喷射雾化最小等效直径的能量判断准则

通过能量分析的方法考虑喷射雾化问题,认为破碎的液滴存在着表面张力势能,这种势能是由于气流对液体的冲击作用提供的,并由此得到了破碎液滴的最小等效直径的能量判断准则.该判断准则在趋势上与经验公式的符合程度是令人满意的.     

压力旋流喷射雾化的喷雾角特性之探讨

本文根据提出的旋流喷射锥膜雾化理论,由旋流喷射的作用力关系,求导出压力旋流喷射雾化的喷雾角特性关系式:θ=tg-12·(1-φ) 并将各种喷雾角关系式同实验数据进行比较。结果表明:本文所导出的喷雾角关系式与实验数据有较好的接近,而且计算式也较简单,有一定的实用价值。     

压力旋流喷雾理论与喷雾粒度特性的探讨

本文试图以实验观察的压力旋流喷雾工况为依据,提出关于压力旋流喷射雾化机理的模型设想——压力旋流喷射锥膜雾化理论。在一些基本假定条件下,导出压力旋流喷雾粒度特性关系式:(d_z的单位为米)

上述理论与所导出的关系式基本上同实际喷雾工况相符合,依此能对实际的喷雾工况进行较好的解释。本文还对国内电厂使用的一些油喷雾咀的喷雾粒度特性进行理论计算,以同实验数相对照,结果表明理论对实践有一定的价值,可供有关方面参考。     

煤粉起动速度与粒径及湿度的关系

在煤码头和车站的煤堆积场上,煤粉的扬起是污染环境的主要原因之一.作者根据力学的基本原理,考虑颗粒和液滴重力、煤粉颗粒尺寸和湿度的影响,利用煤粉颗粒起动时的力平衡条件,推导出在风的作用下,具有一定湿度的煤粉颗粒开始扬起的起动风速的理论表达式.最后.根据煤粉起动速度与煤粉颗粒粒径及湿度的关系,将理论公式和风洞实验结果进行了比较,理论和实验结果符合得相当好.     

PDMS表面上的液滴蒸发实验研究

利用粒子跟踪测速(particle tracking velocimetry, PTV) 技术,研究了聚二甲基硅氧烷（polydimethylsiloxane, PDMS）表面上的液滴蒸发行为.发现固液界面上的荧光微球首先向固液界面中心移动,而后发生运动反转,向三相接触线移动.其原因是由于接触线钉扎时接触线附近的蒸发通量较小,从而引起向液滴中心的流动,这种流动将微球带向液滴中心.理论分析了三相接触线的移动特征,发现其移动速度理论值与实验值在同一量级,而移动加速度的实验值较理论值偏大,造成这种偏差的原因是三相接触线处的荧光微球削弱了基底对三相接触线的钉扎作用.     

乙醇液滴撞击高温壁面蒸发过程的模拟预测研究北大核心CSCD

采用CLSVOF方法,引入描述壁面润湿特性的动态接触角,建立了乙醇液滴撞击高温壁面的数值模型,对乙醇液滴撞击高温壁面后的沸腾蒸发过程展开了研究,并与实验数据进行了对比验证.研究表明:在相同液滴温度下,壁面温度越高,亲水性越强,乙醇液滴的撞击速度越快,液滴的沸腾时间越早,蒸发完成所用时间也越短.在此研究基础上,基于机器学习算法,建立了液滴蒸发预测模型,对乙醇液滴撞击高温壁面后蒸发剩余量随时间的变化进行了预测研究,并通过将不同机器学习算法的预测结果与模拟结果对比,选出最优预测模型.     

二元海水液滴对心碰撞过程数值模拟

为研究海水循环冷却系统中液滴碰撞的基本规律及碰撞结果预测模型,采用流体体积函数(volume of fluid,VOF)方法捕捉两相交界面,利用动态网格自适应技术提高求解精度,对二元海水液滴的对心碰撞过程进行直接数值分析与模拟.首先对氮气中正十四烷液滴的碰撞实验进行数值模拟,验证了数值模型的可靠性.开展了常温常压下等尺寸二元海水液滴对心碰撞数值研究,分析了液滴碰撞过程流场结构及流动机理,研究了不同液滴直径和不同海水浓度对碰撞过程的影响规律,得到了聚合和自反分离两种碰撞结果类型以及二者的临界Weber数.总结出不同Ohnesorge数下海水液滴碰撞结果诺模图.     

表面张力对非传播孤立波的影响

本文在研究非传播弧立波时仔细考虑了表面张力的影响,把表面张力和液体深度的参数平面划分为三个区域,发现其中两个区可产生呼吸弧立波。到目前为止,所有理论和实验文章中提到的呼吸弧立波的参数都在一个参数区内,我们首先报道了另一个参数区并被我们的实验证实.在第三个参数区中,理论分析得到的解是纽结孤立波,但是在我们的实验中除了得到纽结孤立波之外,过得到了一种类似于呼吸孤立波的非传播孤立波.     

