Local Film Thickness Research Articles

Measurement of Liquid Film Thickness in Vertical Multiphase Slug and Churn Flows Using Distributed Ultrasonic Method

A distributed ultrasonic method for measuring the liquid film thickness of vertical multiphase slug flow and churn flow widely existed in industrial production is proposed in this paper. First, according to the proportional relationship between transit time of ultrasonic echo and liquid film thickness, we use the twin-crystal ultrasonic probe to measure the local thickness of liquid film. In view of the non-uniform circumferential distribution of the liquid film, three ultrasonic probes are installed on the same pipe section to measure the circumferential distribution of liquid film thickness. Then, a high-speed data acquisition system suitable for multi-channel and a data transmission system based on gigabit network are designed. The experiments of gas-liquid two-phase flow and oil-gas-water three-phase are carried out in a vertical pipe with a diameter of 20 mm. The fluctuation of local liquid film thickness has been measured by the distributed ultrasonic sensor, and the average thickness of liquid film of slug flow and churn flow under different flow conditions is obtained. The result indicates that the average liquid film thickness of slug flow and churn flow gradually thickens with the increase of liquid superficial velocity, and the distributed ultrasonic sensor can accurately measure the change of film thickness.

Characteristics of vertically falling liquid film affected by cross gas flow in a narrow flat channel

Gas crossflow on a falling liquid film occurs in many practical cases in industries, particularly, in the downcomer of the APR1400, when accidents such as damage to the cold legs occur, hot steam crossflows over the vertical falling water film formed by the water impingement on the wall through a direct-vessel injection nozzle. Such crossflow may degrade the heat transfer ability of the liquid film. To investigate the film characteristics of downward flowing water, across which air was blowing horizontally, an experimental setup was designed and assembled. Experiments were conducted under different water flow rates (2, 3, 4, and 5 L/min) and different air velocities (0, 3.4, 4.9, 6.9, 8.4, and 9.8 m/s) to determine their effect on the film structure. A confocal chromatic sensor was employed to detect film thickness, film width, and wave amplitude. It was found that the base film thickness and wave amplitude do not vary when superficial air velocity is not larger than 4.9 m/s. As air velocity increases, the base film thickness exhibits a decreasing trend and wave amplitude exhibits an increasing trend. In particular, wave amplitude shows a different tendency from that found in annular flow. Moreover, local base film thickness and wave amplitude decreased and increased along the film flowing direction, respectively. Furthermore, the average film thickness was found constant throughout the whole air velocity range. In addition, film width expanded nonlinearly as the water flow rate increased, and the slope of the film centerline under different superficial gas velocities was presented for the first time.

Thin liquid film formation on hemispherical and conical substrate

AbstractThe deposition and coating of thin films onto curved rigid substrate, involving displacement of air by a liquid, has numerous applications within the technology sectors but faces two major challenges: (i) control of the local film thickness; (ii) ensuring that the coating remains stable. The work reported here investigates the full coverage of three‐dimensional curved geometries, of hemispherical and conical shape, by a continuously fed, gravity‐driven, thin liquid layer. The modelling approach adopted utilises a first integral formulation [1,2] of the Navier‐Stokes equations leading to a variational formulation in the case of steady flow and an advantageous re‐formulation of the dynamic boundary condition at the free surface [3]. Asymptotic analysis, underpinned by the long‐wave approximation, enables analytic solutions for the local film thickness to be obtained.

Experimental investigation of gas-liquid flow characteristics in slit rectangular and circular channels

This paper aims at the experimental study of the flow characteristics of the ethanol- nitrogen mixture in vertical and horizontal channels. The 95% ethanol and nitrogen were used as working liquid and gas accordingly in slit rectangular microchannel with cross-section of 200 × 2000 μm and in a circular channel with ID=2.2 mm. The hydraulic focusing and T-shape mixers were used for gas-liquid flow formation. Experimental data were obtained using highspeed visualization, laser scanning, μLIF, and PLIF methods. Using method μLIF the nonuniform thickness of the liquid film was obtained at the cross-section of the rectangular microchannel. For the circular channel, the measurements of the local film thickness using the PLIF method confirmed accordance of the experimental data with Taylor law.

