Theoretical study of liquid droplet dispersion in a venturi scrubber

The droplet concentration distribution in an atomizing scrubber was calculated based on droplet eddy diffusion by a three-dimensional dispersion model. This model is also capable of predicting the liquid flowing on the wall. The theoretical distribution of droplet concentration agrees well with experimental data given by Viswanathan et al. for droplet concentration distribution in a venturi-type scrubber. The results obtained by the model show a non-uniform distribution of drops over the cross section of the scrubber, as noted by the experimental data. While the maximum of droplet concentration distribution may depend on many operating parameters of the scrubber, the results of this study show that the highest uniformity of drop distribution will be reached when penetration length is approximately equal to one-fourth of the depth of the scrubber. The results of this study can be applied to evaluate the removal efficiency of a venturi scrubber.     

Mathematical Modeling of SO2 Absorption in a Venturi Scrubber

Abstract

A three-dimensional mathematical model was used to predict the removal efficiency of a venturi scrubber for SO₂ absorption into a water and alkaline solution. In order to obtain better results, nonuniform droplet concentration distribution was considered. The results of the model with nonuniform droplet concentration distribution are compared with several sets of experimental data, as well as with prediction data of a mathematical model with uniform droplet concentration distribution. Without exception, all comparisons indicated that including nonuniformity of droplet concentration distribution in the model will significantly improve the agreement between the experimental data and predicted values.     

喷嘴数量对文丘里除尘器性能影响的模拟

选用欧拉-欧拉多相流模型和RNG k-ε湍流模型对文丘里除尘器流动特性进行数值模拟,首先对2个喷嘴时不同液体速度下的压力降和喉管液体分布情况进行模拟,并与实验值进行了比较,得到了较好的一致性;然后比较了2个喷嘴和4个喷嘴的情况下不同液体速度时文丘里除尘器的压力降和液体分布情况。模拟结果表明：欧拉-欧拉多相流模型可准确地模拟文丘里除尘器内部流场;喷嘴数量对文丘里除尘器内部的压力降和液体分布有较大影响,当相对射流深度为0.2,喉管气速为64 m·s-1,喷嘴数量由2增加为4时,压力降增加了17.17%,在喷嘴数目为4,喉管气速为74 m·s-1,液体速度为12.5 m·s-1时可使液体在文丘里除尘器内部得到较均匀的分布。     

球柱形旋风除尘器分离性能数值模拟与实验

针对传统柱锥形旋风除尘器存在细颗粒分离效率低的问题,提出了一种新型球柱形旋风除尘器。采用数值模拟和实验研究手段,分析了柱段高度对球柱形旋风除尘器分离特性的影响。模拟结果表明：当球柱形旋风除尘器柱段高度不为零时,随着柱段高度的增加,静压力逐渐变小;球柱形旋风除尘器内流体的切向速度均呈“M”型分布;流体轴向速度随着半径的减小,其绝对值先增大后减小,在中心轴线处又开始增大;流体径向速度均关于中心轴线对称。实验结果表明,当球柱形旋风除尘器柱段高度为150 mm时,总分离效率最高,可达到92.01%。研究结果可为旋风除尘器中细小颗粒分离应用提供指导,对提高5 μm以下颗粒分离效率具有重要意义。     

基于剪切乳化机的采油污水再现配置与分散相破碎乳化行为

以剪切乳化机为主体,设计、加工了一套采油污水室内再现配置装置,探讨了含油量、剪切强度(与流量同比变化,下同)等不同配置条件下的再现采油污水粒度分布、乳化度、Zeta电位和粘度等性能参数以及乳化机前后的压力损失.结果表明,该采油污水再现配置装置,能满足不同性质采油污水的配置要求,剪切乳化机的压力损失随着剪切强度的增大而明显升高,而随含油量增大没有明显的变化趋势,呈现出基本稳定的状态.较大的剪切强度创造了湍流流体条件,强化了分散相颗粒之间的碰撞聚结行为,因而再现采油污水的乳化度随剪切强度增大而降低,这种降低的程度又受含油量的影响:低含油量时剪切强度的影响明显,高含油量时这种影响逐渐减弱并趋于平缓.鉴于高含油量时趋于形成大粒径油珠,再现采油污水的乳化度随含油量增大而呈现略微的下降趋势,这种下降趋势的程度同时受剪切强度的影响:低剪切强度时含油量的影响较为明显,而高剪切强度时的剪切条件削弱了含油量的影响.Zeta电位随着剪切强度增大而不断增大,使得分散相颗粒带电绝对值降低,乳化体系有脱稳的趋势,导致颗粒间的碰撞聚结机会变大,与乳化度降低的趋势相吻合.剪切强度和含油量对再现采油污水粘度的影响不大.     

