Measurement of locally heated liquid film thickness by a double-fiber optical probe

An experimental investigation of thermocapillary deformations in a film of 10% ethyl alcohol solution in water, flowing down a plate with a heater of length 6.7 mm and width 68 mm, is performed. Heating of the film results in the formation of a horizontal liquid bump at the top edge of the heater. On the heater the flow divides into vertical rivulets with a thin film between them. Film deformations in the bump and the thin film between the rivulets are investigated. Local film thickness is measured by means of a double-fiber optical probe. The method is based on the dependence of the intensity of reflected light on the distance between the probe and the reflecting surface. The measurement results are compared to those previously obtained using the schlieren method. The experiment is controlled by three parameters. They are, with their respective values, the plate inclination angle (4–90°), the Reynolds number (0.15–62) and the heat flux density (0–4.5 W/cm²).     

Comparison of different theoretical approaches to experiments on film flow down an inclined wavy channel

We study the flow of a liquid down an inclined channel with a sinusoidal bottom profile. We show how wavy bottom variations, which are long compared with the film thickness or the amplitude, modify the flow with respect to that down a flat inclined channel. We consider different perturbation analyses. Their results are compared with experimental data on the velocity profiles and on the film thickness. We discuss the effect of waviness, inclination angle, film thickness, and Reynolds number.     

Instabilities of a gas-liquid flow between two inclined plates analyzed using the Navier–Stokes equations

The paper is devoted to a theoretical analysis of a counter-current gas-liquid flow between two inclined plates. We linearized the Navier–Stokes equations and carried out a stability analysis of the basic steady-state solution over a wide variation of the liquid Reynolds number and the gas superficial velocity. As a result, we found two modes of the unstable disturbances and computed the wavelength and phase velocity of their neutral disturbances varying the liquid and gas Reynolds number. The first mode is a “surface mode” that corresponds to the Kapitza's waves at small values of the gas superficial velocity. We found that the dependence of the neutral disturbance wavelength on the liquid Reynolds number strongly depends on the gas superficial velocity, the distance between the plates and the channel inclination angle for this mode. The second mode of the unstable disturbances corresponds to the transition to a turbulent flow in the gas phase and there is a critical value of the gas Reynolds number for this mode. We obtained that this critical Reynolds number weakly depends on both the channel inclination angle, the distance between the plates and the liquid flow parameters for the conditions considered in the paper. Despite a thorough search, we did not find the unstable modes that may correspond to the instability in frame of the viscous (or inviscid) Kelvin–Helmholtz heuristic analysis.     

Local film thickness and temperature distribution measurement in wavy liquid films with a laser-induced luminescence technique

Heat transfer in falling liquid film systems is enhanced by waviness. Comprehension of the underlying kinetic phenomena requires experimental data of the temperature field with high spatiotemporal resolution. Therefore a non-invasive measuring method based on luminescence indicators is developed. It is used to determine the temperature distribution and the local film thickness simultaneously. Results are presented for the temperature distribution measurement in a laminar-wavy water film with a liquid side Reynolds number of 126 flowing down a heated plane with an inclination angle of 2° at two positions in flow direction. The measured temperature distributions are used to calculate the local heat transfer coefficient for solitary waves at two positions in flow direction.     

Experimental investigation of mixed convection in a rectangular duct with a heated plate in the middle of cross section

The flow and heat transfer in an inclined and horizontal rectangular duct with a heated plate longitudinally mounted in the middle of cross section was experimentally investigated. The heated plate and rectangular duct were both made of highly conductive materials, and the heated plate was subjected to a uniform heat flux. The heat transfer processes through the test section were under various operating conditions: Pr ≈ 0.7, inclination angle ϕ = −60° to +60°, Reynolds number Re = 334–1,911, Grashof number Gr = 5.26 × 10²–5.78 × 10⁶. The experimental results showed that the average Nusselt number in the entrance region was 1.6–2 times as large as that in the fully developed region. The average Nusselt numbers and pressure drops increased with the Reynolds number. The average Nusselt numbers and pressure drops decreased with an increase in the inclination angle from −60° to +60° when the Reynolds number was less than 1,500. But when the Reynolds number increased to over about 1,800, the heat transfer coefficients and pressure drops were independent of inclination angles.     

