Stability of an annular viscous liquid jet in compressible gases with different properties inside and outside of the jet

A spatial linear instability analysis is conducted on an annular viscous liquid jet injected into compressible gases and a three-dimensional model of the jet is developed. The model takes into account differences between the velocities, densities of the gases inside and outside of the liquid jet. Theoretical analysis reveals that there exist 9 dimensionless parameters controlling the instability of the liquid jet. Numerical computations reveal some basic characteristics in the breakup and atomization process of the liquid jet as well as influences of these relevant parameters. Major observations and findings of this study are as follows. The Mach number plays a destabilizing role and the inner Mach number has a greater effect on the jet instability than the outer Mach number. The Reynolds number always tends to promote the instabilities of the liquid jet, but its influence is very limited. The Weber number and the gas-to-liquid density ratio also have unstable effects and can improve the atomization of liquid jets. Furthermore, the effects of the Weber number and gas-to-liquid density ratio on the maximum growth rates of axisymmetric and non-axisymmetric disturbances and corresponding dominant wave numbers are manifested in a linear way, while that of the Mach number is non-linear. The effect of Reynolds on the maximum growth rates is non-linear, but the dominant wavenumber is almost not affected by the Reynolds number.     

液体射流结构特征的理论分析

本文利用线性不稳定性理论，在较大的韦伯数（Ｗｅ）和雷诺数（Ｒｅ）范围内对液体射流的结构特征进行了理论分析，研究结果表明：在一定韦伯数和雷诺数条件下，射流的高阶模式将成为最不稳定模式，射流的结构也将相应表现出该模式的特征，雷诺数减小对各阶模式有不同程度的抑制作用；圣零阶模式抑制较大，对高阶模式抑制较小，这是导致高阶模式成为最不稳定模式的原因；在小韦伯数和大雷诺数情况下，各阶模式几乎同样不稳定，这时射     

液体射流在旋转气流中的雾化机理研究

对旋转气流中的空心柱形液体射流的雾化过程进行了数值模拟。结果表明：Reynolds数Re、Weber数We、空心柱内部及外部气液密度比Q和Qh及喷嘴内半径与液膜厚度比Ah，在射流的雾化过程中均起不稳定性的作用；空心柱射流内部的气流旋转强度Ea在雾化过程中加强了自由面上的轴对称扰动和非轴对称扰动；空心柱射流外部的气流旋转强度Eb削弱了自由表面上的轴对称扰动强度，却加强了自由表面的非轴对称扰动。此外，Eb的增加，能大大增加扰动种类。     

受激液体射流的非轴对称模式

利用液体射流破碎的线性不稳定性理论，研究了受激液体射流特有的“局部频段非轴对称模式占优”现象，由一系列理论计算结果，得到了Re-We平面上的相应临界曲线，并根据这些曲线得出了受激液体射流出现非轴对称模式结构特征的物理条件。计算结果还表明，受激液体射流和自由液体射流的临界曲线有明显不同。初步的实验观察结果证实了理论的预测。     

加热条件下液体燃料射流破碎机理的研究

利用线性热不稳定性理论,对黏性液体射入高温气体介质模型所对应的色散方程进行了数值求解。利用所得到的计算结果,研究了加热条件下轴对称模式扰动液体射流破碎机理,探讨了表征各种影响射流破碎作用力的无量纲Weber数（We）、密度比（Q）、Marangoni数（Ma）和Ohnesorge数（Z）对液体射流破碎最大扰动增长率及占优波数的影响。研究结果表明,液体和气体介质之间的温度梯度对液体射流稳定性有着非常显著的影响,表明热毛细力对于液体射流的破碎有促进作用,这种作用对处于Taylor模式下的液体射流尤为显著,并且这种热力作用可使液体射流从一种模式进入另一种模式,并可以大大改变射流的破碎尺序。     

