Numerical investigation of separation efficiency of the cyclone with supercritical fluid–solid flow

The utilization of hydrogen is gaining increasing attention due to its high heating value and environmentally friendly combustion product. The supercritical water circulating fluidized bed reactor is a promising and potentially clean technology that can generate hydrogen from coal gasification. Cyclone is a vital part of the reactor which can separate incomplete decomposition of pulverized coal particles from mixed working fluid. This paper aims to gain in-depth understanding of the cyclone separation mechanisms under supercritical fluid by computational fluid dynamics (CFD). Although the amount of supercritical carbon dioxide in mixed working fluid is minor, it obviously influences the flow fields and separation efficiency of a cyclone. The simulation results suggest that both the decreasing content of supercritical carbon dioxide and adding the extra dipleg cause the promoting performance of cyclones. Research findings could refine the design of supercritical fluid–solid cyclones.     

Non-uniform distribution of gas–solid flow through six parallel cyclones in a CFB system: An experimental study

In large-scale circulating fluidized bed (CFB) boilers, it is common to use multiple cyclones in parallel for the capture of solids, assuming that gas–solid flow to be the same in the cyclones. This article presents a study investigating gas–solid flow through six parallel cyclones in a CFB cold test rig. The six cyclones were located asymmetrically on the left and right walls of the riser. Solid volume fraction and particle velocity profiles at the riser outlets and in the horizontal ducts were measured using a fiber optical probe. Cyclone pressure drop and solid circulating rate were measured for each individual cyclone. Measurements showed good agreement as to the non-uniform distribution of the gas–solid flow, which occurred mainly across the three cyclones on one side: the middle cyclones on both sides had higher particle velocities. Conversely, the solid volume fractions, solid fluxes and solid circulating rates of the middle cyclones were lower than those of the other four cyclones. The apparent reason for the flow non-uniformity among the cyclones is the significant flow non-uniformity at the riser outlets. Under typical operating conditions, the solid volume fractions at the riser outlets had a deviation of up to 26% whereas the solid circulating rates at the stand pipes, 7%. These results are consistent with most other studies in the literature.     

Numerical study of short-circuiting flow and particles in a gas cyclone

Short-circuiting flow is an important secondary flow in gas cyclones, which has a negative impact on the separation performance. To improve the understanding of the short-circuiting flow and guide the optimization of gas cyclones, this paper presents a numerical study of a cyclone using computational fluid dynamics. Based on the steady flow field, three methods were adopted to investigate the formation mechanism and characteristics of the short-circuiting flow and particles. The temporal variation of the tracer species concentration distribution reveals that the formation mechanism of the short-circuiting flow is the squeeze between the airflows entering the annular space of the gas cyclone at different times. The short-circuiting flow region, distinguished through the spatial distribution of the moments of age, is characterized by a small mean age and a large coefficient of variation. The proportion of the short-circuiting particles increases with the increase of the inlet velocity only for small particles. But with the increase of particle size, the proportion of the short-circuiting particles decreases faster at higher inlet velocities, resulting in significant differences in collection efficiency curves.     

旋风分离器压力损失及减阻杆的研究

本文介绍通过安装旋风分离器减阻杆前后压力损失及分离效率的测定，了解减阻杆对分离器性能的影响．根据流场测定结果，分析了减阻杆的减阻机理及减阻杆减阻时可保证分离效率的原因．     

Effect of inlet volute wrap angle on the flow field and performance of double inlet gas cyclones

In response to the divergent understanding of double inlet cyclone performance in the literature, the effect of inlet volute wrap angle on the performance and flow field of double inlet cyclone separator was studied by Computational Fluid Dynamic (CFD) method. The results showed that the inlet volute wrap angle can affect the comparison results of the single and double inlet gas cyclones with the same total inlet cross-sectional area and velocity. 0° and 90° volute double inlet improved the efficiency mainly by separating particles below 10 μm, while 180° volute double inlet had no separation advantage for any particles, so the symmetrical double inlet does not always improve the efficiency, and the appropriate inlet volute wrap angle should be selected according to the actual situation, otherwise, the expected performance requirements of the symmetrical double inlet cyclone cannot be achieved. Compared with the flow field, it is found that the inlet volute wrap angle changed the tangential velocity of the symmetrical double inlet cyclone separator, thus changing the performance.     

