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...     

两相射流穿透研究

前言 射流穿透在工程技术上有广泛的应用.液体燃油喷入高速热气流的油雾射流穿透对喷气发动机燃烧室中燃油的散布,对燃烧性能有重要的影响。穿透深度直接关系到燃油是否碰壁,燃油在空间的分布与浓度场以及稳定器表面集油率等。射流穿透的研究对于多相流理论和计算分析方法的发展也有意义。     

超声速气流中液体横向射流的非定常特性与振荡边界模型

针对液体圆柱射流垂直喷入超声速横向气流中的非定常分布特性开展实验研究, 并建立穿透深度方向上的射流振荡分布模型. 利用脉冲激光背景成像方法“冻结”拍摄马赫2.1(Ma=2.1)气流中煤油射流/喷雾瞬态图像, 结合最大类间方差法(Otsu)和Canny算法提取瞬态图像特征, 基于统计方法并引入间歇因子(γ)定量描述射流振荡分布特性; 通过研究多参数协同作用下的射流振荡分布规律, 提出振荡分布数学模型, 研究的参数变量包括超声速来流总压(642-1010 kPa)、 液体喷注压降(0.36-4.61 MPa)、液体喷嘴流道直径 (0.48 mm/1.0 mm/1.25 mm/1.52 mm)、距离喷嘴的流向距离(10-125 mm)以及液气动量通量比(0.11-7.49). 研究中利用射流振荡分布模型成功预测出水射流在Ma=2.1气流中的的振荡分布, 预测分布与实验结果符合良好.     

液体横向射流在气膜作用下的破碎过程

为了研究液体横向射流在气膜作用下的破碎过程,采用背景光成像技术及VOF TO DPM方法进行了实验研究和仿真研究,模拟介质为水和空气.研究结果表明,液体射流在气膜作用下主要存在两种破碎过程:柱状破碎和表面破碎.Rayleigh-Taylor(R-T)不稳定性产生的表面波是液体射流发生柱状破碎的主要原因,气流穿透表面波的波谷导致射流柱破碎,破碎后的液丝沿流向逐渐发展呈带状分布.Kelvin-Helmholtz(K-H)不稳定性产生的表面波是液体射流发生表面破碎的主要原因,液丝和液滴从射流表面剥离.局部动量比对液体横向射流的破碎过程具有重要影响,当局部动量比较低时,液体射流的破碎由K-H不稳定性主导;随着局部动量比的增大液体射流的破碎逐渐由R-T不稳定性主导.液体射流的破碎长度及穿透深度均随局部动量比的增大而增大.     

煤油氧气脉冲爆震火箭发动机爆震特性

脉冲爆震火箭发动机(PDRE)是一种利用脉冲式爆震波产生高温、高压燃气发出的冲量来产生推力的推进系统.与常规液体火箭发动机相比,脉冲爆震火箭发动机具有更高的性能,并且结构更简单.本文以航空煤油为燃料、氧气为氧化剂、压缩氮气为隔离气体,并利用电磁阀控制燃料、氧化剂和隔离气体的间歇式供给.利用低的点火能量(50mJ),在内径50mm,长度1.1m的爆震管内进行了大量的多循环爆震试验,研究煤油氧气电磁阀脉冲爆震火箭发动机的爆震波特性.研究结果为进一步研究气液两相多次爆震燃烧机理提供了依据,为研制工程应用的PDRE提供理论和实践基础.     

横向气膜作用下液体射流在近喷孔区域的破碎雾化特性

为了深入理解横向气膜作用下液体射流的破碎雾化特性,设计了一种以空气和水为模拟介质的针栓喷注单元,并通过两相流大涡模拟和背景光成像对大气环境下其近喷孔区域内的液体射流破碎过程和动态特性进行研究.通过大涡模拟液体射流的表面波主导破碎过程得到了针栓喷注单元近喷孔区域的喷雾场建立过程,亚声速气流离开狭缝后膨胀加速为超声速气流,经过液体射流上游的脱体弓形激波后减速增压.而在液体射流上下游之间的压差作用下,射流往下游发生弯曲的同时迎风面出现Rayleigh-Taylor (R-T)不稳定表面波;随着表面波的发展,气流穿透表面波的波谷位置导致连续射流发生断裂.正交分解(proper orthogonal decomposition,POD)方法可有效重构瞬时喷雾图像,POD模态表明近喷孔区域的低频和高频喷雾振荡分别由喷雾场的整体扩张/收缩过程和液块或者液雾团在迎风面的“撞击波”型运动引起,而后者的形态属于受连续液体射流断裂前的R-T不稳定表面波影响而产生的行波结构,其无量纲行波波长与韦伯数呈幂次律关系.     