实际射流参数对加热条件下液体燃料射流不稳定性的影响

利用线性热不稳定性理论,对黏性液体射入高温气体介质所对应的色散方程进行了数值求解．利用所得到的计算结果,研究了加热条件下射流速度、气液密度比、液体黏度、温度梯度及液体种类等实际射流参数对射流最大扰动增长率及占优波数的影响规律．研究结果表明：对于加热条件下Reyleigh模式的液体射流,气液密度比、温度梯度是射流破碎的失稳因素,而射流速度、液体黏度则是液体射流破碎的促稳因素;对于Taylor模式的液体射流,射流速度、温度梯度、气液密度比是射流破碎的失稳因素,而液体黏度是液体射流破碎的促稳因素．研究结果同时证明了液体种类的改变对射流不稳定性的影响是多种因素共同作用的结果．     

液体燃料射流最不稳定频率的理论分析（1）——液体燃料射流的最不？…

利用线性不稳定性理论,对粘性液体燃料射流射入气体介质的射流模型所对应的色散方程进行了数值求解。利用计算结果,分析了液体燃料射流不稳定性研究中的３个最重要的无量纲参数参韦韦伯数Ｗｅ、雷诺数Ｒｅ及密度比Ｑ,对分别处于瑞利模式和泰勒模式下的液体射流的最不稳定频率及对应的最大扰动增长率的影响。     

可压缩气体中的三维黏性液体射流雾化机理

建立了可压缩气体中的三维黏性液体射流雾化数学模型,在射流雾化过程中起控制作用的参数主要有气液速度、气液密度、气液界面表面张力、液体黏性、喷嘴直径及音速.采用线性空间稳定性分析方法详细分析了这些参数在高速射流雾化过程中不稳定性的作用.结果是：增加液体射流速度、气体密度及喷嘴直径;减少液体密度、液体黏性及表面张力,可使射流不稳定性增强.此外,当气流与液体射流反向时增加气体流速也可以使流动不稳定性增强,但当气流与液体射流同向时结果相反.气体可压缩性的增加使流动变得不稳定,但它的影响是很小的.     

射流参数对旋流雾化的影响

本文利用线性不稳定性理论,分析了无粘旋转液体射流射入无界气体介质中的破碎机理。通过改变各项射流参数,着重分析了液体旋流数对射流分裂雾化的影响,同时也探讨了与各种射流条件相对应的射流分裂雾化的主导模式。其研究结果可为旋流在工程实际中的应用,特别是在直喷式汽油机中的应用提供了一定的理论依据。     

加热条件下粘性液膜射流破碎机理的研究

利用线性不稳定性理论，对具有温度梯度的粘性液膜射流模型所对应的色散方程进行了数值求解。利用计算结果，研究了加热条件下反对称模式和对称模式粘性液膜射流大、小尺度破碎模式的破碎机理，探讨了韦伯数(We)、密度比(Q)、Marangoni数(Mα)和Ohnesorge数(x)对液膜射流表面波的最大扰动增长率及占优波数的影响。     

Impingement heat transfer in an under-expanded axisymmetric air jet

This paper is concerned with the heat transfer that occurs when an underexpanded jet impinges onto a heated surface. The heat transfer in the impingement zone is extremely high and when the surface interferes with the expansion of the jet, the radial distribution of the heat transfer coefficient becomes more complex. If the jet impinges upon the surface before the core of the jet has decayed, there is no longer a maximum stagnation heat transfer coefficient on the geometric axis of the jet, instead a stagnation ‘ring’ is formed with a radius of about one nozzle diameter. Experimental results are presented for nozzle pressure ratios up to 5.08, and nozzle-to-surface spacings of 3, 6 and 10 nozzle diameters. In addition to the measured data, a clear outcome of the work is that the usual method of representing Nusselt number as a function of Reynolds number is inadequate in compressible flows where the dimensional analysis shows that the nozzle Mach number, or pressure ratio, should also be included.     

Liquid jet in a subsonic gaseous crossflow: Recent progress and remaining challenges

This article reviews published literature on the characteristics of a liquid jet injected transversally into a subsonic gaseous crossflow. The review covers the following aspects: (і) liquid jet primary breakup regimes, (іі) liquid jet trajectory and penetration, (ііі) liquid jet breakup length, and (іv) droplets features and formation mechanisms. The focus is on analyzing the role of different prominent parameters which include gaseous and liquid properties, and liquid injector geometry. The review revealed that gas Weber number plays a crucial role in defining non-turbulent primary breakup regimes, while liquid jet Weber number is of great importance for the transition to turbulent primary breakup. Jet-to-crossflow momentum flux ratio is the most important parameter for predicting the trajectory, penetration, and breakup length of a liquid jet in a crossflow. The characteristics of droplets disintegrated during the primary breakup are mostly influenced by the nozzle exit conditions, whereas the characteristics of droplets produced via the secondary breakup are strongly dependent on the velocity of cross airflow. Although the review revealed that substantial progress has been made in understanding this complex two-phase flow phenomenon, there still remain several shortcomings which require further research.     

