来流脉动对自激振荡脉冲流的影响

基于瞬变流和流体网络理论建立了由往复柱塞泵,管路和自激振荡呀嘴组成的装置系统的数学模型,分析了泵源脉动对自激振荡脉冲射流振荡特性的影响,表明当来流脉动频率小于或等于嘴嘴装置的固有频率时,能提高射流的振荡,冲蚀效果,理论分析与实验结果吻合,其结论对合理设计自激振荡射流喷嘴有一定的指导意义。     

围压下自振空化射流冲蚀性能实验研究

自振空化射流是利用瞬态流和水声学原理调制而成的一种新型射流,为研究围压下自激振荡空化射流的冲蚀破碎规律,利用高压釜装置测量了1.0mm出口直径的风琴管自振空化喷嘴在各种射流参数情况下冲蚀铝试样的冲蚀质量,并与同等条件下锥形喷嘴冲蚀效率进行了对比。测量结果表明,冲蚀质量基本与射流压力成正比;存在最优喷距和围压,使得冲蚀效果最佳,在本实验条件,分别为喷嘴出口直径的5～7倍和2MPa左右;相同条件下,自振空化喷嘴冲蚀质量约为同等条件下锥形喷嘴冲蚀的1～2倍,这为自振空化射流提高钻井速度等实际应用提供了实验依据。     

淹没条件下不同结构参数的自激吸气式脉冲射流喷嘴压力变化试验研究

运用自行研制的试验装置对淹没条件下的自激吸气式脉冲射流喷嘴特性进行了大量的试验,研究了吸气对喷嘴内的压力变化和脉冲射流峰值打击力的影响。通过研究淹没条件下10-16-125-75和8-14-85-60结构参数喷嘴在不同吸气根数下的腔套内各测点压力及峰值打击力的变化,得出不同结构喷嘴的压力和峰值打击力随吸气量的增大而逐渐提高,存在最优吸气量使脉冲射流峰值打击力最大。通过研究淹没条件下结构参数分别为8-14-85-60、10-16-105-75、14-28-125-105的喷嘴在不吸气及吸气根数为4情况下的射流峰谷差及峰值打击力,得出三种喷嘴在吸气时的射流峰值打击力分别提高45%~78%、40%~46%、22%~38%。研究表明:对于不同结构参数喷嘴,吸气可提高射流压力波动值和峰值打击力,随上喷嘴直径和振荡腔内容积的增大,该吸气方式对射流打击力的提高程度呈减小趋势。结果对进一步研究淹没条件下自激吸气式脉冲射流喷嘴特性具有指导意义。     

自振脉冲气液射流振动分析

从提高射流利用率出发, 在自振脉冲水射流喷嘴振荡腔上开孔自吸入空气, 以形成一种自振脉冲气液射流, 研 究其振荡频率以及打击力的实际影响因素.基于水声学与流体动力学原理建立自振脉冲气液射流频率模型, 并以悬臂梁为靶物, 通过考察射流冲击悬臂梁时的振动特性获得其时域及频域特征.结果表明, 自振脉冲气液射流的振荡频率随腔长增加单调递减, 随含气率变化存在极小值, 且在初始段发生骤变, 实测值与理论值相吻合;对应自振脉冲气液射流产生最大谐振峰值的最优腔长略小于自振脉冲水射流, 且前者最大谐振峰值较后者明显提高.     

超临界CO2直旋混合射流破岩特性的实验研究

超临界二氧化碳(CO₂)射流破岩既能降低岩石门限压力又能有效保护储层,直旋混合射流兼具直射流和旋转射流特点可提高破岩效率,基于此提出了超临界CO₂直旋混合射流的破岩方法。为了揭示超临界CO₂直旋混合射流破岩特性,设计加工出叶轮式直旋混合射流喷嘴,通过岩石定点冲击破碎实验对比了该射流与常规水射流的破岩效果,并研究了叶轮长度、叶轮中心孔直径、混合腔长度、喷射距离、射流压力等重要参数对超临界CO₂直旋混合射流破岩效果的影响。结果表明:相同实验条件下,该射流方法的平均破岩能力比常规水射流提高了42.9%;超临界CO₂直旋混合射流破岩易出现较大体积岩屑崩落现象;随着叶轮长度、混合腔长度、喷射距离的增大破岩效果均先增强后减弱,实验条件下上述参数存在最优范围值;叶轮中心孔直径的增大会导致岩石破碎孔深度增加、直径减小;随着射流压力的升高,超临界CO₂直旋混合射流破岩效果有着较为明显的提升。研究结果可为超临界CO₂直旋混合射流破岩方法的进一步研究提供实验依据。     