可压缩气体中带电Newton流体平面液膜的不稳定性分析

液体雾化被广泛应用于工业、农业等领域,雾化过程中液体首先形成液膜,液膜在气液边界层内出现不稳定,导致液膜破碎。因此液膜的不稳定性对液体雾化起着非常重要的作用,有必要对液膜的不稳定性进行研究。采用数值计算的方法对可压缩气体中黏性平面液膜的不稳定性进行了研究,首先对黏性平面液膜和气体的速度分布进行推导,在此基础上采用谱方法对黏性平面液膜的不稳定性进行了研究。研究发现：在正弦和曲张模式下,施加电场会加速平面液膜的破碎;正弦模式比曲张模式更加不稳定,说明正弦模式在平面液膜不稳定中起主导作用;气体Mach数、电Euler数、气体Reynolds数、 Weber数和动量通量比的增加会加速液膜的破裂;随着气体边界层与液膜厚度比和液体Reynolds数的增加,扰动波的增长速率降低,液膜变得更加稳定。     

往复搅拌流场中气体射流两相流数值研究

应用欧拉-欧拉双流体模型,与动网格技术相耦合数值分析往复搅拌流场中气体射流两相流的流动特性及气泡在往复流场内的分布规律。通过调整过程中吹气头的往复运动频率、气体射流速度来分析这些因素对于气泡分布和流场流动特性的影响。数值模拟结果表明,往复搅拌流场中气体射流两相流呈现与搅拌频率一致的周期性,气泡分布随吹气头位置的移动呈螺...     

水煤浆多级气动喷嘴的喷雾特性研究

本文研究了一种多级气动喷嘴对水煤浆燃料的喷雾特性。采用实验的方法研究了水煤浆性质（动态表面张力和表观粘度）、喷嘴操作工况和喷嘴几何结构对射流雾化细度的影响,给出喷嘴雾炬轴向粒度分布,并对相关结果进行了分析。     

氢-空气斜爆震波的极曲线分析

本文建立了一种以温度为单变量进行迭代,求解真实气体模型下的斜爆震波波后参数非线性方程组的方法。并通过以加热量和气流偏转角为变量,计算出对应斜爆震波角度和斜爆震波的驻定范围。对于给定的来流条件,基于真实气体模型的斜爆震波的驻定范围比量热完全气体的宽,所以在斜爆震波的计算中必须考虑热物性的影响。随着氢气和空气化学当量比的增加,斜爆震波驻定范围逐步变小。而来流马赫数越大,斜爆震波的驻定范围越大,其CJ爆震波对应的平面偏转角越小。来流初温的升高则使斜爆震波的驻定范围变宽。斜爆震波的驻定需要来流条件与斜面角度等因素的相互匹配。     

Calculation of shock wave propagation in water containing reactive gas bubbles

The entry of a shock wave from air into water containing reactive gas (stoichiometric acetylene–oxygen mixture) bubbles uniformly distributed over the volume of the liquid has been numerically investigated using equations describing two-phase compressible viscous reactive flow. It has been demonstrated that a steady-state supersonic self-sustaining reaction front with rapid and complete fuel burnout in the leading shock wave can propagate in this bubbly medium. This reaction front can be treated as a detonation-like front or “bubble detonation.” The calculated and measured velocities of the bubble detonation wave have been compared at initial gas volume fraction of 2 to 6%. The observed and calculated data are in satisfactory qualitative and quantitative agreement. The structure of the bubble detonation wave has been numerically studied. In this wave, the gas volume fraction behind the leading front is approximately 3–4 times higher than in the pressure wave that propagates in water with air bubbles when the other initial conditions are the same. The bubble detonation wave can form after the penetration of the shock wave to a small depth (~300 mm) into the column of the bubbly medium. The model suggested here can be used to find optimum conditions for maximizing the efficiency of momentum transfer from the pressure wave to the bubbly medium in promising hydrojet pulse detonation engines.     

