Effect of drainage channel dimensions on the performance of wave-plate mist eliminators

We investigated the effects of drainage channel dimensions on droplet removal efficiency and pressure drop of the gas droplet flow in a wave-plate mist eliminator. Droplet dispersion in turbulent gas flows is numerically simulated using eddy interaction model (EIM) and Eulerian-Lagrangian method. Reynolds stress transport model (RSTM) with enhanced wall treatment and shear stress transport (SST) k-ω model are used for simulating the turbulent airflow. Comparison between the numerical simulations and available experimental data shows that eddy lifetime constant (C _L ) can affect the results significantly, and by selecting suitable values of the eddy lifetime constant, both turbulence models give reasonable predictions of droplet removal efficiency. Simulations of gas droplet flow in the eliminators with various drainage channel dimensions show that the drainage channel length (L _DC ) has a greater effect on droplet removal efficiency than the drainage channel width (W _DC ).     

Comparative efficiency evaluations of four types of cooling tower drift eliminator, by numerical investigation

This work presents a comparative evaluation of the performance of four types of wave-plate drift eliminator commonly used in mechanical cooling towers, with similar morphology. The droplet collection efficiency and the coefficient of pressure drop are numerically calculated, for values of inlet velocity and droplet diameter , with Reynolds number and inertial parameter roughly in the ranges 650≤Re≤8500 and 0.05≤P_i≤2.5, respectively. The numerical model has been validated through comparisons with numerical and experimental results taken from the literature, including additional configurations of horizontal wave-plate mist separators. The effects of considering the turbulence dispersion of droplets are studied, as well as the influence of the inertial parameter and the aspect ratio of the eliminator sample channel. Best results are obtained by using the SST k−ω turbulence model, with values of non-dimensional scaled distance to wall y⁺ comprised in the range 0.2–0.5, including the turbulent dispersion of droplets. A global correlation for the collection efficiency is proposed, as a function of the inertial parameter and the removal geometric parameter, which is introduced in this work. Finally, it is developed a procedure focused on establishing the overall efficiency for each type of eliminator, based in a key power function.     

螺旋片导流式分离器分离性能的数值模拟与试验研究

应用计算流体力学（CFD）方法对不同结构参数的螺旋片导流式气液分离器在湍流状态下的流体流动场进行了数值模拟,研究了螺旋个数和螺距对螺旋片导流式气液分离器分离性能的影响,并与试验测定结果对比,由此验证基于数值模拟方法设计螺旋片导流式气液分离器分离效率的可行性与准确性.分析结果表明,数值模拟结果与试验数据基本一致,可以作为螺旋片导流式气液分离器设计的有效工具.     

The influence of airflow on fuel spray characteristics from a slit injector

Optimization of fuel spray, airflow, and their interaction with the cylinder and piston wall is crucial to achieve stable combustion of stratified charge with minimum emissions in direct-injection spark-ignition (DISI) engines. In this study, the interaction between air and fuel spray from slit injector was investigated in a steady airflow system generated by a wind tunnel under atmospheric conditions. Both Mie scattering images and phase Doppler anemometry (PDA) measurements of the spray were analyzed for different air velocities. Three-dimensional computational fluid dynamics (3-D CFD) have been employed to further explain the mutual interaction between air and spray. It was found that increasing the airflow velocity across the spray results in a significant change in the bottom part of the spray while a slight change was observed close to the nozzle exit. The variation in spray geometry, which is mainly attributed to an aerodynamic effect and to extracted droplets from the main spray by the airflow effect, was evaluated and presented for different air velocities. The spray droplet size redistribution within the spray plume was investigated, and regions with smaller and larger droplets were identified and discussed. The results indicate that the effect of airflow pattern on droplet size distribution within the spray is a considerable factor in the optimization of airflow and spray together. This could be considered in achieving a limited ignitable region without much diffusion of smaller droplets to the non burning zone during the part load operation of DISI engines.     

