Gravity Effect on the Locally Heated Liquid Film Driven by Gas Flow in an Inclined Minichannel

Thin nonisothermal liquid film flowing under action of gravity force and co-current gas flow, which create the tangential force on the gas-liquid interface, in an inclined minichannel is considered. 3D time dependant mathematical model has been developed. Effects of surface tension, temperature dependent viscosity and thermocapillarity are taken into account. The effect of gravity as well as the effect of gas speed has been studied to define main features of the film dynamics. In calculations vector of gravitational acceleration is oriented along the flow and is equal to the normal Earth gravity and Lunar gravity. Our investigations have shown that gravity has a significant effect on the film deformations. At the lower gravity conditions 3D liquid film pattern changes noticeably in spanwise direction and a middle stream between two main lateral waves appears. Also speed of film deformation is higher and stabilization time is longer. Variation of gas Reynolds number from 543 to 2000 does not change noticeably film pattern at normal gravity. At lower gravity conditions increasing of gas Reynolds number decreases significantly the width of the thermocapillary deformations and leads to a film stabilization.     

Shear-Driven Flow of Locally Heated Viscous Liquid Film in a Minichannel

This paper considers a flow of a liquid sheared by gas in a flat mini-channel with two identical heaters arranged in a row one after another in a streamwise direction at the bottom wall. The present study is focused on the investigation of influence of local heaters arrangement and size on thermocapillary deformations in a viscous film, gravity effect is also investigated. 3D one-sided model is considered, viscosity of the liquid is supposed to be temperature dependent. Numerical analysis reveals that interaction and mutual influence of 3D structures takes place. Film pattern changes qualitatively depending on the heaters arrangement and form. For rectangular heaters a middle stream exists. Minimum film thickness value increases and its location moves to heater edges for rectangular heaters. A critical backlash between two heaters, at which film thinning is the largest, exists. Gravity significantly affects on the film deformations. Decreasing of gravity level leads to a flow destabilization and film deformations, especially film thinning, essentially increases.     

Liquid Rivulets Moved by Shear Stress of Gas Flow at Altered Levels of Gravity

Experiments with gas shear-driven rivulet flows in a minichannel of height 1.4 mm and of width 30 mm were conducted during several parabolic flights campaigns organized by the European Space Agency (ESA). Rivulet flow is defined as a particular case of liquid film flow when the film occupies only a part of the substrate without touching to the lateral walls of the channel. A high frequency schlieren technique has been used for visualization of the two-phase flows. It is shown that surface tension becomes the dominating force with decreasing of the gravity level, which results in reducing of the rivulet width. The width increases with the gravity level and with the liquid flow rate growth and reduces with the gas flow rate growth.     

The Effect of Gravity on the Stability of an Evaporating Liquid Film

Evaporation of a Dichloromethane liquid film is explored with an evolution equation describing film dynamics. The film is subject to different initial conditions, smooth and uniform random perturbation. Two different gravity environments (Earth and zero gravity) and two different domain shapes (square and rectangular) have been used. The occurrence of long wave instabilities affecting film dynamics is noted in each of these cases. The evaporating Dicholormethane liquid film is destabilized via long wave instabilities in zero gravity. The thermocapillary patterns formed due to long wave destabilization show a coupling to the initial conditions and domain shape. A criterion for the occurrence of long wave instabilities based on the growth rate of perturbations is described. This equation considers a non-stationary film thickness. It predicts that long wave instabilities are always present in zero gravity environments with a growth rate that increases as the film thickness decreases due to evaporation. Our equation for growth rate of long wave instabilities may be used as an engineering design tool to confine operating parameters of zero gravity heat transfer equipment, that include or harness phase change, to safe limits.     

Effect of Gravity During Condensation of R134a in a Circular Minichannel

A number of steady-state simulations of condensation of R134a inside a 1 mm i.d. circular minichannel at two far different mass flux values are proposed. The VOF method is used to track the vapour-liquid interface. The first simulations are run at G?=?100 kg m^???2 s^???1 and G?=?800 kg m^???2 s^???1 assuming that the channel displays horizontal orientation. The effects of interfacial shear stress, gravity and surface tension are all taken into account in this case and the results are validated by means of experimental data already available. As a further step, the same simulations have been run under normal gravity conditions but vertical downflow and finally assuming zero-gravity conditions. The condensation process is found to be gravity dominated at low mass flux, and thus very different results are obtained when neglecting gravity at this mass flux. An opposite result is achieved at high mass flux, as expected from the increased relative importance of interfacial shear stress in this case. The present results also allow to verify the influence of the surface tension effect during condensation in the circular cross section minichannel.     