Numerical study on condensation annular flow of R1234ze(E) inside an inclined tube

The condensation characteristic of R1234ze(E) inside an inclined circular tube (D = 4.57 mm) were numerically investigated. The mass flux covered from 300 to 800 kg·m−2 s−1 and the inclination angle varied from −90° to 90°. The inclination effects on film distribution, local film thickness, local heat transfer coefficient and velocity profile were detailed presented. Inclining the tube has a negligible effect on heat transfer coefficients at high mass fluxes and vapor qualities, but may cause an increase or decrease in heat performance when decreasing the mass flux and vapor quality. At the same inclination angle, slightly higher heat transfer coefficients were observed for the down-flow configuration. Increasing the inclination angle would thicken the film thickness of thin film region while reducing the film thickness of stratification region, which affected local heat transfer performance. Compared with heat transfer coefficients, the pressure gradients were more sensitive to the inclination angle. The local pressure gradients generally increased with the inclination angle for down-flow configuration. While the reverse was true for up-flow configuration. The simulated heat transfer coefficients and pressure gradients were compared with predictions of well-known correlations. The pressure drop models developed for horizontal tubes were not able to precisely predict the data of inclined tubes.

Controlled Spacing between Nanopatterned Regions in Block Copolymer Films Obtained by Utilizing Substrate Topography for Local Film Thickness Differentiation.

Various types of devices require hierarchically nanopatterned substrates, where the spacing between patterned domains is controlled. Ultraconfined films exhibit extreme morphological sensitivity to slight variations in film thickness when the substrate is highly selective toward one of the blocks. Here, it is shown that using the substrate's topography as a thickness differentiating tool enables the creation of domains with different surface patterns in a fully controlled fashion from a single, unblended block copolymer. This approach is applicable to block copolymers of different compositions and to different topographical patterns and thus opens numerous possibilities for the hierarchical construction of multifunctional devices.

Local liquid film behavior of annular two-phase flow on rod-bundle geometry – II. Modeling and verification

Local liquid film behavior of annular flow on rod bundle geometry is vital for accurate prediction of dryout. This paper is the Part II of a two-part study devoting to modelling and prediction of local liquid film behavior of annular flow on rod bundle geometry based on the unique feature of experimental phenomenon in Part I of this work. In this paper, a mechanism model is developed to predict the local liquid film thickness of annular two-phase flow on rod bundle geometry, in which the spatial distribution of gas flow in the cross-section of rod bundle is specified, and the local velocity of liquid film is obtained by employing the triangular relationship that taking into account properly the momentum and mass balances for the liquid film and the gas core. Ordinary differential equation (ODE) for the liquid film thickness is developed according to the Navier-Stokes equations for momentum balance of film flow on a rod to analyze the spatial characteristic of film thickness, in which the droplet entrainment-deposition process and the effect of circumferential shear stress are considered as boundary conditions during mass transfer at the gas-liquid interface. The present model predicts the experimental local liquid film thickness with the maximum relative error of ±10%. Circumferential coherent wave structure is formed by the combination effect of transversal stress gradient in the wave, droplet redeposition and film flow by circumferential shear stress, and the circumferential shear stress reduces the relative importance of the droplet redeposition with respect to the transversal stress gradient making the peripheral distribution of film thickness more uniform.

Draining of films on a quasivertical plate using viscous dissipation

This work focused on obtaining an improved and expanded general theoretical analysis of a two-dimensional film draining on a quasivertical plate, solving rigorous mass, momentum, and energy balances. A dimensional analysis and scaling was used to simplify the mathematical description, and a generalized Newtonian fluid was assumed as the film-forming material. A new quantity that governs the draining flow and film characteristics, called viscous dissipation, was proposed as part of the novel analytical expressions obtained in this work. Velocity profile, average velocity, flow rate, and local and average film thickness expressions can be obtained, allowing to simplify the overall calculation complexity and to find new potential analytical expressions using more complex rheological models.