CFB cyclones at high temperature: Operational results and design assessment

Pressure drop and cut size measurements are reported for a full scale cyclone operating within a 58 MWth CFB-combustor unit at 775℃.The paper reviews the vast number of equations to calculate the pressure drop and separation efficiency of cyclones, generally for operation at ambient temperature and at low Cs[0.5]. None of the literature correlations predicts the pressure drop with a fair accuracy within the range of experimental operating conditions. The cut size d50 can be estimated using direct empirical methods or using the Stokes number, Stk50. Both methods were used to compare measured and predicted values of d50. With the exception of Muschelknautz and Krambrock, none of the equations made accurate predictions.Finally, an alternative method to determine the friction factor of the pressure drop equation (Euler number, Eu) and of the cut size is proposed. The Eu number is determined from the geometry of common cyclones, and the derived value of Stk50 defines more accurate cut sizes. The remaining discrepancy of less than 5%, when compared with the measured values, is tentatively explained in terms of a reduced cyclone diameter due to the solids layer formed near its wall. Further measurements, mostly using positron emission particle tracking, elucidate the particle motion in the cyclone and both tracking results and the influence of the particle movement on Eu and Stk50 will be discussed in a follow-up paper.     

Confined and Agitated Swirling Flows with Applications in Chemical Engineering

Turbulent, swirling flows are encountered frequently in chemical engineering practice. In this article, experiments and simulations on two classes of swirling flows, viz. agitated flows (stirred tanks), and confined swirling flows are discussed. Results of large-eddy simulations of stirred tank flow are compared with experimental data, mainly phase-resolved LDA data of the flow in the vicinity of the impeller. Next to the average velocity field, also the turbulent kinetic energy, and the anisotropy of the Reynolds stress tensor have been assessed. An important application of confined swirling flow is the cyclone separator (hydrocyclones for the separation of liquids, gas cyclones for gas-solid separation). The flow in a swirl tube geometry exhibiting many of the typical features of swirl flows (e.g. vortex breakdown) is discussed. Furthermore, a large-eddy simulation of the gas flow in a high-efficiency Stairmand cyclone separator is presented. Two examples of process modeling based on flow simulations are briefly treated: orthokinetic agglomeration of crystals in a stirred tank, and particle separation in a cyclone. This revised version was published online in July 2006 with corrections to the Cover Date.     

CFB cyclones at high temperature: Operational results and design assessment

Pressure drop and cut size measurements are reported for a full scale cyclone operating within a 58 MWth CFB-combustor unit at 775 ℃.
The paper reviews the vast number of equations to calculate the pressure drop and separation efficiency of cyclones, generally for operation at ambient temperature and at low Cs [〈0.5]. None of the literature correlations predicts the pressure drop with a fair accuracy within the range of experimental operating conditions. The cut size d50 can be estimated using direct empirical methods or using the Stokes number, Stk50. Both methods were used to compare measured and predicted values of d50. With the exception of Muschelknautz and Krambrock, none of the equations made accurate predictions.
Finally, an alternative method to determine the friction factor of the pressure drop equation （Euler number, Eu） and of the cut size is proposed. The Eu number is determined from the geometry of common cyclones, and the derived value of Stk50 defines more accurate cut sizes. The remaining discrepancy of less than 5%, when compared with the measured values, is tentatively explained in terms of a reduced cyclone diameter due to the solids layer formed near its wall. Further measurements, mostly using positron emission particle tracking, elucidate the particle motion in the cyclone and both tracking results and the influence of the particle movement on Eu and Stk50 will be discussed in a follow-up paper.     

Investigation of operational parameters for an industrial CFB combustor of coal, biomass and sludge

The combustion of coal and/or biomass （sludge, wood waste, RDF, etc.） in a circulating fluidized bed has been a commercial topper for over 20 years, and references to principles and applications are numerous and widespread although few data are presented concerning the operation of large scale CFB-units. The authors studied the CFB-combustion at UPM-Kymmene （Ayr）, a major paper mill relying for its steam production upon the combustion of coal （80-85 %）, wood bark （5-10%） and wastewater treatment sludge （5-10%）. The maximum capacity of the CFB is 58 MWth. A complete diagnostic of the operation was made, and additional tests were performed to assess the operating mode. The plant schematics, relevant dimensions and process data are given. To assess the operation of the UPM-CFB, it is important to review essential design parameters and principles of CFB combustors, which will be discussed in detail to include required data, heat balance and flowrates, operating versus transport velocity, kinetics and conversion （including the possible effect of the Bouduard reaction if carbon is present）. Since the residence time in the riser and the cyclone efficiency determine the burnout of circulating fuel-particles, the UPM-CFB was subjected to a stimulus response technique using nickel oxide as tracer. Results illustrate the efficiency of the cyclone separation and the number of recycle loops for particles of a given size. Results will also be used to assess the cyclone operation and efficiency and to comment upon expected and measured carbon conversion.     

