新型定容燃烧喷雾模拟实验装置的开发及应用

研制成功了一种无需将燃烧弹壳体加热到高温,只需点燃可控预充可燃混合气即可模拟现代柴油机喷油时缸内高温、高压、高密度工作环境下高压燃油喷射特性及油束撞壁混合过程的定容燃烧喷雾模拟实验装置。此装置可以采用平面激光诱导荧光法(PL IF)等多种光学方法对柴油喷雾油束特性及油束撞壁混合过程进行测量。经实验验证,此装置可以模拟实际发动机高达6.5 M Pa、1000 K的缸内高压、高温工作环境,并给出了模拟不同压力和温度时预充气组分各分压的理论计算方法及实验所得的压力—时间曲线,为测量现代柴油机的喷雾特性及油束撞壁混合过程提供了必要的条件。

多层螺旋雾幕技术的喷雾模拟及降尘试验研究

为了解决掘进工作面传统喷雾除尘无法有效抑制粉尘扩散的问题,提出了一种将气动喷嘴呈螺旋状布置的多层螺旋雾幕除尘方法。采用k-ω湍流模型与K-H液滴破碎模型,通过Comsol多物理场耦合数值模拟,得出了多层螺旋雾幕速度场分布和粒子轨迹的变化规律,结合模拟结果搭建试验平台,并将多层螺旋雾幕与传统喷雾的除尘效果进行对比分析。以2层雾幕为例的模拟结果显示,喷雾2 s时就会形成明显的2层螺旋状雾幕,10 s后旋转水雾充满整个模拟区域,并且雾滴粒径较传统喷雾更小。试验表明多层螺旋雾幕的除尘效果明显强于自然降尘与传统喷雾,3 min后可将浓度470 mg/m^3左右的粉尘降到4 mg/m^3以下。

缸内直喷汽油机的喷雾模拟

在定容弹内测量了某直列4缸均质缸内直喷汽油机不同时刻的喷雾油束形状和喷油器附近位置的喷雾液滴直径及速度分布,并在CFD模型中进行了喷雾的标定。分析了原机缸内喷雾、混合情况,研究了喷雾锥角、喷孔布置对缸内混合气均匀性的影响,论述了在低转速、部分负荷时加进气翻板的作用。结果表明,调整喷雾锥角、喷孔布置方式可以改善直喷汽油机缸内空燃比分布的均匀性;采用进气翻板可以提高发动机低转速部分负荷时缸内的滚流比及紊流强度,从而改善缸内混合气质量及加快缸内燃烧速度。

提高喷雾模拟精度的算法

燃油喷雾是直喷式发动机燃烧的关键过程,喷雾模拟的精度决定了燃烧计算的可靠性.然而,在KIVA和商业CFD代码中,传统喷雾模拟技术对网格精度非常敏感.因而,预测的发动机性能和排放依赖于计算网格.导致这个问题的两个主要原因是液滴碰撞算法和气液相间耦合.为了提高喷雾模拟精度,采用交错网格液滴碰撞(CMC)算法和气相速度插值算法对原始KIVA代码进行了修正.在定容燃烧室和直喷式柴油机条件下,分别从喷雾结构、预测平均液滴尺寸和喷雾贯穿距三个方面检验了改进KIVA代码对喷雾模拟精度的改善.结果表明,网格依赖性显著降低.通过这些改进,喷雾结构的失真现象消失.在定容燃烧室计算中,预测的平均液滴尺寸的不确定性从30μm减小到5μm;在发动机模拟中,这种不确定性进一步减小到2μm.在中等和精细网格条件下,发动机模拟预测的喷雾贯穿距也获得了较好的一致性.

多区喷雾模拟——优化重型柴油机喷射参数的有效方法

本研究的目的是考查喷射参数对重型柴油机性能和排放特性的影响。为对采用高压共轨喷射系统的柴油喷射和雾化特性进行分析,对于诸如喷射延迟期、喷射持续期以及喷油率和喷孔数之类的喷射特性,均采用一种准维模型进行研究。在本研究中,喷油率的改变是在各种不同的喷油参数下实现的,同时将对发动机性能和排放进行模拟。最后,将探讨适用于高效、低排放重型柴油机的最佳喷射系统。

乙醇、柴油及其混合燃料的喷雾特性

为了研究乙醇、柴油及其混合燃料的喷雾特性,采用试验和计算的方法获得了乙醇-柴油混合燃料的物性,并将其用于Wave-KH破碎子模型来模拟混合燃料的喷雾过程.模拟结果能够较好地与纹影摄像结果相吻合.研究表明,乙醇及乙醇-柴油混合燃料的喷雾贯穿距小于柴油,喷雾锥角略大于柴油,喷雾形态的分布范围较大,与空气混合更好.