氮气射流抑制氢气喷射火的实验研究

为了探究抑制氢气喷射火的有效方法,并揭示氮气射流对氢气喷射火焰的影响规律,开展了一系列氮气作用下氢气喷射火实验。采用喷嘴直径为3mm、滞止压力为10atm的氮气射流,并改变氮气喷射高度和水平喷射距离,对氢气喷射火进行抑制。选择2种典型的氢喷射火,分别为喷嘴直径为3mm、滞止压力为0.1atm的过膨胀亚音速火焰和喷嘴直径为1mm、滞止压力为8atm的欠膨胀超音速火焰。实验结果表明,在氮气射流作用下火焰发生偏转,氢气喷射火长度衰减率随氮气喷射高度增大而减小。当氮气作用于火焰根部时,能有效地扑灭氢气喷射火。随氮气喷射距离增加,氢喷射火长度衰减率减小。另外,欠膨胀超音速氢气喷射火存在火焰抬升现象,氮气更易使喷射火熄灭,且在较大的水平喷射距离下仍能使火焰熄灭;过膨胀亚音速氢气喷射火由于抬升高度不明显,氮气射流灭火效果较差,仅能够在较小的水平喷射距离下使火焰熄灭。     

高速液体受限射流扩展形态研究

采用一种火药燃烧驱动液体喷射的新装置及其测试系统，研究受限空间中高速惰性液体射流的扩展结构。观察了环境反压、液体粘性、喷嘴结构等参量对射流扩展形态的影响，分析了射流雾化机理。研究结果对改进燃烧室设计及控制燃烧稳定性有一定的指导意义。     

水下爆炸中气泡射流壁压特性实验研究

水下爆炸气泡射流载荷是中近场水下爆炸壁压载荷的重要组成部分, 将水下爆炸气泡射流简化为一段高速水柱来研究水下爆炸气泡射流载荷特性是研究水下爆炸气泡射流载荷的主要手段。本文基于腔内爆炸提出了一种新的高速水射流实验方法,并给出了实验装置设计、实验方法以及实验系统。基于实验系统,开展了不同工况下高速水射流的实验研究,研究了腔口位置、腔深对水射流形态的影响,并对水射流的形态形成因素进行了分析。使用压电型壁压传感器测得了水射流冲击壁压,给出了水射流冲击壁压的特性及其特点。实验结果表明:腔口位置与腔深是影响水射流端面形态的重要因素;生成的高速水射流冲击壁压峰值满足水锤理论。基于腔内爆炸的高速水射流实验方法能够应用于包括水下爆炸气泡射流在内的高速水射流形态、壁压特性的研究。     

脉冲液体射流泵能量平衡试验研究

根据脉冲液体射流泵主要流动部件的能量损失压力比,对其能量平衡进行试验,研究了主要过流部件的能量转化与脉冲频率之间的关系。与恒定液体射流泵试验数据进行对比,证明了脉冲频率、脉冲装置对脉冲液体射流泵的主要流动部件的能量损失压力比有一定的影响;且脉冲射流是提高射流泵传能及传质效率的有效途径。     

等离子射流与渐扩边界中液体工质相互作用特性的模拟实验

为了探索整装式液体工质电热化学炮中药室边界形状对燃烧控制的影响,采用数字高速录像系统对等离子体射流在液体工质中的扩展过程进行了测试,研究了不同放电电压、不同喷嘴直径、不同渐扩边界结构下等离子射流与液体工质的相互作用特性.获得了等离子射流在液体工质中扩展形态的时间序列图,处理出不同工况下Taylor空腔扩展的轴向位移与时...     

Oscillations of the fluid flow and the free surface in a cavity with a submerged bifurcated nozzle

The free surface dynamics and sub-surface flow behavior in a thin (height and width much larger than thickness), liquid filled, rectangular cavity with a submerged bifurcated nozzle were investigated using free surface visualization and particle image velocimetry (PIV). Three regimes in the free surface behavior were identified, depending on nozzle depth and inlet velocity. For small nozzle depths, an irregular free surface is observed without clear periodicities. For intermediate nozzle depths and sufficiently high inlet velocities, natural mode oscillations consistent with gravity waves are present, while at large nozzle depths long term self-sustained asymmetric oscillations occur.For the latter case, time-resolved PIV measurements of the flow below the free surface indicated a strong oscillation of the direction with which each of the two jets issue from the nozzle. The frequency of the jet oscillation is identical to the free surface oscillation frequency. The two jets oscillate in anti-phase, causing the asymmetric free surface oscillation. The jets interact through a cross-flow in the gaps between the inlet channel and the front and back walls of the cavity.     

Effect of nozzle thickness on the self-excited impinging planar jet

The self-excited oscillation of a large aspect ratio planar jet impinging on a flat plate is investigated experimentally at a single transonic jet velocity to clarify the effect of varying the jet thickness on pattern of jet oscillation and frequency of resulting acoustic tone. The study has been performed for a series of jet thicknesses, 1 mm to 4 mm, each of which is tested for the complete range of plate position, i.e. impingement distance, over which acoustic tones are generated. The results reveal that the jet oscillation is controlled by a fluid-dynamic mechanism for small impingement distances, where the hydrodynamic flow instability controls the jet oscillation without any coupling with local acoustic resonances. At larger impingement distances, a fluid-resonant mechanism becomes dominant, in which one of the various hydrodynamic modes of the jet couples with one of the resonant acoustic modes occurring between the jet nozzle and the impingement plate. Within the fluid-resonant regime, the acoustic tones are found to be controlled by the impingement distance, which is the length scale of the acoustic mode, with the jet thickness having only minor effects on the tone frequency. Flow visualization images of the jet oscillation pattern at a constant impingement distance show that the oscillation occurs at the same hydrodynamic mode of the jet despite a four-fold increase in its thickness. Finally, a feedback model has been developed to predict the frequency of acoustic tones, and has been found to yield reasonable predictions over the tested range of impingement distance and nozzle thickness.     

