EXPERIMENTAL STUDY ON SMALL CYCLONES OPERATING AT HIGH FLOWRATES

A new set of experimental data on the particle collection efficiency of small cyclones operating at high flowrates is reported. Seven different cyclones were studied systematically covering a flowrate range 60–110 l min^-1 and a particle size range 0.026–3.6 μm. Special emphasis was given to the effects of the cylinder height and of the exit tube length on the particle collection efficiency. The length ratios of the cylinder height and exit tube length to the cyclone body diameter were varied from 0.75 to 4.5 and from 0.5 to 1.5, respectively. The experimental results suggest that the flowrate plays a significant role in cyclone collection efficiency. High flowrate can help to collect fine particles. As the cylinder height is increased, the collection efficiency increases. However, when the cylinder height is increased excessively, the collection efficiency appears to decrease to certain extent. An optimized exit tube length was also observed. Pressure drop decreased substantially either as the cylinder height became longer or as the exit tube length became shorter. It was also found that the difference between the cyclone cylinder height and the exit tube length affects the particle collection characteristics significantly.     

Experimental study of small virtual cyclones as particle concentrators

Two small cyclones, made of aluminum and glass, with different dimensions were employed to understand the effects of minor flow on cyclone efficiency, and then to evaluate their potential usage as particle concentrators. Minor flow is airflow from the bottom of the cyclone where the dust collects in a conventional cyclone. A cyclone having a minor flow is referred to as a virtual cyclone in this study. The difference between a conventional cyclone and a virtual cyclone is that the latter has a minor flow while a conventional cyclone does not. In order to study their performance systematically, both virtual cyclones were tested under different operating conditions. The major flow efficiency curve, 50% cut-off diameter and pressure drop, were measured as a function of the ratio of the minor flow to the inflow. Compared to a conventional cyclone, the virtual cyclone showed a higher major flow efficiency and a smaller 50% cut-off diameter under the same operational conditions. As a final step, the concentration factor, the factor by which a given size is concentrated in the minor flow, was calculated as a function of particle diameter. It was found that at a fixed inflow rate, the concentration factor first increased and then decreased as particle diameter increased. This study shows that the virtual cyclone has the greatest potential for concentrating particles in the region of 50% cut-off diameter.     

Design and performance evaluation of vacuum cleaners using cyclone technology

A cyclone technology for a vacuum cleaner—axial inlet flow cyclone and the tangential inlet flow cyclone — to collect dusts efficiently and reduce pressure drop has been studied experimentally. The optimal design factors such as dust collection efficiency, pressure drop, and cut-size being the particle size corresponding to the fractional collection efficiency of 50% were investigated. The particle cut-size decreases with reduced inlet area, body diameter, and vortex finder diameter of the cyclone. The tangential inlet twin-flow cyclone has good performance taking into account the low pressure drop of 350 mmAq and the cut-size of 1.5 μm in mass median diameter at the flow rate of 1 m³/min. A vacuum cleaner using tangential inlet twin-flow cyclone shows the potential to be an effective method for collecting dusts generated in the household.     

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.     

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.     

Determining Cyclone Particle Holdup by Pressure Drop for a CFB Boiler

An experimental study was conducted to assess the possibility of determining particle holdup by measuring the pressure drop of a conventional cyclone used in a circulating fluidized bed (CFB) boiler. It was found that within a wide range of inlet solid concentrations, i.e., 0.54–4.42 kg/kg‐gas, the cyclone pressure drop increased linearly with inlet solid concentration at a given gas velocity, while the pressure drop between the dust exit and the vortex finder of the cyclone remained almost constant. Since particle holdup increases virtually linearly with solid flow rate, the particle holdup in the cyclone can be derived from the cyclone pressure drop, and therefore, an equation set was proposed to calculate the particle holdup from the cyclone pressure drop.     

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.     

