在轴向气流作用下液体轴对称抛撒的近场研究

在轴向气流作用下液体轴对称抛撒的研究,是以飞行物体在运动状态下向大气抛撒液体燃料所导致的破碎和雾化为背景的。为了研究在轴向气流作用下液体轴对称抛撒所产生的雾场特性,本文提出了一种新的组合型实验设备。该设备由两台激波管、一套电子同步控制系统组成,可以观察在轴向气流作用下液体轴对称抛撒、破碎和雾化的过程。通过在此设备上的一系列实验,获得了在不同压力和不同气流速度下液体轴对称抛撒的近场纹影照片。通过对照片的研究发现,液体轴对称抛撒具有两个明显的阶段:液核生长阶段和液核稳定阶段。另外近场云雾区轮廓有明显的转折点,此转折点即为液核发生首次破碎的位置。进一步分析表明,轴向气流能促使液体轴对称抛撒首次破碎发生的时间缩短。     

运动状态下液体轴对称抛撒首次破碎的实验研究

在实验室条件下利用组合式激波管设备,对运动状态下液体轴对称抛撒进行了实验研究。通过纹影装置获得其所形成雾化场的外形轮廓照片,测量获得了液核发生首次破碎的位置与对称轴之间的距离。通过对抛撒过程中R-T不稳定性与K-H不稳定性的分析认为,轴向气流作用下液体轴对称抛撒的首次破碎点与对称轴的距离主要由轴向气流的速度、轴向气流的密度、液体轴对称抛撒的出口速度、抛撒液体的表面张力系数、环形喷口的宽度等参数所决定。在此基础上,利用相似性理论和无量纲分析,获得了运动状态下液体轴对称抛撒首次破碎点与对称轴之间的距离与相关参数的无量纲关系式。     

激波作用下液体喷射雾化的实验研究

液体在气流作用下的喷射抛撒过程是流体力学研究中令人感兴趣的重要领域,笔者采用改进后的激波管实现了激波作用下液体的喷射抛撒,并通过阴影照相和激光散射法分别对液体的抛撒状态和抛撒液滴的直径进行了测量,研究表明,在喷射过程中,流场中固定位置所测得的液滴Ｓａｒｔｅｒ平均直径随时间的发展而逐渐减小,在开始时时刻液滴直三小较快而最终渐趋平缓;在对不同抛撒距离雾化场的测量中发现,没位置测得的颗粒最大直 隧测得位     

有约束、轴对称抛撒条件下液体破碎、雾化的远场研究

液体轴对称抛撒在研制云雾爆轰武器中有着重要的实际应用背景和学术研究价值，由于过去这方面大量的工作集中在野外场地中进行，对液体燃料抛撒初期及后期形成的云雾区特性缺少细致的实验研究，本文用垂直无膜激波管与液体抛撒发生器结合起来，用多根隔条形成不同宽度的间隙，利用激光粒子测量仪研究在有约束的条件下，液体破碎、雾化的远场特性，并得到了一些结论。     

《实验力学》2004年（第19卷）目录索引

磁流变弹性体剪切性能的动态实验研究王　桦 ,周刚毅 ,方　生 ,张培强 ( 1 ,1 )…………………………………………杭州大剧院风压分布的风洞试验研究田森源 ,楼文娟 ,王高帆 ,沈国辉 ,孙炳楠 ,周　嵘 ( 1 ,6)………………………在轴向气流作用下液体轴对称抛撒的近场研究杨　磊     

液体环二次破碎所形成云雾颗粒尺寸测量和测试系统的标定

介绍了激光散射法测量颗粒尺寸系统的工作原理和标定结果,并对液体环轴对称抛撒进行了光学测量。实验结果表明,液体环二次破碎产生云雾区的液滴Ｓａｕｔｅｒ平均直径在固定点随时间的增加呈减小的趋势,而云雾区的宽度和云雾区前缘的液滴颗粒的Ｓａｕｔｅｒ平均直径则随测量的距离增加均有所增加。     

轴对称液体环的形成、变形和破碎的研究

为了实现液体环的轴对称径向扩展运动,提出了一种新的实验方法及设备。这种新的实验设备是由垂直的无膜激波管和液体环发生器所组成,可以用来观察轴对称液体环的形成、变形和破碎过程。轴对称液体环抛撒过程的实验研究已在这种实验设备上完成,并得到了相应的一系列流场照片。对实验结果的分析表明,液体表面的不稳定性是造成液体环破碎的主要原因。     