旋风除尘器流场及浓度场实验与模拟

在研究旋风除尘器内气固两相的运动状况及分离机理方面,计算机模拟替代部分实验的方法能够优化设计旋风除尘器结构参数,提高其对微细颗粒的捕集效率,减少运行压力损失。本研究采用RSM模型和随机轨道模型对旋风除尘器内流场及浓度场进行模拟及实验。研究表明,旋风除尘器压力损失模拟结果与实验结果吻合较好,对于大于5μm的颗粒其捕集效率模拟结果与实验结果基本吻合;旋风除尘器外壁的颗粒浓度呈螺旋带状分布;如将排气管管径减少至原直径0.8倍,可使其对2μm颗粒捕集效率提高6.6%,但压力损失提高36.5%;颗粒的凝并作用有利于提高旋风除尘器微细颗粒的捕集效率。     

移动颗粒床除尘器的除尘性能

为进一步了解移动颗粒床除尘器的除尘性能,首先解决了除尘器中容易出现的流动静止区域的消除问题,并实验研究了除尘效率及压降与表观过滤风速和滤料质量流率之间的关系,结果表明当表观过滤风速为0.3 m·s-1,滤料质量流率为300 g·min-1时,除尘效率高达99.8%,压降最大还不及180 Pa。另外,通过数值仿真得到了滤料床层内部气体的流场分布以及灰尘粒子在床层中的运行轨迹,发现除尘器结构部件对气体流场分布以及灰尘粒子轨迹有重要影响。对仿真过程中逃逸出滤料床层的灰尘粒子进行统计,并与实验数据相比较,验证了尘饼对增强细微灰尘捕集能力的重要性。     

Scaling of spontaneous imbibition data with wettability included

Wettability is a dominant parameter governing spontaneous imbibition. However less attention has been paid to the effect of wettability on the scaling of spontaneous imbibition data. Actually few models can include wettability in scaling of spontaneous imbibition data. To this end, a scaling model has been developed for NAPL (oil)-saturated porous media with different wettability based on the fluid flow mechanisms in porous media. Relative permeability, capillary pressure, initial water saturation, and wettability are considered in the scaling model. Theoretically this scaling model is suitable for both cocurrent and countercurrent spontaneous imbibition. The experimental data of countercurrent spontaneous water imbibition at different wettability cannot be scaled using the frequently used scaling model but can be scaled satisfactorily using the scaling model developed in this study. An analytical solution to the relationship between recovery and imbibition time for linear spontaneous imbibition has also been derived in the case in which gravity is ignored. The analytical solution predicts a linear correlation between the recovery by spontaneous water imbibition and the square root of imbibition time, which has been verified against experimental data.     

Aerosol size distribution during a condensational growth process. Measurements and comparison with theory

Changes of a unimodal aerosol size distribution due to condensational particle growth were observed. Water droplets growing in an expansion chamber were illuminated by a laser beam and the scattered light flux was measured under various scattering angles. The droplet size distribution was obtained using a best-fit procedure and has been found to be independent of the choice of condensation nuclei (NaCl-or DOP-particles). The absolute standard deviation of the aerosol size distribution was found to decrease with increasing droplet radius in good agreement with model calculations based upon the first order theory of droplet growth. Thus mutual interactions of the diffusion fields around the growing droplets are negligible under the considered experimental conditions.     

孔道结构对柴油机DPF压降及再生特性的影响

针对微粒捕集器（DPF）内部碳烟及灰分颗粒特征,运用AVL-Fire软件建立了六边形孔道结构柴油机微粒捕集器模型。 针对不同排气流量、进口温度、孔密度、碳烟和灰分沉积量,对六边形孔道及四边形孔道DPF压降特性和碳烟再生特性进行分析,并研究灰分分布形式对不同孔道形状DPF的影响。 结果表明：排气质量流量越大,进口温度越高,不同孔道结构的压降敏感性增大;与传统四边形孔道DPF相比,当碳烟沉积量较低时,六边形孔道DPF压降损失较高;随着碳烟沉积量的增加,六边形孔道DPF压降损失较低,且碳烟承载量较大;灰分在DPF孔道表面层状分布可以有效阻止碳烟深床捕集模式,降低压降损失;六边形孔道DPF能够有效提高碳烟及灰分容量,且碳烟捕集及再生效率较高,再生速率较快,热应力较小,可以降低DPF主动再生频率,延长使用寿命。     