液体燃料射流破碎机理研究中的时间模式与空间模式

利用线性不稳定性理论分析了液体燃料射流在分别采用时间模式与空间模式两种方法下液体表面扰动波的波数、频率及最大扰动增长率之间的关系,讨论了两种模式下液体射流结构特征的差别。应用基于粘性液体射流射入气体介质中的物理模型,计算分析了 Ｒｅ 数、 Ｗｅ 数和密度比 Ｑ对时间模式与空间模式差值的影响,结果发现采用时间模式或空间模式并不改变液体射流各阶模式之间的相互关系, Ｗｅ 数和 Ｑ是影响时间模式与空间模式差值的主要因素,而 Ｒｅ 数影响不大。在一定条件下,时间模式与空间模式具有等效性。     

Effects of Mach number and Reynolds number on jet array impingement heat transfer

Experimental results from the present study show substantial, independent Mach number effects (as the Reynolds number is held constant) for an array of impinging jets. The present discharge coefficients, local and spatially averaged Nusselt numbers, and local and spatially averaged recovery factors are unique because (i) these data are obtained at constant Reynolds number as the Mach number is varied, and at constant Mach number as the Reynolds number is varied, and (ii) data are given for jet impingement Mach numbers up to 0.74, and for Reynolds numbers up to 60,000. As such, results are given for experimental conditions not previously examined, which are outside the range of applicability of existing correlations.     

Investigation on mechanism of critical cavitating flow in liquid jet pumps under operating limits

The critical cavitating flow in liquid jet pumps under operating limits is investigated in this paper. Measurements on the axial pressure distribution along the wall of jet pumps indicate that two-phase critical flow occurs in the throat pipe under operating limits. The entrained flow rate and the distribution of the wall pressure upstream lowest pressure section does not change when the outlet pressure is lower than a critical value. A liquid–vapor mixing shockwave is also observed under operating limits. The wave front moves back and forth in low frequency around the position of the lowest pressure. With the measured axial wall pressures, the Mach number of the two-phase cavitating flow is calculated. It’s found that the maximum Mach number is very close to 1 under operating limits. Further analysis infers a cross-section where Mach number approaches to 1 near the wave front. Thus, the liquid–vapor mixture velocity should reach the local sound velocity and resulting in the occurrence of operating limits.     

The role of jet inlet geometry in impinging jet heat transfer,modeling and experiments

Effects of jet inlet geometry and aspect ratio on local and average heat transfer characteristics of totally nine confined impinging jets have been investigated experimentally using thermochromic liquid crystals and numerically by using a 3-D low Reynolds number k–? model. Experimental study by using liquid crystals for temperature measurement was conducted for three different jet exit geometries (circular, elliptic, rectangular). In addition, simulations were performed at the same mass flow rate for totally nine jet exit geometries including circular, elliptic and rectangular jets with different aspect ratios for dimensionless jet to plate distances 2, 6, and 12.As the aspect ratio of equal cross-sectional area elliptic and rectangular jets increases, heat transfer enhancement in the stagnation region was obtained. As a result higher aspect ratio jets can be used as a passive enhancement technique for localized heating or cooling especially at small jet to plate distances. Wall jet region comprises very large portion of the impinging plate under study and generally lower heat transfer rates were attained for higher aspect ratio jets in this region especially at small jet to plate distances. Therefore as the aspect ratio increases, lower average heat transfer rates were acquired. The effect of aspect ratio on local and average heat transfer decreases with increasing jet to plate distance. Even though the mass flow rate is the same, heat transfer rate of rectangular jets were reduced with increasing the cross-sectional area. With increasing jet to plate distance very similar heat transfer characteristics were observed along the major and minor axis directions.     