三氧化二铝液滴对心碰撞直接数值模拟

为研究固体火箭发动机内三氧化二铝液滴碰撞的物理规律及结果预测模型,针对两个相同尺寸的三氧化二铝液滴对心碰撞,开展了直接数值模拟.首先进行了正十四烷液滴在氮气环境下的对心碰撞数值研究,数值与实验结果基本一致,验证了计算方法的可行性及准确性.针对三氧化二铝液滴开展了6 MPa压强下不同Weber数的对心碰撞数值研究,计算Weber数范围为10~200,Ohnesorge数为0.036 4;获得了反弹、大变形后聚合和自反分离3种结果类型,反弹与大变形后聚合的临界分离Weber数为26,大变形后聚合与自反分离的临界分离Weber数为44.根据临界Weber数对其他流体液滴碰撞模型进行修正,可以获得三氧化二铝液滴的碰撞模型.     

液滴平壁铺展过程中的滞后效应及力学机制研究

研究了液滴平壁铺展过程中的接触角滞后效应,从接触线附近流体的压力、速度以及能量分布等角度考虑滞后效应的成因和变化规律.在此基础之上分析了固体表面粗糙度对滞后效应的影响,并借助部分三维形貌参数(ISO 25178)建立了固体表面形貌与接触角滞后效应之间的量化关系.为了研究以上内容,应用数值仿真软件建立了液滴铺展动力学模型,并结合固体表面上的滞后性实验进行了相关验证.研究结果表明:由于表面粗糙度的存在,液滴铺展至平衡位置时,位于铺展前沿的液体分子被钉扎在固体表面的凹坑或低谷中,使得前沿接触角逐渐增大,后沿接触角逐渐减小,接触角发生滞后;驱动液滴铺展的Laplace压力和自身重力与阻碍液滴铺展的黏性阻力之间的平衡关系,是接触角发生滞后的主要力学机制.另外,实验结果表明接触角滞后效应与固体表面形貌密切相关,具有相同表面粗糙度(Sa)的固体表面,由于表面形貌不同接触角滞后效应可能存在明显的差异.     

热传导对气-液射流界面不稳定性的作用机理研究

研究了平面分层气-液射流在非线性温度分布条件下的界面不稳定性性质.考虑了气体的可压缩性、液体的粘性、以及气体热导率和密度随温度变化等事实.并应用正则模态方法将问题转化为四阶变系数常微分方程,用数值积分和多重打靶法对模型的空间模式进行了计算,研究了不稳定模态随各物理参量的变化趋势.计算表明模型所体现的不稳定性特征与其它模型的计算结果是一致的.同时计算还得出气体和液体的温差越小、雷诺数越大、热导率变大均将有利于液体射流有效雾化的结果.该结论与HJE.Co.Inc(Glens Falls,NY,USA)的实验数据是定性吻合的.     

Theoretical analysis of Rayleigh–Taylor instability on a spherical droplet in a gas stream

A linear analysis of the Rayleigh–Taylor (R–T) instability on a spherical viscous liquid droplet in a gas stream is presented. Different from the most previous studies in which the external acceleration is usually assumed to be radial, the present study considers a unidirectional acceleration acting on a spherical droplet with arbitrary initial disturbances and therefore can provide insights into the influence of R–T instability on the atomization of spherical droplets. A general recursion relation coupling different spherical modes is derived and two physically prevalent limiting cases are discussed. In the limiting case of inviscid droplet, the critical Bond numbers to excite the instability and the growth rates for a given Bond number are obtained by solving two eigenvalue problems. In the limiting case of large droplet acceleration, different spherical modes are asymptotically decoupled and an explicit dispersion relation is derived. For given Bond number and Ohnesorge numbers, the critical size of stable droplet, the most-unstable mode and its corresponding growth rate are determined theoretically.     

Two-fluid spatial instability with velocity defect in the basic flow

The initial state of liquid atomization by a fast gas stream is considered by viscous linear spatial stability analysis for parallel two-fluid flow. The unbounded basic velocity profile is characterized by boundary layers near the interface and different asymptotic velocities. By computing generalized spatial branches we identify conditions for absolute instability when a velocity deficit is introduced to account for the conditions near the nozzle. (© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Linear analysis and energy budget of viscous liquid jets in both axial and radial electric fields

