平面液体层碎裂过程实验研究

小宽厚比喷嘴喷射出的平面水膜进入静止空气中,在不同气流流速环境下对水膜碎裂过程进行了实验研究。结果表明,静止空气中的水膜表面波呈现对称波形,射流的碎裂长度随雷诺数的增大而增大,喷射压力对射流碎裂长度没有直接影响。空气助力作用使平面射流表面波的上、下气液交界面出现相位差。水膜的碎裂长度随空气助力气流速度的增大而减小;空气助力对于低雷诺数水膜射流具有很强的促进碎裂作用,所以会极大地改善低雷诺数射流的一次雾化效果。随着水流雷诺数的提高,空气助力作用对水膜碎裂长度的影响大为减弱;即使在高速助力空气的作用下,水膜仍长期保持较稳定的射流流态,没有出现明显的水膜撕裂现象。说明在小宽厚比喷嘴的瑞利(Rayleigh)模式射流中,高雷诺数射流是水膜的稳定因素。与气液流速比、气流马赫数等无量纲参数相比,液体喷射的雷诺数是射流碎裂的主要影响因素。     

Linear Stability Analysis of a Viscous Liquid Sheet in a High-Speed Viscous Gas

Air-assisted atomizers in which a thin liquid sheet is deformed under the action of a high-speed air flow are extensively used in industrial applications, e.g., in aircraft turbojet injectors. Primary atomization in these devices is a consequence of the onset and growth of instabilities on the air/liquid interfaces. To better understand this process, a temporal linear instability analysis is applied to a thin planar liquid sheet flowing between two semi-infinite streams of a high-speed viscous gas. This study includes the full viscous effects both in the liquid and gas basic states and perturbations. The relevant dimensionless groups entering the non-dimensional Orr–Sommerfeld equations and boundary conditions are the liquid and gas stream Reynolds numbers, the gas to liquid momentum flux ratio, the gas/liquid velocity ratio, the Weber number and the equivalent gas boundary layer to liquid sheet thickness ratio. Growth rates and temporal frequencies as a function of the wave number, varying the different dimensionless parameters are presented, together with neutral stability curves. From the results of this parametric study it is concluded that when the physical properties of gas and liquid are fixed, the momentum flux ratio is especially relevant to determine the instability conditions. It is also observed that the gas boundary layer thickness strongly influences the wave propagation, and acts by damping sheet oscillation frequency and growth. This is especially important because viscosity in the basic gas velocity profile has always been ignored in instability analysis applied to the geometry under study. This revised version was published online in July 2006 with corrections to the Cover Date.     

Bubble generation and transport in a microfluidic device with high aspect ratio

Bubble generation and transport in a micro-device composed of a micro-T-junction and a following serpentine micro-channel was experimentally investigated. It has a rectangular cross-sectional with an aspect ratio of 7.425. Air and water were used as gas and liquid, respectively. Mixtures of water–glycerol and water–Tween-20 were used to study the effects of liquid viscosity and surface tension. Compared with previous T-junction bubble generation, the liquid and gas inlets orientation was switched in this work. The continuous flow was driven from the perpendicular channel and the dispersion flow was from the main channel. It shows that the break-up process has three periodic steps under certain operating conditions. The dimensionless bubble length L/w in the micro-channel with high aspect ratio is much larger than that in square microchannels. A correlation is proposed to correlate L/w with liquid flow rate J_L, gas flow rate J_G, and liquid viscosity μ_L. Surface tension σ can change the bubble shape but almost does not affect the bubble length in this fast break-up process. Additionally, a long bubble may be broken up at the corners at the same time because the locations of gas and liquid are exchanged relative to the concave and convex portions of an elbow after a turn which may result in the change of fluid velocities and gas–liquid pressure drop.     

Evaluation of the dimensionless groups based scale-up of gas–solid spouted beds

In this work, the scale-up methodology of He et al. (1997) that is based on maintaining similar or close dimensionless groups in gas–solid spouted beds has been evaluated. Two geometrically similar spouted beds of 0.152 m and 0.076 m diameter have been used. It has been demonstrated experimentally, that there is non-similarity in the local hydrodynamic parameters such as solids holdup and dimensionless solids velocity, when all the dimensionless groups have been matched or close to each other in the two studied spouted beds. This confirms that the global hydrodynamic parameters should not be used to confirm the validity of or to evaluate the dimensionless groups scale-up based methodologies.     