喷雾降尘用化学除尘剂的实验研究

为了改善水与粉尘的结合能力,提高煤矿喷雾降尘效果,进一步改善矿井工作面的工作环境,通过应用JK99C型表面张力仪和视频光学接触角测量仪对优选的SDS、CAB-35、JFCS、BS-12四种表面活性剂溶液不同浓度下的表面张力和接触角进行了测试,确定其CMC,并以此浓度作为四种表面活性剂溶液的配比浓度;通过两两单体复配的原则,对6种复配溶液进行了煤粉沉降与反向渗透实验,并结合喷雾模拟实验,确定了降尘用表面活性剂的最优配方为质量分数为0.05%的CAB-35+JFCS的复配溶液,具有较好的降尘效果。

燃油粘度与环境压力对喷雾碰撞过程的影响

O’Rourke碰撞模型仅仅考虑了碰撞拉伸分离以及聚合,而忽略了碰撞反弹、反射分离以及高韦伯数下的碰撞二次破碎过程,在碰撞分区过程中也忽略了燃油物性以及碰撞环境条件的影响,使得预测的碰撞分区临界韦伯数偏低。复合碰撞模型根据相关的实验结果对碰撞分区进行油滴粘度以及碰撞环境压力修正,考虑碰撞反弹、反射分离、拉伸分离以及碰撞二次破碎过程,提高了碰撞分区的预测精度。基于复合碰撞模型,分析了油滴粘度与环境压力对喷雾碰撞过程的影响。结果表明环境压力的增加,使得碰撞反弹概率增加而聚合概率下降;燃油粘度的增加使得碰撞聚合比例增加而碰撞分离比例下降。

直喷发动机的喷雾光学标定

为了提高缸内CFD分析喷雾模拟的准确度,运用Converge软件进行了定容弹内的喷雾光学标定和光学发动机的喷雾标定。在催化器起燃工况(1 200 r/min@0.1 MPa)和中速中负荷(2 000 r/min@0.6 MPa)两个工况下,分别验证了喷雾的形态和贯穿度,并且在催化器起燃工况下,进行了机油湿壁的验证。结果表明,利用Converge软件对喷雾模型进行标定,能够与定容弹实验和光学发动机吻合较好。

温度对表面活性剂溶液润湿性影响规律研究

为将煤矿井下开采过程中粉尘的防治与实际环境结合,采用实验方法研究温度对表面活性剂溶液润湿性能的影响,从流变性和黏附性角度解释表面活性剂润湿性改变的原因,并通过喷雾模拟降尘实验得出表面活性剂最佳使用温度范围和除尘效率。结果表明:当温度范围为14~32℃时,温度高的溶液润湿效果更好;温度升高,表面活性剂的流动性、黏附性增强,在煤表面扩散和吸附,润湿效果增强;表面活性剂ɑ-烯烃硫酸钠(AOS)、脂肪酸甲酯乙氧基化物磺酸盐(FMES)、脂肪醇聚氧乙烯醚(JFC)、烷基糖苷(APG)、脂肪醇聚氧乙烯醚(AEO),从14~32℃降尘效率分别提高17.81%、20.23%、17.87%、16.22%和18.43%,温度在18~26℃时,表面活性剂降尘效率提高最明显,可以考虑提高溶液温度增强其润湿性时,添加能够提高溶液抗蒸发性的物质来提升降尘效果。

生物柴油-乙醇-柴油混合燃料的燃烧过程模拟与分析

为了研究生物柴油-乙醇-柴油混合燃料的燃烧特性,使用AVL-FIRE软件对纯柴油和B30E10(含30%体积分数的生物柴油,10%体积分数的乙醇和60%体积分数柴油的生物柴油-乙醇-柴油混合燃料)的缸内燃烧和喷雾过程进行模拟分析,将燃烧室进行切片,比较了两种燃料的速度场、压力场和温度场变化及变化规律。研究结果表明:在燃烧初期,B30E10燃烧比柴油慢,随着燃烧的进行,当燃油堆积于燃烧室凹槽后,B30E10在缸内的燃烧速度、压力和缸内温度均比柴油低。