强冲击波作用下液体抛撒的实验研究

通过强冲击波作用下液体抛撒的系列实验,总结分析了抛撒液体尺寸、爆炸装药量、抛撒液体性质等对液体抛撒运动过程、抛撒半径及液体抛撒作用时间的影响,发现强冲击波作用下液体抛撒速度随时间呈指数衰减,不同的实验参数对衰减系数将产生一定影响。     

阶梯型加速段对旋流喷嘴雾化特性的影响

旋流内芯是压力旋流式喷嘴最主要的旋流发生构件,其几何特征直接影响压力旋流式喷嘴的喷雾特性.目前采用平滑型加速段的旋流内芯导流效率较低.为减小高流量条件下的能量损失,使喷嘴旋流内芯加速段对喷雾介质产生预旋效应,增强旋流强度,本文设计喷嘴旋流内芯加速段为阶梯型,其下段阶梯相对上段阶梯旋转15°,旋向与喷嘴旋流槽方向相同.利用粒子动态分析仪(particle dynamics analysis system,PDA)和高速摄影(charge coupled device,CCD)系统实验研究了加速段结构改进前后喷嘴的喷雾流量、雾场索特尔平均直径(Sauter mean diameter,SMD)、雾滴速度以及喷雾锥角,并分析了SMD、雾滴速度的轴向和径向分布特性.结果表明,背压差0.08~0.46 MPa范围内,阶梯型加速段对喷雾介质具有较好的预旋效果.喷嘴的流量提高了48.0%~51.8%;喷雾的轴向速度提升了31.4%~32.8%,径向速度提升了1.6%~16.8%;喷雾锥角减小了4.21°~6.57°;较高背压差下喷雾下游的SMD减小了9.8%.与平滑型加速段相比,阶梯型加速段的设计有效地提高了喷嘴的雾化质量.     

液体的爆炸抛撒特征

针对液体爆炸抛撒过程设计了实验装置,利用高速摄像仪进行记录。通过研究不同中心装药量和 填充液体的抛撒过程,发现在壳体破裂后,液体沿裂缝处向外飞散。药量较小时,液体分散成树枝状形态,然 后破碎成液滴;药量较大时,则形成液体环状区。对于不同粘度的液体,环状区分别由小液滴及已雾化、汽化 的液体,或大液滴、液体丝及液膜等组成,抛撒过程中其宽度越来越大,大液滴、液体丝及液膜等也逐渐破碎成 细小的液滴。     

On the experimental investigation on primary atomization of liquid streams

The production of a liquid spray can be summarized as the succession of the following three steps; the liquid flow ejection, the primary breakup mechanism and the secondary breakup mechanism. The intermediate step—the primary breakup mechanism—covers the early liquid flow deformation down to the production of the first isolated liquid fragments. This step is very important and requires to be fully understood since it constitutes the link between the flow issuing from the atomizer and the final spray. This paper reviews the experimental investigations dedicated to this early atomization step. Several situations are considered: cylindrical liquid jets, flat liquid sheets, air-assisted cylindrical liquid jets and air-assisted flat liquid sheets. Each fluid stream adopts several atomization regimes according to the operating conditions. These regimes as well as the significant parameters they depend on are listed. The main instability mechanisms, which control primary breakup processes, are rather well described. This review points out the internal geometrical nozzle characteristics and internal flow details that influence the atomization mechanisms. The contributions of these characteristics, which require further investigations to be fully identified and quantified, are believed to be the main reason of experimental discrepancies and explain a lack of universal primary breakup regime categorizations.     

Atomization and spray characteristics of bioethanol and bioethanol blended gasoline fuel injected through a direct injection gasoline injector

The focus of this study was to investigate the spray characteristics and atomization performance of gasoline fuel (G100), bioethanol fuel (E100), and bioethanol blended gasoline fuel (E85) in a direct injection gasoline injector in a gasoline engine. The overall spray and atomization characteristics such as an axial spray tip penetration, spray width, and overall SMD were measured experimentally and predicted by using KIVA-3V code.The development process and the appearance timing of the vortices in the test fuels were very similar. In addition, the numerical results accurately described the experimentally observed spray development pattern and shape, the beginning position of the vortex, and the spray breakup on the spray surface. Moreover, the increased injection pressure induced the occurrence of a clear circular shape in the downstream spray and a uniform mixture between the injected spray droplets and ambient air. The axial spray tip penetrations of the test fuels were similar, while the spray width and spray cone angle of E100 were slightly larger than the other fuels. In terms of atomization performance, the E100 fuel among the tested fuels had the largest droplet size because E100 has a high kinematic viscosity and surface tension.     