Numerical simulation of primary break-up and atomization: DNS and modelling study

This work deals with numerical simulations of atomization with high Weber and Reynolds values. A special attention has been devoted to the modelling of primary break-up. Due to progress of direct numerical simulations (DNS) of two phase flows it is now possible to simulate the primary break-up of a Diesel spray [Menard, T., Tanguy, S., Berlemont, A., 2007. Coupling level set/VOF/ghost fluid methods: validation and application to 3D simulation of the primary break-up of a liquid jet. Int. J. Multiphase Flow 33 (5), 510–524]. The present formulation of the so-called ELSA (Eulerian–Lagrangian Spray Atomization model) [Vallet, A., Borghi, R., 1999. Modélisation Eulerienne de L’atomisation d’un Jet Liquide. C. R. Acad. Sci. Paris Sér. II b 327, 1015–1020] for atomization is presented and evaluated in the dense zone of the spray by comparison to a DNS based on a coupled level set/VOF/ghost fluid method. Once constants and parameters of the model are fixed thanks to comparisons with DNS, the model is tested with experimental data. The liquid and vapour penetrations show a good agreement when they are compared to experiments of Diesel atomization. In particular the influence of the gas temperature is well recovered. For different temperatures, similarly to the experiments, vapour penetrations are unchanged, but the corresponding equivalent ratio fields are strongly modified. Finally, the combustion model ECFM-3Z [Colin, O., Benkenida, A., 2004. The 3-zones extended coherent flame model (ecfm-3z) for computing premixed/diffusion combustion. Oil Gas Sci. Technol. 59 (6) 593–609] is joined to the ELSA model and the effect of gas temperature changes on a Diesel spray flame is reproduced.     

An Analytic Solution for the Frontal Flow Period in 1D Counter-Current Spontaneous Imbibition into Fractured Porous Media Including Gravity and Wettability Effects

Including gravity and wettability effects, a full analytical solution for the frontal flow period for 1D counter-current spontaneous imbibition of a wetting phase into a porous medium saturated initially with non-wetting phase at initial wetting phase saturation is presented. The analytical solution applicable for liquid–liquid and liquid–gas systems is essentially valid for the cases when the gravity forces are relatively large and before the wetting phase front hits the no-flow boundary in the capillary-dominated regime. The new analytical solution free of any arbitrary parameters can also be utilized for predicting non-wetting phase recovery by spontaneous imbibition. In addition, a new dimensionless time equation for predicting dimensionless distances travelled by the wetting phase front versus dimensionless time is presented. Dimensionless distance travelled by the waterfront versus time was calculated varying the non-wetting phase viscosity between 1 and 100 mPas. The new dimensionless time expression was able to perfectly scale all these calculated dimensionless distance versus time responses into one single curve confirming the ability for the new scaling equation to properly account for variations in non-wetting phase viscosities. The dimensionless stabilization time, defined as the time at which the capillary forces are balanced by the gravity forces, was calculated to be approximately 0.6. The full analytical solution was finally used to derive a new transfer function with application to dual-porosity simulation.     

Gas displacement of viscoplastic liquids in capillary tubes

The displacement of viscoplastic liquids in capillary tubes by gas injection is examined. The viscoplasticity alters the flow kinematics and changes dramatically the amount of mass left attached at the tube wall as compared to the Newtonian case, studied experimentally by G.I. Taylor in 1961 [G.I. Taylor, Deposition of a viscous fluid on the wall of a tube, J. Fluid Mech. 10 (1961) 161–165]. Experiments with Carbopol aqueous solutions were performed for different flow rates. A recently proposed viscosity function for viscoplastic liquids was fitted to the rheological data of the Carbopol solutions. A new dimensionless rheological property – the jump number – arises in the dimensionless version of this viscosity function. The results show the effect of the viscoplastic character of the liquid on the free surface shape and on the thickness of the film of liquid left attached to the wall. This thickness decreases with the jump number and increases with the flow rate. It is also observed that there is a critical dimensionless flow rate below which the displacement is apparently perfect, i.e. there is no observable liquid left attached to the wall. This behavior is shown to be directly related to the fully developed flow far ahead the air–liquid interface.     

Stress-free layers in photoinduced deformations of photoelastomer beams

Nematic liquid crystals combined with long molecular chains to form liquid crystal elastomers are capable of large extension. When such liquid crystal elastomers contain azo dyes to constitute photoelastomers, illumination can trigger large contraction. Beams made from such photoelastomers possess a non-uniform illumination and hence photostrain across their cross-section, resulting in bending and highly non-linear stress distribution. Due to the non-linear stress distribution, there can be more than one stress-free layers within the beam. In this paper, we present a dimensionless parametric study of nematic photoelastomer beams under the combined effects of light and mechanical loads. We show how the number of stress-free layers depends on three dimensionless parameters. The paths traced out by the system in the space of dimensionless parameters by varying the different real parameters are investigated, showing how the number of stress-free layers changes when e.g. the thickness or the mechanical load of the elastomer beam is varied. These results are important if the strain induced director rotation is not negligible.     

