超声速气流中液体横向射流雾化过程数值模拟

为了更加深入了解超燃冲压发动机燃烧室中的燃料雾化机理,对来流Mach数为1.94的超声速气流中液体横向射流的雾化过程进行了数值模拟研究.计算采用Euler-Lagrange方法,液滴二次破碎模型采用K-H/R-T模型.计算结果表明:考虑液滴二次破碎时,采用雾化锥模型获得的射流穿透深度以及液滴速度分布与实验结果符合得很好...     

旋转射流雾化的直接数值模拟研究

雾化现象涉及到气液界面复杂的拓扑变化,使得雾化过程极其复杂。目前针对一次雾化的机理研究及模型尚不够准确,由于数值计算方法的快速发展,越来越多的研究者采用数值方法来研究一次雾化。本文采用大规模的直接数值模拟手段和Level Set追踪气液界面的方法模拟旋转射流的一次雾化过程,以揭示旋流雾化的机理和界面不稳定波的贡献。     

关于自由射流计算的两点考虑

本文用Multi-Block算法和标准的κ-ε模型计算轴对称自由射流,计算中发现:(1)不同的纵向网格宽度比(疏密度比)会导致不同的扩张速率的计算结果。(2)在相同的网格疏密度比下,由于不同喷管长度L带来的射流上游进口的湍流结构的变化与下游射流扩张速率有清晰的关联。     

雾化燃烧模拟一体化方法及应用研究

如何模拟液体燃料的雾化、掺混及燃烧过程,一直以来都备受关注。本文试图将无网格MPS(moving particlesemi-implicit rnethod)方法与颗粒轨道模型结合起来描述液体燃料从射流、破碎、雾化、输运全过程,并与基于Euler网格气相方程耦合求解,从而可以获得对雾化燃烧全过程模拟的一体化方法。初步结果显示,其方法和技术路线可行。     

三维可压缩圆孔单相射流近场DNS并行数值模拟研究

本文对圆孔射流近场的湍流统计量进行了DNS模拟研究.模拟的Re=5000,计算网格3000万,计算CPU个数为60.采用MPI并行求解了可压缩的射流守恒方程,包括连续性方程、动量方程、能量方程和理想气体的状态方程.对射流近场(30d以内,d为喷口的直径)的湍流统计量的迁移规律进行了系统的分析.研究了入口湍流条件对射流统...     

液体物性对雾化射流液雾粒径的影响

对不同液体在空气中湍动雾化射流的气液两相流场进行了数值模拟.建立了一次雾化的一维模型,分析了粘度、表面张力和气液质量流量比对液雾粒径的影响趋势,采用基于粒子追踪法的二次雾化三维模型,分析了物性和各种工况对液雾粒径沿轴向分布的影响程度.计算结果和已公开发表的实验数据进行比对,得到了较好的吻合,在此基础上,分析了影响气泡雾化喷嘴雾化质量的主要因素.     

缸内高压直喷柴油/天然气喷射混合过程分析

在天然气发动机中,天然气和柴油射流的喷射及混合过程对燃烧排放性能有重要影响。本文基于AVL FIRE软件,模拟了定容燃烧腔内高压天然气和柴油射流喷射过程,分析了不同喷孔高度、喷孔倾斜角以及喷孔交角下柴油和天然气射流的混合情况,研究结果表明喷孔高度,倾斜角,交角与喷雾贯穿距呈正比,柴油喷雾贯穿距是影响雾化质量的重要因素。喷孔相对位置中喷孔交角因素对射流的影响最大,对柴油喷雾的雾化效果提升明显。     

电极感应熔化气雾化制粉技术中非限制式喷嘴雾化过程模拟

电极感应熔化气雾化(electrode induction melting gas atomization, EIGA)是一种制备超洁净无夹杂物的先进制粉技术,本文以粉末高温合金的氩气雾化过程为研究示例,对现有用于实际生产的国内某厂家提供的EIGA用非限制式喷嘴进行建模,采用商用计算流体力学软件FLUENT,分布采用欧拉-欧拉VOF(volume of fluid)多相流方法与欧拉-拉格朗日DPM (discrete phase model)离散相方法,对非限制式环缝喷嘴主雾化与二次雾化过程进行了数值模拟.通过对主雾化过程中多相流大涡模拟速度流场,主雾化过程中不同阶段高温熔体云图模拟以及二次雾化过程中TAB (Taylor analogy breakup)模型速度流场及TAB模型粒度分布的模拟研究,实现了对EIGA制粉技术中非限制式喷嘴雾化过程的全过程模拟,并预测了雾化后的粉末粒度分布.在此基础上,采用本文模拟使用的非限制式环缝喷嘴,设定与模拟条件一致(进气压力4 MPa,液流直径约4 mm)的实验条件,制备的粉末大部分颗粒的直径大小在100μm左右,该实验结果与模拟得到的粉末直径D50=100μm大小一致,进一步验证了模拟数据的合理性.该方法也适用于非限制式喷嘴里,其他金属或合金的雾化过的模拟研究.     