The Effect of Three-Dimensional Deformations on Local Heat Transfer to a Nonuniformly Heated Falling Film of Liquid

An experimental investigation is performed of heat transfer under conditions of flow of a water film on a vertical surface with a heater 150×150 mm in size in the range of the Reynolds number values from 1 to 45. A map of modes of flow of the liquid film is plotted, and regions of heat transfer are identified. Data are obtained on the longitudinal coordinate dependence of the heater wall temperature and of the local heat flux on the symmetry axis of the heater. Local coefficients of heat transfer are measured. The experimental data are compared with the results of numerical calculations for a smooth film. The effect of the forming of jet flows on heat transfer to the liquid film is analyzed.     

Investigation on Effect of Gravity Level on Bubble Distribution and Liquid Turbulence Modification for Horizontal Channel Bubbly Flow

Bubbly flows in the horizontal channel or pipe are often seen in industrial engineering fields, so it is very necessary to fully understand hydrodynamics of horizontal bubbly flows so as to improve industrial efficiency and to design an efficient bubbly system. In this paper, in order to fully understand mechanisms of phase distribution and liquid–phase turbulence modulation in the horizontal channel bubbly flow, the influence of gravity level on both of them were investigated in detail with the developed Euler–Lagrange two–way coupling method. For the present investigation, the buoyance on bubbles in both sides of the channel always points to the corresponding wall in order to study the liquid–phase turbulence modulation by bubbles under the symmetric physical condition. The present investigation shows that the gravity level has the important influence on the wall–normal distribution of bubbles and the liquid–phase turbulence modulation; the higher the gravity level is, the more bubbles can overcome the wall–normal resistance to accumulate near the wall, and the more obvious the liquid–phase turbulence modulation is. It is also discovered that interphase forces on the bubbles are various along the wall–normal direction, which leads to the fact that the bubble located in different wall–normal places has a different wall–normal velocity.     

Numerical Study on the Effects of Gravity and Surface Tension on Condensation Process in Square Minichannel

A volume of fluid (VOF) method is adopted to simulate the condensation of R134a in a horizontal single square minichannel with 1 mm side length. The effect of gravity, surface tension and gas-liquid interfacial shear stress are taken into account. The result denotes that condensation is first appeared at the corner of channel, and then the condensation is stretched at the effect of surface tension until the whole channel boundary covered. The effect of gravity on the distribution of the liquid film depends on the channel length. In short channel, the gravity shows no significant effect, the distribution shape of steam in the cross section of the channel is approximately circular. In long channel, due to the influence of gravity, the liquid converges at the bottom under the effect of gravity, and the thickness of the liquid film at the bottom is obviously higher than that of the upper part of the channel. The effect of surface tension on condensation is also analysed. The surface tension can enhance the condensation heat transfer significantly when the inlet mass flux is low. Whilst, at high mass flux, the enhancement of surface tension on heat transfer is unobvious and can be neglected.     

不同退火升温速率下PZT铁电薄膜中的残余应力分析

室温下利用射频磁控溅射在Pt/Ti/SiO2/Si(100)上淀积了约200nm厚的PZT铁电薄膜,在N2(纯度为99.999%)气氛中以不同升温速率进行快速热退火,采用X射线衍射法表征其残余应力。结果表明,薄膜中的应力不是二维应力而是三维应力,且除了切应力σ22以外的大部分应力张量的分量为张应力;薄膜中张应力的增大主要是由于氧空位与晶粒尺寸共同作用的结果,而该应力的减小是由于晶粒尺寸在薄膜应力演变过程中占主导地位所致。     

Effect of Liquid Properties on Heat Transfer from Miniature Heaters at Different Gravity Conditions

