电场作用下无黏聚焦射流的时间不稳定性研究

基于电场作用下的流动聚焦实验建立了简化的理论模型,开展了带电同轴液气射流的时间不稳定性分析.在无黏假设下,得到了扰动在时间域内发展演化的解析形式的色散关系,分析了主要控制参数对不稳定模态的影响.结果表明,只有轴对称扰动和第一类非轴对称扰动在时间域内是增长的;液气界面的表面张力对轴对称扰动有着双重影响而对非轴对称扰动起抑制作用;外层气体的流速以及密度的增加均能促进射流的失稳.这些结论与实验结果是定性一致的.结果也表明,在不考虑初始界面电荷密度时,单一的轴向电场能抑制射流的失稳.      

电场作用下流动聚焦的实验研究

通过在流动聚焦的同轴液-气射流区域施加电场, 开展了电场力和气动力共同作用下锥形以及带电射流的不稳定性特性实验研究. 实验在精密设计的流动聚焦装置上完成, 分析了外部电压、气体压力降和液体流量等主要控制参数对流动聚焦过程的影响, 获得了锥形的振动模式和稳定模式及其之间的转换, 得到了射流的滴模式、轴对称模式、共存模式和非轴对称模式及其转换并定量分析了电场对射流尺寸参数的影响. 结果表明, 相比于单一的流动聚焦, 该方法能够增强锥形的稳定性, 促进液体射流雾化, 减小颗粒的直径, 因此在科技领域和工程实际中具有重要的应用价值.      

电场作用下导电射流稳定性的理论与实验研究

建立导电射流在径向电场作用下的线性稳定性粘性模型,通过正则模方法,推导了轴对称和非轴对称模态下的色散关系,通过计算求得增长率随波数及电欧拉数的变化,并在理论上预测了最有可能波长.选用酒精和酒精甘油混合物作实验液体,观察了径向电场对射流不稳定性行为的影响规律,并测量射流表面波的波长.实验结果和理论结果在定性方面取得了较好的一致.但通过与实验比较,理论预测的最有可能波长在非轴对称模态出现较大偏差,普遍比实验结果小.而且,实验表明,最大增长率并不是判断主导模态的好标准,因为在非轴对称的最大增长率小于轴对称的最大增长率情况下,实验显示非轴对称模态要比轴对称模态明显了.因此,对于非轴对称的不稳定机理,需要进一步研究.对轴对称模态,理论给出了较好的预测.     

圆环旋转粘性液膜射流三维不稳定性研究

利用线性稳定性理论进行了射流液体粘性对圆环旋转液膜射流稳定性影响的研究,推导出了三维扰动下具有固体旋涡型速度分布的圆环旋转粘性液膜射流的色散方程;在此基础上进行了类反对称模式与类对称模式下的圆环旋转粘性液膜射流的三维不稳定性分析。研究结果表明,在类反对称模式下,液体粘性超过一定值后,射流最大扰动增长率随液体粘性的增加而迅速减小;轴对称模态的射流特征频率产生一个突降变化;随液体粘性增加,轴对称模态不稳定波数范围减小,非轴对称模态不稳定波数范围呈现出先减小后增大趋势。在类对称模式下,液体粘性对射流最大扰动增长率的影响主要体现在对非轴对称模态的影响上;液体粘性只在粘性较大时才会对非轴对称模态射流特征频率产生一定影响;液体粘性超过一定值后,轴对称模态与非轴对称模态的不稳定波数范围都会快速下降。     

电场驱动射流的弯曲失稳分析

本文对电场驱动射流弯曲问题的动力模型作了理论推导和数值分析.在假定射流具有牛顿粘性并作一维流动的基础上,建立了射流运动的控制方程,采用此方程来研究当带电流体穿过电场时受小扰动而产生的失稳现象.将方程无量纲化后,利用简正模态法对控制方程进行了稳定性分析,而后采用伽辽金法求解方程.结果表明,外电场增强、表面电荷量进一步增多(电荷量较大时)、射流轴向流速增大(其值较大时)、射流半径减小以及电荷的分布不均匀性增大,都可以显著增强射流的短波不稳定性;射流偶数阶频率导致了不稳定性,而且阶数越高,不稳定性增长得越快;频率具有虚部,意味着模态中相差的存在;并且射流在短时问内会失去稳定性.     