An optimizing reduced order FDS for the tropical Pacific Ocean reduced gravity model

Proper orthogonal decomposition (POD) and singular value decomposition (SVD) methods are used to study a finite difference discretization scheme (FDS) for the tropical Pacific Ocean reduced gravity model. Ensembles of data are compiled from transient solutions computed from the discrete equation system derived by FDS for the tropical Pacific Ocean reduced gravity model. The optimal orthogonal bases are used to reconstruct the elements of the ensemble with POD and SVD. Combining the above approach with a Galerkin projection procedure yields a new optimizing FDS model of lower dimensions and high accuracy for the tropical Pacific Ocean reduced gravity model. An error estimate of the new reduced order optimizing FDS model is then derived. Numerical examples are presented illustrating that the error between the POD approximate solution and the full FDS solution is consistent with previously obtained theoretical results, thus validating the feasibility and efficiency of POD method. Copyright © 2007 John Wiley & Sons, Ltd.     

NUMERICAL SIMULATION OF THREE DIMENSIONAL GAS-PARTICLE FLOW IN A SPIRAL CYCLONE

The three-dimension gas-particle flow in a spiral cyclone is simulated numerically in this paper. The gas flow field was obtained by solving the three-dimension Navier-Stokes equations with Reynolds Stress Model (RSM). It is shown that there are two regions in the cyclone, the steadily tangential flow in the spiral channel and the combined vortex flow in the centre. Numerical results for particles trajectories show that the initial position of the particle at the inlet plane substantially affects its trajectory in the cyclone. The particle collection efficiency curves at different inlet velocities were obtained and the effects of inlet flow rate on the performance of the spiral cyclone were presented. Numerical results also show that the increase of flow rate leads to the increase of particles collection efficiency, but the pressure drop increases sharply.     

Numerical simulation of a dense solid particle flow inside a cyclone separator using the hybrid Euler–Lagrange approach

This paper presents a numerical simulation of the flow inside a cyclone separator at high particle loads. The gas and gas–particle flows were analyzed using a commercial computational fluid dynamics code. The turbulence effects inside the separator were modeled using the Reynolds stress model. The two phase gas–solid particles flow was modeled using a hybrid Euler–Lagrange approach, which accounts for the four-way coupling between phases. The simulations were performed for three inlet velocities of the gaseous phase and several cyclone mass particle loadings. Moreover, the influences of several submodel parameters on the calculated results were investigated. The obtained results were compared against experimental data collected at the in-house experimental rig. The cyclone pressure drop evaluated numerically underpredicts the measured values. The possible reason of this discrepancies was disused.     

Experimental and numerical study of a gas cyclone with a central filter

This paper studies a novel gas cyclone with a cylindrical filter face installed in the center from the vortex finder to the bottom hopper. The experimental results show that this composite cyclone has a higher collection efficiency and a lower pressure drop than the original cyclone. The mechanisms for the improvement are analyzed by both physical experiments and numerical simulations. By measuring dust samples collected at different places it is revealed that the center filter can prevent fine particles from entering the inner vortex and escaping, which accounts for the increase of the collection efficiency. In addition, the flow field of the composite cyclone is simulated by computational fluid dynamics and compared with that of the original cyclone. The analysis shows that with the filter layer installed, the swirling flow disappears in the vortex finder, which decreases the kinetic energy dissipation and hence lowers the pressure drop.     

Investigation of mini-hydrocyclone performance in removing small-size microplastics

Large amounts of microplastics (MPs) have been found in rivers and oceans, bringing great harm to aquatic animals, plants, even human beings. However, the effective removal method of MPs, especially those with small sizes (5–20 μm) is still lacking. This work presents mini-hydrocyclones to remove 10 μm (average size) diameter MPs. The removal performance of nine mini-hydrocyclones with different diameters of spigot and vortex finder is examined experimentally and numerically. The performance of the designed cyclones is evaluated in terms of recovery, water split, concentration ratio and pressure drop. The results show that mini-hydrocyclones are applicable to removing small-size MPs with the maximum concentration ratio at 2.16 and the particle recovery at 51%. The flow characteristics inside the mini-hydrocyclones are analyzed in detail. It is shown that the distributions of water axial velocity and radial velocity could collectively affect the behaviors of small-size MPs in mini-hydrocyclones. Specifically, a larger amount of water split could entrain more fine particles to underflow. Meanwhile, a less frequent alternation of radial velocity between the positive and negative directions on the same side of the cyclone should benefit the removal of small-size MPs.     