This work presents experiments for the estimation of heat transfer from sub-millimeter spheroidal heaters at varying gravity conditions. Experiments were performed during the 50^th Parabolic Flight Campaign of European Space Agency (May 2009). Heat pulses of varying strength were given to a miniature heater submerged in the liquid while its thermal response was registered during heating. Tests were conducted in water and FC-72, two liquids with distinctly different thermophysical properties. Runs were also conducted with packed beds and dense suspensions of two size classes of polystyrene particles in water. The contribution of natural convection in heat transfer was estimated from differences observed when the acceleration varied from 0g to 1.8g. Although the analysis of data is not finished yet, it is evident that natural convection is more profound in FC-72 than water. In addition, closely packed particles suppress entirely natural convection but this is not so for dense particle suspensions.     

The Effect of Stress Triaxiality on the Local Cleavage Fracture Stress in a Granular Bainitic Weld Metal

The local cleavage fracture stress σ_F measured at the cleavage initiation sites of a granular bainitic weld metal is only an apparent fracture parameter dependent on stress triaxialities at the local sites in the propagation controlled cleavage regime. This dependence can be explained by the stress triaxiality modified Smith equation in which the intrinsic cleavage fracture stress σ _F ⁰ is introduced, considered to be an invariant characterizing a material's fracture property. In the nucleation controlled cleavage regime σ_F is temperature dependent and coincident with the local yield stresses defined by Von Mises criterion at the local sites. It is suggested that the modified Smith model would provide a unified physical basis for the intrinsic correlation among various macroscopic fracture properties as well as their temperature transition behaviors from the temperature dependence of material's yield strength. This revised version was published online in August 2006 with corrections to the Cover Date.     

The Stability of a Condensate Film Moving under the Effect of Gravity and Turbulent Flow of Vapor

The linear stability of a condensate film moving along a vertical isothermal plate under the effect of gravity and turbulent vapor flow is investigated. The cases of both cocurrent and countercurrent motion of phases are treated with regard for phase transformation. An analytical solution for the distribution of film thickness along the plate taking into account the film inertia is obtained using the integral method. A two-wave equation is deduced for the film thickness, and dispersion relations are derived. The effect of moving vapor on the film stability in a wide range of flow parameters is shown.     

Effect of Gravity on Film Boiling Heat Transfer and Rewetting Temperature during Quenching

This paper describes the experimental strategy developed to improve the modeling of liquid-vapor flows during the chill down of rocket engines by cryogenic fluid in microgravity. A similarity analysis is performed to determine the relevant dimensionless numbers for the design of an experiment similar to engine flows. A literature review on reduced gravity quenching experiments, and on rewetting temperature and film boiling heat transfer shows the lack of validated models for microgravity. Experimental results obtained with the quenching of a glass tube by FC72 during parabolic flight are presented. Especially the impact of gravity and subcooling on rewetting temperature and film boiling heat transfer is investigated. Results show an increase in rewetting temperature, and a decrease in film boiling heat transfer under reduced gravity in agreement with the literature. The comparison of 0 g flow pattern with corresponding tests on ground points out a behavior at 0 g closest to 1 g upflow than 1 g downflow.     

The Effect of Wave Formation and Wetting Angle on the Thermocapillary Breakdown of a Falling Liquid Film

An experimental investigation is performed of the breakdown of a liquid film flowing down a vertical plate with a heater sized 150×150 mm. The main parameters which are varied in the experiment are the Reynolds number Re = 0.47 to 331 and the heat flux q = 0 to 1.92 W/cm². It is found that the effect of the heat flux on the wave motion of the liquid film causes the formation of a jet flow. Dry spots are formed in the region of thin film between the jets. For the purpose of investigating the effect of wave formation on the film breakdown, the distance between the nozzle and heater is varied from 41.5 to 200 mm. It is found that the distance between the nozzle and heater defines the hydrodynamics of the liquid at relatively low heat fluxes, but has no appreciable effect on the heat flux at which the film breakdown occurs. Different working liquids and coatings of the working surface are used in the experiments to investigate the effect of the wetting angle on the film breakdown. The equilibrium wetting angle is measured by the "bubble" method. No effect of the equilibrium wetting angle on the nonisothermal breakdown of the film was revealed.     