Raising the maximum power density of nanoporous catalyst film-based polymer-electrolyte-membrane fuel cells by laser micro-machining of the gas diffusion layer

Although nanoporous Pt film has been shown to be an effective catalyst for polymer electrolyte membrane (PEM) fuel cells, the maximum power density of the cell is limited by the optimal film thickness. When the Pt film thickness exceeds the optimal value, regions with good gas transport (the side near the gas diffusion layer (GDL)) separate from regions with good proton transport (the side near the PEM), so the current density and the power density drop with increasing film thickness. Here we demonstrate that this obstacle can be overcome by laser micro-machining the GDL. The picosecond laser fabricates grooves on the GDL surface to greatly increase the effective surface area for Pt deposition, thereby reducing the local Pt film thickness. A nearly two-fold increase in the power density is achieved by using laser micro-machined periodic grooves of 20 μm depth, reaching a 0.6-V power density of 853 mW cm−2 and a maximum power density of 1.2 W cm−2 with a cathode Pt loading of 200 μg cm−2. The results also indicate that further enhancement may be achieved by increasing the surface modulation depth/period ratio and by implementing a better way to fill the grooves with polymer electrolyte.

Influence of piston ring profiles and oil temperature distribution on cylinder liner lubrication of a marine two-stroke engine

In the paper a comprehensive model of a piston-ring-cylinder (PRC) system has been presented. The local thickness of the oil film can be compared to height of the combined surface roughness of a cylinder liner and piston rings. Equations describing the mixed lubri-cation problem based on the empirical mathematical model formulated in works of Patir, Cheng and Greenwood, Tripp have been ap-plied. The main parts of the model have been experimentally verified abroad by the author at the marine engine designing centre. In contrast to the previous papers of the author concerning the PRC system of combustion engines, new calculation results for a ma-rine two-stroke engine have been presented. Firstly the right selection of barrel-shaped sliding surface of piston rings has been analysed. Secondly the influence of oil temperature distribution along the cylinder liner on the lubrication of the PRC system has been assessed. The developed model and software can be useful for optimization of the PRC system design.

Local liquid film behavior of annular two-phase flow on rod-bundle geometry-I. Experimental phenomenon and analysis

Local liquid film behavior of annular flow on rod bundle geometry is vital for accurate prediction of dryout. This paper is the first part of a two-part study concerning measurement and analysis of local liquid film behavior of annular flow on rod bundle geometry. In this paper, air-water annular flow experiments have been performed at different gas and liquid superficial velocity conditions in a 3 × 3 simulating PWR fuel rod-bundle test-section, and a conductivity-based technique is developed to measure the local dynamical liquid film thickness. Large amplitude disturbance waves appear at all low liquid phase superficial velocity conditions through down-burst mechanism caused by the highly turbulent gas flow and strong inter-subchannel interaction. Wave shapes do not change drastically by the variation of superficial liquid velocity since liquid flow tends to fill out the rod bundle corner due to the nature of annular flow. For disturbance wave dominating the film flow case, secondary flow in subchannel produces a circumferential force to shear off the liquid film, and the liquid droplets are redistributed, resulting in the liquid film thickness for the central region being thicker than that of gap region. Cross-correlation analysis indicates that gap and central regions are in possession of circumferential coherent wave structure with the same interfacial velocity, and the wave position for the central region is a little ahead of that for gap region since a higher wave has a longer leeward and upwind faces. The current study on local liquid film behaviors for rod bundle and the possible mechanisms that causing the discrepancies in local liquid film structures can be further used for the mechanism model development for the liquid film flow on the rod bundle. Part II of this study will introduce a mechanism model as an alternative approach to local liquid film thickness prediction for annular flow in rod bundle geometry.

Nonuniform Quantum-Confined States and Visualization of Hidden Defects in Pb(111) Films

The spatial distribution of the differential conductance for ultrathin Pb films grown on Si(111)7x7 substrate is studied by means of low-temperature scanning tunneling microscopy and spectroscopy. The formation of the quantum--confined states for conduction electrons and, correspondingly, the appearance of local maxima of the differential tunneling conductance are typical for Pb films; the energy of such states is determined mainly by the local thickness of Pb film. We demonstrate that the magnitude of the tunneling conductivity within atomically flat terraces can be spatially nonuniform and the period of the small-scale modulation coincides with the period of Si(111)7x7 reconstruction. For relatively thick Pb films we observe large-scale inhomogeneities of the tunneling conductance, which reveal itself as a gradual shift of the quantized levels at a value of the order of 50 meV at distances of the order of 100 nm. We believe that such large-scale variations of the tunneling conductance and, respectively, local density of states in Pb films can be related to presence of internal defects of crystalline structure, for instance, local electrical potentials and stresses.