Viscous liquid film flow down an inclined corrugated surface. Calculation of the flow stability to arbitrary perturbations using an integral method

Viscous liquid film flow along an inclined corrugated (sinusoidal) surface has been studied. Calculations were performed using an integral model. The stability of nonlinear steady-state flows to arbitrary perturbations was examined using the Floquet theory. It has been shown that for each type of corrugation there is a critical Reynolds number for which unstable perturbations occur. It has been found that this value greatly depends on the physical properties of the liquid and geometric parameters of the flow. In particular, in the case of film flow down a smooth wall, the critical waveformation parameter depends only on the angle of inclination of the flow surface. The values of the corrugation parameters (amplitude and period) were obtained for which the film flow down a wavy wall is stable to arbitrary perturbations up to moderate Reynolds numbers. Such parameter values exist for all investigated angles of inclination of the flow surface.     

固液润湿性对流体动压润滑薄膜的影响

利用自行开发的微型面接触润滑油膜测量系统,研究了固液润湿性对流体动压润滑油膜厚度的影响.试验中以静止的微型滑块平面和旋转的光学透明圆盘平面形成润滑副.固液的润湿性通过接触角判定,不同材料的微滑块平面和润滑液体形成不同的界面.在保持载荷和面接触楔形角不变的条件下对油膜厚度-速度关系进行了测量.结果表明:对于固液润湿性强的界面,形成的油膜厚度与经典润滑理论有较好的一致性;当固液润湿性明显降低时,测量得到的油膜厚度减小.对于试验中观察到的界面效应,应用界面滑移理论进行了初步分析.     

Study of the thermocapillary effect on a wavy falling film using a fiber optical thickness probe

The wave flow of a water film down a vertical plate with a 150×150 mm heater has been experimentally studied. The effect of the heat flux on the film flow leads to the formation of periodically flowing rivulets and thin film between them due to the action of thermocapillary forces in spanwise direction. The local film thickness between rivulets is measured by means of a noncontact fiber optical probe. As the heat flux grows, the average film thickness continuously decreases but upon reaching about 50% of the initial thickness, the film spontaneously breaks down. It is found that the decrease of the wave amplitude between rivulets is caused by the reduction of the local Reynolds number and is in a qualitative agreement with the laws of the hydrodynamics for the isothermal case. That is, no appreciable effect of streamwise thermocapillary forces on the wave amplitudes is detected. The experimental results are in good agreement with recently published data obtained by the capacitance method.     

Crown behavior and bubble entrainment during a drop impact on a liquid film

Physical and mathematical models are established to simulate a single liquid drop impinging onto a liquid film using the coupled level set and volume of fluid method. The crown liquid sheet after impact is obtained, which coincides well with the experimental results in literatures. Influence of Weber number, Reynolds number and the dimensionless film thickness on the crown diameter and height is discussed quantitatively. Results indicate that the crown diameter is independent of the two non-dimensional numbers, while it can be increased by reducing the dimensionless film thickness. The crown height increases with the increasing of Weber number, but Reynolds number has small effect on it. Mechanism of the jet formation process is revealed by analyzing pressure distribution and velocity field in the liquid. It is found that both pressure difference in the neck region and velocity discontinuity can greatly affect the jet formation. Besides, the bubble entrainment phenomenon during a liquid drop impact on a liquid film is successfully captured with this numerical method. It is found that the increase in both impact Weber number and the drop diameter contributes to the emerging of bubble rings.     

Luminescence technique for the measurement of local concentration distribution in thin liquid films

Experimental data of the concentration field with high spatiotemporal resolution is required for the comprehension of mass transfer increasing kinetic phenomena in falling liquid films. For this purpose a non-invasive measuring method based on luminescence indicators is developed. It is used to determine the concentration distribution and the local film thickness simultaneously. First results are presented for the oxygen absorption into a laminar-wavy water film flowing down a plane with an inclination angle of 4° and a liquid side Reynolds number of 177. With the measured concentration distributions the effective diffusion coefficients are calculated at three points in a single wave of the film.     

Heat transfer from a flat surface to an inclined impinging jet

An experimental study was carried out to investigate the effect of the inclination jet on convection heat transfer to horizontal flat plate. Local heat transfer determined as a function is of three parameters including inclination angle of the air jet relative to the plate in range of 90° ≤ θ ≤ 45°, jet-to-plate spacing in range of 2 ≤ L/D ≤ 8 and Reynolds number in range of 1,500 ≤ Re ≤ 30,000. The results show that the maximum heat transfer point moves towards the uphill side of the plate and the maximum heat transfer decreases as the inclination angle decreases. The correlations were conducted to predict maximum and local Nusselt number as a function of Re, θ, L/D, and x/D for a specific three regions.     