SIMULATION OF TYPHOON'S ANOMALOUS TRACK (Ⅰ)──RANKINE VORTEX MODEL

I.IntroductionTropicalcycloneisoneoftheseverestnaturaldisasterswhichcausescatastrophicdamages.StatisticshaveshownthatthenumberoftheinjuredandeconomicallossescausedbytyphooninChinarankthefirstandthedeathtollthesecond.Soitisimportanttoilllprovethetbrecastingaccuracyofthetyphoon'strackandalleviatethedisastersespeciallytarChina.Foralongtime.forecastingerrorintrackisalwaysilroUnd400---500kilometerill72hours.Scientistsilssumethatsucherrormaybeuttributedtotiletyphoon'sinnerilsymmctricttIstructure,…     

SIMULATION OF TYPHOON’S ANOMALOUS TRACK (Ⅰ)──RANKINE VORTEX MODEL

This paper proposes a method for simualting symmetric and simulating symmetric and asymmetric typhoonby using Rankine vortex model. Considering similarity between tropical cyclone andthe Rankine vortex,the paper has qualitatively discussed the feasibility of the methods In order to decide quantitatively Rankine vortex’s parameters to simulate typhoon’s structure, the paper has dealt with TCM data.for Yancy Typhoon (9012) as initial fields.. These results are considered as a foundation for further studying typhoon motion by CD approach.     

热带气旋的基本热力学分析

通过对热带气旋的热力学分析与计算, 发现热带气旋的能量主要来源于水蒸气凝结所释放的潜热. 气旋内外的温度差异使其产生了做功能力, 形成了一个热力循环, 该热力循环的效率在一定高度范围内随温度的增高有一个最大值: 对于一定的气旋底部温度,在饱和空气温度较低时, 其效率随着高度的增加存在极大值; 而当温度较高时, 则其效率随着高度一直增加. 当海面环境温度在33oC~34oC时, 效率达到最大值, 约为7.4%. 而输出功则随着温度的增加而一直增加, 因而气旋强度随着环境温度的增高而增大.      

STUDY OF AXIAL VELOCITY IN GAS CYCLONES BY 2D-PIV, 3D-PIY, AND SIMULATION

The axial velocity distribution in a gas cyclone has been examined with two-dimensional particle image velocimetry (2D-PIV) and three-dimensional particle image velocimetry (3D-PIV) experiments in this study. Due to the limitation of 2D-PIV configuration, the contamination generated by the strong tangential velocity in the cyclone can be registered in the axial velocity detected by 2D-PIV. Efficient methods are proposed in this work to remove this contamination. The contamination-removed 2D-PIV data agree well with 3D-PIV results. The distributions of the axial velocity are also computed by the Reynolds stress model (RSM) and verified using the PIV experimental results. Reasonable agreements are obtained.     

On the effect of the nozzle design on the performances of gas–liquid cylindrical cyclone separators

This paper constitutes an experimental study of the separation performances of a gas–liquid cylindrical cyclone (GLCC) separator that interests the oil industry. The global hydrodynamics behavior in the GLCC is characterized by flow visualization under various inflow operating conditions. The effect of the inlet nozzle design on the performances of the separator is studied by using three different nozzles, and it proves to be a key parameter. With an insufficient nozzle restriction, low swirl intensity is imparted to the flow. Due to inadequate centrifugal effects, liquid is prematurely carried over by the gas as flooding occurs in the separator upper part. High amounts of gas are also carried under by the liquid stream. On the other hand, with a too severe nozzle convergence, the important drag applied by the gas leads to liquid “short circuiting” the cyclone toward the gas outlet. In addition to the nozzle design, the separator performances are influenced by phenomena such as liquid bridging or the occurrence of the slug flow regime at the cyclone inlet. This paper leads to a better understanding of the links between the hydrodynamics in the GLCC and its operational limits, which is necessary to enable reliable scaling up tools.     

Aerodynamics and heat transfer in cyclones with particle-laden gas flow

Experiments were performed on a 204 mm diameter water-cooled cyclone to measure the pressure drop and heat transfer in different sections of the cyclone. Hot gas at 250°C entered the cyclone with and without suspended particles. Heat transfer and pressure drop in solids-free gas flow were compared with those measured for particle-laden gas flow of different solids.