Rupture of a subcooled liquid film falling down a heated grooved surface

This paper presents an experimental study of rupture of a subcooled water film falling down an 1 m long heated copper plate with longitudinal grooves of 0.5×0.15 mm² cross sectional area and 2 mm spacing. It was found that the threshold heat flux at which an initial stable dry patch forms on the grooved surface is about two times higher than that on a smooth surface. Furthermore, the grooves prevent dry patches from spreading over the total heated surface thus essentially delaying the onset of the heat transfer crisis. The main governing parameters of the experiment and their respective values are: initial film temperature (20–95°C), heat flux (0–1.26 W/cm²) and volumetric flow rate (11.1–38.2 l/h) (Re=56.2–653.2).     

水平管降膜式蒸发器管间流动模式的研究

对应用于空气调节和制冷方面的水平管降膜式蒸发器原理进行简述，在分析设计水平管降膜式蒸发器时，需要考虑制冷剂在水平光管上流动模式。给出影响制冷剂在水平光管上流动模式的关键参数。     

水平管降膜蒸发器的研究进展

介绍应用于制冷空调系统的水平管降膜蒸发器的基本原理与结构,给出管间流动模式及传热预测模型的研究现状,分析布液器及管阵布置方式对其性能的影响,为设计制造降膜蒸发器提供一定的参考。     

An experimental study of the effects of surfactant on a falling liquid film

Addition of surface active agents to a falling liquid film changes the fluid mechanical behavior of the film. In this study we describe, both by observations and by calculations, what happens with the wave pattern after an addition of surfactant. The experiments were carried out in a 6-m high falling film column with a mixture of water and glycerol. The changes in wave pattern were detected by a video camera. Addition of surfactant will influence the film differently for different surfactant concentrations. Surfactant concentrations under the saturation limit destabilize the film and concentrations over the limit stabilize the film. The surface excess concentration, the concentration of surfactant in the surface per square meter, plays a key role in the influence of surfactant on the behavior of falling liquid films.     

太阳能溴化锂吸收式制冷系统溶液降膜吸收流动分析

溴化锂降膜式吸收器能在较小液流量和较小温差下获得较高的热流密度和传热传质系数,尤其是当液膜沿着水平管外作降膜流动时,传热传质效果更佳。为此建立溴化锂降膜吸收器溶液吸收过程流动的物理模型,通过对模型假设简化,对其进行数值求解,从而进行流动分析。与实验结果分析相结合,使得对吸收式制冷系统的分析更加全面。     

A numerical study of wave structures developed on the free surface of a film flowing on inclined planes and subjected to surface shear

In this work, we determine the different patterns of possible wave structures that can be observed on a thin film flowing on an inclined plane when at the free surface a shear force (surface traction) is applied. Different wave structures are obtained dependening on the selected combination of downstream and upstream boundary conditions and initial conditions. The resulting initial boundary value problems are solved numerically using the direct BEM numerical solution of the complete two‐dimensional Stokes system of equations. In our numerical results, the initial discontinuous shock profiles joining uniform fluid depths are smoothed due to the two‐dimensional character of the Stokes formulation, including the effect of the gravitational force as well as the interfacial surface tension force. In this way, physically feasible continuous surface profiles are determined, in which the initial uniform depths are joined by smooth moving wave structures. Numerical solutions have been attained to reproduce the different patterns of possible wave structures previously reported in the literature and extended to identify some other new structures and features defining the behaviour of the surface patterns. Copyright © 2006 John Wiley & Sons, Ltd.     

Wave characteristics of falling liquid film on a vertical circular tube

A falling film with waves on a vertical circular tube has been analyzed using the integral approach. A theoretical model of free surface deflection has been developed. The nonlinear equation obtained in the present work is similar to the deflection equation of a wave on a flat plate. It becomes exactly the same as the wave equation on a flat plate in the case of an infinite radius. This study shows that the wave characteristics depend on the parameters such as wave number, dimensionless wave velocity, tube radius and Reynolds number. As the tube radius decreases, the intensity of the wavy processes increases. The velocity distribution of the falling film was investigated in the present work. The cylindrical model appears to be more appropriate over the Cartesian model when the film thickness to tube diameter ratio is large. Calculated values are in good agreement with other published data.     

降膜式蒸发器用高效传热管换热性能试验研究

通过试验对降膜式蒸发器用高效传热管的换热性能进行研究,并将其与之相对应的池沸腾换热性能进行比较.由比较数据可知:样管池沸腾换热性能均随热流密度的增大而增强,降膜蒸发性能在一定热流密度下随喷淋流量的增大而增强;在恒定热流密度和恒定喷淋流量下,光管降膜燕发性能低于池沸腾性能,强化管降膜蒸发性能高于池沸腾性能;池沸腾性能高的强化管降膜蒸发性能也强.     

