Predictive models for characterizing the atomization process in pyrolysis of methyl ricinoleate

Pyrolysis of methyl ricinoleate(MR) can produce undecylenic acid methyl ester and heptanal which are important chemicals. Atomization feeding favors the heat exchange in the pyrolysis process and hence increases the product yield. Herein, predictive models to characterize the atomization process were developed. The effect of spray distance on Sauter mean diameter(SMD) of atomized MR droplets was examined, with the optimal spray distance to be 40–50 mm. Temperature mainly affected the physical properties of feedstock, with smaller droplet size obtained at increasing temperature. In addition, pressure had significant influence on SMD and higher pressure resulted in smaller atomized droplets. Then, a model for SMD prediction, combining temperature, pressure, spray distance, and structural parameters of nozzle, was developed through dimensionless analysis. The results showed that SMD was a power function of Reynolds number(Re), Ohnesorge number(Oh), and the ratio of spray distance to diameter of swirl chamber in the nozzle(H/dsc), with the exponents of -1.6618,-1.3205 and 0.1038, respectively. The experimental measured SMD was in good agreement with the calculated values, with the error within ±15%. Moreover, the droplet size distribution was studied by establishing the relationship between the standard deviation of droplet size and SMD. This study could provide reference to the regulation and optimization of the atomization process in MR pyrolysis.     

Droplets diameter distribution using maximum entropy formulation combined with a new energy-based sub-model

The maximum entropy principle (MEP) is one of the first methods which have been used to predict droplet size and velocity distributions of liquid sprays. This method needs a mean droplets diameter as an input to predict the droplet size distribution. This paper presents a new sub-model based on the deterministic aspects of liquid atom-ization process independent of the experimental data to provide the mean droplets diameter for using in the maximum entropy formulation (MEF). For this purpose, a theoretical model based on the approach of energy conservation law entitled energy-based model (EBM) is presented. Based on this approach, atomization occurs due to the kinetic energy loss. Prediction of the combined model (MEF/EBM) is in good agreement with the avail-able experimental data. The energy-based model can be used as a fast and reliable enough model to obtain a good estimation of the mean droplets diameter of a spray and the combined model (MEF/EBM) can be used to wel predict the droplet size distribution at the primary breakup.     

CFD modeling of immiscible liquids turbulent dispersion in Kenics static mixers: Focusing on droplet behavior

The present study is concerned with the computational fluid dynamics(CFD) simulation of turbulent dispersion of immiscible liquids, namely, water–silicone oil and water–benzene through Kenics static mixers using the Eulerian–Eulerian and Eulerian–Lagrangian approaches of the ANSYS Fluent 16.0 software. To study the droplet size distribution(DSD), the Eulerian formulation incorporating a population balance model(PBM) was employed. For the Eulerian–Lagrangian approach, a discrete phase model(DPM) in conjunction with the Eulerian approach for continuous phase simulation was used to predict the residence time distribution(RTD) of droplets.In both approaches, a shear stress transport(SST) k-ω turbulence model was used. For validation purposes, the simulated results were compared with the experimental data and theoretical values for the Fanning friction factor, Sauter mean diameter and the mean residence time. The reliability of the computational model was further assessed by comparing the results with the available empirical correlations for Fanning friction factor and Sauter mean diameter. In addition, the influence of important geometrical and operational parameters, including the number of mixing elements and Weber number, was studied. It was found that the proposed models are capable of predicting the performance of the Kenics static mixer reasonably well.     

Numerical simulation of heavy fuel oil atomization using a pulsed pressure-swirl injector

It is known that increasing the injection pressure reduces the breakup length and the droplet size. Adding pulses, on the other hand, helps to atomize the liquid into finer droplets, similar to air-assisted injectors but without altering the air-to-fuel concentration.To further reduce the droplet size and breakup length, a novel injector type, called ‘‘Pulsed Pressure-Swirl"(PPS), is introduced in this work, which is a combination of pressure-swirl and ultrasonic pulsed injectors. A pressure-swirl atomizer was designed and fabricated specifically for Mazut HFO(Heavy Fuel Oil). The droplet formation process and droplet size distribution have been studied experimentally(by shadowgraphy high speed imaging) and numerically(with the open-source Volume-of-Fluid code Gerris).Changing liquid injection pressure effect on the spray angle and film thickness has been quantified.These simulations have been used to study the primary breakup process and quantify the droplet size distributions, using different injection pulse frequencies and pressures.The numerical results have revealed that the new injector concept successfully produces finer droplets and results in a decrease in the breakup length, especially when applying high pulse frequencies, with no significant changes in the spray angle.     

