超高速自由液体射流的动力学研究

自由射流的稳定性一直是材料加工、能源利用领域的重要基础问题,超高压射流对稳定性研究提出了新的要求。从湍流的角度阐述了超高压射流流动的特殊性及相关研究面临的挑战。运用相位多普勒(PDA)技术,设计实验对射流横截面的动力参数进行测量。围绕速度、速度脉动及液滴直径分析了射流内部存在的流动现象并针对实验潜在的局限性展开讨论.结果表明:流向速度沿径向呈1/7幂次分布,破碎段速度均匀但核心区存在逆向浓度梯度;雾化段具有明显的核心、剪切层结构,流动参数呈轴对称分布.PDA解析的液滴尺寸仅为全部尺度的0.1%。     

喷孔直径对柴油射流雾化影响的实验研究

本文利用可视化高压定容弹,就常压环境下高速射流在破碎过程中的近场及远场的雾化特性随喷孔直径变化的规律进行实验研究。研究结果表明:对于远场喷雾来说,随着喷油压力和喷孔直径的增大,导致流动阻力的减小,从而导致贯穿加速度增大;而在近场区域,利用长焦显微技术拍摄的微观图像显示,随着喷雾的发展,液柱逐渐失稳,出现类似不稳定波的拟序结构;近场喷雾锥角随着喷孔直径的增大呈现增长的趋势,这是主要是因为长径比的减小导致流动阻力的减少,进而引起湍动能的增加,最终体现在法向湍流脉动速度的增加引起了锥角的增大。     

400MPa超高压水射流结构的分析研究

在超高压水射流切割过程中,射流结构是设备工作效率及损耗程度的决定性因素,本文针对400MPa范围内超高压水射流结构进行了数值模拟及实验研究,得到了相关因素对其具体影响。结果表明:超高压水射流的流束可分为几个特征各不相同的区域;射流的发散程度随着压力和宝石孔径的增大而增大;喷嘴形状对射流结构具有显著的影响,渐扩的锥形出口及倾斜的出口端面有助于增强射流的集束性;宝石孔内的空化直接影响了射流的形成;空化加剧了宝石的磨损,随着宝石孔入口的磨损程度提高,水射流趋于发散。     

围压条件下空化磨料射流的冲蚀特性与机制分析

 磨料射流在水利水电、石油工程和海洋资源开发等领域有着广泛的应用,研究射流的冲蚀特性和破坏物料机制对提高水射流利用效率具有重要意义。利用自行研制的实验装置,研究了围压条件下自振空化、文丘里和锥形3种喷嘴形成的磨料射流冲蚀特性,分析了空化磨料射流冲蚀物料的机制。结果表明：自振空化喷嘴和文丘里喷嘴的冲蚀效果优于锥形喷嘴,空化作用有助于提高磨料射流的冲蚀效果;磨料射流的冲蚀效率随围压的增大而降低,主要原因是围压对纯水射流和空化磨料射流的压力脉动和冲击力具有抑制作用。分析认为,空化磨料射流冲蚀破碎物料的机制主要包括冲蚀、气蚀和共混磨蚀3 种作用。     

超高压水射流冲蚀切割岩石断口微观断裂机理实验研究

 由于岩石破碎过程的复杂性，目前对水射流的破岩机理的认识仍然不十分清楚。先采用超高压万能水射流切割机冲蚀切割岩石，然后用扫描电镜对水射流切割岩石断口形貌进行观测，分析了岩石在超高压水射流作用下的破坏形式。观测分析表明，超高压射流切割岩石形成的切槽主要有长形规则和漏斗状两种形状。岩石在水射流作用下的破坏机制主要有拉伸破坏和剪切破坏两种，剥落岩块以穿晶断裂为主，其微观断裂机制是脆性拉伸破坏；切槽凹侧面主要是剪切错动，其微观破坏机制是剪脆性破坏。实验为水射流破岩机理分析提供了实测依据。     

水下超声速气体射流诱导尾空泡实验研究

两相射流与空化问题对采用喷气推进的水下高速运载器而言不可避免.本文通过水洞实验,探究了回转体在水流场中由亚声速及超声速气体射流诱导形成尾空泡的形态特征,发现了四种不同类型的诱导尾空泡,并探讨了相应的形成机理和控制条件.通过高速图像采集及数字处理技术,得到了不同弗劳德数和通气流量系数下诱导尾空泡的瞬时及时间平均形态.通过气体射流数值解及射流耦合空泡闭合理论模型与实验图像的对比分析,得到如下结论:根据形态特征,将观察到的射流诱导尾空泡划分为泡沫状、完整、部分破碎和脉动泡沫状四类,其中诱导产生的部分破碎尾空泡在形态上与超空泡存在明显差异,脉动泡沫状则为诱导空泡所特有;气体射流受到空泡阻挡发生回射后对应的实际通气流量系数是控制空泡形态的关键;诱导空泡类型转变可以通过Paryshev提出的射流空泡耦合模型预测,但必须在考虑射流空间结构和流动损失的前提下;进行上述修正后,诱导尾空泡形态变化规律与理论模型估算得到的实际流量系数相符合.     