制冷空调系统水平管降膜蒸发研究

在制冷剂面临严峻的环境保护控制要求的背景下,研究表明,降膜蒸发由于其较低的制冷剂充注量在制冷空调行业具有重大的应用潜力。本文主要从降膜蒸发管外液膜的流动模式和强化传热2个方面,对水平管降膜蒸发的国内外研究现状进行研究分析。分析表明,管外液膜存在滴状流、射（柱状）流和片状流等模式,而且强化传热须区分高低热流密度情况。为了进一步探究降膜蒸发的机制,从换热和可视化的角度开展强化管外降膜蒸发的特性实验研究。     

The interfacial turbulence in falling film absorption: effects of additives

The results of vertical falling film experiments on the absorption of water vapor to aqueous lithium bromide solutions with an additive, 2-ethyl-1-hexanol, are reported. During the absorption, the film becomes highly turbulent. Consequently, the heat and mass transfer is significantly enhanced by turbulent mixing. In addition, the instability mechanisms are detailed. In the vicinity of water absorption, surface-tension gradients due to the lower LiBr concentration, the lower additive concentration, and the higher temperature at the interface, can favor instability of the falling film.     

Experimental investigation of heat transfer in a horizontal tube falling film absorber with aqueous solutions of LiBr with and without surfactants

To learn about the physical background of the absorption process, the accompanying heat transfer was studied experimentally. A horizontal tube heat exchanger, similar to commercial absorbers, was built. Two different types of tubes were tested, one with a plain, the other with a knurled surface. The solution volume flow, the temperatures of the cooling water and of the solution, and the concentration of the solution were varied to search for correlations of the heat-transfer coefficient with the physical properties of the solution. The heat-transfer coefficients decrease with increasing viscosity and augmenting surface tension. They also depend on the fluid regime of the solution film, i.e. they increase with growing solution volume flow. The influence of two surfactants — 1-octanol and 2-ethyl-1-hexanol in various concentrations — on the absorption process was examined quantitatively. An increase of 60–140% was measured for the heat-transfer coe coefficients.     

基于铀的激光氮化机理研究和数值模拟

针对工件铀的表面激光氮化工艺,考虑铀的固-液相变过程中各物理参数随温度的变化,结合计算流体软件FLUENT中流体体积函数(VOF)模型计算气-液相界面处的降膜解吸传质过程。进行了铀的瞬态激光氮化传热传质耦合数值仿真,仿真分析研究了不同工艺参数下瞬态温度场和流场的分布,同时获得渗氮量在铀表面和深度上的分布。分析结果表明,因激光局部加热引起的表面张力梯度导致的Marangoni对流对铀表面氮化过程中的传热和传质有很大的影响。其中渗氮量在不同工艺参数下的数值结果与试验结果相吻合,验证了数值模型的可行性,为激光氮化的理论分析和工艺指导提供了理论和方法。     

Dynamics of a thin viscoelastic film on an inclined plane

A theory for two-dimensional long and stationary waves of finite-amplitude on a thin viscoelastic fluid (weakly elastic) layer flowing down an inclined plane is investigated. A set of exact averaged equations for the viscoelastic film flow system is described and linearised stability analysis of the uniform flow is performed using normal-mode formulation and the critical condition for linear instability is obtained. The linearised instability for the permanent wave equation, consistent to the second order in (, – unperturbed film thickness, L – characteristic length) is examined and the eigenvalue properties of the fixed points are classified in various parametric regimes. The possible domains of heteroclinic orbits and the regions of possible nonlinear bifurcations are analysed for different values of viscoelastic parameter Γ. Numerical integration of the permanent wave equation as a third order dynamical system is carried out. While wave transitions in real life involve complex spatio-temporal dynamics and many of these transitions lead to chaotic waves that are not stationary traveling waves, bifurcation of stationary traveling waves has been examined as a preliminary study of the more complex transitions. Different bifurcation scenarios leading to multiple hump solitary waves or leading to chaos are exhibited in the parametric space. The results are compared and contrasted with the Newtonian results. A summary of the bifurcation scenarios in the We versus cot θ/Re plane is obtained for different values of viscoelastic parameter Γ, when Re ≈ 13.33 and Re = 100.     

Experimental correlation of falling film absorption heat transfer on micro-scale hatched tubes

The objectives of this paper are to investigate the effect of surface roughness of micro-hatching tubes on the absorption performance and to develop an experimental correlation of Nusselt number as a function of the roughness. Two different micro-scale hatched tubes and a bare tube are tested to quantify the effect of the surface roughness on the absorption performance. The roughness of the micro-scale hatched tubes ranges 0.386–11.359 μm. The working fluid is H₂O/LiBr solution with inlet concentration of 55, 58 and 61 wt.% of LiBr. The absorber heat exchanger consists of 24 horizontal tubes in a column, liquid distributor at the liquid inlet and the liquid reservoir at the bottom of the absorber. It was found that absorption performance of micro-hatching tubes was improved up to two times compared with the bare tube. An experimental correlation of Nusselt number was developed as a function of the film Reynolds number and the roughness with an error band of ±25%.     

