Local Liquid Film Research Articles

Studying macro- and mesoscopic wetting dynamics of a spreading oil droplet using multiple wavelength interferometry

In this study we present an interferometric technique based on multiple wavelengths to capture the transient free surface contour of nanolitre drops spreading on a wettable surface, in particular close to the three-phase contact line. Various data analysis procedures are evaluated in terms of error and noise sensitivity. The technique allows an unambiguous determination of the local liquid film thickness for optical path differences up to Δs≈3.19μm\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\Delta s \\approx 3.19\\,\\upmu \ ext {m}$$\\end{document} without the need of a known reference height. Film thicknesses as low as 0.1μm\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$0.1\\,\\upmu \ ext {m}$$\\end{document} can be measured with the present optical configuration. The entire three-dimensional droplet shape is investigated for different capillary numbers, allowing also reliable measurements of the time-resolved contact angle.

Annular liquid film thermohydraulic and intermittent dryout in microchannel flow boiling

The annular flow regime in microchannels is commonly described as the steady flow of a vapor core with a surrounding liquid film along the channel length. However, limited studies have suggested temporal discontinuity in the liquid film coverage of the microchannel wall. The lack of experimental tools to directly interrogate the liquid flow characteristics and the local surface heat flux has resulted in proliferation of different hypotheses concerning the physics of this process. Surface de-wetting due to high shear stresses at the vapor–liquid interface and liquid film instability and rupture due to perturbation growth at the vapor–liquid interface have been suggested as the underlaying causes of this phenomenon. In this study, a recently developed microsensor array capable of measuring the surface local heat flux and liquid film thickness and velocity is utilized to decipher thermohydraulic characteristics of the annular flow regime and intermittent dryout with unprecedented details. The studies are conducted in a 600 μm square cross-section microchannel using FC-72 as the test fluid at 70–80% exit vapor quality. The results show that liquid film thinning because of evaporation rather than de-wetting and film rupture is responsible for intermittent surface dryout. Studies at different mass and heat fluxes suggest that for a given surface heat flux, there is a mass flux threshold above which intermittent surface dryout vanishes. Furthermore, the experimental liquid and vapor flow velocities and liquid film thickness are used to calculate the liquid–vapor interfacial shear stress. The results suggest that under the studied conditions, the liquid–vapor interfacial shear stress has little impact on the surface intermittent dryout. This study enhances predictable and reliable design and operation of two-phase heat sinks at high exit vapor qualities.

Effect of high-frequency beam oscillation on microstructures and cracks in laser cladding of Al-Cu-Mg alloys

This paper provides a new method to suppress solidification cracks in laser cladding of Al-Cu-Mg alloy by high-frequency beam oscillation without additional auxiliary measures. The effects of oscillation frequency on crack suppression were investigated by analyzing the microstructure evolution. The results show that the intercrystalline precipitated phase changed from coarse and continuous line to spiderweb-like morphology with the oscillation frequency (f) increased from 50 Hz to 500 Hz, and the thermal cracking tendency was completely reduced when the density of coarse intergranular precipitates was <300 μm/mm2 (f ≥ 200 Hz). The coarse Al2Cu and Al0.92Cu1.08Mg low-melting eutectic phases formed by the Cu element segregation between the columnar dendrites constituted the crack source and crack expansion channel. The rapid and periodic motion of a high-frequency oscillating laser can promote the uniform energy distribution and drive the melt flow, which will induce the columnar to equiaxed transition. The web-like eutectic liquid film between equiaxed dendrites has a smoother reflux channel and coordinated deformation ability. When the solidification shrinkage force pulls the local liquid film, the rest of the liquid phase can return to heal the crack in time, which is the main mechanism for the solidification cracks to be reduced.