Mathematical modelling of a hydrocyclone for the down-hole oil–water separation (DOWS)

In this study, a mathematical model is developed to predict the efficiency of a down-hole oil–water separation hydrocyclone. In the proposed model, the separation efficiency is determined based on droplet trajectory of a single oil droplet through the continuous-phase. The droplet trajectory model is developed using a Lagrangian approach in which single droplets are traced in the continuous-phase. The droplet trajectory model uses the swirling flow of the continuous-phase to trace the oil droplets. By applying the droplet trajectory, a trial and error approach is used to determine the size of the oil droplet that reaches the reverse flow region, where they can be separated. The required input for the proposed model is hydrocyclone geometry, fluid properties, inlet droplet size distribution and operational conditions at the down hole. The model is capable of predicting the hydrocyclone hydrodynamic flow field, namely, the axial, tangential and radial velocity distributions of the continuous-phase. The model was then applied for some case studies from the field tested DOWS systems which exist in the literature. The results show that the proposed model can predict well the split ratio and separation efficiency of the hydrocyclone. Moreover, the results of the proposed model can be used as a preliminary evaluation for installing a down-hole oil–water separation hydrocyclone system in a producing well.     

Optimal design of drainage channel geometry parameters in vane demister liquid–gas separators

Vane liquid–gas demisters are widely used as one of the most efficient separators. To achieve higher liquid disposal and to avoid flooding, vanes are enhanced with drainage channels. In this research, the effects of drainage channel geometry parameters on the droplet removal efficiency have been investigated applying CFD techniques. The observed parameters are channel angle, channel height and channel length. The gas phase flow field was determined by the Eulerian method and the droplet flow field and trajectories were computed applying the Lagrangian method. The turbulent dispersion of the droplets was modeled using the discrete random walk (DRW) approach. The CFD simulation results indicate that by applying DRW model, the droplet separation efficiency predictions for small droplets are closer to the corresponding experimental data. The CFD simulation results showed that in the vane, enhanced with drainage channels, fewer low velocity sectors were observed in the gas flow field due to more turbulence. Consequently, the droplets had a higher chance of hitting the vane walls leading to higher separation efficiency. On the other hand, the parameters affect the liquid droplet trajectory leading to the changes in separation efficiency and hydrodynamic characteristic of the vane. To attain the overall optimum geometry of the drainage channel, all three geometry parameters were simultaneously studied employing 27 CFD simulation cases. To interpolate the overall optimal geometry a surface methodology method was used to fit the achieved CFD simulation data and finally a polynomial equation was proposed.     

除雾器内雾滴运动特性与除雾效率

利用流体动力学计算方法对湿法脱硫折流板除雾器内气液两相流动进行数值模拟.分析了除雾器叶片间距、板型及流速对不同粒径雾滴的分级除雾效率和总除雾效率的影响,获得了不同粒径雾滴的运动和捕集规律.研究结果表明,粒径小于10 μm的雾滴去除效率随流速增加呈现不规律的波动,随板间距增加而下降的趋势不明显,不受叶片形状变化影响;粒径大于16.3 μm的雾滴去除效率随流速增加而增大,随板间距增加而显著下降;在板间距为38 mm时,梯形板除雾效率大于三角形板,在板间距较小的情况下两种板型的性能相差不大;流速小于3 m·s^-1时,粒径小于20 μm的雾滴的去除对气流均匀性要求较高,气流扰动增加利于小雾滴的碰撞聚并;流速高于3 m·s^-1时,气流扰动增强增加了小雾滴运动的随机性,不利于小雾滴的碰撞聚并.     

Vaporizing Microdroplet Inhalation,Transport, and Deposition in a Human Upper Airway Model

Evaluation of injuries from inhalation exposure to toxic fuel requires detailed knowledge of inhaled aerosol transport and deposition in human airways. Focusing on highly toxic, easily volatized JP-8 fuel droplets, the three-dimensional airflow, temperature distributions, and fluid-particle thermodynamics, i.e., droplet motion as well as evaporation, are simulated and analyzed for laminar as well as locally turbulent flow conditions.

Specifically, using a commercial finite-volume software with user-supplied programs as a solver, the Euler-Lagrange approach for the fluid-particle thermodynamics is employed with: (1) a low Reynolds number k-ω model for laminar-to-turbulent airflow, and (2) a stochastic model for random fluctuations in the droplet trajectories with droplet evaporation. Presently, the respiratory system consists of two major segments of a simplified human cast replica, i.e., a representative oral airway from mouth to trachea (Generation 0) and a symmetric four-generation upper bronchial tree model (G0 to G3). Experimentally validated computational fluid-particle thermodynamics results show that evaporation of JP-8 fuel droplets is greatly affecting deposition in the human airway. Specifically, droplet deposition fractions due to vaporization decrease with increasing ambient temperatures and decreasing inspiratory flow rates. It is also demonstrated that assuming idealized velocity profiles and particle distributions in or after the trachea may greatly overpredict particle deposition efficiencies in the upper bronchial tree.     