Experimental study of a heavy fuel oil atomization by pressure-swirl injector in the application of entrained flow gasifier

The available SMD(Sauter mean diameter) correlations on pressure-swirl injectors predict droplet sizing very different from each other, especially for heavy fuels. Also there was a lack in the literature for comparing available correlations. So an experimental study was conducted on a heavy fuel oil(HFO) spray, Mazut 380. A pressure swirl injector was designed and fabricated. The experiments for Mazut at 40 °C and 80 °C were compared with the results for water, including spray half cone angle, breakup length and mean droplet diameter,at different injection pressures. Lower spray angle, higher breakup length and larger droplets were observed for lower injection pressures and higher liquid viscosity. SMD was about 75 μm for water and about 87 μm for Mazut at 80 °C. The results for droplet mean diameter were also compared with correlations from previous studies on pressure swirl atomizers. The SMD results show that for water spray, LISA method was in good agreement,also Babu and Ballester correlations were successful when high viscosity fluid was injected.     

Numerical simulation and experimental study of the characteristics of packing feature size on liquid flow in a rotating packed bed

Rotating packed bed has high efficiency of gas–liquid mass transfer. So it is significant to investigate fluid motion in rotating packed bed. Numerical simulations of the effects of packing feature size on liquid flow characteristics in a rotating packed bed are reported in this paper. The particle image velocimetry is compared with the numerical simulations to validate the turbulent model. Results show that the liquid exists in the packing zone in the form of droplet and liquid line, and the cavity is droplet. When the radial thickness of the packing is less than 0.101 m, liquid line and droplets appear in the cavity. When rotational speed and radial thickness of the packing increase, the average diameter of the droplets becomes smaller, and the droplet size distribution becomes uniform. As the initial velocity of the liquid increases, the average droplet diameter increases and the uniformity of particle size distribution become worse. The droplet velocity increases with the radial thickness of the packing increasing, and gradually decreases when it reaches the cavity region. The effect of packing thickness is most substantial through linear fitting. The predicted and simulated values are within ±15%. The cumulative volume distribution curves of the experimental and simulated droplets are consistent with the R-R distribution.     

基于离散颗粒法模拟搅拌釜气含率分布

The discrete particle method was used to simulate the distribution of gas holdup in a gas-liquid standard Rushton stirred tank. The gas phase was treated as a large number of bubbles and their trajectories were tracked with the results of motion equations. The two-way approach was performed to couple the interphase momentum exchange. The turbulent dispersion of bubbles with a size distribution was modeled using a stochastic tracking model, and the added mass force was involved to account for the effect of bubble acceleration on the surrounding fluid. The predicted gas holdup distribution showed that this method could give reasonable prediction comparable to the reported experimental data when the effect of turbulence was took into account in modification for drag coefficient.     

Experimental evaluation and modeling of liquid jet penetration to estimate droplet size in a three-phase riser reactor

In this work,the effects of injecting an evaporating liquid jet into solid–gas flow are experimentally investigated.A new model(SHED model) and a supplementary model(spray model) have also been proposed to investigate some flow-field characteristics in three-phase fluidized bed with the mean relative error 4.3% between model and measured results.Some experiments were conducted to study the influences of flow-field parameters such as liquid volumetric flow rate,injection velocity,jet angle and gas superficial velocity as well as solid mass flux on the jet penetration depth(JPD).In addition,independent variables were experimentally employed to propose two empirical correlations for JPD by using multiple regression method and spray cone angle(SCA) by using dimensional analysis technique.The mean relative errors between the JPD and SCA correlations versus experimental data were 7.5% and 3.9%,respectively.In addition,in order to identify the variable effect,a parametric study was carried out.Applying the proposed model can avoid direct use of expensive devices to measure JPD and to predict droplet size.     

