气泡夹带对壁面油膜流动特性的影响

油-气润滑系统工作过程中,润滑油膜受微油滴冲击和压缩空气扰动影响易形成气泡夹带现象,气泡夹带行为将对壁面润滑油膜层的形成及流动过程产生重要影响。基于VOF数值模拟方法,对含气泡油膜沿倾斜壁面的流动行为进行研究,考察了气泡的存在对油膜形态和流动速度的影响规律,以及气泡破裂阶段空腔邻域内流体压力变化特性。研究表明,油膜夹带气泡的形变和迁移诱发气泡周围微流场的速度扰动现象,导致气液界面处产生非均匀速度梯度分布,进而引发油膜表面的形态波动。气泡发生破裂时,油膜空穴部位发生明显的正负压力波动现象,气泡附近壁面将承受一定的交变载荷作用。     

The analysis of a mechanism of liquid replenishment and draining in horizontal two-phase flow

There is a regime of two-phase flow in which large waves or surges pass rapidly along a horizontal tube accompanied by splashing, wave-breaking and entrainment with the result that water is thrown to the upper surface of the tube. Between surges the film on the top surface is depleted by draining under gravity and by evaporation if the tube is heated. If the interval between surges is sufficiently long a dry patch may begin to form. In this paper, theory is given for the calculation of the film thickness left behind on the top surface and for the calculation of the time to dryout. The theory includes both the effect of the boundary layer developement during replenishment of the film and also the effect of the axial deceleration of the film at the point where the liquid replenishment ceases. Finally, the predicted variation of film thickness is compared with experimental film thickness traces obtained in this type of horizontal two-phase flow. The agreement is found to be very satisfactory. This analysis is of interest in connection with the prevention of permanent and intermittent dryout at low qualities in nuclear power station evaporators.     

Entrainment for horizontal annular gas-liquid flow

Measurements of entrainment are presented for air and water flowing in horizontal 2.54 and 5.08 cm pipelines. After the initiation of atomization, entrainment increases with the third power of the gas velocity. At very high gas velocities a fully entrained oendition in reached for which further increases in the gas velocity do not cause a decrease in the flow rate of the wall film. Gas density bas a small effect provided comparisons are made at the same gas velocity rather than at the same mass flowrate. The results are interpreted by asauming that the rate of deposition of droplets on the wall film varies linearly with the concentration of droplets and that the rate of atomization of the wall film varies linearly with its flow rate.     

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.     

A simultaneous PIV and heat transfer study of bubble interaction with free convection flow

Whole field velocity and point temperature and surface heat flux measurements were performed to characterise the interaction of a single rising ellipsoidal air bubble with the free convection flow from a heated flat surface immersed in water at different angles of inclination. Two thermocouples and a hot film sensor were used to characterise heat transfer from the surface, while a time-resolved digital particle image velocimetry technique was used to map the bubble induced flow in a plane parallel to the surface. Heat flux fluctuations, preceding and following the bubble passage, were shown to correlate with the variation in both local flow velocities and fluid temperatures. The largest increases in heat transfer were recorded when both flow and temperature effects combined to enhance the convective cooling simultaneously. Such conditions were shown to be most likely met when the block was inclined at 45°, thus forcing the bubble to slide closer to the heated surface and hence to the thermal boundary layer.     

台阶式溢洪道掺气特性的试验研究

通过试验研究了台阶式溢洪道上的掺气现象以及掺气区域的划分，试验表明，台阶式溢洪道沿程掺气可以分为非掺气区、掺气发展区和掺气充分发展区等三个区域；断面掺气分为台阶影响的含气区、楔形清水区、水中气泡区和空中水滴区。试验还测量了台阶式溢洪道上的断面掺气浓度及其沿程分布，计算了断面含水率，提出了初始掺气点的计算方法。与光滑溢洪道相比较，台阶式溢洪道具有初始掺气点前移，掺气发生早，发展快，掺气浓度大等特点。     