In their countless industrial applications, axisymmetric and non-axisymmetric instabilities are respectively responsible for electrospraying and electrospinning. A linear method and energy budget have been applied in this study to investigate the instability of viscous jets under both the axial and radial electric fields; the liquid was taken to be a leaky dielectric and the gas a perfect dielectric; the effect of a parabolic velocity profile was considered and compared to that of a uniform velocity, and the energy analysis explained the physical mechanisms to an extent. The liquid viscosity and parabolic velocity profile had a combined effect on jet instability. Work induced by the parabolic velocity profile consisted of two parts: the energy transferred from the basic flow to the disturbances, and the influence of the corresponding shear stresses. At low viscosities, these influences were positive enough to prevail over the viscous dissipation, enhancing axisymmetric instability. However, the parabolic velocity profile functioned differently in small and large wavenumber regions, and the response to the axial electric fields varied in different regions, accounting for the dual effects of axial electric fields on axisymmetric instability. Also, under the interplay between the strong axial electric fields and the parabolic velocity profile, two distinct unstable regions emerged for the non-axisymmetric mode. The effects of the radial electric fields were less sensitive, whether or not the parabolic velocity profile was considered. In summary, the parabolic velocity profile was significant–especially for charged jets with weak viscosity and strong axial electric intensity. The effects of axial electric fields in the atomization zone, and the effects of fluid permittivity coupled with the axial electric fields, were also investigated here.     

Hartmann共振管及超音速雾化喷嘴流场的数值模拟

采用基于Roe解法的有限体积法,对Hartmann共振管中的气体流场进行了数值模拟,研究了当喷嘴轴线处存在针型激励器的情况下流场的振动情况,数值计算的结果与理论和相关的实验结果符合得较好．计算结果表明移除或引入激励器,将会使Hartmann共振管的共振模式发生转换．通过对超音速雾化喷嘴流场的数值模拟,研究了其中Hartmann共振腔和二级共振腔共同作用下的振动现象以及各物理参数对振动的影响,并对喷嘴中气流从亚音速向超音速的转变机理进行了研究．     

Finite element analysis of flood discharge atomization based on water–air two-phase flow

Flood discharge atomization is a phenomenon of water fog diffusion caused by the discharge of water from a spillway structure, which brings strong wind and heavy rainfall. These unnatural winds and rainfall are harmful for the safe operation of hydropower stations with high water heads. Compared to the method of prototype observations, physical models and mathematical models, which are semi-theoretical and semi-empirical, numerical simulation methods have the advantage of being not limited by a similar scale and are more economical. A finite element model is presented to simulate flood discharge atomization based on water–air two-phase flow in this paper. Equations governing flood discharge atomization are composed of partial differential equations of mass and momentum conservation laws with unknowns for pressure, velocity and the water concentration. The finite element method is used to solve the governing equations by adopting appropriate solution strategies to increase the convergence and numerical stability. Then, the finite element model is applied to a practical project, the Shuibuya hydropower station, which experienced a flood discharge in 2016. Simulation results show that the proposed model can simulate flood discharge atomization with efficient convergence and numerical stability in three dimensions, and good agreement was observed between numerical simulations and prototype observational data. Based on the simulation results, the mechanism of flood discharge atomization was analyzed.     

Temporal analysis of a power-law liquid sheet in the presence of acoustic oscillations

The instability of a non-Newtonian liquid sheet in the presence of acoustic oscillations is investigated theoretically. The power-law model is used to describe the viscosity of the non-Newtonian liquid. The corresponding dispersion relation is obtained by linear analysis. The effects of the mean velocity of the gas, the oscillation amplitude, the oscillation frequency, and the gas density on the instability of the power-law liquid sheet are studied. The results show that the shear-thickening liquid sheet is more unstable than Newtonian and shear-thinning liquid sheets when the effects of acoustic oscillations are considered. In particular, a second unstable region appears on the shear-thickening liquid sheet at a low oscillation frequency. Especially, for the shear-thinning liquid sheet, there is a second unstable region in the dispersion curve at a high mean gas velocity. A third unstable region appears on the shear-thinning liquid sheet at a high gas density in the presence of acoustic oscillations. The unstable range of the Newtonian liquid is always the widest among these liquids.     

A minimum-stencil difference scheme for computing two-dimensional axisymmetric gas flows: Examples of pulsating flows with instabilities

A minimum-stencil difference scheme for computing two-dimensional axisymmetric gas flows is described. The scheme is explicit, conservative, and second-order accurate in space and time. The numerical results obtained for pulsating flows and contact discontinuity instabilities are discussed. The mechanisms of flow pulsation and instability generation are described.     

液滴的脉动雾化

应用稳定性分析,本文显示一球形液滴可以由参数共振而促使形成比原液滴直径小得多的小微滴。表面张力和周围气体的密度将延缓这一雾化过程。与达朗伯表观体力有关的两重迭平面流体层的界面不稳定性与参数共振引起的界面不稳定性有着密切的关系。