单喷雾射流在受限空间内的错流混合(英文)

In order to achieve uniform mixing between spray droplets and crossflow, cold-model experiment of a hollow-cone water spray in an air crossflow is investigated via a numerical simulation. The simulation cases are designed by using the orthogonal design method. The Eulerian-Lagrangian formulation is employed for modeling the droplets-crossflow two-phase flow while the realizable k-ε turbulence model is used to describe the turbulence. A new index, mixedness quality, is proposed to assess the overall mixing of the droplets in the crossflow. The simulation results demonstrate that the counter-rotating vortex pair (CVP) imposes a more significant impact on the spatial distribution than on the size distribution of the droplets. Pairs of CVP with smaller scales are preferable for achieving a better mixing. The influencing factors are listed in the following order in terms of the degree of their impact from the greatest to the least: the Sauter diameter of the initial droplets, the mixing tube diameter, the spray angle, the velocity of the inlet crossflow, and the vertical velocity of the initial droplets. A moderate droplet diameter, a smaller tube diameter, a moderate spray angle, a greater crossflow velocity and a moderate vertical velocity of the droplet are favorable for achieving a higher mixedness quality of the jet spray in a confined crossflow.     

Spray and mixing characteristics of liquid jet in a tubular gas–liquid atomization mixer

For the design and optimization of a tubular gas–liquid atomization mixer,the atomization and mixing characteristics of liquid jet breakup in the limited tube space is a key problem.In this study,the primary breakup process of liquid jet column was analyzed by high-speed camera,then the droplet size and velocity distribution of atomized droplets were measured by Phase-Doppler anemometry (PDA).The hydrodynamic characteristics of gas flow in tubular gas–liquid atomization mixer were analyzed by computational fluid dynamics (CFD) numerical simulation.The results indicate that the liquid flow rate has little effect on the atomization droplet size and atomization pressure drop,and the gas flow rate is the main influence parameter.Under all experimental gas flow conditions,the liquid jet column undergoes a primary breakup process,forming larger liquid blocks and droplets.When the gas flow rate (Q_g) is less than 127 m~3·h~(-1),the secondary breakup of large liquid blocks and droplets does not occur in venturi throat region.The Sauter mean diameter (SMD) of droplets measured at the outlet is more than 140μm,and the distribution is uneven.When Q_g127 m~3·h~(-1),the large liquid blocks and droplets have secondary breakup process at the throat region.The SMD of droplets measured at the outlet is less than 140μm,and the distribution is uniform.When 127Q_g162 m~3·h~(-1),the secondary breakup mode of droplets is bag breakup or pouch breakup.When 181Q_g216 m~3·h~(-1),the secondary breakup mode of droplets is shear breakup or catastrophic breakup.In order to ensure efficient atomization and mixing,the throat gas velocity of the tubular atomization mixer should be designed to be about 51 m·s~(-1)under the lowest operating flow rate.The pressure drop of the tubular atomization mixer increases linearly with the square of gas velocity,and the resistance coefficient is about 2.55 in single-phase flow condition and 2.73 in gas–liquid atomization condition.     

喷淋塔液滴粒径分布及比表面积的实验研究

以水和空气为实验介质,通过拍照法获得喷淋塔内液滴粒径分布,考察了不同喷淋量及空塔气速对塔内不同高度处液滴Sauter平均直径(SMD)的影响,并对液滴粒径分布进行了理论分析.结果表明,喷淋塔顶部液滴分布密集,底部稀疏,液滴群在下落过程中,平均粒径减小且趋于均匀化;塔顶处液滴SMD随喷淋量的增加而增大,处在塔中下部的液滴SMD则随喷淋量增大而减小,提高空塔气速,可减小平均粒径;理论分析认为,液滴粒径减小主要是由于发生了碰撞破碎的缘故,而塔内液滴大小不一是碰撞的主要原因;通过量纲1化拟合得到喷淋塔内液滴SMD经验关联式,其计算结果与实验值吻合较好;考虑液滴破碎的喷淋塔比表面积比不考虑破碎的比表面积大70%左右.     