聚变堆相关的液态金属膜流初步实验研究

在磁约束核聚变堆的面对等离子部件设计中,液态金属锂膜流因具有带走杂质、保护面对等离子固壁等优点而被认为是优选方案之一. 然而,如何克服聚变堆中强磁场环境下产生的磁流体力学效应并形成大面积均匀铺展锂膜流动是目前亟需解决的问题.本文通过搭建室温液 态镓铟锡回路和高温液态锂回路,开展了两种不同特性的液态金属膜流实验, 并采用传统可视化方法获得了展向磁场存在时镓铟锡和锂在导电底板形成的液膜流动表面特征.实验结果 表明: 无磁场时,两种液态金属膜流流动表面波动特性与常规流体膜流均一致, 即随着流动雷诺数的增加表面波动变得更为混乱; 而展向磁场存在时,镓铟锡膜流表面波动变得更为规则, 且沿着磁场方向平行排列,表现为拟二维波动的特征; 而锂膜流却产生了明显的磁流体 力学阻力效应,表现为在流动方向局部产生锂滞留现象, 且滞留点随雷诺数增大向下游移动. 最后通过膜流受力分析,进一步阐述了锂膜流受到比镓铟锡膜流更为严重磁流体力学效应影响的原因.      

Natural Convection Heat Transfer Augmentation Factor with Square Conductive Pin Fin Arrays

Natural convection heat transfer from a vertical isothermal plate with pin fins is numerically studied by solving the Navier–Stokes equations along with the energy equation. The average Nusselt number for the plate with different configurations of pin fins is obtained. The average Nusselt number is found to increase with increasing aspect ratio of the fin and to decrease with increasing angle of fin inclination with respect to the plate. There is only a minor difference between the average Nusselt numbers for in-line and staggered arrangement of fins for the range of parameters studied in the present work. A correlation is developed to predict the average Nusselt number of the plate as a function of fin spacing in the streamwise and spanwise directions, aspect ratio of the fin, and its angle of inclination.     

Numerical investigation of natural convection inside an inclined parallel‐walled channel

Steady two‐dimensional natural convection in an inclined parallel‐walled channel was investigated numerically. The full elliptic forms of conservation equations were solved together and the velocity vectors, temperature contours and local and average Nusselt number distribution were obtained. The comparisons of local and average Nusselt number with published experimental and numerical results indicate very good agreement. Results are presented for a single aspect ratio, L/b=24, over the range of Rayleigh number of 3–1000 and angle of inclination 0–90°. The results indicate that the overall channel average Nusselt number is reduced as the inclination angle is increased. Significant reductions in the overall Nusselt number are exhibited at high angle of channel inclination. Copyright © 2005 John Wiley & Sons, Ltd.     

Numerical study of condensing a small concentration of vapour inside a vertical tube

The purpose of this study is to analyse the combined heat and mass transfer of liquid film condensation from a small steam–air mixtures flowing downward along a vertical tube. Both liquid and gas stream are approached by two coupled laminar boundary layer. An implicit finite difference method is employed to solve the coupled governing equations for liquid film and gas flow together with the interfacial matching conditions. The effects of a wide range of changes of three independent variables (inlet pressure, inlet Reynolds number and wall temperature) on the concentration at exit tube, local Nusselt and Sherwood numbers, film thickness, accumulated condensate rate and temperature are carefully examined. The numerical results indicate that in the case of condensing a small concentration of vapours from a mixture, the resistance to heat and mass transfer by non-condensable gas becomes very intense. The comparisons of average Nusselt number and local condensate heat transfer coefficient with the literature results are in good agreement.     

下游泵送螺旋槽密封空化试验及性能分析

基于自主设计的可视化试验装置及膜温和膜厚测量方法,对下游泵送螺旋槽密封空化特征及性能参数进行试验研究.探讨了油压和转速对不同螺旋槽密封液膜中空穴发生位置、空穴分布及空穴边界的影响,拟合了不同螺旋槽密封空穴边界的试验关系式,并对不同空化模型的理论泄漏量和膜厚与相应试验值进行了对比分析.结果表明:内槽型和中槽型密封的空穴均发生在螺旋槽内,但两者的空穴区形状明显不同;油压的增加有助于抑制液膜中空穴的发生,而转速的增加反之;尤其是内槽型,油压对其空穴发生影响更为显著;在低转速或高油压时,Reynolds和JFO两空化模型均可用于预测泄漏量和膜厚;在高转速或低油压时,JFO空化模型预测值更准确,而Reynolds空化模型预测值偏大.     