Transport and Removal of Expiratory Droplets in Hospital Ward Environment

The transport and removal characteristics of expiratory droplets at different supply airflow rates and “coughing” orientations were investigated both numerically and experimentally in a three-bed hospital ward setting. A Lagrangian-based particle-tracking model with near-wall correction functions for turbulence was employed to simulate the fate of the expiratory droplets. The model was tested against experimental droplet dispersion data obtained in an experimental hospital ward using Interferometric Mie Imaging and a light-scattering aerosol spectrometer. The change in airflow supply rate had insignificant effect on the transport and deposition of very large droplets (initial sizes ≥ 87.5 μm) due to the dominance of gravitational settling on these behaviors. Smaller droplets (initial sizes ≤ 45 μm) exhibited certain airborne behaviors. The effect of thermal plumes from heat sources was observed only when the supply airflow was low and when the droplet size was small, as observed in the vertical mixing patterns of the droplets of various sizes. Larger droplets tended to settle lower and lateral dispersion of the droplets became weak at the low supply airflow rate. The deposition characteristics for different surfaces in the room are described. The heat plumes seemed to obstruct small droplets from being deposited onto heated surfaces. More deposition was predicted in the lateral injection case compared with the vertical injection case. Adopting near-wall correction for turbulence in the model reduced the predicted deposition removal fraction by 25% for 1.5 μm droplets. This reduction became less significant for larger droplets due to the smaller dependence on turbulent diffusion in their deposition.     

W-FGD中折板式除雾器性能的数值模拟

采用CFD软件对湿法烟气脱硫系统中广泛使用的折板式除雾器的主要性能进行了数值模拟.在模拟过程中,建立了数学模型,对气相采用基于雷诺时均方程的SST湍流模型封闭N-S方程,对液相采用基于Euler-Lagrange方法的DPM方法.通过调节除雾器的结构参数和工作参数,揭示了气固两相流动的流场,分析了结构参数和工作参数对除雾器分离效率和工作压力降的影响,得出相关性规律,可用于湿法烟气脱硫系统中折板式除雾器的设计和优化.     

Multi-objective optimization of drainage-plates in wave-plate mist eliminators using experiment and data-driven modelling for lower water loss and energy requirement

Wave-plate mist eliminators are widely employed as gas–liquid separation devices to prevent the liquid escaping from thermal power plants or other cooling towers. In this study, the wave-plate mist eliminator with drainage plates was numerically analyzed and the effects between geometrical variables on two objectives, namely, pressure drop (ΔP) and separation efficiency (η), were revealed. Plate spacing, width, and length, as well as the relative position of the drainage plate, were thoroughly investigated. A combined strategy was developed for multi-objective optimization of the wave-plate mist eliminator by integrating computational fluid dynamics (CFD) simulation, response surface methodology (RSM), non-dominated sorting genetic algorithm-II (NSGA-II), and a technique for order of preference by similarity to ideal solution (TOPSIS) method. The results demonstrated that the relative position of drainage plates has a greater impact on the overall performance, whereas the width of drainage plates has the minimum effect. With the implementation of NSGA-II and the TOPSIS method, an optimal solution for the design of the mist eliminator was obtained. After comparing with the baseline case, the optimized case presents promising characteristics with high separation efficiency (enhanced by 3.6%~9.06%) and a low energy consumption coefficient (reduced by 72.30% at η = 45%).     

折流板式除沫器性能分析及研究进展

针对生产实践中遇到液滴夹带影响设备及工艺的情况介绍了高效率、低压降、体积小的折流板式除沫器的应用背景.分析了工业上常用的折流板式除沫器类型、转折角度、叶片间距和级数等设计参数,临界分离粒径dcr和临界气流速度等特性参数对分离效率、压降和处理量等主要性能指标的影响情况.对折流板式除沫器国内外研究进展、存在的问题及发展方向进行了综述,指出通过建立数学模型结合理论分析,推导出除沫器效率理论计算式显得十分重要.对除沫器的力学性能进行分析,得出一些改进措施可使除沫器整体性能得到优化;采用高速摄影等先进实验设备对折板除沫器壁面液滴的运动及之后的沉积过程进行研究仍属较新的方向.     