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.     

On the instability of microjets

The instability of an axisymmetric viscous liquid jet in a gas or in a vacuum is examined using the interface formation theory. This model allows for variable surface tension at constant temperature, generalising the classical continuum formulation by using irreversible thermodynamics. Steady-state solutions are determined and found to be unstable to a travelling wave that propagates down the liquid jet, causing the jet to break-up into drops. The linear instability results are compared to those of the classical formulation. These are especially found to differ when the jets are on the micron scale. This will give rise to significantly revised predictions in some parameter ranges for the break-up length and droplet sizes produced by microjets. Comparisons with molecular dynamics simulations are also presented, with encouraging results. Finally, the dependence of the results on the initial conditions is discussed. PACS 68.03.Cd     

Numerical investigation of heat and mass transfer processes accompanying crystallization under conditions when body forces produce weak accelerations

A numerical investigation has been made into the development of the flow of a liquid and distribution of an dopant during crystallization under conditions when weak body forces act. It is shown that the development and structure of the flow, and also the distribution of the dopant over the height of the liquid volume and along the radius near the crystallization boundary are influenced by the thermal boundary conditions and the wettability of the ampoule wall by the liquid. The mean (over the height of the ampoule) radial inhomogeneity of the dopant distribution is found as a function of the time and the regime parameters (the Grashof, Marangoni, and Schmidt numbers).Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 1, pp. 186–192, January–February, 1981.     

Measurement of liquid droplet disintegration mechanisms

An experimental study is made of deformation mechanisms and break-up duration of liquid droplets due to an air stream. A relationship between flow conditions and the type of droplet deformation mechanisms has been obtained as well as some data concerning duration of disintegration. The investigations have been carried out for several liquids of various viscosity.     

Quasi-steady diffusion-controlled droplet evaporation and condensation

This theoretical investigation deals with the vaporization or condensation rate of a motionless liquid droplet using the quasi-steady diffusion-controlled model. A single-component liquid droplet is considered to be undergoing a phase change within a binary mixture of ideal gases (vapor plus noncondensable gas). Droplet vaporization rates corresponding to specified ambient conditions have been calculated by numerical solution of the variable-property governing equations. Results are presented for water and a series of pure hydrocarbon liquids for a range of ambient conditions of interest. A dimensionless correlation is given for the hydrocarbon vaporization and condensation rates. The pressure variation in the region surrounding a droplet undergoing vaporization or condensation has been investigated by numerical integration of the momentum equation. The resulting calculations indicate that the pressure decreases with increasing distance from the droplet for condensation as well as vaporization. Finally, criteria are given for estimating when the pressure gradient and viscous dissipation may be of significance in the energy equation.     

Laminar natural convection in inclined enclosures bounded by a solid wall

Laminar natural convection has been studied in enclosures bounded by a solid wall with its outer boundary at constant temperature while the opposing side has a constant heat flux. Two-dimensional equations of conservation of mass, momentum and energy, with the Boussinesq approximation are solved using a finite difference method. The numerical procedure adopted is based on the SIMPLER algorithm. Various parameters were: Rayleigh number (from 10³ to 10⁶), dimensionless conductivity of bounding wall (from 1 to 10) and dimensionless wall width (from 0.15 to 0.5), aspect ratio (from 0.5 to 1) and the inclination angle (from 30^° to 180^°). The results are reduced in terms of the normalized Nusselt number as a function of the Rayleigh number, and other dimensionless parameters. The isotherms and streamlines are produced for various Rayleigh numbers and geometrical conditions. It is found that the heat transfer is an increasing function of the Rayleigh number, wall to fluid conductivity ratio, enclosure aspect ratio and a decreasing function of the wall thickness. It passes from a maximum for the inclination angle of about 80^°.     

Heat transfer and interfacial drag in countercurrent steam-water stratified flow

Local condensation heat transfer coefficients and interfacial shear stresses have been measured for countercurrent stratified flow of steam and subcooled water in rectangular channels over a wide range of inclination angles (4–87°) at two aspect ratios. Dimensionless correlations for the interfacial friction factor have been developed that show that it is a function of the liquid Reynolds number only. Empirical correlations of the heat transfer coefficient, based upon the bulk flow properties, have also been set up for the whole body of data encompassing the different inclination angles and aspect ratios. These indicate that the Froude number as a dimensionless gas velocity is a better correlating parameter than the gas Reynolds number. As an alternative approach, a simple dimensionless relationship for the beat transfer coefficient was obtained by analogy between heat and momentum transfer through the interface. Finally, a turbulence-centered model has been modified by using measured interfacial parameters for the turbulent velocity and length scales, resulting in good agreement with the data.     