Numerical Study on R32 Flow Condensation in Horizontally Oriented Tubes with U-Bends

In this paper, the flow condensation heat transfer characteristics of R32 in a horizontally oriented tube with a horizontal U-bend (HUB tube) and a horizontally oriented tube with a vertical U-bend (VUB tube) were numerically investigated. The volume of fluid (VOF) multiphase model with the mass transfer assumption by Lee is adopted to simulate the flow condensation behavior of R32, which are validated by well-known empirical correlations. The influence of structural parameters, mass flux and vapor quality on the heat transfer coefficient (HTC) and liquid film distribution is investigated. Meanwhile, the local liquid film thickness (LLFT) and local heat transfer coefficient (LHTC) are also depicted. The phase transition at the U-bend section is captured. Results show that the U-bend has remarkable disturbance on the flow pattern and LHTC due to the effect of centrifugal force where the LLFT is changed, inducing strong secondary flow. The LHTC is increased by a maximum of 8.47% and 11.86% in VUB tube and HUB tube, respectively, when compared to the case in a horizontally straight oriented tube at the same operating conditions.

Effect of part-length rods for different BWR subchannel geometries

Despite the beneficial effects of part-length rods (PLRs) in BWR fuel, they also introduce inhomogeneities to the coolant flow. These effects are hard to simulate and might affect cooling efficiency, dryout margin and CRUD deposition. So far, no fundamental experimental research was conducted to explore the details of their influence to the liquid film behavior in the dominating annular flow regime.To investigate the fluid dynamical effects of PLRs, we analyzed three different types of subchannel geometries. We varied the positions of PLRs in a pair of adjacent subchannels as well as the liquid and gas flow rates, staying within the annular flow regime. The adiabatic air-water test section was equipped with two types of sensors measuring the liquid film thickness distribution on half a fuel rod and on a fuel element wall. The measurement frequency was 10 kHz and the lateral resolution in the two-dimensional measuring domain was 2 mm. This corresponds to 64 × 16 and 16 × 32 measuring points, for rod and wall sensor respectively.Closely downstream of a PLR, we found a recirculation zone with less shear driving the liquid film. This is reflected by an increase of the average film thickness and a decrease of the wave velocity. A cross-flow of gas and liquid towards the open space downstream of the PLR was detected. The side of neighboring fuel rods facing this open space sees an increase of liquid flow. On the PLR-averted side, the coolant flow decreases, reducing the dryout margin. Further downstream, more gas tends to traverse into the open space left behind the PLR, which again increases the driving shear stress on the liquid film. The film becomes thinner and faster. The precise channel geometry featuring PLRs have a strong and complex influence on the local average liquid film thickness and the wave velocities. Their qualitative distribution at different flow conditions however, is similar.

Experimental study of the gas-liquid flow characteristics in a rectangular channel with a large aspect ratio

This paper experimental studies the gas-liquid flow in a rectangular slit microchannel with cross-section of 200 × 2045 μm. Ethanol and nitrogen are used as working liquids and gas, respectively. The microchannel with an internal symmetric hydraulic focusing mixer is used for the study of the flow pattern for elongated bubble and transition flows. For the study of the wave patterns in the liquid film, the microchannel with an external mixer is applied. Experimental data on flow patterns are obtained using high-speed visualization and laser-induced fluorescence (LIF) methods. Using method LIF for elongated bubble and transition flows the local film thickness is measured and compared with Taylor law. For microchannel with an external mixer, the local liquid film thickness is measured for a wavy film at high gas velocity, and the wave influence on local film distribution is discussed.

Effect of asymmetric airflow on liquid film behavior and emergency core coolant bypass in the downcomer geometry of a nuclear reactor pressure vessel

In this study, experimental work was carried out using a 1/10 reduced-scale downcomer of a nuclear reactor pressure vessel to examine the liquid film behavior under emergency core coolant (ECC) bypass conditions. It is caused by the interaction between the transverse gas flow and the falling liquid film. The experiments were conducted considering the asymmetric gas velocity inside the downcomer. An air–water mixture was used as the working fluid in the experiment, forming a complex liquid film flow in the annular test section of the downcomer. To measure the local liquid film thickness, an electrical conductance sensor fabricated on a flexible printed circuit board was applied. The effect of the asymmetric airflow on the liquid film behavior was investigated using the liquid film thickness measurements. Results showed that the asymmetry influences the distribution of the liquid film; the larger the asymmetry, the closer is the liquid film is to the break with the same gas outlet velocity. It influences the trend of the ECC bypass fraction with respect to the gas velocity. The present study provides detailed understanding of the ECC bypass phenomenon under asymmetric airflow conditions; this enables determining how the liquid film behavior is changed by the gas flow and how it affects the ECC bypass.