Prediction of amount of entrained droplets in vertical annular two-phase flow

Prediction of amount of entrained droplets or entrainment fraction in annular two-phase flow is essential for the estimation of dryout condition and analysis of post dryout heat transfer in light water nuclear reactors and steam boilers. In this study, air–water and organic fluid (Freon-113) annular flow entrainment experiments have been carried out in 9.4 and 10.2 mm diameter test sections, respectively. Both the experiments covered three distinct pressure conditions and wide range of liquid and gas flow conditions. The organic fluid experiments simulated high pressure steam–water annular flow conditions. In each experiment, measurements of entrainment fraction, droplet entrainment rate and droplet deposition rate have been performed by using the liquid film extraction method. A simple, explicit and non-dimensional correlation developed by Sawant [Sawant, P.H., Ishii, M., Mori, M., 2008. Droplet entrainment correlation in vertical upward co-current annular two-phase flow. Nucl. Eng. Des. 238 (6), 1342–1352] for the prediction of entrainment fraction is further improved in this study in order to account for the existence of critical gas and liquid flow rates below which no entrainment is possible.Additionally, a new correlation is proposed for the estimation of minimum liquid film flow rate at the maximum entrainment fraction condition. The improved correlation successfully predicted the newly collected air–water and Freon-113 entrainment fraction data. Furthermore, the correlations satisfactorily compared with the air–water, helium–water and air–genklene experimental data measured by Willetts [Willetts, I.P., 1987. Non-aqueous annular two-phase flow. D.Phil. Thesis, University of Oxford]. However, comparison of the correlations with the steam–water data available in literature showed significant discrepancies. It is proposed that these discrepancies might have been caused due to the inadequacy of the liquid film extraction method used to measure the entrainment fraction or due to the change in mechanism of entrainment under high liquid flow conditions.     

垂直上升气液环状流流动参数的数值计算

提出了一个新的气核-液膜耦合模型来求解垂直上升气液环状流在充分发展段的流动参数.本模型考虑了液膜、气核以及它们之间的相互影响和作用.模型中基本的气核区域和液膜区域的质量和动量方程由Fluent6.3.26进行求解,而液滴方程以及相界面上的夹带和沉积作用通过用户自定义接口函数UDF(User Defined Functi...     

Simulation and measurement of 3D shear-driven thin liquid film flow in a duct

Three-dimensional flow behavior of thin liquid film that is shear-driven by turbulent air flow in a duct is measured and simulated. Its film thickness and width are reported as a function of air velocity, liquid flow rate, surface tension coefficient, and wall contact angle. The numerical component of this study is aimed at exploring and assessing the suitability of utilizing the FLUENT-CFD code and its existing components, i.e. Volume of Fluid model (VOF) along with selected turbulence model, for simulating the behavior of 3D shear-driven liquid film flow, through a comparison with measured results. The thickness and width of the shear-driven liquid film are measured using an interferometric technique that makes use of the phase shift between the reflections of incident light from the top and bottom surfaces of the thin liquid film. Such measurements are quite challenging due to the dynamic interfacial instabilities that develop in this flow. The results reveal that higher air flow velocity decreases the liquid film thickness but increases its width, while higher liquid flow rate increases both its thickness and width. Simulated results provide good estimates of the measured values, and reveal the need for considering a dynamic rather than a static wall contact angle in the model for improving the comparison with measured values.     

Time and spatially resolved measurements of interfacial waves in vertical annular flow

Film thickness measurements have been performed in a vertical air/water annular flow in a pipe of 0.05 m diameter. A sensor has been built which allows to measure the film thickness evolution in time at 320 positions, such that the interface of the vertical annular flow can be reconstructed. The large-scale structures moving on the interface are described statistically, with a special attention to the disturbance waves. Probability density functions and mean statistics are given for the height, length, velocity, frequency and spatial distribution of the disturbance waves. In particular, it is shown that the disturbance waves are three-dimensional structures with large height fluctuations in the circumferential and axial direction, giving a meandering path between the maximum height around the circumference. It is also shown that the disturbance waves can flow with a slight inclination with respect to the axial direction. Finally, the disturbance waves are shown to be located randomly in space, within a Gamma distribution whose order only depends on the liquid superficial velocity. Due to the nature of the Gamma distribution, it could indicate that the spatial distribution of the disturbance waves results from a cascade of coalescence processes between the original disturbance waves on the film.     