Flow and breakup characteristics of elliptical liquid jets

This paper presents the results of an experimental study on liquid jets discharging from elliptical orifices into still ambient air. The experiments were conducted with a set of elliptical orifices of approximately same area of cross section but varying orifice aspect ratio using water and water–glycerol mixture as experimental fluids. The flow behavior of liquid jets was analyzed using their photographs captured by an imaging system. The measurements obtained for the elliptical liquid jets were compared with the circular liquid jets discharging from a circular orifice of the same area of cross section. Elliptical geometry of the orifice results in a flow process by which the emanating liquid jet periodically switches its major and minor axes as it flows downstream of the orifice. In this paper, we attempt to characterize the axis-switching process through its wavelength and amplitude. For a given elliptical orifice, the axis-switching process is dominantly seen in a particular range of flow conditions. The effects of the orifice aspect ratio and liquid viscosity on the axis-switching process are revealed through this study. The experimental results on jet breakup show that axis-switching process has a destabilizing effect on elliptical liquid jets within a particular range of flow conditions and it results in shorter breakup lengths compared to the circular jet. The extent to which axis-switching destabilizes the jet is dictated by the viscosity of liquid. An increase in orifice aspect ratio destabilizes elliptical liquid jets with low viscosity like water; however, this behavior seems to get obscured in water–glycerol mixture elliptical jets due to high viscosity.     

Catastrophe analysis of cylindrical lithium ion battery

The thermal runaway reactions in a lithium ion battery cause its temperature and pressure to increase sharply and even as a result explode in the worst conditions. This kind of explosion is thought of as a catastrophe phenomenon. The energy conservation equation for the discharging process of lithium ion battery was produced, to disclose the catastrophic mechanism of thermal runaway explosion. By the dimensionless method, the swallowtail catastrophe potential function of the lithium ion battery was obtained. The control variables of the potential function were discussed further and the thermal runaway zones and non-thermal runaway zones were obtained. The results indicate that the thermal runaway of lithium ion battery is a swallowtail catastrophe in essence, and thus the control methods of lithium ion battery thermal runaway can be designed from the view point of catastrophes in the future.     

Numerical and experimental studies of natural convective heat transfer from vertical and inclined narrow isothermal flat plates

Natural convective heat transfer from an isothermal narrow flat plate embedded in a plane adiabatic surface and inclined at moderate positive and negative angles to the vertical has been numerically and experimentally studied. The solution has the Rayleigh number, the dimensionless plate width, the angle of inclination, and the Prandtl number as parameters. Attention was restricted to a Prandtl number of 0.7. The numerical results have been obtained for Rayleigh numbers between 10³ and 10⁷ for dimensionless plate widths of between 0.3 and 1.2 and for angles of inclination between +45° and −45°. In the experimental study, results have been obtained for Rayleigh numbers between 4 × 10² and 10⁵ for dimensionless plate widths of 0.4 and 2.5 and for angles of inclination between +45° and −45° to the vertical. Empirical equations for the heat transfer rate have been derived.     

Liquid film break-up in a model of a prefilming airblast nozzle

 The paper describes the atomisation process of a liquid in an axissymmetric shear layer formed through the interaction of turbulent coaxial jets (respectively, inner and outer jets), with and without swirl, in a model airblast prefilming atomiser. The atomisation process and spray quality was studied using different visualisation techniques, namely laser shadowgraphy and digital image acquisition. The experiments were conducted for different liquid flow rates, Reynolds numbers ranging from 6600 to 66000 and 27300 to 92900 for the inner and outer air flows, respectively, for different outer flow swirl levels, and two liquid film thicknesses −0.2 and 0.7 mm. All the tests were carried out at atmospheric pressure and using water. The results include the analysis of the film structure at break-up and of the break-up length, and suggest that the deterioration of the liquid film close to the atomising edge exhibits a periodic behaviour and is mainly dependent on the inner air velocity. Film thickness strongly affects the time and length scales of the break-up process for the lower range of air velocities. For higher inner air velocities, the break-up length and time become less dependent on liquid flow rate and initial film thickness. Received: 14 March 1997/Accepted: 27 October 1997