Surface waves on viscoelastic magnetic fluid film flow down a vertical column

Long-wave perturbation method is employed to solve the nonlinear hydromagnetic stability of a thin electrically conductive viscoelastic fluid film flowing down on the outside surface of a vertical cylinder. As the viscoelastic effect and the magnetic force are introduced into governing equation, the results are more compatible with the practice situation. The modeling results display that the degree of instability in the film flow is further intensified by the lateral curvature of cylinder. It is also observed that by increasing the effect of the magnetic field and by decreasing the viscoelastic effect tends to enhance the stability as traveling down along the vertical cylinder. Moreover, the instability state could be neutralized by controlling the applied magnetic field. The optimum conditions can be found through the use of a system to alter stability of the film flow by controlling the applied magnetic field.     

Longitudinal Strain Solitary Wave in a Two‐Layered Polymeric Bar

Abstract: Both theoretical investigations and successful experimental research were performed recently, confirming the existence and demonstrating the main properties of bulk strain solitary waves in nonlinearly elastic solid wave guides. Our current research is devoted to nonlinear wave processes in layered elastic wave guides with inhomogeneities modelling damage/delamination. Here, we present first experimental and theoretical results, demonstrating the change in the amplitude and width of a strain solitary wave propagating in an inhomogeneous two‐layered wave guide made of Polymethyl Methacrylate (PMMA). Parameters of such waves in layered structures may be useful for the assessment of their operational integrity and robustness.     

Experimental study on enhancement of ammonia-water falling film absorption by adding nano-particles

Based on the preparation of Al₂O₃, Fe₂O₃ and ZnFe₂O₄ nanofluid, the comparative experiments on the falling film absorption between ammonia-water and ammonia-water with various kinds of nano-particles are carried out. Experimental results show that the sorts and mass fraction of nano-particles, the viscosity and stability of nanofluid, as well as the mass fraction of ammonia in the basefluid are considered as the key parameters. The absorption of ammonia is weakened by only adding surfactants or adding poorly dispersed nano-particles. The increase of mass fraction of nano-particles with matched surfactants can improve the absorption rate of ammonia under the condition that the viscosity of nanofluid does not increase remarkably, and there is an optimal mass fraction for each kind of nano-particles and surfactant. With the increase in ammonia mass fraction of initial nanofluid, the absorption potential capacity decline, but the enhancing effect induced by the nanofluid is more obvious compared to that without nano-particles. The effective absorption ratio can be increased by 70% and 50% with Fe₂O₃ and ZnFe₂O₄ nanofluid respectively when the initial ammonia mass fraction is 15%. The absorption enhancement by the nanofluid is attributable to the heat transfer enhancement and the decrease in viscosity of nanofluid, which are strongly proved by the temperature differences in cooling water and nanofluids as well as the falling film flowing time.     

Effect of a low-thermal-conductive coating on the dynamics of rewetting of overheated plate by falling liquid film

The experimental results of rewetting of overheated vertical copper heater with a low-thermal-conductive coating by a falling nitrogen film are presented. It is shown that the presence of the coating affects significantly the feature of the temperature curves and decreases the total plate cooling time by more than fourfold. The experimental data obtained earlier showed that the rewetting front propagating over the copper plate without coating was one-dimensional. It is revealed by comparison of the experimental data that the presence of the low-thermal-conductive coating results in the appearance of the front of a cellular structure with regular boiling jets, characteristic of rewetting of overheated thin plates. The synchronized measurements of the plate temperature and high-speed video recording of the transition processes show that the higher rate of cooling in the presence of the coating is associated with the development of intense boiling in the rewetting front at a much higher temperature of the plate.     

Hamiltonian formulation for solitary waves propagating on a variable background

Solitary waves propagating on a variable background are conventionally described by the variable-coefficient Korteweg-de Vries equation. However, the underlying physical system is often Hamiltonian, with a conserved energy functional. Recent studies for water waves and interfacial waves have shown that an alternative approach to deriving an appropriate evolution equation, which asymptotically approximates the Hamiltonian, leads to an alternative variable-coefficient Korteweg-de Vries equation, which conserves the underlying Hamiltonian structure more explicitly. This paper examines the relationship between these two evolution equations, which are asymptotically equivalent, by first discussing the conservation laws for each equation, and then constructing asymptotically a slowly-varying solitary wave.     

FeNHf软磁薄膜中磁各向异性对太赫兹波调控的研究

本文制备了具有磁各向异性的纳米FeNHf软磁薄膜,表征了FeNHf薄膜的微结构、磁性能、微波磁动力学行为和磁各向异性对太赫兹波传输特性的影响.FeNHf薄膜的难轴方向具有410的磁导率,易轴方向没有磁导率信号,磁各向异性场为2537.65 A/m.FeNHf薄膜在1.04 THz时出现了共振吸收峰,当调控磁化强度方向分...