The effect of orifices on the liquid distribution in annular two-phase flow

The effect of using orifices to disrupt the water film in air-water annular two-phase flow has been studied experimentally in a vertical tube by measuring the wall film flowrate at various distances upstream and downstream of several different sizes of orifice. The orifices cause a temporary reduction in the downstream water film flowrate, which returns to its equilibrium value further downstream. The experimental results have been used, together with those of other investigators, to compare the effects of orifices to those of swirl tapes, and further to compare the processes of entrainment and deposition within annular two-phase flow.     

Numerical study on the effect of initial flow velocity on liquid film thickness of accelerated slug flow in a micro tube

Numerical simulation of air–water slug flows accelerated from steady states with different initial velocities in a micro tube is conducted. It is shown that the liquid film formed between the gas bubble and the wall in an accelerated flow is significantly thinner than that in a steady flow at the same instantaneous capillary number. Specifically, the liquid film thickness is kept almost unchanged just after the onset of acceleration, and then gradually increases and eventually converges to that of an accelerated flow from zero initial velocity. Due to the flow acceleration, the Stokes layer is generated from the wall, and the instant velocity profile can be given by superposition of the Stokes layer and the initial parabolic velocity profile of a steady flow. It is found that the velocity profile inside a liquid slug away from the bubble can be well predicted by the analytical solution of a single-phase flow with acceleration. The change of the velocity profile in an accelerated flow changes the balance between the inertia, surface tension and viscous forces around the meniscus region, and thus the resultant liquid film thickness. By introducing the displacement thickness, the existing correlation for liquid film thickness in a steady flow (Han and Shikazono, 2009) is extended so that it can be applied to a flow with acceleration from an arbitrary initial velocity. It is demonstrated that the proposed correlation can predict liquid film thickness at Re < 4600 within the range of ±10% accuracy.     

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.     

Planar laser-induced fluorescence (PLIF) measurements of liquid film thickness in annular flow. Part II: Analysis and comparison to models

Film thickness distributions in upward vertical air–water annular flow have been determined using planar laser-induced fluorescence (PLIF). Film thickness data are frequently used to estimate interfacial shear and pressure loss. This film roughness concept has been used in a number of models for annular flow of varying complexity. The PLIF data are presently applied to the single-zone interfacial shear correlation of Wallis; the more detailed model of Owen and Hewitt; and the two-zone (base film and waves) model of Hurlburt, Fore, and Bauer. For the present data, these models all under-predict the importance of increasing liquid flow on pressure loss and interfacial shear. Since high liquid flow rates in annular flow induce disturbance wave and entrainment activity, further modeling in these areas is advised.     

Counter-current gas–liquid flow in a vertical narrow channel—Liquid film characteristics and flooding phenomena

Results are reported of an experimental investigation of gas–liquid counter-current flow in a vertical rectangular channel with 10 mm gap, at rather short distances from liquid entry. Flooding experiments are carried out using air and various liquids (i.e., water, 1.5% and 2.5% aqueous butanol solutions) at liquid Reynolds numbers Re_L < 350. Visual observations and fast recordings suggest that the onset of flooding at low Re_L (<250) is associated with liquid entrainment from isolated waves, whereas “local bridging” is dominant at the higher Re_L examined in this study. Significant reduction of flooding velocities is observed with decreasing interfacial tension, as expected. Instantaneous film thickness measurements show that under conditions approaching flooding, a sharp increase of the mean film thickness, of mean wave amplitude and of the corresponding RMS values takes place. Film thickness power spectra provide evidence that by increasing gas flow the wave structure is significantly affected; e.g., the dominant wave frequency is drastically reduced. These data are complemented by similar statistical information from instantaneous wall shear stress measurements made with an electrochemical technique. Power spectra of film thickness and of shear stress display similarities indicative of the strong effect of waves on wall stress; additional evidence of the drastic changes in the liquid flow field near the wall due to the imposed gas flow, even at conditions below flooding, is provided by the RMS values of the wall stress. A simple model is presented for predicting the mean film thickness and mean wall shear stress under counter-current gas–liquid flow, below critical flooding velocities.     