CFD study of Marangoni condensation heat transfer of vapor mixture on a horizontal tube

Over the past few decades, experimental studies have been conducted on the condensation heat transfer of vapor mixtures. However, the local heat transfer characteristics of vapor mixture affected by surface tension have not been fully investigated. The present work, for the first time, suggested the CFD simulation method for the Marangoni condensation of ethanol-vapor mixture and analyzed the calculated result with flow structure. A CFD study has been performed for the condensation heat transfer of steam–ethanol mixtures on a horizontal tube to elucidate the Marangoni condensation phenomenon and the consequent heat transfer mechanism, which are difficult to fully obtain experimentally. In the present study, the numerical predictions of condensation heat transfer using a liquid film model agree well with those of the experimental results. The CFD analysis of steam–ethanol mixture compares with pure steam to clearly show the influence of surface tension on the heat transfer. In particular, when a small amount of ethanol is added to pure steam, the condensation heat transfer is enhanced because the local liquid film thickness is changed because of the surface tension gradient on the gas–liquid interface, which is caused by a concentration difference.

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.

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.

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.

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.

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.

Dryout-type critical heat flux in vertical upward annular flow: effects of entrainment rate, initial entrained fraction and diameter

This study aims to reveal the effects of liquid entrainment, initial entrained fraction and tube diameter on liquid film dryout in vertical upward annular flow for flow boiling. Entrainment and deposition rates of droplets were included in mass conservation equations to estimate the local liquid film mass flux in annular flow, and the critical vapor quality at dryout conditions. Different entrainment rate correlations were evaluated using flow boiling data of water and organic liquids including n-pentane, iso-octane and R134a. Effect of the initial entrained fraction (IEF) at the churn-to-annular flow transition was also investigated. A transitional Boiling number was proposed to separate the IEF-sensitive region at high Boiling numbers and the IEF-insensitive region at low Boiling numbers. Besides, the diameter effect on dryout vapor quality was studied. The dryout vapor quality increases with decreasing tube diameter. It needs to be pointed out that the dryout characteristics of submillimeter channels might be different because of different mechanisms of dryout, i.e., drying of liquid film underneath long vapor slugs and flow boiling instabilities.

Research of vertical falling film behavior in scrubbing-cooling tube

Experimental results of the thickness and velocity distribution of the vertical falling film of a gas–liquid two-phase flow in a scrubbing-cooling tube are reported. The liquid Reynolds numbers ranges from 1.0×104 to 3.1×104. A 3D numerical model of the scrubbing-cooling ring and tube has also been established. The simulation results agree well with the experimental data. The research shows that the average liquid film thickness is largest at the 0° circumferential position. A part of the liquid is separated from the local liquid film and drips in the gas core when Reynolds number≥1.5×104. Circumferentially the liquid film thickness is distributed more evenly in the lower section of the tube. When the liquid Reynolds number increases, both of the global liquid film thickness and velocity increase, and the thickness distribution becomes unevener. The liquid film velocity increases with the increase of flow distance at small Reynolds number and decreases at large Reynolds number.

Study of the impacts of droplets deposited from the gas core onto a gas-sheared liquid film

The results of an experimental study on droplet impactions in the flow of a gas-sheared liquid film are presented. In contrast to most similar studies, the impacting droplets were entrained from film surface by the gas stream. The measurements provide film thickness data, resolved in both longitudinal and transverse coordinates and in time together with the images of droplets above the interface and images of gas bubbles entrapped by liquid film. The parameters of impacting droplets were measured together with the local liquid film thickness. Two main scenarios of droplet-film interaction, based on type of film perturbation, are identified; the parameter identifying which scenario occurs is identified as the angle of impingement. At large angles an asymmetric crater appears on film surface; at shallow angles a long, narrow furrow appears. The most significant difference between the two scenarios is related to possible impact outcome: craters may lead to creation secondary droplets, whereas furrows are accompanied by entrapment of gas bubbles into the liquid film. In addition, occurrence of partial survival of impacting droplet is reported.