喷雾仿真中的网格依赖性

基于离散油滴模型,比较了原始喷雾模型与修正喷雾模型的网格依赖性.计算结果表明:原始喷雾模型由于采用ORourke碰撞模型以及最近节点法耦合算法,使得预测结果出现明显的网格依赖性.相反,修正喷雾模型采用独立的拉格朗日碰撞单元降低碰撞过程的网格依赖性,联合使用拉格朗日插值算法以及气相射流模型降低气液耦合过程对于网格的依赖性.因此修正喷雾模型能获得较好的低网格依赖性从而保证粗大网格下的计算精度.

Simulation of Droplet-gas Flow in the Effervescent Atomization Spray with an Impinging Plate

Abstract A comprehensive three-dimensional model of droplet-gas flow was presented to study the evolution of spray in the effervescent atomization spray with an impinging plate. For gas phase, the N-S equation with the κ-ε turbulence model was solved, considering two-way coupling interaction between droplets and gas phase. Dispersed droplet phase is modeled as Lagrangian entities, accounting for the physics of droplet generation from primary and secondary breakup, droplet collision and coalescence, droplet momentum and heat transfer. The mean size and sta- tistical distribution of atomized droplets at various nozzle-to-plate distances were calculated. Some simulation resuits were compared well with experimental data. The results show that the existence of the impinging plate has a pronounced influence on the droplet mean size, size distribution and the droplet spatial distribution. The air-to-liquid ratio has obvious effects on the droplet size and distribution.

Numerical simulation of temperature and velocity fields in plasma spray

Based on the turbulence jet model, with respect to Ar-He mixture plasma gas injecting to ambient atmosphere, the temperature filed and velocity field under typical working conditions were investigated. Given the conditions of I=900 A, FAr= 1.98 m^3/h, FEe=0.85 m^3/h, it is found that both the temperature and the velocity undergo a plateau region near the nozzle exit （0-10 mm） at the very first stage, then decrease abruptly from initial 13 543 K and 778.2 m/s to 4 000 K and 260.0 m/s, and finally decrease slowly again. Meanwhile, the radial temperature and radial velocity change relatively slow. The inner mechanism for such phenomena is due to the complex violent interaction between the high-temperature and high-velocity turbulent plasma jet and the ambient atmosphere. Compared with traditional methods, the initial working conditions can be directly related to the temperature and velocity fields of the plasma jet by deriving basic boundary conditions.

An Investigation of Error Sources in Computational Fluid Dynamics Modelling of a Co-current Spray Dryer

The paper is focused on identifying error sources in computational fluid dynamics(CFD) predictions of a spray drying process. Seven groups of drying and atomisation parameters were selected for analysis and 13simulation trials were performed. The theoretical results were compared with experimental data and sensitivity of the simulation results to the analysed factors was determined. The following parameters affecting the accuracy of CFD spray modelling were found: gas turbulence model, particle dispersion, atomising air, initial parameters of atomisation and heat losses to the environment. A major difference in the errors committed during modelling of spray drying process for fine and coarse sprays was observed.

Advances in Large-eddy Simulation of Two-phase Combustion (I) LES of Spray Combustion

Spray combustion is widely used in power, transportation, chemical and metallurgical, iron and steel making, aeronautical and astronautical engineering. In recent years, large-eddy simulation (LES) becomes more and more attractive, because it can give the instantaneous flow and flame structures, and may give more accurate statistical results than the Reynolds averaged Navier-Stokes (RANS) modeling. In this paper, the present status of the studies on LES of spray combustion is reviewed, and the future research needs are discussed.