PIV对射流煤粉火焰流场特性的分析

为研究湍流煤粉射流进入高温环境后的流动及混合特性,采用PIV测量平流射流煤粉燃烧器中心射流区域的流场特性。通过对不同示踪粒子特性的分析,选定SiC表征气相流场,选定煤粉表征煤粉射流流场。实验结果表明;煤粉具有较好的散射特性,因而采用PIV可获得合理的煤粉颗粒流场;相同射流速度下,冷态射流比热态有更强的湍流脉动特征,主要原因是热态条件下气流黏度增加2~3倍,显著降低射流Re。     

喷嘴结构调节毛细超高压射流流动特性

要实现超高压水射流的稳定性调控首先必须对其流动特性展开探索。设计了四种喷嘴出口流道结构,对其产生的射流进行了测量,并结合数值模拟对实验反映出的物理现象进行了诠释。结果显示:圆柱形出口流道喷嘴,其射流流动形态受雷诺数变化影响不明显,射流与环境气体界面近似为圆锥。而圆柱与圆锥组合型出口流道喷嘴,其射流结构形态随雷诺数变化显著。这表明具有特定结构,不与射流直接接触的出口流道,也能对气液边界层湍流产生显著影响,从而对射流流动特性起到调节作用。在雷诺数120000~130000范围内,提出能显著提高射流能量集中性的最佳结构形式及参数:锥角为90°,圆锥与圆柱组合型出口流道。这些发现为最终实现射流流动控制提供了研究方向的引导。     

超高压水射流形成过程中的压力损失研究

 对超高压水射流形成过程中的压力损失进行了理论研究,并且对不同条件下超高压水射流速度的变化规律进行了实验研究,最后确定了不同条件下超高压水射流的初始参数。结果显示,在超高压水射流形成过程中,压力损失主要是在水流从高压管道进入高压喷嘴时由于流道发生断面收缩而造成的,压力损失大约为最终超高压水射流动能密度的49%;随着喷嘴口径的增大,压力损失减小,整个系统的能量利用率越高;随着水射流压力的增大,压力损失增大,但增大的程度会减小。     

高速水射流集束性的数值模拟及试验研究

本文采用VOF多相流模型,在400 MPa工作压力下,针对3种不同结构喷嘴产生的自由水射流流场进行了数值模拟和试验研究。结果表明:喷嘴出口处空气向喷嘴内部卷吸,一定程度上提高了其集束性;射流核心区轮廓呈现粗细交替的波动状,是诱发射流破碎的重要因素;射流上游的湍动能大小及旋涡强度对射流的集束性影响显著;该条件下射流轮廓的高速数码摄像照片定性地验证了数值模拟的结果。     

Utilization of ultrasound to enhance high-speed water jet effects

Continuous high-speed water jets are presently used in many industrial applications such as cutting of various materials, cleaning and removal of surface layers. However, there is still a need for further research to enhance the performance of pure water jets. An obvious method is to generate water jets at ultra-high pressures (currently up to 700 MPa). An alternate approach is to eliminate the need for such high pressures by pulsing of the jet. This follows from the fact that the impact pressure on a target generated by a slug of water is considerably higher than the stagnation pressure of a corresponding continuous jet. Ultrasonically forced modulation of a continuous stream of water represents the most promising method of pulsed jet generation because of its simplicity and practicality. A pulsed jet is generated by modulating a continuous stream of water by ultrasonic waves. A velocity transformer connected to a piezoelectric transducer is located axially inside a nozzle to induce longitudinal pulsations in the water. An extensive laboratory research program is in progress to understand the basic principles of the process and to optimize the nozzle design for several applications. The results reported in this paper show that the performance of such a pulsed jet is far superior to that of a continuous jet operating at the same parameters. Experimental results obtained with the ultrasonic vibration of a tip situated inside the nozzle indicate that using this technique one can achieve performance of the jet even order of magnitude higher in comparison to continuous jet at the same hydraulic parameters. Performance of ultrasonically modulated jets in cutting of various materials was tested in laboratory conditions. In this paper, results of measurement of dynamic pressure in the nozzle and force effects of modulated jets are presented together with results obtained in cutting of various materials using ultrasonically modulated water jets. The results are compared with those obtained with continuous jets at the same operating parameters. Potential of forced modulation of the jet in applications of cleaning, paint and coating removal from surfaces and concrete cutting in the process of repair of concrete structures is mentioned.     