面接触润滑油膜测量系统滑块倾角的快速计算

在面接触润滑油膜厚度的光干涉测量中,需要确定滑块和玻璃盘所形成的微小倾角.一般通过目测干涉条纹的数量计算倾角,效率低且误差大.根据滑块表面干涉条纹等距,光强呈周期性变化的特点,对滑块面的干涉光强曲线进行傅里叶变换,综合采用Rife-Jane和重叠FFT相位差法进行频率插值,较好地消除了频谱泄漏,较准确计算干涉条纹数量,从而得到滑块与玻璃盘间的倾角.试验证明:当条纹数量大于5时该方法的测量相对误差在1%以下,具有很强的抗干扰能力.对不同倾角下的油膜厚度进行了测量,得出了润滑油膜的无量纲承载量-滑块收敛比曲线.结果表明:膜厚较高时,无量纲载荷最大值出现在收敛比为1.2左右的位置,但当膜厚较低时,无量纲载荷最大值出现在收敛比大于2的位置,偏离了经典理论.     

Experimental investigation of inclined liquid water jet flow onto vertically located superhydrophobic surfaces

In this study, the behaviour of an inclined water jet, which is impinged onto hydrophobic and superhydrophobic surfaces, has been investigated experimentally. Water jet was impinged with different inclination angles (15°–45°) onto five different hydrophobic surfaces made of rough polymer, which were held vertically. The water contact angles on these surfaces were measured as 102°, 112°, 123°, 145° and 167° showing that the last surface was superhydrophobic. Two different nozzles with 1.75 and 4 mm in diameters were used to create the water jet. Water jet velocity was within the range of 0.5–5 m/s, thus the Weber number varied from 5 to 650 and Reynolds number from 500 to 8,000 during the experiments. Hydrophobic surfaces reflected the liquid jet depending on the surface contact angle, jet inclination angle and the Weber number. The variation of the reflection angle with the Weber number showed a maximum value for a constant jet angle. The maximum value of the reflection angle was nearly equal to half of the jet angle. It was determined that the viscous drag decreases as the contact angle of the hydrophobic surface increases. The drag force on the wall is reduced dramatically with superhydrophobic surfaces. The amount of reduction of the average shear stress on the wall was about 40%, when the contact angle of the surface was increased from 145° to 167°. The area of the spreading water layer decreased as the contact angle of the surface increased and as the jet inclination angle, Weber number and Reynolds number decreased.     

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。     

Characteristics of solitary waves on a falling liquid film sheared by a turbulent counter-current gas flow

We report an experimental investigation of a falling water film sheared by a turbulent counter-current air flow in an inclined rectangular channel. Film thickness and wave velocity measurements associated with visual observation are conducted to study the influence of the air flow on controlled traveling waves consisting of a large wave hump preceded by capillary ripples. First, we focus on the variation of the shape, amplitude and velocity of the waves as the gas velocity is gradually increased. We demonstrate that the amplitude of the main hump grows substantially even for moderate gas velocities, whereas modification of the wave celerity becomes significant above a specific gas velocity around 4 m/s, associated with an alteration of the capillary region. The influence of the gas flow on 3D secondary instabilities of the solitary waves detected in a previous study Kofman et al. (2014), namely rugged or scallop waves, is also investigated. We show that the capillary mode is damped while the inertial mode is enhanced by the interfacial shear. Next, the gas velocity is increased until the onset of upstream-moving patterns referred to as flooding in our experiments. At moderate inclination angles (typically < 7 ^○), flooding occurs for a gas velocity around 8 m/s and is initiated at the scallop wave crests by a backward wave-breaking phenomenon preceded by the onset of ripples on the flat residual film separating two waves. At high inclination angle, a rapid development of solitons is observed as the air velocity is increased preventing the waves to turn back. Finally, at high liquid Reynolds number, sudden and intermittent events are triggered consisting of very large amplitude waves that go back upwards very fast. These “slugs” either extend over the whole width of the channel or are very localized and can thus potentially evolve towards atomization.