More general capillary pressure and relative permeability models from fractal geometry

More general capillary pressure and relative permeability models were derived theoretically from fractal modeling of a porous medium. It was found that the new capillary pressure model could be reduced to the frequently-used Brooks–Corey capillary pressure model and the Li–Horne imbibition model when the fractal dimension of a porous medium takes specific values. This also demonstrates that the Brooks–Corey model and the Li-Horne model have a further confirmed theoretical basis. Capillary pressure data measured using mercury intrusion techinque were used to verify the model. The results demonstrated that the new capillary pressure model could represent the capillary pressure curves in those rocks with fracures or with great heterogeneity while the existing models cannot. The new relative permeability models can be reduced to the Brooks–Corey relative permeability model in a specific case. It has been proved theoretically that the relative permeability of each phase in a smooth fracture is only a linear function of its own saturation. Relative permeability data were calculated using the new models and the model results were compared with experimental data measured using a steady-state technique. The comparison demonstrated that the relative permeability models and experimental results were consistent with each other.     

湿法烟气脱硫系统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。建立了数学模型,对气体相采用基于雷诺时均方程的SST ｋ-ω湍流模型封闭N-S方程,对液滴相采用基于Euler-Lagrange的DPM方法。通过调节除雾器的结构参数和工作参数,揭示了气液两相流动的流场,分析了结构参数对除雾器分离效率和工作压力降的影响,可用于湿法烟气脱硫系统中折板式除雾器的设计和优化。     

The effects of drop size and formulation upon the spread of pesticide droplets impacting on water‐sensitive papers

Abstract

The effects of drop size and formulation upon the spread of pesticide droplets impacting on water‐sensitive papers (WSPs) was investigated. Droplets of diameter 70–350 μm, of four permethrin formulations, were produced using a monosize droplet generator. The droplets were collected on WSPs and in Dow Corning fluid and their diameters measured using a binocular microscope. Spread factors, upon the WSPs, for each dropsize/formulation combination were then calculated. Spread factors varied with both formulation and droplet size and for droplets of 200 and 300 μm diameter spread was significantly correlated with the dynamic surface tension of the formulation. The results suggest that proper calibration of WSPs is necessary for effective droplet sizing.     

新型离心式脱硫塔气液两相流数值模拟

利用FLUENT软件和SIMPLE算法对新型旋流脱硫塔的气液两相流场进行了数值模拟。计算中气相采用了RSM湍流模型,颗粒相采用了Lagrange坐标系下的随机轨道模型。分析结果表明,气相流场具有强旋流特性;喷射液滴的直径、喷淋量和烟气流速影响其在塔内的分布:喷射液滴粒径越大、喷射量越小、烟气流速越大,入口段降温越少;塔体上方截面平均浓度随液滴粒径的增加而降低,随液气比的增加而增加,随烟气流速的增加会先增加至最高值然后降低。喷淋液滴在其他运行参数不变时,平均粒径范围为0.5～1 mm,会对进口烟气起到较好的净化与降温的作用,并使塔体上方喷淋液滴在截面z=4.15 m处浓度分布均匀且覆盖率高;在保证液滴粒径较小时,通过降低烟气流速或增加喷淋量可提高液滴喷淋覆盖率,使得烟气与喷淋充分接触。计算得到的气相流场分布与实测值吻合较好,证明了数学模型的合理性,为进一步优化分离器结构提供了可靠依据。     

废水液滴在低温烟气中的蒸发特性数值研究

建立了液滴在低温低速烟气中运动和蒸发的数学模型,数值研究了液滴在不同的烟气速度和烟气温度环境中的蒸发特性,获得了液滴速度、温度和直径随时间的变化规律,以及烟气速度和温度对液滴蒸发特性的影响。计算结果表明：气相速度越大,液滴初速度对液滴运动速度的影响越小,液滴速度越容易达到烟气速度。对于初始直径和速度一定的液滴,气液初始相对速度越大,液滴达到临界蒸发温度所需的时间越短,完全蒸发时间也越短。在文中研究的烟气速度范围内,整体上气相速度的改变对液滴完全蒸发时间的影响很小。气相温度的改变对液滴达到临界蒸发温度的时间以及完全蒸发时间的影响很大。烟气从较高温度和较低温度分别下降相同幅度的温度时,液滴在烟气中的完全蒸发时间按不同的倍数在增加,烟气温度越低,液滴完全蒸发时间增加的越多。对于文中研究的液滴,无量纲完全蒸发时间随烟气温度变化呈指数递减规律。     