气泡群的动态物理特性研究

假设气泡周围流场为无黏、无旋、不可压缩的理想流体,建立气泡群相互作用的三维数值模型.将多极快速傅里叶变换方法(FFTM)与高阶边界元法(HOBEM)相结合求解气泡群的运动,在达到同样计算精度时显著加快了边界积分方程的求解速度,可以在合理的时间内模拟气泡群的动态物理特性.同时为维持气泡群模拟过程中的数值稳定性,引入了弹性网格技术(EMT),并用算例验证了数值模型及算法的有效性.基于建立的数值模型,研究了不同组合的气泡群之间的相互作用,模拟和解释了各类气泡运动的物理现象,讨论了影响气泡群膨胀、坍塌、迁移及射流 关键词： 气泡群 FFTM 射流 三维     

平行射流大涡模拟及SGS模型的比较

高精度算法和亚网格尺度(SGS)模型是大涡模拟的关键,对声源流场和气动声场的准确预测有着重要的影响。本文采用前人具有部分DNS数据的平板射流为考核算例,使用具有4阶空间精度和3阶时间精度的高精度算法,系统地比较Smagorinsky模型(SM)、选择多尺度模型(SMSM)、以及前人采用动态Smagorinsky模型(DSM)的计算结果。对射流速度、剪切层、湍流脉动以及涡结构的多尺度特性的结果分析表明,Smagorinsky模型的黏性耗散过大,而选择多尺度模型则能揭示转捩过程中的黏性变化,其结果比DSM更接近DNS的数据。同时,SMSM是局部模型,在大规模分布式内存的并行计算方面易于编程,具有良好的应用前景。     

Effects of fuel viscosity on the primary breakup dynamics of a high-speed liquid jet with comparison to X-ray radiography

One of the major concerns in combustion engines is the sensitivity of engine performance to fuel properties. Recent works have shown that even slight differences in fuel properties can cause significant changes in performance and emission of an engine. In order to design the combustion engines with multi-fuel flexibilities, the precise assessment of fuel sensitivity on liquid jet atomization process is a prerequisite since the resulting fuel/air mixture is critical to the subsequent combustion process. The present study is focusing on the effect of physical fuel properties, mostly viscosity difference, on the breakup process of the liquid jet injected into still air. Two different jet fuels, CAT-A2 and CAT-C3, are considered here as surrogates for a fossil-based fuel and a bio-derived high-viscosity alternative fuel. The simulations are performed using the volume-of-fluid (VoF) interface tracking method coupled to Lagrangian particle method in order to capture the breakup instabilities of jets and the resulting droplets. The investigations take the actual geometry of the injector into account to resolve the unsteady flow phenomena inside the nozzle that impact the turbulence transition and atomization. The simulation results are compared to the experimental measurement using X-ray radiography. Both simulation and X-ray measurements consistently describe the effects of different fuels on the fundamental properties of atomization including the breakup length, transverse liquid volume fraction and the droplet sauter-mean-diameter. The application of a Detailed Numerical Simulation approach complemented by unique X-ray diagnostics is novel and providing new understanding and research directions in engine spray dynamics.     

Detailed simulation of primary atomization mechanisms in Diesel jet sprays (isolated identification of liquid jet tip effects)

In this study, the atomization characteristics of Diesel jet front tip have been investigated to elucidate the physical mechanisms by detailed numerical simulation. The computations are carried out with the finest grid resolutions ever that can resolve the final droplet generation by surface tension. The numerical methods are based on level-set interface tracking. The methods were validated by test cases and the grid resolution survey shows that the resolutions for the present study are sufficient. The present flow setup excludes nozzle disturbances to investigate how the disturbances from the liquid jet front would lead to atomization where the liquid jet impacts against the quiescent gas. The liquid jet front becomes an umbrella-like shape. From the front umbrella tip edge, ligament breakup first occurs. Ligament breakup is strongly correlated with the gas motion in the vicinity. The gas region behind the front is highly disturbed by atomization. By the gas recirculation motion here, air and some droplets are entrained and mixed. Also, the disturbances are fed back to the front umbrella by this motion and become synchronized with the breakup. Droplet pinch-off is mainly in the short-wave mode, but some ligaments are elongated by local gas stretch to finally have a long-wave mode shape, namely a mode shift occurs. The above findings of liquid jet front umbrella formation, atomization at the umbrella edge, mixing and atomization loop in the recirculation flow region and droplet generation mode give an insight to the modeling of droplet generation in actual sprays.     

