产生类似于顶端进气口的现象,即二次流、灰环和短路流,降低了小于6μm颗粒的分离效率。当回流气速略大于主进气速时,可以完全消除主进气口附近的二次流,使得所有粒径颗粒的分离效率都较高。模拟结果与实验结果从定性的角度符合较好。     

高入口浓度下PV型旋风分离器的分离效率计算

基于Muschelknautz 分离模型,以PV型旋风分离器为对象,针对高入口浓度的分离效率的计算,将旋风分离器分离空间的气固分离过程划分为2个区域,提出了串级分离模型。当入口浓度大于临界入口浓度时,旋风分离器内有器壁附近的颗粒支配区和中心区域的气体支配区。颗粒支配区内颗粒速度大于气体速度,颗粒夹带气体沿器壁螺旋下行进入灰斗被全部捕集,形成了颗粒的一级分离;气体支配区内气体速度大于颗粒速度,气体携带颗粒做旋转运动进行离心分离过程,形成了颗粒的二级分离。旋风分离器总的气固分离过程是一级分离和二级分离的叠加。通过高入口浓度的实验对串级分离模型进行了验证,基于串级分离模型给出的PV型旋风分离器的分离效率与实测值较吻合。研究表明旋风分离器临界入口浓度对总效率的计算影响较大。串级分离计算模型包含了结构参数和气、固相物性等参数,具有很好的通用性,可以满足PV型旋风分离器的工程计算和设计要求。     

环流式旋风除尘器内颗粒运动的数值模拟

采用CFD软件Fluent提供的雷诺应力模型（RSM）和随机轨道模型,对环流式旋风除尘器内颗粒运动轨迹进行了数值模拟研究。预测了不同粒径颗粒的运动轨迹和分离效率。结果表明：颗粒在环流式旋风除尘器内的运动路径比常规除尘器长;特殊的流路设计,避免了常规旋风除尘器易产生的上灰环和颗粒短路问题,使除尘效率大幅度提高;除尘器内颗粒运动有较强的随机性,尤其对于小颗粒,受气流湍动影响显著。对不同粒径颗粒分离效率的预测表明：环流式旋风除尘器的分割粒径为1．25μm。     

DIMENSIONLESS REPRESENTATION OF PARTICLE SEPARATION CHARACTERISTIC OF CYCLONES

In particular the collection efficiencies were measured as a function of flow rate, cyclone dimensions and particle size. For this purpose a fast, accurate and problem adapted measuring technique has been used, which enables the determination of grade efficiency curves by measuring the size distributions in the cyclone up- and downstream with optical particle counters. The extended experimental data from this parameter study were analysed by the methods of dimensional analysis and theory of models. An evaluation of all measuring results for two cyclone designs has been resulted in an empirical, nondimensional correlation of the collection characteristic, a dimensionless grade efficiency curve. Deviating from geometric similarity this correlation includes a variation of cyclone outlet diameter. Grade efficiencies of the cyclones are a definite function of the dimensionless numbers Stokes and Reynolds number and of the dimensionless cyclone outlet diameter. Analysis of own and published data has shown that this experimental correlation includes the influence of the temperature and that cyclone body diameter do not influence efficiency. The influence of cyclone height on flow behaviour and collection characteristic could be quantified as well. The range, in which prediction of collection efficiencies is possible, is marked in a state diagram Reynolds number versus dimensionless cyclone height.     

旋风分离器内颗粒运动规律的数值模拟

采用单颗粒动力学模型计算了旋风分离器内颗粒的运动轨迹。计算结果表明：进入旋风分离器的颗粒绝大多数在环形空间内就被甩向器壁；从入口上部进入的部分较小颗粒由于局部二次流作用而上升形成顶灰环；还有少量颗粒进入下部分离空间。在下部分离空间内，有灰斗返气夹带上来的颗粒，处于内旋流的颗粒则有可能随上升气流而逃逸，所以一定的分离空间高度有利用颗粒的二次再分离。     

A Mathematical Model to Compare the Efficiency of Cyclones

Tangential cyclones modified to form spiral cyclones are very efficient in separating solid particles from dust laden gases. They offer a lower gas pressure drop and higher particle separation efficiency when compared to basic tangential cyclones. Their high performance is believed to be related to their special structure. A mathematical model is introduced to explain why a spiral cyclone is more efficient than a tangential one. An experimental apparatus is designed to compare both the performances of spiral and tangential cyclones and to check the effectiveness of the model equations. The experimental data were found to be consistent with the model predictions.     