Investigation on emergency core coolant bypass with local measurement of liquid film thickness using electrical conductance sensor fabricated on flexible printed circuit board

An experimental work was conducted in a reduced-scale downcomer annulus of a nuclear reactor pressure vessel in order to investigate the behavior of liquid films under emergency core coolant (ECC) bypass conditions and to obtain high definition data for validating two-phase flow prediction capability of computational fluid dynamics (CFD) codes. Air and water at ambient pressure and temperature were used as a working fluid, creating a multidimensional two-phase flow in the test section. The main instrumentation was an electrical conductance sensor that was developed to measure the local thickness of the liquid film. The sensor has an array of 24 × 24 measuring points, each of which has 12 mm × 6 mm of the sensing area. The fabrication of the electrodes on a flexible printed circuit board enabled the sensor to be installed on the curved surface. The time-averaged liquid film thickness, which was measured by the developed sensor, shows the influence of the lateral air flow on the liquid film flow, and it was compared with visual observations. As the air velocity increased, a droplet appeared that was created in the thick part of the liquid film, and the thinned liquid film after entrainment could be confirmed from the measured thickness distribution. In this study, qualitative and quantitative analyses of the measurement results show the reliability of the developed sensor and help to understand the liquid film behavior in the ECC bypass phenomenon. Furthermore, the measured thickness distribution can contribute to validating the CFD codes, which have not been validated sufficiently because of a lack of local measurement data for the ECC bypass condition.

Influence of a single microstructure on local mass transfer in liquid film flows

The influence of a single microstructure on liquid-side controlled mass transfer is studied for liquid film flow over an inclined plate with a film height of less than 1 mm. Planar Laser Induced Fluorescence (PLIF) is adapted and used to measure oxygen absorption at three gas flow rates. The liquid is a mixture of water, ethanol and glycerol. A ruthenium complex is used as a fluorescence indicator. By combining PLIF with dynamic fluorescence quenching, local concentration fields are visualized and determined through image processing. The results show that the local mass transfer coefficient decreases in the flow direction on a smooth inclined plate. Compared to a completely smooth surface, the local mass transfer tends to be enhanced in the direct vicinity of a single microstructure. The impact of the microstructure on local mass transfer depends on the gas flow conditions and the measurement position. The highest local mass transfer coefficients are measured, where the local film thickness is lowered due to the structure and with lower gas flow rates.

Characteristics of churn and annular flows in a large diameter vertical riser

This paper presents the results of a series of experimental studies conducted to investigate the characteristics of churn and annular flows in an 11 m long riser with the internal diameter of 127 mm. A series of experiments in churn–annular flow regime were performed for a range of gas superficial velocities (3.5–16.1 m/s) and liquid superficial velocities of 0.02–0.33 m/s. Statistical analysis involving average, standard deviation and skewness, dominant frequency and structure velocity of the time series data was carried out as a quantitative approach to discriminate between churn and annular flows and to describe the transition between them. The variation of average liquid film thickness against gas superficial velocity showed a minimum at gas superficial velocity of 12.4 m/s. This is in close agreement with the minima at gas superficial velocity of 13 m/s reported in the literature. The results of the comparison between average liquid film thicknesses from the conductance ring probes agree well with those obtained by using local liquid film thickness probes and conductance ring probes in the literature. The results of the comparison of dominant frequency obtained from the conductance ring probes agree well with those obtained by using wire mesh sensor (WMS) in the literature. A comparative analysis of the current experimental data with those previously published experimental results confirms good agreement.