Numerical Simulation of Atomization due to Flood Discharges of Hydropower Stations

To mitigate the damages produced by flood discharges of hydropower stations, a three-dimensional numerical model of the aerated water jet restricted by gravity, air resistance and air buoyancy is proposed. Based on theoretical analysis and prototype data, a three-dimensional stochastic model is constructed using Monte Carlo method to evaluate the range of atomization and intensity of rainfall in gorges, which are strongly affected by complex terrain and various wind conditions. The prototype data observed from two hydropower stations are selected in the feedback and verification analysis to verify the proposed model. The result shows that the computational solutions fit the prototype data well. This model can be used to forecast the atomization of a hydropower station that is being designed or constructed.

Modeling the influence of forced ventilation on the dispersion of droplets ejected from roadheader-mounted external sprayer

In order to reveal the influence of forced ventilation on the dispersion of droplets ejected from roadheader-mounted external sprayer,the paper studies the air-flowing field and the droplet distribution under the condition of gentle breeze and normal forced ventilation in heading face using the particle tracking technology of computational fluid dynamics(CFD).The results show that air-flowing tendency in the same section presents great comparability in the period of gentle breeze and forced ventilation,and the difference mainly embodies in the different wind velocity.The influence of ventilation on the dispersion of droplets is faint under the gentle breeze condition.The droplet can be evenly distributed around the cutting head.However,under the normal forced ventilation,a large number of droplets will drift to the return air side.At the same time,droplet clusters are predominantly presented in the lower part of windward side and the middle of the leeward side around the cutting head.In contrast,the droplet concentration in other parts around cutting head decreases a lot and the droplets are unable to form close-grained mist curtain.So the dust escape channel is formed.In addition,the simulation results also reveal that the disturbance of air flow on the droplet distribution can be effectively relieved when using ventilation duct with Coanda effect(VDCE).Field experiment results show that the dust suppression efficiency of total dust and respirable dust increases respectively by 10.5%and 9.3%when using VDCE,which proves that it can weaken the influence of airflow on droplet dispersion.

Coupling of 3D Eulerian and Lagrangian Spray Approaches in Industrial Combustion Engine Simulations

The Lagrangian DDM （discrete droplet model） is state-of-the-art for CFD （computational fluid dynamics） simulations of mixture formation and combustion in industrial engines. A commonly known drawback of the DDM approach is the attenuated validity in the dense spray, where the bulk liquid disintegrates into droplets. There the assumption of single droplets surrounded by a homogenous gas field is not reasonable. In this region, the Eulerian-Eulerian multi-phase approach performs better because instead of parcels the spray is represented by the volume fractions of one bulk liquid and several droplet size class phases. A further drawback of the DDM approach is that increasing the spatial resolution of the computational grid leads to a reduced statistical convergence, since the number of spray parcels per computational cell becomes smaller. It is desirable to combine the benefits of both spray approaches in coupled CFD simulations. Therefore, the dense spray region is simulated separately with the Eulerian spray approach on a highly resolved mesh covering only the region close to the nozzle orifice. The entire engine domain with combustion and emission models is simulated with the Eulerian-Lagrangian spray approach for the dilute spray region. The two simulations are coupled through exchange of boundary conditions and model source terms. An on-line coupling interface manages the data transfer between the two simulation clients, i.e., Eulerian spray and engine client. The aim of this work is to extend the coupled spray approach in terms of exchanging combustion related heat and species sources, and consequently creating the link between Eulerian spray and combustion models. The results show mixture formation and combustion in real-case engine simulations, and demonstrate the feasibility of spray model combination in engineering applications.

Large-eddy Simulation of Ethanol Spray-Air Combustion and Its Experimental Validation

Large-eddy simulation of spray combustion is under its rapid development.Different combustion models were used by different investigators.However,these models are less validated by detailed experimental data.In this paper,large-eddy simulation(LES)of ethanol spray-air combustion was made using an Eulerian-Lagrangian approach,a subgrid-scale kinetic energy stress model,and a filtered finite-rate combustion model,neglecting the sub-grid scale reaction rate.The simulation results are compared with experimental dada in the literature and validated in detail.The LES obtained statistically averaged gas temperature is in much better agreement with the experimental results than Reynolds averaged(RANS)modeling using the most complex probability density function(PDF)equation combustion model.The instantaneous LES results show the coherent structures of the shear region near the high-temperature flame zone and the fuel vapor concentration map,indicating that the droplets are concentrated in this shear region.The instantaneous temperature,oxygen and carbon dioxide concentration maps show the close interaction between the coherent structures and the combustion reaction.