Heat transfer to air–water annular flow in a horizontal pipe

Two-phase air–water flow and heat transfer in a 25 mm internal diameter horizontal pipe were investigated experimentally. The water superficial velocity varied from 24.2 m/s to 41.5 m/s and the air superficial velocity varied from 0.02 m/s to 0.09 m/s. The aim of the study was to determine the heat transfer coefficient and its connection to flow pattern and liquid film thickness. The flow patterns were visualized using a high speed video camera, and the film thickness was measured by the conductive tomography technique. The heat transfer coefficient was calculated from the temperature measurements using the infrared thermography method. It was found that the heat transfer coefficient at the bottom of the pipe is up to three times higher than that at the top, and becomes more uniform around the pipe for higher air flow-rates. Correlations on local and average Nusselt number were obtained and compared to results reported in the literature. The behavior of local heat transfer coefficient was analyzed and the role of film thickness and flow pattern was clarified.     

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.     

Preventing snow accumulating upon inclined substrates using falling water film

This paper has concerned preventing snow accumulating upon inclined substrates using falling water film. A cluster of physical models for a gas–water–snow system was developed and solved with a control-volume finite-difference procedure. The effects of various parameters on heat transfer and water film temperature decrease were investigated. The results revealed that it is feasible to use falling water film to prevent snow accumulation upon inclined substrates as long as the suitable initial film flow rate and temperature are adopted. It was clarified that among the influential factors of water film temperature decrease, the snowfall intensity, initial water film thickness and the substrate inclination play an important role. The intensity of the film temperature decrease mainly depends on the snowfall intensity and the initial heat capacity of the water film flow, which is a function of temperature and mass flow rate. An intensive heat transfer occurs in a very thin layer near the free surface of the water film. The numerical results were correlated in terms of effective Nusselt number, and Reynolds, Prandtl and Gukhman numbers. The maximum deviation of the numerical data from the correlation is approximately 15.6%, which only causes an uncertainty less than 3% in the simple prediction. Received on 26 April 1999     

A model for droplet entrainment in heated annular flow

The ability to accurately predict droplet entrainment in annular two-phase flow is required to effectively calculate the interfacial mass, momentum, and energy transfer, which characterizes nuclear reactor safety, system design, analysis, and performance. Most annular flow entrainment models in the open literature are formulated in terms of dimensionless groups, which do not directly account for interfacial instabilities. However, many researchers agree that there is a clear presence of interfacial instability phenomena having a direct impact on droplet entrainment. The present study proposes a model for droplet entrainment, based on the underlying physics of droplet entrainment from upward co-current annular film flow that is characteristic to light water reactor safety analysis. The model is developed based on a force balance and stability analysis that can be implemented into a transient three-field (continuous liquid, droplet, and vapor) two-phase heat transfer and fluid flow systems analysis computer code.     

Entrainment in condensing annular flow

Experiments were carried out on low pressure, steam-water, condensing annular flow in a 38.1 mm i.d. horizontal tube. The velocity of the steam at inlet was in the range 97–186 m/s.Measurements of the liquid film flow rate at the end of the test section, which arose as a result of condensation, entrainment, and deposition, were made by extracting the film through a porous sinter bush. The liquid flow rate in the vapour core at exit was deduced from these measurements together with a heat balance on the condenser section.These results were compared with three correlations for entrainment developed from air-water studies. On the basis of the experimental data available, there was sufficient agreement in one case to warrant further investigation.