Effect of tube shape on the condensation patterns of R1234ze(E) in horizontal mini-channels

In this paper, the condensation characteristic of R1234ze(E) in horizontal circular, square and triangle channels were numerically investigated. The detailed liquid-vapor interfaces, local film thickness and stream traces were presented for better understanding the condensation process in circular and non-circular tubes. Under the same hydraulic diameter, the void fraction in square and triangle tubes was lower than that in circular tubes at the same vapor quality. The square tube exhibited higher heat transfer coefficients than that of the circular tube, while the triangle tube showed similar heat transfer performance with the circular tube. The tube shape had no significant effects on the pressure gradients. The gravity played a dominant role in local film distribution in the circular tube, but was almost negligible in the square tube. Both the surface tension and gravity played an important role in film distribution in the triangle tube. The number and structure of vortexes in the square and triangle tube were very different from that in the circular tube. The tube shape has a significant effect on the liquid phase velocity at different circumference positions. Under the same external perimeter, either the heat transfer coefficients or the pressure gradients of the square or triangle tube were larger than that of the circular tube. The numerical data of heat transfer coefficients and pressure gradients were also compared with the predicted values from the available well-known correlations. The results of this paper are important for the industrial applications of alternative refrigerants.

Experimental study of high-speed gas-film flows in curvilinear channels with drop injection of liquid

The paper investigates the local thickness distribution of a wall liquid film in a curvilinear channel. The research technique on experimental water–air stand with a curvilinear working channel is described. The results of measurements of the local thickness of the liquid film on the upper and lower walls of the channel for two flow velocities are obtained at various flow rates.

Experimental Investigation of Local Film Thickness and Velocity Distribution Inside Falling Liquid Films on Corrugated Structured Packings

Understanding the complex fluid dynamics of the liquid phase in structured packing is crucial for the prediction of the separation performance of packed tower and for further optimization of the packing geometry. Detailed numerical investigations are useful to study a wide range of parameters, but require high computational effort and validation is required. Therefore, in this study, experimental investigations on the gravity driven liquid film flow phenomena over corrugated structured packing geometries are presented and evaluated. Due to the fragile nature of thin liquid films, nonintrusive optical measurements where performed through the backside of transparent moldings of packing geometries, thereby reducing optical distortion. To enable the investigation of three-dimensional flow phenomena a µ-Stereoscopic Particle Image Velocimetry (µSPIV) was adapted for thin liquid films. Measurements are performed on MONTZ-Pak B1.250 macro-structure with a smooth surface or a micro-structured surface at different locations. Values of film thicknesses and velocity distributions inside the falling liquid film are discussed in detail.

Condensation flow patterns and model assessment for R1234ze(E) in horizontal mini/macro-channels

Knowledge regarding the condensation flow patterns of environment friendly refrigerant R1234ze(E) in mini/macro-channels was important for alternative industrial applications. In this paper, the condensation heat transfer and pressure drop characteristic of R1234ze(E) in horizontal circular channels with diameter ranged from 0.493 to 4.57 mm were numerically investigated and compared with that of R134a. The detailed liquid film distribution, local film thickness and velocity field were presented for better understanding the condensation process. Both the heat transfer coefficients and pressure gradients increased with the mass flux, vapor quality and the decrease of the tube diameter. The heat transfer coefficients of R1234ze(E) were smaller than that of R134a, but the pressure gradients of R1234ze(E) were larger than that of R134a. The difference in heat transfer and pressure drop performance was found to be smaller in macro-channels. The effects of surface tension and gravity on liquid film distribution are significantly influenced by the tube diameter. The liquid film distribution was very close for R1234ze(E) and R134a for their similar physical properties. The average film thickness of R1234ze(E) was thinner than that of R134a. The axial velocity of the vapor phase decreased with the vapor quality, while the variation of the liquid phase velocity was associated with the circumferential position for the gravity effect. The simulated heat transfer coefficients and pressure gradients were compared with predicted values from the available well-known correlations. A new pressure drop correlation was proposed for practical application.

Analysis of annular flow disturbance wave frequency in two different installation conditions

The dynamic local liquid film thickness of annular flow was measured with a near infrared (NIR) equipment. Basing on the power spectral density (PSD) to analyze this signal, the frequency of disturbance wave was obtained. The experiment was carried out for both horizontal and vertical annular flow. Considering the effects of flow velocity and gas pressure, the results showed that the wave frequency increases with the increase of flow velocity and gas density in both installation conditions. For the numerical correlation, the frequencies of the vertical is more regular than those of the horizontal.