Spray features in the near field of a flow-blurring injector investigated by high-speed visualization and time-resolved PIV

In a flow-blurring (FB) injector, atomizing air stagnates and bifurcates at the gap upstream of the injector orifice. A small portion of the air penetrates into the liquid supply line to create a turbulent two-phase flow. Pressure drop across the injector orifice causes air bubbles to expand and burst thereby disintegrating the surrounding liquid into a fine spray. In previous studies, we have demonstrated clean and stable combustion of alternative liquid fuels, such as biodiesel, straight vegetable oil and glycerol by using the FB injector without requiring fuel pre-processing or combustor hardware modification. In this study, high-speed visualization and time-resolved particle image velocimetry (PIV) techniques are employed to investigate the FB spray in the near field of the injector to delineate the underlying mechanisms of atomization. Experiments are performed using water as the liquid and air as the atomizing gas for air to liquid mass ratio of 2.0. Flow visualization at the injector exit focused on a field of view with physical dimensions of 2.3 mm × 1.4 mm at spatial resolution of 7.16 µm per pixel, exposure time of 1 µs, and image acquisition rate of 100 k frames per second. Image sequences illustrate mostly fine droplets indicating that the primary breakup by FB atomization likely occurs within the injector itself. A few larger droplets appearing mainly at the injector periphery undergo secondary breakup by Rayleigh–Taylor instabilities. Time-resolved PIV is applied to quantify the droplet dynamics in the injector near field. Plots of instantaneous, mean, and root-mean-square droplet velocities are presented to reveal the secondary breakup process. Results show that the secondary atomization to produce fine and stable spray is complete within a few diameters from the injector exit. These superior characteristics of the FB injector are attractive to achieve clean combustion of different fuels in practical systems.     

Parametric study on the fuel film breakup of a cold start PFI engine

In order to provide more insight on improving the cold start fuel atomization for reducing unburned hydrocarbon emissions, the liquid fuel film breakup phenomenon in the intake valve/port region was investigated in depth for port-fuel-injected engines. Experiments were conducted using high-speed high-resolution imaging techniques to visualize the liquid film atomization and airflow patterns in an axisymmetric steady flow apparatus. The impact of valve/port seat geometry, surface roughness, and fuel properties on airflow separation and fuel film breakup were determined through a parametric study. CFD simulations were also performed with FLUENT to help understand the airflow behavior inside the intake port and valve gap region and its potential impact on fuel film atomization.     

Unsteadiness of the internal flow in an effervescent atomizer nozzle

Liquid atomization system has been extensively applied as the most significant process in many industrial fields. In the internal combustion engine, the combustion phenomenon is strongly influenced by the spray characteristics of the fuel given by the atomization process. In order to completely understand the whole atomization process, a detail investigation of relations between the liquid jet characteristics and the breakup phenomenon is required. In this study, a non-intrusive method called as laser tagging method by photochromic dye has been developed with aim to study the breakup process of liquid sheet in detail, covering from the behavior in film until disintegrated into ligament and droplets. The laser tagging method by photochromic dye is based on a shift in the absorption spectrum of photochromic dye molecules tagged by ultraviolet laser. The shift results a color change at the tagged region of liquid containing the dye. In this study, the motions of the dye traces were analyzed as the liquid surface velocity. As a result, liquid sheet was found to keep its velocity constantly in film before suddenly increase around broken point. However, it then decreased after broken into droplets. By forming a set of four points of dye traces on the liquid sheet, the change of relative position of the set enabled the measurement of deformation and rotational motion of the liquid sheet. As a result, the normal strain of the liquid sheet parallel to the flow direction depended on the flow behavior of ligament formation.     

A study on the fuel injection and atomization characteristics of soybean oil methyl ester (SME)

The spray atomization characteristics of an undiluted biodiesel fuel (soybean oil methyl ester, SME) in a diesel engine were investigated and compared with that of diesel fuel (ultra low sulfur diesel, ULSD). The experimental results were compared with numerical results predicted by the KIVA-3V code. The spray characteristics of the spray tip penetration, spray area, spray centroid and injection delay were analyzed using images obtained from a visualization system. The Sauter mean diameter (SMD) was analyzed using a droplet analyzer system to investigate the atomization characteristics.It was found that the peak injection rate increases and advances when the injection pressure increases due to the increase of the initial injection momentum. The injection rate of the SME, which has a higher density than diesel fuel, is higher than that of diesel fuel despite its low injection velocity. The high ambient pressure induces the shortening of spray tip penetration of the SME. Moreover, the predicted spray tip penetration pattern is similar to the pattern observed experimentally. The SMD of the SME decreases along the axial distance. The predicted local and overall SMD distribution patterns of diesel and SME fuels illustrate similar tendencies when compared with the experimental droplet size distribution patterns.