scholarly journals Contact Angles in Two-Phase Flow Images

2020 ◽  
Vol 135 (3) ◽  
pp. 535-553
Author(s):  
Hamid Hosseinzade Khanamiri ◽  
Per Arne Slotte ◽  
Carl Fredrik Berg
Keyword(s):  
Two Phase Flow ◽  
Contact Angles ◽  
Image Features ◽  
Phase Flow ◽  
Two Phase ◽  
Line Energy ◽  
Triangulated Surfaces ◽  
Experimental Image ◽  
Large Clusters ◽  
Voxel Grid

AbstractIn this work, we calculate contact angles in X-ray tomography images of two-phase flow in order to investigate the wettability. Triangulated surfaces, generated using the images, are smoothed to calculate the contact angles. As expected, the angles have a spread rather than being a constant value. We attempt to shed light on sources of the spread by addressing the overlooked mesh corrections prior to smoothing, poorly resolved image features, cluster-based analysis, and local variations of contact angles. We verify the smoothing algorithm by analytical examples with known contact angle and curvature. According to the analytical cases, point-wise and average contact angles, average mean curvature and surface area converge to the analytical values with increased voxel grid resolution. Analytical examples show that these parameters can reliably be calculated for fluid–fluid surfaces composed of roughly 3000 vertices or more equivalent to 1000 pixel2. In an experimental image, by looking into individual interfaces and clusters, we show that contact angles are underestimated for wetting fluid clusters where the fluid–fluid surface is resolved with less than roughly 500 vertices. However, for the fluid–fluid surfaces with at least a few thousand vertices, the mean and standard deviation of angles converge to similar values. Further investigation of local variations of angles along three-phase lines for large clusters revealed that a source of angle variations is anomalies in the solid surface. However, in the places least influenced by such noise, we observed that angles tend to be larger when the line is convex and smaller when the line is concave. We believe this pattern may indicate the significance of line energy in the free energy of the two-phase flow systems.

scholarly journals Modeling of Two-Phase Flow in Rough-Walled Fracture Using Level Set Method

Geofluids ◽  
2017 ◽  
Vol 2017 ◽  
pp. 1-11 ◽  
Author(s):  
Yunfeng Dai ◽  
Zhifang Zhou ◽  
Jin Lin ◽  
Jiangbo Han
Keyword(s):  
Crude Oil ◽  
Level Set Method ◽  
Level Set ◽  
Two Phase Flow ◽  
Volume Fraction ◽  
Contact Angles ◽  
Immiscible Fluids ◽  
Water Volume ◽  
Phase Flow ◽  
Two Phase

To describe accurately the flow characteristic of fracture scale displacements of immiscible fluids, an incompressible two-phase (crude oil and water) flow model incorporating interfacial forces and nonzero contact angles is developed. The roughness of the two-dimensional synthetic rough-walled fractures is controlled with different fractal dimension parameters. Described by the Navier–Stokes equations, the moving interface between crude oil and water is tracked using level set method. The method accounts for differences in densities and viscosities of crude oil and water and includes the effect of interfacial force. The wettability of the rough fracture wall is taken into account by defining the contact angle and slip length. The curve of the invasion pressure-water volume fraction is generated by modeling two-phase flow during a sudden drainage. The volume fraction of water restricted in the rough-walled fracture is calculated by integrating the water volume and dividing by the total cavity volume of the fracture while the two-phase flow is quasistatic. The effect of invasion pressure of crude oil, roughness of fracture wall, and wettability of the wall on two-phase flow in rough-walled fracture is evaluated.

Micro/Meso Scale Modelling of Two-Phase Flow on Functional Surfaces: A Numerical Simulation of Water Droplets on Natural Hydrophobic Surfaces With Micro Roughness

2009 ◽  
Author(s):  
Y. Y. Yan
Keyword(s):  
Two Phase Flow ◽  
Contact Angles ◽  
Phase Flow ◽  
Fluid Interface ◽  
Water Droplets ◽  
Hydrophobic Surfaces ◽  
Two Phase ◽  
Partial Wetting ◽  
Scale Modelling ◽  
Meso Scale

A micro/meso scale modelling of two-phase droplets move on hydrophilic/hydrophobic surfaces with micro roughness is reported. The physical model is basically of two-phase flow interacting with the surfaces of different hydrophobicity or wettability. Numerical modelling based on the lattice Boltzmann method (LBM) is developed and applied to the computational calculation and simulation. The LBM modelling deals with surface tension dominated behaviour of water droplets in air spreading on a hydrophilic surface with hydrophobic strips of different sizes and contact angles under different physical and interfacial conditions, and aims to find quantitative data and physical conditions of the biomimetic approaches. The current LBM can be applied to simulate two-phase fluids with large density ratio (up to 1000), and meanwhile deal with interactions between a fluid-fluid interface and a partial wetting wall. In the simulation, the interactions between the fluid-fluid interface and the partial wetting wall with different hydrophobic strips such as single strip, intersecting stripes, and alternating & parallel stripes, of different sizes and contact angles are considered and tested numerically; the phenomena of droplets spreading and breaking up, and the effect of hydrophobic strips on the surface wettability or self-cleaning characteristics are simulated, reported and discussed.