同轴旋转可压缩流动中液体射流稳定性

基于线性稳定性理论,建立了描述同轴旋转可压缩流动中超空化条件下液体射流稳定性的数学模型,并对数学模型及其求解方法进行了验证;在此基础上,对模型中考虑的射流及气体可压缩性、气体同轴旋转以及超空化等因素对射流稳定性的影响进行了分析. 分析结果表明,模型中考虑射流及气体的可压缩性后,与不考虑可压缩性相比,计算得到的射流稳定性明显变差,最小液滴直径减小,分裂液滴直径变化范围变宽,且小液滴数量增多. 气体的同轴旋转在轴对称与非轴对称扰动下对射流稳定性的影响完全相反;轴对称扰动时,气体旋转使射流稳定性增强,而非轴对称扰动时则正好相反;气体旋转有可能导致影响射流稳定性的扰动模式发生根本性变化. 超空化使射流稳定性变差;超空化程度较弱时,超空化使分裂液滴最小直径减小,分裂液滴直径变化范围增大;而超空化达到一定程度后,进一步提高超空化程度,分裂液滴最小直径几乎保持不变.     

欠膨胀超声速射流不稳定性机理的数值研究

应用NND有限差分格式求解轴对称可压缩N-S方程,研究了不同驻室与环境压力比条件下欠膨胀超声速射流近场的失稳特性.计算结果表明欠膨胀超声速射流的失稳机制根据射流激波结构的特征可分为3种失稳模式:具有规则反射激波结构和单一剪切层特征的射流不稳定性;带有马赫反射激波结构和双剪切层特征的射流不稳定性;具有弯曲马赫杆和高度欠膨胀射流的不稳定性.对于欠膨胀超声速射流,沿射流方向重复出现拟周期性的射流激波结构是射流稳定发展的特征,这种射流激波结构的消失是射流开始失稳的标志.     

基于扰动方程的超音速轴对称射流马赫波辐射研究

超音速不稳定波是导致剪切流失稳和转捩的主要不稳定模态,这种模态以马赫波的形式辐射到远场,从而产生强烈的声场。采用线性稳定性理论和非线性扰动方程(NLDE)分析,计算超音速轴对称射流不稳定波的扰动演化(Ma=2.1),对马赫波辐射进行研究,包括马赫波辐射方向、辐射源位置,以及随斯特劳哈尔数的变化情况。研究结果表明,在超音速轴对称射流中,马赫波沿固定方向辐射向远方,不稳定波相位沿另一方向传播,这两个方向相互正交;马赫波辐射源位置位于不稳定波压力幅值最大处;斯特劳哈尔数St越大,马赫波辐射的能力越强,辐射区域越集中。     

低速轴对称层流射流流动形态和失稳机制的实验研究

本文应用染色液和悬浮粒子显示方法,进一步实验研究轴对称层流射流的流动形态及其失稳机制。首次成功地在从一定口径的喷嘴流出的低速轴对称层流射流中观察到环形回流流动。给出了射流随速度演化及实验容器边界对其流动形态影响的显示照片,发现实际射流的轴对称波动及失稳过程正好对应射流在容器底部产生的环形旋涡的生长和破碎过程。本文认为由于实验空间有限尺度对流动的限制改变了原来射流的流动形态和流场空间的拓扑性质,射流与实际边界的相互作用对实际射流的失稳和转捩有重要的影响。     

流动聚焦及射流不稳定性

流动聚焦是一种有效的微细射流产生方法,其原理可以描述为从毛细管流出的流体由另一种高速运动的流体驱动,经小孔聚焦后形成稳定的锥–射流结构,射流因不稳定性破碎成单分散的液滴.自从1998年流动聚焦被提出以来,陆续发展了单轴流动聚焦、电流动聚焦、复合流动聚焦和微流控流动聚焦等毛细流动技术.这些技术稳定、易操作、没有苛刻的环境条件的要求,能够制备单分散性较好的微纳米量级的液滴、颗粒和胶囊,在科学研究和实际应用中具有重要价值.流动聚焦涉及了多尺度、多界面和多场耦合的复杂流体力学问题,其中稳定的锥形是形成稳定射流的先决条件,过程参数是影响射流界面扰动发展的关键因素,而射流不稳定性分析是揭示射流破碎的最主要理论工具.该文回顾了近二十年来不同结构流动聚焦的研究进展,概述这些技术涉及的过程控制、流动模式、尺度律和不稳定性分析等关键力学问题,总结射流不稳定性的研究方法和已取得的成果,最后展望流动聚焦的研究方向和应用前景.     