Assessment of wall friction model in multi-dimensional component of MARS with air–water cross flow experiment

Recently, high precision and high accuracy analysis on multi-dimensional thermal hydraulic phenomena in a nuclear power plant has been considered as state-of-the-art issues. System analysis code, MARS, also adopted a multi-dimensional module to simulate them more accurately. Even though it was applied to represent the multi-dimensional phenomena, but implemented models and correlations in that are one-dimensional empirical ones based on one-dimensional pipe experimental results. Prior to the application of the multi-dimensional simulation tools, however, the constitutive models for a two-phase flow need to be carefully validated, such as the wall friction model. Especially, in a Direct Vessel Injection (DVI) system, the injected emergency core coolant (ECC) on the upper part of the downcomer interacts with the lateral steam flow during the reflood phase in the Large-Break Loss-Of-Coolant-Accident (LBLOCA). The interaction between the falling film and lateral steam flow induces a multi-dimensional two-phase flow. The prediction of ECC flow behavior plays a key role in determining the amount of coolant that can be used as core cooling. Therefore, the wall friction model which is implemented to simulate the multi-dimensional phenomena should be assessed by multidimensional experimental results. In this paper, the air–water cross film flow experiments simulating the multi-dimensional phenomenon in upper part of downcomer as a conceptual problem will be introduced. The two-dimensional local liquid film velocity and thickness data were used as benchmark data for code assessment. And then the previous wall friction model of the MARS-MultiD in the annular flow regime was modified. As a result, the modified MARS-MultiD produced improved calculation result than previous one.

Unified Modeling Suite for Two-Phase Flow, Convective Boiling, and Condensation in Macro- and Microchannels

This paper focuses on the unified modeling suite for annular flow that the authors have developed and continue to develop. The annular flow suite currently includes models to predict the void fraction, the entrained liquid fraction, and the wall shear stress and pressure gradient, and a turbulence model for momentum and heat transport inside the annular liquid film. The turbulence model, in particular, allows prediction of the local average liquid film thicknesses and the local heat transfer coefficients during convective evaporation and condensation. The benefit of a unified modeling suite is that all the included prediction methods are consistently formulated and are proven to work well together, and provide a platform for continued advancement based on the other models in the suite. First the unified suite of methods is presented, illustrating in particular the most recent updates. Then results for the pressure drop and the heat transfer coefficient during convective evaporation and condensation are presented and discussed, covering both water and refrigerants flowing through circular tubes and noncircular multi-microchannel configurations for microelectronics cooling.

Influence of Triggered Seed Bubbles on Flow Boiling in Microchannels

Combined with a simple phase change model, the Volume of Fluid (VOF) method is used for simulating the seed bubble triggered boiling in a single circular microchannel with 6.5mm length. Simulation results show that the introduction of seed bubbles improves the heat transfer performance and suppresses the flow instability simultaneously. The seed bubbles are triggered into the microchannel forf=2000 Hz, passing by only about five numbers of periods, the wall superheats remarkably decreased, a complete thermal equilibrium between the liquid and vapor phases is approached, the flow instability can be controlled successfully. At the stable flow stage, the heat transfer which can reach more than 3 times the single liquid phase heat transfer dominated by the local liquid film thickness, which is the actual heat transfer length-scale in the liquid film regime. However, the liquid bridge region existing between the bubbles produces a week heat transfer mechanism.

Study of the hydrodynamic characteristics of a free flowing liquid film in open inclined microchannels

A widely used device for continuous contacting in microdevices is the Falling Film Micro-Reactor (FFMR). The liquid films that are formed in the channels of a FFMR are quite stable and have a typical thickness of less than 100μm. In the present work, a μ-PIV system was used for measuring velocity profiles and for estimating the local liquid film thickness in three open inclined microchannels (with width of 1200, 600 and 300μm). The film thickness measurements were utilized for reconstructing the shape of the interface, while the effect of various fluid parameters (e.g. physical properties, flow rate) as well as geometrical characteristics of the conduit and the inclination angle of the microchannels, on the geometrical characteristics of the liquid phase has been also experimentally investigated. As the correlations that are valid in the macroscale are generally not applicable in the microscale, the aim of the present study is to develop a means for predicting the geometrical characteristics (i.e. liquid film thickness, shape of the gas/liquid interface) of the liquid phase, as well as its mean residence time. Generalized correlations for predicting film thickness and meniscus shape are proposed (accuracy±15%) while, by combining these correlations an estimation of the mean residence time can be also made.