Full-field measurements of self-excited oscillations in momentum-dominated helium jets

Flow structure of momentum-dominated helium jets discharged vertically into ambient air was investigated using a high-speed rainbow schlieren deflectometry (RSD) apparatus operated at up to 2,000 Hz. The operating parameters, i.e., Reynolds number and Richardson number were varied independently to examine the self-excited, flow oscillatory behavior over a range of experimental conditions. Measurements revealed highly periodic oscillations in the laminar region at a unique frequency as well as high regularity in the flow transition and initial turbulent regions. The buoyancy was shown to affect the oscillation frequency and the distance from the jet exit to the flow transition plane. Instantaneous helium concentration contours across the field of view revealed changes in the jet flow structure and the evolution of the vortical structures during an oscillation cycle. A cross-correlation technique was applied to track the vortices and to find their convection velocity. Time traces of helium concentration at different axial locations provided detailed information about the oscillating flow.     

An experimental study on the coherent structures and chaotic phenomena in the axisymmetric countercurrent shear flow

The coherent structures and the chaotic phenomena in the transition of the axisymmetric countercurrent mixing shear flow were investigated experimentally. Two kinds of self-excited oscillation modes could exist in the axisymmetric countercurrent mixing shear flow. One is the shear layer self-excited oscillation mode corresponding to the high Reynolds number regime and the other is the jet column self-excited oscillation mode corresponding to the low Reynolds number regime in the case of the velocity ratio ranging from 1 to 1.5. Analyzing the auto-power spectrum, self-correlation-function and three dimensional reconstructed phase trajectory, the route to chaos through three Hopf bifurcations intercepted by an intermittence of the dynamical system corresponding to the axisymmetric countercurrent mixing shear flow was discovered when the velocity ratio is equal to 1.32.     

Analytical and experimental investigations of the pulsed air–water jet

This paper describes a new way of generating pulsed air–water jet by entraining and mixing air into the cavity of a pulsed water jet nozzle. Based on the theory of hydro-acoustics and fluid dynamics, a theoretical model which describes the frequency characteristic of the pulsed air–water jet is outlined aimed at gaining a better understanding of this nozzle for generating pulses. The calculated result indicates that as the air hold-up increases, the jet oscillation frequency has an abrupt decrease firstly, and then reaches a minimum gradually at α (air hold-up)=0.5, finally it gets increased slightly. Furthermore, a vibration test was conducted to validate the present theoretical result. By this way, the jet oscillation frequency can be obtained by analyzing the vibration acceleration of the equal strength beam affected by the jet impinging. Thereby, it is found that the experimental result shows similar trend with the prediction of the present model. Also, the relationship between vibration acceleration and cavity length for the pulsed water jet follows a similar tendency in accord with the pulsed air–water jet, i.e. there exists a maximum for each curve and the maximum occurs at the ratio of L/d₁ (the ratio of cavity length and upstream nozzle diameter) =2.5 and 2.2, respectively. In addition, experimental results on specimens impinged by the pulsed water jet and pulsed air–water jet show that the erosion depth increases slightly with air addition within a certain range of cavity length. Further, this behavior is very close to the vibration test results. As for erosion volume, the air entrained into the cavity significantly affects the material removal rate.     

Development of a feedback model for the high-speed impinging planar jet

This article experimentally investigates the self-excited impinging planar jet flow, specifically the development and propagation of large-scale coherent flow structures convecting between the nozzle lip and the downstream impingement surface. The investigation uses phase-locked particle image velocimetry measurements and a new structure-tracking scheme to measure convection velocity and characterize the impingement mechanism near the plate, in order to develop a new feedback model that can be used to predict the oscillation frequency as a function of flow velocity ( $U_o$ ), impingement distance ( $x_o$ ) and nozzle thickness ( $h$ ). The resulting model prediction shows a good agreement with experimental tone frequency data.     

SELF-SUSTAINED OSCILLATION OF A SUBMERGED JET IN A THIN RECTANGULAR CAVITY

Self-sustained oscillating jet flow of water in a rectangular cavity, having thickness which is small relative to its width, is measured using LDA and PIV, and predicted using a transient two-dimensional computational fluid dynamic model which incorporates a resistance coefficient for cross-flow. The basic geometry represents a scale model of a mould typical of thin slab steel casting. The frequency of the oscillation was found to be independent of cavity thickness. It also increased as the cavity width decreased down to some critical value, after which the oscillation ceased. The frequency was observed to increase with nozzle diameter and was found to decrease with increasing length/width ratio of the cavity. The numerical model, with a fixed dimensionless cross-flow resistance coefficient, was shown to predict the Strouhal number of the oscillation and the dimensionless mean velocity profiles in the jet extremely well.