Nonlinear instability of an annular liquid sheet exposed to gas flow

Nonlinear instability and breakup of an annular liquid sheet has been modeled in this paper. The liquid sheet is considered to move axially and is exposed to co-flowing inner and outer gas streams. Also, the effect of outer gas swirl on sheet breakup has been studied. In the developed model a perturbation expansion method has been used with the initial magnitude of the disturbance as the perturbation parameter. This is a comprehensive model in that other geometries of planar sheet and a coaxial jet can be obtained as limiting cases of very large inner radius and inner radius equal to zero, respectively. In this temporal analysis, the effect of liquid Weber number, initial disturbance amplitude, inner gas-to-liquid velocity ratio, outer gas-to-liquid velocity ratio and outer gas swirl strength on the breakup time is investigated. The model is validated by comparison with earlier analytical studies for the limiting case of a planar sheet as well as with experimental data of sheet breakup length available in literature. It is shown that the linear theory cannot predict breakup of an annular sheet and the developed nonlinear model is necessary to accurately determine the breakup length. In the limiting case of a coaxial jet, results show that gas swirl destabilizes the jet, makes helical modes dominant compared to the axisymmetric mode and decreases jet breakup length. These results contradict earlier linear analyses and agree with experimental observations. For an annular sheet, it is found that gas flow hastens the sheet breakup process and shorter breakup lengths are obtained by increasing the inner and the outer gas velocity. Axially moving inner gas stream is more effective in disintegrating the annular sheet compared to axially moving outer gas stream. When both gas streams are moving axially, the liquid sheet breakup is quicker compared to that with any one gas stream. In the absence of outer gas swirl, the axisymmetric mode is the dominant instability mode. However, when outer gas flow has a swirl component higher helical modes become dominant. With increasing outer gas swirl strength, the maximum disturbance growth rate increases and the most unstable circumferential wave number increases resulting in a highly asymmetric sheet breakup with shorter breakup lengths and thinner ligaments.     

Axisymmetric and non-axisymmetric instability of an electrically charged viscoelastic liquid jet

A linear analysis is performed to investigate the competition between axisymmetric and non-axisymmetric instability of an electrically charged viscoelastic liquid jet. The liquid is assumed to be a dilute polymer solution modeled by the Oldroyd-B constitutive equation. As to its electric properties, the liquid is assumed to be of finite electrical conductivity and is described by the Taylor–Melcher leaky dielectric theory. An analytical dispersion relation is derived and the temporal growth rate is solved numerically. Two viscoelastic liquids, i.e. a PEO aqueous solution and a PIB Boger fluid, are taken as examples to study the effects of electric field and electrical conductivity on jet instability. The result shows that electric field basically destabilizes both the axisymmetric and the non-axisymmetric mode. On the other hand, the effect of electrical conductivity on the modes is quite limited. An energy analysis shows that elasticity enhances both axisymmetric and non-axisymmetric jet instability and its destabilization effect on the axisymmetric mode is more profound. For viscoelastic jets of high Deborah numbers the combined effect of viscosity and elasticity is possibly characterized by an equivalent Reynolds number related only to the viscosity of solvent.     

Linear instability characteristics of incompressible coaxial jets

The present study deals with the local linear instability of axisymmetric coaxial jets with a duct wall separating the two streams. The flow is assumed to be locally parallel, inviscid and incompressible. The objective of the work is to understand how the various parameters describing this flow geometry (i.e. boundary layers thicknesses at the exit, velocity ratio, wall thickness) may influence the instability of the flow and, in particular, the convective/absolute instability transition. A specific family of profiles is chosen for the modelling of the mean undisturbed flow and a spatial stability analysis is performed in order to identify the unstable modes and to assess how they are affected by the wake region behind the wall. An absolutely unstable mode is found, and its characteristics, depending on the velocity ratio and shear layers thicknesses, are determined. Results show that the absolute unstable mode is present only for a limited range of velocity ratios and that the corresponding frequency is almost constant if normalized with the mean velocity and wake thickness. This frequency value and the extension of the range of velocity ratios is similar to those found in the experiments on a similar geometry. Finally, a specific velocity ratio is found that maximizes the region at the jet exit for which an absolute instability behind the wall is present. This may increase the possibility for the onset of a global mode that may sustain the instability of the whole jet, enhancing considerably the mixing and entrainment characteristics between the two streams.     