燃油粒度对两相PDE爆震波速的影响

两相爆震燃烧的研究近来取得了较大的进步,但是仍有很多问题需要解决,诸如燃油的喷射、雾化和蒸发,燃油和氧化剂的混合,两相可爆混合物的短距离起爆等等．本文利用激光喷雾粒度分析仪分别就直射喷嘴与气动喷嘴研究了汽油的雾化情况,随着汽油流量的增加,两种喷嘴的雾化变化趋势相反．结合汽油、空气PDE模型机多循环爆震试验,发现汽油的粒度对模型机的爆震波速有较大的影响,粒度减小,波速增大,同时波速具有循环效应．     

Effects of the penetration height of ethylene transverse jets on flame stabilization behavior in a Mach 2 supersonic crossflow

Effects of fuel jet penetration height on supersonic combustion behaviors were investigated experimentally in a supersonic combustion ramjet model combustor at a Mach speed of 2 and at a stagnation temperature of 1900 K. The jet-to-crossflow momentum flux ratio was varied to control the fuel-jet penetration height, using several injectors with different orifice diameters: 2, 3, and 4 mm. First, transverse nitrogen jets were observed to identify a relationship between the fuel jet penetration height and the momentum flux ratio by focusing Schlieren photography. Then, supersonic combustion behaviors of ethylene were investigated through combustion pressure measurements. Simultaneously, time-resolved images of CH* chemiluminescence and shadowgraphs were recorded with high-speed video cameras. Furthermore, a morphology of supersonic combustion modes was investigated for various equivalence ratios and fuel penetration heights in a two-dimensional latent space trained by the shared Gaussian process latent variable models (SGPLVM), considering CH* chemiluminescence images and the shock parameters. The results indicated that the penetration height of nitrogen jets was a function of the jet momentum flux ratio; this function was expressed by a fitting curve. Five typical combustion modes were identified based on time-resolved CH* chemiluminescence images, shadowgraphs, and pressure profiles. Even for a given equivalence ratio, different combustion modes were observed depending on the fuel penetration height. For an injection diameter of 3 and 4 mm, cavity shear-layer and jet-wake stabilized combustions were observed as the scram modes. On the other hand, although the cavity shear-layer and lifted-shear-layer stabilized combustions were observed, no jet-wake stabilized combustion was observed for an orifice diameter of 2 mm. Fuel penetration heights above the cavity aft wall were expected to affect the combustion behavior. Finally, a morphology of the supersonic combustion modes was clearly shown in the two-dimensional latent space of the SGPVLM.     

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.     

进口条件对超燃冲压发动机氢气燃烧流场特性的影响分析

对不同进口条件下的超燃冲压发动机燃烧室内氢气喷流超声速燃烧流动特性进行了数值模拟与分析.宽范围超燃冲压发动机是吸气式高超声速飞行器推进系统设计中的热点问题之一,受实验设备硬件条件及实验技术限制,数值模拟技术仍然是超燃冲压发动机燃烧室内燃气燃烧特性及流场特性的主要研究手段.采用基于混合网格技术的多组元N-S方程有限体积方法求解器,在不同进口Mach数及压强条件下,对带楔板/凹腔结构的燃烧室模型氢气喷流燃烧流场进行了数值模拟,对比分析了氢气喷流穿透深度、喷口前后回流区结构、掺混效率及燃烧效率等流场结构与典型流场参数的变化特性及影响规律.研究成果可为宽范围超燃冲压发动机喷流燃烧流动特性分析提供参考.      

Coaxial turbulent jet flames: Scaling relations for measured stoichiometric mixing lengths

Previous studies have quantified the fuel–air mixing processes within jet flames having a central fuel jet surrounded by an oxidizer coflow, where the mixing primarily occurs in the far field. The present work instead quantifies mixing within coaxial jet flames having a central jet of oxygen and a surrounding finite thickness coflow of hydrogen. These flames are relatively short and the primary mixing occurs in the near field. The stoichiometric mixing length (L_S) was measured, which is the distance along the centerline at which the stoichiometric condition occurs. Values of L_S were measured for a wide range of velocity ratios for both reacting and nonreacting cases using Planar Laser Induced Fluorescence (PLIF). Acetone PLIF was utilized for nonreacting cases, while OH PLIF was used for reacting. In nonreacting cases the use of a nondimensional momentum flux ratio collapses the nonreacting coaxial jet values of L_S to a single curve, which confirms previous theory. It also was found that the reacting and nonreacting coaxial jet data collapses approximately to a single curve, if one uses both the nondimensional momentum flux ratio and an effective outer flow gas density which is predicted by the analysis of Tacina and Dahm.     

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.