甘露醇喷雾干燥过程中液滴粒度分布变化的群体粒数衡算模拟和实验研究

利用群体粒数衡算（population balance，PB）计算机模拟和实验研究了甘露醇水溶液的喷雾干燥过程中液滴的粒度分布的变化规律。液滴干燥过程中的颗粒粒度的萎缩速率，在群体粒数衡算模型中描述为液滴的逆（或负）生长项，通过单个液滴反应动力学方法（reaction engineering approach，REA）获得。基于单个液滴干燥的反应工程方法模型REA和群体粒数衡算模型PB集成建立了PBREA模型。PBREA 模型的求解是通过高分辨率数值方法。本文模拟研究了不同工况下，不同粒径液滴的干燥时间、液滴平均含湿量以及液滴粒度分布随时间的变化。结果显示，液滴粒径越大，干燥时间越长，模型预测的颗粒平均粒径为实验值的1.0~1.5倍，粒度分布跨度是实验值的0.61~0.89倍。模拟误差主要来源于液滴及颗粒粒径分布统计精度、单个静止液滴与群体运动液滴干燥的差异、热导率及扩散系数是经验值3个方面。在使用Buchi 290 小型喷雾干燥仪进行的实验中，使用了图像采集和分析方法得到了液滴及颗粒的数密度分布，并和模拟结果做了对比。结果表明该模型可以有效地预测喷雾干燥过程中干燥颗粒的平均粒度及分布跨度。     

APPLICATION OF LASER DIAGNOSTIC TECHNIQUES FOR THE EXAMINATION OF LIQUID FUEL SPRAY STRUCTURE

Results are presented on the spatial distribution of droplet mean size and number density obtained from a hollow-cone kerosene spray, introduced into nonswirling and swirling flow fields. An ensemble light scattering technique, based on measurement of the polarization ratio, has been employed to determine local droplet characteristics in both dense and dilute regions of the spray. The measurements are complemented with Lorenz-Mie calculations of the scattering characteristics for a polydispersion of droplets; the calculations were carried out for different mean sizes and refractive indices. The results reveal that the degree of swirl imparted to the surrounding air flow has a strong influence on spray structure. For all conditions examined the droplet mean size is found lo be larger on the spray boundary than towards the centerline. Droplet mean size is also found to increase with axial distance at all radial positions of the spray; this trend is attributed to the vaporization of smaller droplets and/or possible coalescence between the droplets.

In addition to the ensemble technique, measurements have also been obtained with the phase/Doppler interferometry and light intensity deconvolution techniques under identical experimental conditions. The droplet mean sizes obtained with the ensemble approach are in general smaller than those measured with the phase/Doppler technique; however, general features of the radial profiles obtained with both techniques are similar. The deconvolution technique also indicates the presence of smaller size droplets and supports the results obtained with the ensemble technique. The selective sensitivity of these sizing techniques to different ranges of droplet size and number density is discussed.     

An atomization model for splash plate nozzles

A model for the atomization and spray formation by splash plate nozzles is presented. This model is based on the liquid sheet formation theory due to an oblique impingement of a liquid jet on a solid surface. The continuous liquid sheet formed by the jet impingement is replaced with a set of dispersed droplets. The initial droplet sizes and velocities are determined based on theoretically predicted liquid sheet thickness and velocity. A Lagrangian spray code is used to model the spray dynamics and droplet size distribution further downstream of the nozzle. Results of this model are confirmed by the experimental data on the droplet size distribution across the spray. © 2009 American Institute of Chemical Engineers AIChE J, 2010     

Droplet collision processes in an inter-spray impingement system

The main objectives of the present article are in developing and evaluating a new model for the droplet collision, and in discussing the physical feature for the mutual impinging sprays. The new model is originally devised to be capable of predicting the droplet collision-induced breakup accompanied with the formation of satellite droplets. From the well-established conservation equations, the several formulae are newly proposed by consideration of some important regimes such as bounce, coalescence, and stretching and reflexive separation. This formulation makes it possible to predict the number of satellite droplets, and the droplet sizes and velocities. The extensive experiments are conducted for the inter-spray impingement systems under the various conditions to investigate the influence of impinging angle and impingement distance upon the spray characteristics. The droplet sizes and velocities are measured by the phase Doppler particle analyzer system and the distribution of liquid volume fraction is also measured by the self-manufactured patternator. In addition, numerical predictions by the new model and the O’Rourke model are compared with experimental data for the various parameters. It is found that the SMD (Sauter mean diameter) measured in the downstream region after inter-spray impingement are 40% smaller than those for the case of single nozzle. The new model shows vigorous breakup during collision, which is no longer seen in the O’Rourke model.     