气-液组合横向射流中气体种类的影响

吸气式旋转爆震发动机中燃料的良好雾化对爆震燃烧的组织极其重要,气-液组合横向射流是其中一种重要的燃料喷注技术。为获取气体种类在超声速来流中对其的影响,通过数值方法对液体射流穿透深度和辅助射流气体分布进行研究。采用Euler-Lagrange方法研究不同气体射流对液体射流的作用规律,结果表明：气体射流通过激波结构降低局部来流动量通量提升液体射流穿透深度,相同的来流条件下气体射流动量通量越大,液体射流穿透深度越高,且主要影响扩张段之前的流场区域,气体种类的变化对于后场液体射流穿透深度和雾化特性几乎无影响。     

Novel low voltage EHD spray nozzle for atomization of water in the cone jet mode

Due to the high surface tension and high conductivity, water is unsuitable for electrohydrodynamic (EHD) atomization using a DC electric field in air. The high local electric field, that is required to atomize water, is likely to generate corona discharge and consequently destabilize the atomization process. This study describes a novel low voltage EHD spray nozzle that can be used to atomize water and weak saline solutions in the stable cone jet mode. The properties of the atomization have been investigated together with the generated droplet size distribution. The nozzle operates at very low flow rates (0.5–4.0 μl/min). Due to the high dielectric constant of water and the low flow rate, the atomization takes place outside the applicability range of the scaling laws. The experimental results show that the droplet size is approximately constant when the flow rate is increased from 0.5 to 4.0 μl/min. The atomization of water was numerically simulated using computational fluid dynamics (CFD). The simulation results agree reasonably well with the experimental results with respect to the liquid cone shape and droplet size.     

Numerical simulation of electrohydrodynamic (EHD) atomization

The objective of the present work was to use a commercial Computational Fluid Dynamics (CFD) code to simulate the electrohydrodynamic (EHD) atomization process. Although the physics of the atomization and cone formation has been discussed in numerous publications, a comprehensive theory has not been presented. Some of the previous approaches are discussed below. A CFD model can give a unique capability to describe and simulate the liquid cone formation and atomization. The approach in this work was to simultaneously solve the coupled (EHD) and electrostatic equations. The heat conduction equation, solved by the CFD solver, has been modified to solve the electrostatic field equations. From the electrostatic field, the electric body forces have been determined and included in the Navier–Stokes equations. The model does not include any current. The key liquid property for the coupling is the permittivity. The predicted velocity fields for heptane and ethanol and the operating window of heptane were found to be consistent with published results. The model does not include a droplet break-up model. If the jet is cylindrical, the droplet size can be calculated from the jet diameter. The droplet size of ethanol was predicted and compared well with experiments.     

Process modeling gas atomization of close-coupled ring-hole nozzle for 316L stainless steel powder production

The paper aims at modeling and simulating the atomization process of the close-coupled ring-hole nozzle in vacuum induction gas atomization(VIGA) for metallic powder production. First of all, the primary atomization of the ring-hole nozzle is simulated by the volume of fluid(VOF) coupled large eddy simulation(LES) model. To simulate the secondary atomization process, we use the method of selecting the droplet sub-model and the VOF model. The results show that the ring-hole nozzle forms a gas recirculation zone at the bottom of the delivery tube, which is the main reason for the formation of an annular liquid film during the primary atomization. In addition, the primary atomization process of the ring-hole nozzle consists of three stages: the formation of the serrated liquid film tip, the appearance and shedding of the ligaments, and the fragmentation of ligaments. At the same time, the primary atomization mainly forms spherical droplets and long droplets, but only the long droplets can be reserved and proceed to the secondary atomization. Moreover,increasing the number of ring holes from 18 to 30, the mass median diameter(MMD, d50) of the primary atomized droplets decreases first and then increases, which is mainly due to the change of the thickness of the melt film. Moreover, the secondary atomization of the ring-hole nozzles is mainly in bag breakup mode and multimode breakup model, and bag breakup will result in the formation of hollow powder, which can be avoided by increasing the gas velocity.     

基于格子Boltzmann方法的平板射流大涡模拟

应用多GPU技术,将格子Boltzmann方法与大涡模拟相结合(LBM-LES),使用1.12×10⁸网格,对雷诺数Re=4 000,倾斜角α=30°,吹风比M=0.5工况下的平板单孔射流进行了大规模高性能数值模拟研究.合理的定性与定量结果验证了LBM-LES模拟平板射流的有效性与可行性.使用上亿的计算网格捕捉了精细的湍流拟序结构,有利于主流与射流之间的掺混机理研究.此外,使用6个K20M GPU并行计算,模拟了71 680 LBM时间步长,仅耗时15 402秒,计算性能达到521.24MLUPS,即每秒更新5.212 4×10⁸个网格点的数据.