多效旋风分离器性能的实验研究

多效旋风分离器通过采用2级螺旋管预分离含尘气体、螺旋形顶盖板导流、筒体中心稳流锥稳流和吸气回流系统防止粉尘返混等措施,解决了在旋风流场中分离微米及亚微米级颗粒的难题。文中通过实验研究了直径为0.25 m的多效旋风分离器的压降、分离效率和进口风速的关系,实验物料粒径范围为0.1—23μm,平均粒径为7.59μm。结果表明:在10—14 m/s入口风速时,对0.1—3μm颗粒的分离效率大于90%,对大于5μm颗粒的分离效率接近100%,压降在500—1 000 Pa。风速大于16 m/s时,对0.1—2μm颗粒的分离效率大于75%。     

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。     

Modelling of the Pressure Drop in Tangential Inlet Cyclone Separators

This article introduces a new mathematical model that predicts the pressure drop in a tangential inlet cyclone. The model calculates the pressure drop from the frictional losses in the cyclone body, using a wall friction coefficient based on the surface roughness and Reynolds number. The entrance and exit losses are also included in the model by defining new geometrical parameters. The pressure drop coefficient is obtained as a function of cyclone dimensions and operating conditions. The model is validated by studying 12 different cyclones presented in the literature. Comparison of the model results with predictions and measurements published in the literature show that the new model predicts the experimental results quite well for a wide range of operating conditions covering a flow rate of 0.3–220 l/s and a temperature range of 293–1200°K, in different cyclones. The pressure drop coefficient is also examined in view of the outlet pipe diameter, friction coefficient, surface roughness, and Reynolds number.     

Numerical and experimental study on the effect of solid particle sphericity on cyclone pressure drop

The continuous flow inside cyclone separator is usually simulated by solving the Reynolds averaged Navier–Stokes equations in Eulerian reference frame whereas the dispersed phase is modeled using Lagrangian approach. Although these methods have had a remarkable success, more advanced ideas are needed to model particulate phase in cyclones, especially the non-spherical shaped particles. Numerical simulation is verified with experimental results for the gas-solid flow in a cyclone separator. Reynolds Averaged Navier–Stokes equations (RANS) employing the RNG-based k–ε turbulence model are used to simulate the gas phase. 3-D particle tracking procedure is used for the solid phase. Three different equations for the drag coefficient are utilized in the numerical modeling to acquire more understanding of the behavior of non-spherical particles in cyclones. Computations resulted in the difference of pressure between the inlet and exit of the cyclone, and results are compared with experimental data. Experiments included measuring the separation efficiency of different shapes and sizes of particles. The results indicate that the CFD simulation can effectively reveal the pressure drop behavior as well as separation efficiency of gas-non-spherical particle flow in cyclone.     

Characteristics of the collection efficiency for a double inlet cyclone with clean air

Two single inlet cyclones and a double inlet cyclone were designed and fabricated to evaluate, and compare, their collection efficiencies. Two single inlet cyclones had different inlet sizes and vortex finder diameters. The double inlet cyclone had two inlet parts that divided the cyclone inlet in two. Clean air was introduced to the inlet near the cyclone wall, and particle-laden air was introduced to the inlet away from the cyclone wall. This double inlet made the clean air swirl in the region near the vortex finder, and the particle-laden air swirl in the region close to cyclone wall. The performance of the double inlet cyclone was evaluated at various clean air flow rates, keeping the particle-laden air flow rate constant.The collection efficiency of the double inlet cyclone was found to be 5–15% greater than that of the single inlet cyclone with the same inlet size and vortex finder diameter. As the flow rate of clean air was increased, the collection efficiency increased. This result indicates the possibility of achieving higher collection efficiencies with a double inlet cyclone.     

A CFD study of the effect of cone dimensions on sampling aerocyclones performance and hydrodynamics

This work presents a Computational Fluid Dynamics calculation to evaluate the effects of cone dimensions on the performance, hydrodynamics and centrifugal forces of sampling aerocyclones (gas cyclones). The problem of modeling highly swirling flow is overcome by means of an algebraic turbulence model. The axial and tangential velocities in a cyclone are successfully simulated. The refined mesh on the cyclone cone was also applied to ensure a better prediction on the effect of cone tip diameter to its performance, centrifugal forces and hydrodynamics. The pressure drop, grade efficiency and cut-off size of a cyclone of different cone dimensions was predicted very well with average deviation of about 2.9%, 5% and 2.1% respectively from experimental data presented in the literature. The findings suggest that the higher peak of tangential and axial velocity in a cyclone of a small cone lead to a higher collection efficiency and pressure drop. This helps to assess the benefit of enlarging or reducing the cone of a given cyclone. Results obtained from the computer modeling have demonstrated that CFD is suitable for modeling an effect of cyclone dimension on its performance.     