微型超高压宝石喷嘴内部的空化与磨损

宝石喷嘴是影响超高压水射流切割系统工作效率的重要部件,而宝石内部的空化直接影响射流的形成,也是宝石磨损的重要原因之一。对400 MPa压力范围内宝石孔内部的空化两相流进行了数值模拟,阐述了射流在宝石内的形成过程,分析了长径比、压力和入口形状对宝石内空化的影响,并在相应压力下对宝石喷嘴的磨损进行了实验研究。结果表明:宝石内部的空化发展程度随着长径比的增大而减弱;在一定的长径比范围内,空化可以发展到喷嘴出口,并最终使射流的初始直径小于喷嘴直径,且在此条件下当压力升高时,射流的初始直径增大;良好的入口形线可以降低空化的发展程度;宝石入口的磨损较出口更显著,空蚀和高压水的冲蚀造成了宝石孔边缘形状的破坏,这种破坏随着压力的升高而加剧,选择合适的长径比是减少冲蚀磨损的有效途径。     

矩形喷口欠膨胀超声速射流对撞的实验研究

在不同喷口间距和射流压力下开展了矩形喷口欠膨胀超声速射流对撞实验并与自由射流进行了对比. 实验表明:超声速射流对撞的辐射噪声中存在四种不同的啸音模式, 且随喷口距离和射流压力的变化在不同模式间切换. 在射流压力大于0.5 MPa且喷口间距小于50 mm时, 射流对撞面在两个喷口外形成两道正激波之间, 啸音基频维持在3 kHz左右. 随喷口间距的增大或射流压力的降低, 射流对撞面在一侧喷口外的弓形激波与另一侧喷口外的正激波之间. 对撞面也有可能出现在两个弓形激波之间, 对应的啸音基频约为9 kHz, 但容易受扰动而回到喷口一侧或是在喷口之间大幅度振荡. 当射流压力小于0.36 MPa且喷口间距大于70 mm后, 对撞面在两个喷口之间大幅度振荡, 产生基频在1 kHz左右并随射流压力的降低和喷口间距的增大而降低的啸音. 关键词： 超声速射流 啸音 射流对撞 激波     

Experimental evidence of a Rayleigh-plateau instability in free falling granular jets

A granular jet falling out of a funnel shaped container, subjected to small vertical vibrations, develops an instability farther downstream as may happen for ordinary liquid jets. Our results show that this instability is reminiscent of the Rayleigh-Plateau capillary instability leading to breakup of the jet at large scales. The first stages of this instability are captured in detail allowing a determination of the dispersion relation. Surface tensions measured in this unstable regime (of the order of mN/m) are in agreement with previously reported measurements carried out at much smaller scales. This instability and the breakup of the jet can be inhibited when the effect of the surrounding medium (air) is reduced by enclosing the jet in an evacuated chamber, showing that the effective surface tension measured is the result of a strong interaction with the surrounding air.     

Oscillation modes of supersonic multijets exhausting from very adjacent multiple nozzles.

The oscillation modes of a supersonic circular twin jet and multijet with square configuration where the center-to-center spacing of nozzles was fixed to 1.4 times the nozzle diameter were investigated experimentally. It is found that the twin jet oscillates simultaneously in three different oscillation modes for the pressure ratios of about 4.1. From the acoustical observation, these oscillation modes are identified as one lateral oscillation mode perpendicular to a plane composed of the twin jet axes and two lateral oscillation modes parallel to this plane. For a multijet with a square configuration, only one lateral oscillation mode was observed. In this case, it is observed that when the spacing between two facing sides of the square formed by four jets is stretched, the spacing between the other two sides shrinks and vice versa.     

Self-similar patterns of subsatellites formation at the capillary breakup of viscous jets

Formation of subsatellites at the capillary breakup of viscous liquid jets has been studied. The proposed analytical model qualitatively describes subsatellites formation. The obtained patterns for the formation of subsatellites’ nuclei and dynamics of their separation from a jet well agree with the results of numerical modeling.     

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.     

Air ingestion by a buckled viscous jet of silicone oil impacting the free surface of the same liquid

As frequently observed in common life, a jet of a viscous liquid impacting on a horizontal surface does not remain straight but instead buckles and folds periodically. We report experiments with planar (ribbonlike) jets of silicone oil impacting the free surface of the same liquid and describe the way in which jet folds incorporate air. It is shown that air ingestion proceeds through different modes, each of them acting as a source of monodisperse bubbles and featuring a threshold in jet height. These sources result from the breakup of remarkable cuspidal structures, produced by the recession of air domains within liquid folds.     

Ministarters and mini blue jets in air and nitrogen at a pulse-periodic discharge in a laboratory experiment

Miniature analogs of starters and blue jets that are observed in the upper atmosphere of the Earth and have dimensions of tens of kilometers are formed and studied in air and nitrogen at pressures of tens and hundreds of Torr. Ministarters and mini blue jets have been obtained in laboratory experiments owing to the application of a pulse-periodic discharge with a plasma jet referred to as apokamp. The velocities of propagation of apokamps have been measured at various pressures. It has been found that the average values of these velocities are about the velocities of propagation of starters and blue jets in the atmosphere of the Earth. It has been shown that jets (apokamps) with the maximum length are observed in the pressure range corresponding to the altitudes of appearance and propagation of starters and blue jets.     

On the stability of a nonaxisymmetric charged jet of a viscous conducting liquid

A dispersion equation is derived for axisymmetric and nonaxisymmetric capillary oscillations in a jet of viscous conducting liquid subjected to a constant potential. It is shown that conditions arising when the surface charge density in the jet is high cause the instability of nonaxisymmetric, rather than axisymmetric, modes with the resulting disintegration of the jet into drops of various sizes. This theoretical finding allows one to correctly interpret of experimental data for the spontaneous disintegration of charged jets.