酸再生烟气净化旋流器的分离性能

盐酸再生系统吸收塔出口的烟气中含有大量酸性雾滴和粉尘,严重影响尾气排放。为了减少排放,提出利用微型旋流器将酸雾和粉尘进行分离,通过研究旋流器的压降、分离效率以及含液浓度等关键因素,确定符合实际工况的最佳工艺参数。研究结果表明,旋流器分离效率随压降增大先快速增加再逐渐减小,压降和分离效率随进口含液浓度的增大而略微增加。氯化氢和氧化铁高效工作区分别为18~33 m3·h-1和18~35 m3·h-1,最高分离效率达到92%和89%,烟气净化效果显著。     

纤维膜过滤特性的数值模拟

一个高性能过滤膜除了与纤维自身结构参数有关联外,过滤时的操作条件对其也有重要影响.为研究纤维膜过滤效率、过滤压降与过滤速度、孔隙率以及过滤膜厚度之间的关系,以更好地对实际过滤进行优化,本研究从纤维膜的过滤机理进行出发,采用FLUENT多孔介质离散相模型对纤维过滤器的流场进行模拟.结果表明,膜前后压降会随着过滤速度和膜厚度的增大而增大;一定范围内纤维膜孔隙率适度增加,截留率变化不大,但是膜压差会逐渐减小;同时过滤速度对颗粒截留率有影响,速度增大时,粒子轨迹发生偏移,导致过滤膜一部分过早发生堵死,而一部分作用减弱,影响了过滤效果.     

Development of a measuring technique for simultaneous in situ detection of nanoscaled particle size distributions and gas temperatures

Point measurements of time-resolved LII signals have been performed in sooting premixed low pressure flames. Soot particle size distribution and gas temperature in these flames are known from independent measurements. This data is used to validate parameters of an improved LII model, where special emphasis is taken on the accurate modelling of mass and heat transfer rates. Using this model particle size distributions and gas temperatures can be estimated from time-resolved LII signals using non-linear regression. Standard numerical methods are applied. An experimental setup is presented, which allows measuring one-dimensional maps of particle size distribution and gas temperature. The technique is based on the one-dimensional and time-resolved detection of LII signals using a Streak camera.     

湿雾抑尘内混式空气雾化喷嘴的选型

针对工业粉尘控制问题,在稳态喷雾实验台上对多孔空气雾化喷嘴和扇形空气雾化喷嘴的雾化性能进行实验研究。结果发现,在喷嘴入口水压和空气压力一定的实验条件下,扇形喷嘴具有更大的喷雾锥角,但多孔喷嘴的液滴索特平均直径SMD明显低于扇形喷嘴;多孔喷嘴的雾化锥角和SMD随水流量变化基本保持稳定,液体质量在抑尘平面位置分布均匀,而扇形喷嘴的雾化锥角和SMD随水流量增加而明显增大,在抑尘位置液滴质量分布差异显著。综合来看,多孔空气雾化喷嘴的雾化效果优于扇形空气雾化喷嘴。     

Modelling dispersion of traffic pollution in a deep street canyon: Application of CFD and operational models

In this study, numerical modelling of the flow and concentration fields has been undertaken for a deep street canyon in Naples (Italy), having aspect ratio (i.e. ratio of the building height H to the street width W) H/W = 5.7. Two different modelling techniques have been employed: computational fluid dynamics (CFD) and operational dispersion modelling. The CFD simulations have been carried out by using the RNG k–? turbulence model included in the commercial suite FLUENT, while operational modelling has been conducted by means of the WinOSPM model. Concentration fields obtained from model simulations have been compared with experimental data of CO concentrations measured at two vertical locations within the canyon. The CFD results are in good agreement with the experimental data, while poor agreement is observed for the WinOSPM results. This is because WinOSPM was originally developed and tested for street canyons with aspect ratio H/W ≌ 1. Large discrepancies in wind profiles simulated within the canyon are observed between CFD and OSPM models. Therefore, a modification of the wind profile within the canyon is introduced in WinOSPM for extending its applicability to deeper canyons, leading to an improved agreement between modelled and experimental data. Further development of the operational dispersion model is required in order to reproduce the distinct air circulation patterns within deep street canyons.