CFD analysis of flow field in square cyclones

In this study, computational fluid dynamic method is used to predict and evaluate the flow field inside a square cyclone. The flow field is calculated using 3D Reynolds-averaged Naveir-Stokes equations. The Reynolds stress transport model (RSTM) is used to simulate the Reynolds stresses. The Eulerian-Lagrangian computational procedure is implemented to predict particle trajectory in the cyclone. The Newton's second law is used to study the particle trajectory with modeling the drag and gravity forces acting on the particles. The velocity fluctuations are simulated using the discrete random walk (DRW). Two square cyclones which have different geometries are studied. The cyclones are simulated at different flow rates. The details of the flow field are studied in the cyclones and the effect of varying the flow rates is observed. Tangential velocity is investigated in different sections inside the square cyclone. Contour of pressure and turbulence intensity is shown for different inlet velocities inside the cyclones. It is observed that different geometries, also different inlet velocities, could affect on the pressure drop. The collection efficiency and the flow patterns obtained numerically are compared with the experimental data and good agreement is observed.     

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。     

Development and application of a novel swirl cyclone scrubber—(1) Experimental

Conventional cyclones have a lower collection efficiency for smaller particles and conventional wet scrubbers have significant clogging and fouling problems by salt formation at the tip, the inside and outside of the nozzles, the tubes and the walls of scrubbers. Also, many companies and manufacturing sites have been in trouble for collecting their adhesive particulates. The novel swirl scrubber that we have developed consists mainly of a cyclone and a swirl scrubber with an impact cone and plates. This study reports the collection efficiency of particulates and the application of the novel swirl scrubber. The particle collection efficiency as a function of particle size was investigated with changes of plate angles, nozzle size and pressure, and volumetric flow rate of scrubbing medium. The particle collection efficiency increased with a decrease in plate angle, an increase in pressure of scrubbing medium at the nozzle tip, and an increase in volumetric flow rate of the scrubbing medium. The collection efficiency of PM₁₀ by scrubbing effect was much higher than that by cyclonic effect. In particular, the total increase in particle collection efficiency by scrubbing effect was significant (around 2.5 μm) in particle aerodynamic diameter. The developed novel swirl scrubber can be used for significantly increasing the collection efficiency of TSP, PM₁₀, and PM_2.5, in particular, which have adhesive characteristics. The costs for installation, operation and maintenance of the scrubber system are much cheaper than those of cyclones and scrubbers or other particulate collecting devices.     

Development of a symmetrical spiral inlet to improve cyclone separator performance

Three cyclone separators with different inlet geometry were designed, which include a conventional tangential single inlet (CTSI), a direct symmetrical spiral inlet (DSSI), and a converging symmetrical spiral inlet (CSSI). The effects of inlet type on cyclone performance characteristics, including the collection efficiency and pressure drop, were investigated and compared as a function of particle size and flow rate in this paper. Experimental result indicated that the symmetrical spiral inlet (SSI), especially CSSI inlet geometry, has effect on significantly increasing collection efficiency with insignificantly increasing pressure drop. In addition, the results of collection efficiency and pressure drop comparison between the experimental data and the theoretical model were also involved.     

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.     

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。     

颗粒负荷对小型旋风器性能影响的模拟分析

为探究颗粒负荷对小型旋风器内气固两相流动的影响,基于雷诺应力模型（RSM）和欧拉-欧拉方法的混合流模型（Mixture）进行气体-颗粒、颗粒-颗粒的相间耦合计算。采用粒径为0.5～5μm的颗粒组在40L/min、60L/min和80L/min的入口流量下模拟0~3kg/m³的5种不同颗粒浓度工况,通过对比旋风器内纯气相流场和颗粒负荷流场的不同,研究了颗粒的存在对流场的影响;探究了入口流量和浓度变化对旋风器内分离效率和压降特性的影响。基于模型有效性验证的数值模拟结果表明：较高颗粒浓度负荷使旋风器内的气相流场发生显著变化。随着入口流量的增大,旋风器的分离效率先增大后减小,压降呈非线性增大。随着颗粒浓度的增大,旋风器的分离效率逐渐增大,压降先减小后增大。