丙烯腈装置气液分离器的出口结构

气液分离器是丙烯腈生产装置的关键设备之一,用于除去气体产品中的酸雾. 分离器的出口结构是影响分离效率的关键因素之一. 本研究借助通用流体分析软件PHOENICS对气液分离器内流场进行了分析,结果发现,增大液滴向器壁运动机会和减弱二次夹带效应均有利于提高分离效率. 基于此原则,对气体出口结构进行了优化实验,并提出了最佳的分离器双层套筒出口结构. 这种结构与其他几种结构相比除雾效果更佳,不管在低空速还是高空速下都能保持较高的除雾效率,操作弹性较好. 工业应用也表明,采用双层套筒出口结构的气液分离器具有较高的除酸雾效果,优于原分离器.     

Effects of the Inlet Section Angle on the Flow Field of a Cyclone

The gas flow fields of a cyclone with different inlet section angles have been studied numerically. The gas flow fields were simulated by means of the Reynolds Stress Transport Model (RSTM). The velocities and pressure drop profiles of these cyclones were investigated. The shortcut flow rates at the bottom of the vortex finder were calculated with different inlet section angles. To analyze the relationship between the inlet section angle and the vortex finder insertion deepness, this paper details the shortcut flow rates at the bottom of the vortex finder for three vortex finder insertion depths. The results indicate that the inlet section angle can decrease the shortcut flow from the bottom of the vortex finder, which has practical importance for the improvement of the separation efficiency. The inlet section angle can also decrease the pressure coefficient of a cyclone. When the inlet section angle is 45 °, the level of decrease is up to 30 %. However, the effect of the inlet section angle on the separation performance is related to the dimension of the vortex finder, i.e., the insertion depth and diameter of the vortex finder, and the effect is different when the cyclone has different vortex finder insertion depths.     

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.     

Numerical Simulation of the Separating Performance of Hydrocyclones

The flow behavior in hydrocyclones is quite complex. The Computational Fluid Dynamics (CFD) method was used to simulate the flow fields inside a hydrocyclone in order to improve its separation efficiency. The RSM turbulent model (Reynolds Stress Model), which abandons the isotropic eddy‐viscosity hypothesis, was used to analyze the highly swirling flow fields in hydrocyclones. The ASM Model (Algebraic Slip Mixture Model) was used to simulate the separation performance. The volume fraction distribution and grade efficiency curve are given. The separating efficiency for 60 μm water particles is more than 90 %. The majority of 60 μm water particles are carried to the underflow. An increase in particle size will improve the efficiency by increasing the centrifugal force on the particles. Based on the simulation, the effects of the overflow tube dimensions on the separation performance were studied. The overflow tube dimensions of the hydrocyclone were modified, and the results showed that the Reynolds Stress Model successfully predicted the characteristics of the flow, and the simulated performances were in good agreement with those obtained by tests.     

海水淡化用汽液分离元件研究进展

汽液分离元件是热法海水淡化装置中至关重要的组成部分,它的主要作用是除去蒸汽中的盐雾液滴,保证淡化产品水水质。通过优化研究汽液分离元件的结构和操作工况可以提高其分离性能,达到提升装置效率、提高产水水质和节约生产成本的目的。本论文综述了应用于海水淡化装置的折流板式和丝网式汽液分离元件的性能特点,总结了国内外研究进展及部分工程案例,并对其发展前景进行了展望。     

Prediction of strongly swirling flow within an axial hydrocyclone using two commercial CFD codes

Two commercial CFD codes were used to simulate the strongly swirling single-phase flow with core recirculation within an axial hydrocyclone. Both packages used a Differential Reynolds Stress Model with default constants for the turbulence closure. The effect of omitting wall reflection terms was also investigated and it was generally found that this lead to better agreement with experiment. Interestingly, the predicted velocity profiles from the two CFD codes did not agree with each other. Possible reasons for this are different turbulence modelling approaches with different terms for the turbulent diffusion and rapid pressure-strain terms.     

折板式汽液分离元件的性能

采用空气-水体系,对折板式汽液分离元件进行冷态模拟实验,研究了气速、液滴粒径、折板形式、间距及安装方式等参数对折板分离效率和压降的影响. 结果表明,气速控制在1~6 m/s,液滴粒径控制在50~80 mm,折板采用立式和卧式安装,形式采用V形和W形,间距为10~50 mm,对分离性能均有一定的影响,折板间距为20 mm的分离元件具有较高的分离效率. 通过对冷态实验数据进行数学回归分析,获得描述折板性能的经验公式,与实验值误差在10%内,同时在海水淡化中试试验装置中验证了折板式汽液分离元件的分离性能.