HAN基液体推进剂喷雾场液滴尺寸分布的简化模型

为了建立HAN基液体推进剂喷雾场液滴尺寸分布的简化模型,基于最大熵原理,在质量守恒与概率和为定值的约束条件下,用拉格朗日因子法则推导了液滴尺寸分布函数,将该函数应用于HAN基液体推进剂模拟工质喷雾场液滴尺寸的预估;采用相位多普勒粒子动态分析仪(PDA)测量了HAN基液体推进剂模拟工质在1.8~2.6MPa喷射压力下对撞式喷嘴雾化液滴尺寸分布;通过最小二乘法优化Nukiyama-Tanasawa分布函数式中的q值对数目微分分布进行了修正,拟合得到q值与喷射压力p之间的函数关系为q(p)=-0.344p2+1.525p+1.268。结果表明,计算得出的液滴尺寸数目微分分布与HAN基液体推进剂喷雾场实测数据变化趋势相符,表明建立的液滴尺寸分布模型是合理的。     

Aerodynamic characteristics of FGD spray towers

On the basis of the balance of forces acting on droplets falling down inside a co- or counter-current spray tower, a model of scrubber aerodynamic characteristics was developed. Relations between pressure drop caused by spray, droplet concentration and residence time distribution of the dispersed phase were found analytically for slurry droplet motion in the range of 400<Re<2500. The differences between the simplified model solutions and complete solutions are presented. The model was verified by the experimental data collected in two large pilot plant scrubbers applied for flue gas desulfurization (FGD).     

转炉一次除尘新OG系统高效喷淋塔喷嘴雾化特性的模拟

利用离散相模型对转炉一次除尘新OG系统高效喷淋塔内喷嘴的雾化特性进行模拟,分析了喷射角度、喷射压力、喷射流量及喷嘴水平间距等因素对雾化场索太尔平均直径(SMD)和蒸发效率的影响. 结果表明,在一定范围内随喷射角度增加,液滴在雾化场中的覆盖面增大,液滴驻留时间变长,蒸发效率增加,雾化场SMD减小,喷射角度大于60o时,SMD值减小缓慢. 随喷射压力增大,液滴蒸发效率增加,雾化场SMD减小,压力大于1.0 MPa时对SMD的影响较小. 随喷射流量增加,液滴蒸发效率减小,雾化场SMD增加,流量小于0.15 kg/s时,SMD增加幅度偏小. 两喷嘴水平间距越大,液滴分布面积越大,但对雾化场SMD影响较小. 在一定条件下,喷嘴间距约为800 mm时,截面速度分布较均匀.     

双液滴碰撞行为及调控机制的研究进展

双液滴碰撞行为广泛存在于雨滴形成、燃油喷雾、喷雾冷却、喷墨印刷、农药喷洒等自然现象与工业应用过程中,其碰撞结果会受到液滴参数及气相环境等因素的综合影响,研究双液滴的碰撞行为规律及调控机制一直是该领域的热点。结合目前双液滴碰撞的实验进展和数值模型,将围绕着碰撞行为的主控因素与调控机制展开综述,具体介绍了碰撞参数、液滴理化性质、气相环境等因素对液滴碰撞行为的影响规律与调控结果,并展望了液滴碰撞理论及应用的发展趋势和方向。     

Development of an axisymmetric population balance model for spray drying and validation against experimental data and CFD simulations

An incremental model for spray drying, including a full droplet size distribution, has been implemented in a flowsheeting package incorporating tracking of distributed particle properties. Results were compared with expected trends based on standard theory and with results from a laboratory-scale spray dryer with a two-fluid nozzle for atomization. Predicted trends were as expected, with larger droplets giving substantially longer drying times and higher final moisture content. Predicted final moisture content was lower than measured values, as the very short residence times for fine particles were inadequately represented by first-order falling-rate drying kinetics. Dryer gas flow patterns were simulated by computational fluid dynamics. Calculated droplet residence times were much lower than for a plug-flow or fully mixed gas flow, because a high-velocity gas flow zone from the two-fluid atomizer persists down a substantial part of the dryer.