Integral solutions for selected turbulent quantities of small-scale hydrogen leakage: A non-buoyant jet or momentum-dominated buoyant jet regime

In this paper, the integral method is used to derive a complete set of results and expressions for selected physical turbulent properties of a non-buoyant jet or momentum-dominated buoyant jet regime of small-scale hydrogen leakage. Several quantities of interest, including the cross-stream velocity, Reynolds stress, velocity-concentration correlation (radial flux), dominant turbulent kinetic energy production term, turbulent eddy viscosity and turbulent eddy diffusivity are obtained. In addition, the turbulent Schmidt number is estimated and the normalized jet-feed material density and the normalized momentum flux density are correlated. Throughout this paper, experimental results from Schefer et al. [Schefer RW, Houf WG, Williams TC. Investigation of small-scale unintended releases of hydrogen: momentum-dominated regime. Int J Hydrogen Energy 2008;33(21):6373–84] and other works for the momentum-dominated jet resulting from small-scale hydrogen leakage are used in the integral method. For a non-buoyant jet or momentum-dominated regime of a buoyant jet, both the centerline velocity and centerline concentration are proportional with z⁻¹. The effects of buoyancy-generated momentum are assumed to be small, and the Reynolds number is sufficient for fully developed turbulent flow. The hydrogen–air momentum-dominated regime or non-buoyant jet is compared with the air–air jet as an example of non-buoyant jets. Good agreement was found between the current results and experimental results from the literature. In addition, the turbulent Schmidt number was shown to depend solely on the ratio of the momentum spread rate to the material spread rate.     

粘性平面液膜喷入可压缩气流中的线性稳定性分析

对粘性平面液膜射流喷射进入可压缩气体环境的线性稳定性进行了理论分析。应用纳维-斯托克斯控制方程组,代入运动学和动力学边界条件,推导得到了线性化和量纲一化的色散关系式。编制Fortran语言数值计算程序,运用穆勒方法求得液膜射流表面波增长率随表面波数变化关系的数值解。分析了气液流速比之差、韦伯数、雷诺数、欧拉数、马赫数等量纲一参数对液膜射流碎裂过程的影响。结果表明,上述量纲一参数均是液膜碎裂的促进因素。     

柴油机喷孔内部空化效应的可视化实验研究

柴油机喷孔内空化现象显著影响燃油雾化质量.针对目前广泛应用的VCO型喷嘴,设计大尺度透明喷嘴稳态实验系统,全面研究了无量纲数、雷诺数、喷嘴射流参数和结构参数对喷嘴内空化过程的影响.结果表明,空化数对空化剧烈程度影响显著,雷诺数对空化剧烈程度影响小于空化数;射流参数直接影响流体流动状态,增大入口压力、减小出口压力和针阀升程能使喷孔内的空化现象显著增强;结构参数决定喷孔内流动形式,增大喷孔倾角、直径和长径比,采用入口圆角等措施会抑制空化的产生.     

A numerical study of the unsteady flow and heat transfer in a transitional confined slot jet impinging on an isothermal surface

A numerical finite-difference approach was used to compute the steady and unsteady flow and heat transfer due to a confined two-dimensional slot jet impinging on an isothermal plate. The jet Reynolds number was varied from Re=250 to 750 for a Prandtl number of 0.7 and a fixed jet-to-plate spacing of H/W=5. The flow was found to become unsteady at a Reynolds number between 585 and 610. In the steady regime, the stagnation Nusselt number increased monotonically with Reynolds number, and the distribution of heat transfer in the wall jet region was influenced by flow separation caused by re-entrainment of the spent flow back into the jet. At a supercritical Reynolds number of 750 the flow was unsteady and the net effect in the time mean was that the area-averaged heat transfer coefficient was higher compared to what it would have been in the absence of jet unsteady effects. The unsteady jet exhibited a dominant frequency that corresponded to the formation of shear layer vortices at the jet exit. Asymmetry in the formation of the vortex sheets caused deformation or buckling of the jet that induced a low-frequency lateral jet “flapping” instability. The heat transfer responds to both effects and leads to a broadening of the cooled area.