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.     

The Effect of Gravity and Shear Stress on a Liquid Film Driven in a Horizontal Minichannel at Local Heating

The present study is focused on the investigation of gravity effect on thermocapillary deformations in a film flowing under action of co-current gas flow, which creates the tangential force on the gas–liquid interface. The influence of local heating intensity on the heater at a substrate is also investigated. Effects of surface tension, temperature dependent viscosity and thermocapillarity are taken into account. Investigations have shown that gravity has a significant effect on the film deformations and pattern. Decreasing of gravity level leads to a flow destabilization. 3D liquid film pattern noticeably changes in spanwise direction. Increasing of heat flux leads to increasing of liquid film deformations. Dependence of film thinning on heat flux is strongly nonlinear. The most dangerous deformations (regions of minimum film thickness with possible disruption of liquid) take place behind the downstream edge of the heater at any gravity conditions.     

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.     

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.     

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.     

Effects of Gravity on Ignition and Combustion Characteristics of Externally Heated Polyethylene Film

The objective of this research is to investigate the effects of gravity on the ignition and the combustion characteristics of the Polyethylene (PE) film by outer heating. Combustion experiments of PE film were carried out in a normal gravity field and the microgravity field. In the microgravity experiments, it was carried out in 50 m-class drop facility. Here it can be realized 10^??4G microgravity field in about 2.5-3.0 second. The PE film is heated by the inserted high-temperature chamber. In the experiments, the PE was used film type. The chamber temperature was fixed at 900 K and 1000 K. In the case of microgravity field, the ignition delay period has become about 50 percent shorter than that in the case of the normal gravitational field. In the normal gravity field, since the PE surface layer is cooled by natural convection, the ignition delay period is considered to be longer than that in the microgravity field. The combustion time in the normal gravity was about 0.8 sec. In the microgravity field, the combustion time was more than 2 sec, and it could not be measured during the free fall period.     

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.     

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.     

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.     

Surface Factor in Incompressible Liquid Film Flow

The results of investigation of the influence of a solid surface on the film flow of a viscous incompressible liquid in a gravitational field are analyzed. Using the dimensional method, a generalized similarity criterion for a film flow has been obtained. The laboratory facility and the method for investigating the film flow for various types of packing elements are described.     

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.     

Experimental Study on Boiling Flow of Liquid Nitrogen in Inclined Tubes—Liquid Slug Length Distribution

An experimental study was carried out to understand the phenomena of the boiling flow of liquid nitrogen in inclined tubes with closed bottom by using the high speed motion analyzer. The tube in the experiment is 1.0 m length and 0.018 m and 0.014 m inner diameter respectively. The range of the inclination angles is 45–90° from the horizontal. The statistical method is employed to analyze the experiment data. The experiment focused on the effect of the inclination angle on the distributions of the liquid slug length along the tube. The experimental results show that the Taylor bubble is easier to coalescence with inclination angle decreasing, but coalescence lessening at 45°. The liquid slug length distributions extend to much larger values for the large tube when x/D is 40 and 50. The mean liquid slug lengths increased first, and then decreased with decreasing θ for two tubes, maximum at 60°.     

Heat Transfer Enhancement in a Heated Liquid Film under the Action of External Artificial Perturbations

Heat transfer in a heated film of distilled water under the action of external artificial perturbations with “the most dangerous wavelength” and variation in the Reynolds number from 300 to 500 has been studied. It has been shown that the action of artificial perturbations with an increase in the heat flux density leads to a change in the rivulet-wave structure of the flow, increase in wave amplitudes, and, finally, heat transfer enhancement.     

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.     

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.     

喉嘴段收缩半角对液气射流泵流场特性影响的数值研究

基于计算流体动力学,对不同的液气射流泵喉嘴腔收缩半角进行了三维数值计算,根据数值计算结果,分析了喉嘴腔收缩半角对液气射流泵内部流场情况的影响,以及对液气射流泵吸气效率的影响关系。研究结果表明:随着喉管收缩半角的增大,流量比先略有升高然后略微减小随后保持不变,压力比先急剧增大,达到峰值后缓慢减小,存在最大值;射流泵的效率随收缩半角的变化趋势与压力比的变化趋势相似,存在最大值。根据最大吸气效率减小5%来确定最优吸气效率区间,相应的喉管收缩半角的最优范围为13.5°~17.1°。     

Bubble Coalescence in Gas–Liquid Flow at Microgravity Conditions

Bubbly flow at microgravity conditions is investigated experimentally. The evolution of bubble size has been measured in tubes of various diameters (6 to 40 mm). It is shown that the bubble size significantly increases along the tube due to coalescence. To predict the bubble size distribution along the tube a mechanistic model based on the transport equations of the moment of the bubble-diameter distribution has been developed. In these equations the coalescence rate is modeled in taking into account the two mechanisms of bubble coalescence encountered in microgravity: the turbulence-induced coalescence and the shear-induced coalescence. The results of the model are then compared to the experimental data.     

溶解工艺对离子液体法纤维素膜力学性能的影响

利用1-丁基-3-甲基咪唑氯盐([BMIM]Cl)为溶剂制备纤维素膜,探讨了浆粕种类、浆粕质量分数、溶解条件等对纤维素膜力学性能的影响。研究表明:聚合度为1460,质量分数为6%的针叶木浆,在溶胀8h,90℃溶解条件下制得的纤维素膜有较好的力学性能。     

Heat Transfer Through the Interface and Flow Regimes in Liquid Bridge Subjected to Co-Axial Gas Flow

We present results of an extensive numerical study on the thermocapillary (Marangoni) convection and a heat transfer through the interface in a liquid bridge of Pr?=?68. The geometry of the physical problem is a cylindrical and non-deformable liquid bridge concentrically surrounded by an annular gas channel under conditions of zero gravity. The gas flow is co- or counter-directed with respect to the Marangoni flow. The forced gas flow along the interface provides two actions: via shear stresses and heat exchange. Usually the cooling of the interface enhances the flow while the heating slows down. This general trend may not hold when shear and thermocapillary stresses are comparable. The results show that when gas enters from the cold side the heat transfer through the interface is considerably larger than that when gas enters from the hot side.