Simulation of Patterned Glass Film Formation in the Evaporating Colloidal Liquid under IR Heating

The paper theoretically studies the method of evaporative lithography in combination with external infrared heating. This method makes it possible to form solid microstructures of the required relief shape as a result of evaporation of the liquid film of the colloidal solution under the mask. The heated particles are sintered easier, so there are no cracks in the obtained structure, unlike the structure obtained employing the standard method of evaporative lithography. The paper puts forward a modification of the mathematical model which allows to describe not only heat and mass transfer at the initial stage of the process, but also the phase transition of colloidal solution into glass. Aqueous latex is taken as an example. The resulting final form of solid film is in good agreement with the experimental data of other authors.     

退火温度对溅射Al膜微结构及应力的影响

用直流溅射法在Si基片上制备了 2 5 0nm厚的Al膜 ,并在不同退火温度下进行退火处理 ,用X射线衍射和光学干涉相位移法对薄膜的微结构及应力随退火温度的变化进行了研究。结构分析表明 :退火后的Al膜均呈多晶状态 ,晶体结构仍为面心立方 ;随着退火温度由 2 0℃升高到 4 0 0℃ ,薄膜的平均晶粒尺寸由 2 2 8nm增加到 2 5 1nm ;薄膜晶格常数在不同退火温度下均比标准值 4 0 4 96 A稍小。应力分析表明 :随退火温度的升高 ,Al膜应力减小 ,30 0℃时平均应力减小为 2 730× 10 8Pa且分布均匀 ;在 4 0 0℃时选区范围内应力差仅为 3 82 8× 10 8Pa。     

Comparison of the Effect of Horizontal Vibrations on Interfacial Waves in a Two-Layer System of Inviscid Liquids to Effective Gravity Inversion

We study the waves at the interface between two thin horizontal layers of immiscible liquids subject to high-frequency tangential vibrations. Nonlinear governing equations are derived for the cases of two- and three-dimensional flows and arbitrary ratio of layer thicknesses. The derivation is performed within the framework of the long-wavelength approximation, which is relevant as the linear instability of a thin-layers system is long-wavelength. The dynamics of equations is integrable and the equations themselves can be compared to the Boussinesq equation for the gravity waves in shallow water, which allows one to compare the action of the vibrational field to the action of the gravity and its possible effective inversion.     

Effect of Static Deformation on Basic Flow Patterns in Thermocapillary-Driven Free Liquid Film

A series of terrestrial, parabolic-flight and on-orbit experiments on thermocapillary-driven flows in free liquid films are carried out. We focus on the basic flow patterns induced in the film formed in a rectangular hole by varying the film volume in order to make a comparison with the results of the fluid physics experiments under microgravity conditions conducted by one of the authors, Pettit, on the International Space Station. The free liquid film is formed in a rectangular hole of O(0.1 mm) in thickness under a designated temperature difference between the end walls. The temperature dependence of the surface tension results in a non-uniform surface tension distribution over the free surfaces. A liquid generally has a negative temperature coefficient of surface tension; i.e., the fluid over a free surface is driven from a higher-temperature region to a lower-temperature region. In the case of a thin free liquid film with two free surfaces, however, an unusual flow pattern is realized. That is, the fluid seems to be driven toward the heated region from a colder region. In order to understand the physical mechanism of this behavior in the free liquid film, a series of on-orbit and ground experiments were conducted. We indicate several flow patterns in the film and corresponding film profiles as well as the surface temperature distribution. We also try to illustrate the cross-sectional flow structures in the thin free liquid film with two free surfaces.     

Nonlinear Wave Simulation on a Surface of Liquid Film Entrained by Turbulent Gas Flow at Weightlessness

The article considers the joint flow of a liquid film entrained by turbulent gas. Since liquid velocity is small in comparison with gas velocity, the problem is reduced to the calculation of pressure and shear stresses produced by the gas flowing over a wavy wall with small amplitude. Further, these data are used at the boundary conditions, when the flow of a liquid film is considered separately. As a result, we obtain a new system of equations for modeling the dynamics of long-wave perturbations on the surface of a viscous liquid film entrained by turbulent gas at microgravity. At small Reynolds numbers typical to the condition of microgravity, it was proved that this system is reduced to one evolution equation for the film thickness. Some numerical solutions of this equation have been received in this work.