Formation and stability of a charged jet in a strong electric field

The shape of a charged jet is determined in the approximation of a strong electric field. The stability of the jet with respect to both axisymmetric and nonaxisymmetric perturbations of the sinusoidal type is investigated in the linear approximation. The domains of predominance of the axisymmetric and bending modes and the longitudinal partition mode are determined. Experimental data on the longitudinal partition of a polymeric jet into several daughter jets are given.     

The breakup and atomization of a viscous liquid jet

Based on the linear analysis of stability, a dispersion equation is deduced which delineates the evolution of a general 3-dimensional disturbance on the free surface of an incompressible viscous liquid jet. With respect to the spatial growing disturbance mode, the numerical results obtained from the solution of the dispersion equation reveal that a dimensionless parameterJ _e exists. AsJ _e>1, the axisymmetric disturbance mode is most unstable; and whenJ _e<1, the asymmetric disturbances come into being, their growth rate increases with the decrease, ofJ _e, till one of them becomes the most unstable disturbance. The breakup of a low-speed liquid jet results from the developing of axisymmetric disturbances, whose instability is produced by the surface tension; while the atomization of a high-speed liquid jet is brought about by the evolution of nonaxisymmetric disturbance, whose instability is caused by the aerodynamic force on the interface between the jet and the ambient gas. The project supported by the National Natural Science Foundation of China     

Instability of a viscoelastic liquid jet with axisymmetric and asymmetric disturbances

The temporal instability behavior of a viscoelastic liquid jet in the wind-induced regime with axisymmetric and asymmetric disturbances moving in an inviscid gaseous environment is investigated theoretically. The corresponding dispersion relation between the wave growth rate and the wavenumber is derived. The linear instability analysis shows that viscoelastic liquid jets are more unstable than their Newtonian counterparts, and less unstable than their inviscid counterparts, for both axisymmetric and asymmetric disturbances, respectively. The instability behavior of viscoelastic jets is influenced by the interaction of liquid viscosity and elasticity, in which the viscosity tends to dampen the instability, whereas the elasticity results in an enhancement of instability. Relatively, the effect of the ratio of deformation retardation to stress relaxation time on the instability of viscoelastic jets is weak. It is found that the liquid Weber number is a key measure that controls the viscoelastic jet instability behavior. At small Weber number, the axisymmetric disturbance dominates the instability of viscoelastic jets, i.e., the growth rate of an axisymmetric disturbance exceeds that of asymmetric disturbances. When the Weber number increases, both the growth rate and the instability range of disturbances increase drastically. The asymptotic analysis shows that at large Weber number, more asymmetric disturbance modes become unstable, and the growth rate of each asymmetric disturbance mode approaches that of the axisymmetric disturbance. Therefore, the asymmetric disturbances are more dangerous than that of axisymmetric disturbances for a viscoelastic jet at large Weber numbers. Similar to the liquid Weber number, the ratio of gas to liquid density is another key measure that affects the viscoelastic jet instability behavior substantially.     

Temporal stability of a particle-laden jet

The temporal instability of a particle-laden jet was investigated numerically which took into consideration the parametric effects of jet parameter, B, jet Reynolds number, Re_j, particle mass loading, Z and Stokes number, St. The linear stability theory was used to derive the instability equations of a viscous particle-laden jet flow. The single-phase instability of a top-hat jet was then calculated and compared with the available analytical theories. The numerical results agree well with the analytical results for both the axisymmetric (n = 0) and first azimuthal (n = 1) modes. The results show that the first azimuthal mode disturbance is usually more unstable than that of the axisymmetric mode. But the axisymmetric mode disturbance can be more unstable when Z is high enough (i.e., Z ? 0.1). The higher B and Re_j are, the more unstable the particle-laden jet will be. The existence of particles enhances the flow stability. With the increasing of Z, the jet flow will grow more stable. The inviscid single-phase jet is the most unstable. The wave amplification, c_i first decreases with the increasing of St and then increases afterwards. There exist certain values of St, at which the jet is the most stable.