Small Electrocyclone Performance

Cyclone Separators and Electrostatic Precipitators (ESPs) are both effective particle separators. The former are more efficient at removing the larger particles, while the latter more suited to removing the smaller size classes. We explore the performance of an “Electrocyclone”, constructed by simply retrofitting an electrode coaxially to a small existing “Whitby” cyclone. Tests were performed with respect to particle size, resitivity, loading and various other operating parameters. Non‐electrical separation efficiencies ranged from 71 to 75 % and with the application of additional electrical forces the increase in separation efficiency was between 17 and 21 % at a cyclone Reynolds number of 19000, with the most conductive particle most easily separated. Further parametric testing correlated the effects of dust loading, electrocyclone Reynolds Number and particle cut upon separation efficiency. In particular we show that the separation of the smallest size cuts (D < 38 μm) of the dust sample almost doubled upon application of the corona. We conclude, based on this initial study of small devices, the range of use of cyclones may be extended significantly by the application of additional electrophoretic separation.     

Effect of geometric configuration on the collection efficiency of axial flow cyclones

The particle collection efficiencies of axial flow cyclones with eight different geometric configurations, operated at 50 lpm aerosol flowrate, have been evaluated in this study. The geometric variation of test cyclones includes the optional addition of an upside-down cup, two vortex finder lengths, and two cyclone base shapes. Under various configurations, the cutoff aerodynamic particle size of axial flow cyclones changed from 272 to 448 nm. Our study shows that configuration effects on the collection efficiency of axial flow cyclones are different from those of tangential flow cyclones. The observation of different geometric effects on particle collection by axial and tangential flow cyclones is attributed to the flow pattern difference between cyclones of two types. It is further concluded that the optimal configuration for axial flow cyclones is with an abrupt contraction base, without an upside-down cup and with an increased vortex finder length. A simple model combining the model of Leith and Licht (1972) and the tubing loss in 90° bends at high Reynolds numbers has also been proposed to predict the particle collection efficiency curve of the optimal axial flow cyclone among those tested.     

排气管偏置分离器分离性能的数值模拟

采用雷诺应力模型（Reynolds stress model,RSM）对旋风分离器排气管中置和偏置时的气相流场进行了数值模拟,并用拉格朗日法模拟了分离器内颗粒的运动轨迹。气相流场模拟结果与实验结果吻合得较好。结果表明：排气管偏置后,分离器内沿排气管偏置方向的切向速度有所提高,排气管下部短路流较小,颗粒离心运动更加强烈,提高了分离效率的同时压降变化不大,分离器经济性更好。     

Effects of Surface Roughness on the Performance of Tangential Inlet Cyclone Separators

This study is carried out to investigate the effects of surface roughness on the flow field and cyclone performance. The flow inside the cyclone separator is modeled as a three-dimensional turbulent continuous gas flow with solid particles as a discrete phase. The continuous gas flow is predicted by solving the governing equations by using the Reynolds Stress turbulence model, and the modeling of the particle motions is based on a Lagrangian approach. The results of the numerical simulations are compared with experimental data as well as with the results of mathematical models. Analysis of computed results shows that increase of relative roughness due to corrosion, wear, or accumulation of particles on the inner walls considerably influences the tangential velocity, cyclone separation efficiency, and cyclone pressure drop especially for high inlet velocities. Decreases in cyclone collection efficiency and pressure drop with the increase in surface roughness are found to be more pronounced for high values of relative roughness.