Two-Dimensional Simulations of Gas-Liquid Two-Phase Flow in Mini channels of PEM Fuel Cell Flow Field

2010 ◽  
Vol 8 (1) ◽  
Author(s):  
Wei Du ◽  
Lifeng Zhang ◽  
Xiaotao T Bi ◽  
David Wilkinson ◽  
Jürgen Stumper ◽  
...  
Keyword(s):  
Fuel Cells ◽  
Liquid Flow ◽  
Two Phase Flow ◽  
Strong Dependence ◽  
Pem Fuel Cells ◽  
Gas Diffusion ◽  
Contact Angles ◽  
Phase Flow ◽  
Two Phase ◽  
Mini Channels

Effective water management is one of the key strategies for improving the performance and durability of PEM fuel cells. Phenomena such as membrane dehydration, catalyst layer flooding and two-phase flow in flow-channels are all determined by the distribution and movement of water during cell operation. In this study, gas-liquid flow in mini-channels relevant to fuel cells was numerically studied using a CFD two-phase flow model in combination with a volume of fluid (VOF) method. The results show that the surface wettability of the channel wall can greatly affect the flow pattern, especially when the channel walls and the gas diffusion layer (GDL) surface possess different contact angles. When the channel walls are more hydrophobic, more water is accumulated on the GDL. An increase in the surface tension results in a slight increase of slug frequency and a slight decrease in slug length. The onset of slugging along the channel is determined by the gas-liquid mixture velocity, gas-to-liquid flow ratio and the way water is introduced into the gas flow channel. Furthermore, the calculated pressure drop fluctuations show a strong dependence on the channel liquid content and the slug length.

Wetting Effects on Two-Phase Flow in a Microchannel

2007 ◽  
Author(s):  
Alexandru Herescu ◽  
Jeffrey S. Allen
Keyword(s):  
Surface Tension ◽  
Contact Angle ◽  
Water Flow ◽  
High Speed ◽  
Two Phase Flow ◽  
Contact Angles ◽  
Phase Flow ◽  
Two Phase Flows ◽  
Two Phase ◽  
Number Systems

In the recent years there has been an increasing interest in the study of two-phase flows in low Bond number systems (where capillary forces are important relative to gravitational forces). Such systems include capillary tubes and microchannels as well as the gas flow channels of a PEM fuel cell. At the capillary scale, surface tension forces play an important role in two-phase flow regime transitions, pointing out the need to take into account the geometry of the cross section and the surface properties (wettability). Surface tension is generally considered in flow transitions, but the wetting properties of the fluid-surface material pairs (contact angle) are rarely given any importance. The researchers investigating two-phase flows should take extreme care when choosing the material of the test sections, as the flow morphology and the the pressure drop accordingly can vary widely with contact angle. In order to show these morphological changes high speed visualization experiments of air-water flow through 500 μm square and round microchannels were conducted. For the round channels, contact angles of less than 20° (wetting) and 105° (non-wetting) were investigated. For the square section, things are complicated by the presence of the corners. According to the Concus-Finn criterion, the liquid will wick into (wet) the corner if the contact angle is less then 45°, or will de-wet the corner if the contact angle is above 45°. A new case not previously mentioned in the literature arises for a contact angle of 45° ≤ θ ≤ 90°, for which the liquid is wetting the walls but dewetting the corners. Three contact angles of less than 20°, 80° and 105° are considered to investigate the possible morphologies in the square geometry. Images aquired with a high speed camera depicting the different flow morphologies that exist at the same air-water flow rates for each of the considered contact angle and geometry are presented.

Investigation on the Effect of Surface Wettability on a Two-Phase Flow in a Compressor Cascade

2018 ◽  
Vol 140 (10) ◽  
Author(s):  
Niklas Neupert ◽  
Janneck Christoph Harbeck ◽  
Franz Joos
Keyword(s):  
Mach Number ◽  
Gas Turbines ◽  
Two Phase Flow ◽  
Contact Angles ◽  
Surface Wettability ◽  
Phase Flow ◽  
Compressor Cascade ◽  
Two Phase ◽  
Hydrophobic Coatings ◽  
Transonic Compressor

In recent years overspray fogging has become a powerful means for power augmentation of industrial gas turbines. Despite the positive thermodynamic effect on the cycle droplets entering the compressor increase the risk of water droplet erosion. Further deposited water leads to a higher sensitivity toward fouling due to an increased stickiness of the blades. Therefore, erosion resistant hydrophobic coatings are applied to the first stages of compressors. Although some patents claim the use of such coatings the aerodynamic impact of a different wettability is not regarded so far. This issue was addressed in the field of aerodynamic efficiency of wings in heavy rain showing higher penalty for hydrophobic coatings. In this study, the issue of a different blade surface wettability in a linear transonic compressor cascade is addressed. Different coatings are applied resulting in contact angles of 51–95 deg. The inflow Mach number was fixed at design inflow Mach number, and the inflow angle was varied over a broad range. The effect on the water film pattern is analyzed in terms of position of film breakup, rivulet width, and totally wetted surface. The performance of the cascade under two-phase flow was analyzed using laser Doppler anemometry/phase Doppler anemometry measurement technique in terms of loss coefficient based on wake momentum thickness and flow turning. It is shown that the wettability of the surface has significant effects on the film structure leading to a lower fraction of wetted surface with increasing contact angle. The influence on performance is limited to effects in the proximity of the surface and is dependent on operation point. While in design conditions hydrophilic coating show lower losses, the trend is vice-versa for off-design conditions. The data represent first experimental work on the influence of surface wettability in a droplet-laden flow supporting positive features for hydrophobic coatings in gas turbine compressors.

Surface Wettability Effects in Heat and Fluid Flow

2006 ◽  
Author(s):  
Yasuyuki Takata
Keyword(s):  
Heat Flux ◽  
Contact Angle ◽  
Two Phase Flow ◽  
Contact Angles ◽  
Bubble Nucleation ◽  
Surface Wettability ◽  
Water Repellent ◽  
Phase Flow ◽  
Two Phase ◽  
Wide Range

Effects of surface wettability on liquid-vapor phase change phenomena and single- and two-phase flow in tube have been studied in wide range of contact angles using superhy drophilic (SH) and super-water-repellent (SWR) surfaces. Heat transfer in falling film evaporation on a TiO2-coated SH surface is tremendously enhanced due to very thin stable film. In pool bioling, critical heat flux (CHF) and minimum heat flux (MHF) increase with the decrease in contact angle. Wetting limit temperature of water drop on heated surface increases with the decrease in contact angle. In pool boiling on SWR surface, bubble nucleation and film boiling occur in extremely small superheating. Drag reduction was observed in water flow in tube with SWR coating in laminar flow region, and on the other hand, in two-phase flow pressure drop for the SH wall is smaller than that for the SWR wall.

Investigation on the Effect of Surface Wettability on a Two-Phase Flow in a Compressor Cascade

2017 ◽  
Author(s):  
Niklas Neupert ◽  
Janneck Christoph Harbeck ◽  
Franz Joos
Keyword(s):  
Mach Number ◽  
Gas Turbines ◽  
Two Phase Flow ◽  
Contact Angles ◽  
Surface Wettability ◽  
Phase Flow ◽  
Compressor Cascade ◽  
Two Phase ◽  
Hydrophobic Coatings ◽  
Transonic Compressor

In recent years overspray fogging has become a powerful means for power augmentation of industrial gas turbines. Despite the positive thermodynamic effect on the cycle droplets entering the compressor increase the risk of water droplet erosion. Further deposited water leads to a higher sensitivity towards fouling due to an increased stickiness of the blades. Therefore erosion resistant hydrophobic coatings are applied to the first stages of compressors. Although some patents claim the use of such coatings the aerodynamic impact of a different wettability is not regarded so far. This issue was addressed in the field of aerodynamic efficiency of wings in heavy rain showing higher penalty for hydrophobic coatings. In this study the issue of a different blade surface wettability in a linear transonic compressor cascade is addressed. Different coatings are applied resulting in contact angles of 51–95°. The inflow Mach number was fixed at design inflow Mach number and the inflow angle was varied over a broad range. The effect on the water film pattern is analyzed in terms of position of film breakup, rivulet width and totally wetted surface. The performance of the cascade under two-phase flow was analyzed using LDA/PDA measurement technique in terms of loss coefficient based on wake momentum thickness and flow turning. It is shown that the wettability of the surface has significant effects on the film structure leading to a lower fraction of wetted surface with increasing contact angle. The influence on performance is limited to effects in the proximity of the surface and is dependent on operation point. While in design conditions hydrophilic coating show lower losses the trend is vice-versa for off-design conditions. The data represent first experimental work on the influence of surface wettability in a droplet-laden flow supporting positive features for hydrophobic coatings in gas turbine compressors.

scholarly journals Pore-Scale Modeling of the Effect of Wettability on Two-Phase Flow Properties for Newtonian and Non-Newtonian Fluids

Polymers ◽  
2020 ◽  
Vol 12 (12) ◽  
pp. 2832
Author(s):  
Moussa Tembely ◽  
Waleed S. Alameri ◽  
Ali M. AlSumaiti ◽  
Mohamed S. Jouini
Keyword(s):  
Contact Angle ◽  
Two Phase Flow ◽  
Dynamic Contact Angle ◽  
Dynamic Contact ◽  
Contact Angles ◽  
Phase Flow ◽  
Pore Scale ◽  
Flow Properties ◽  
Two Phase ◽  
Oil Saturation

The Darcy-scale properties of reservoir rocks, such as capillary pressure and relative permeability, are controlled by multiphase flow properties at the pore scale. In the present paper, we implement a volume of fluid (VOF) method coupled with a physically based dynamic contact angle to perform pore-scale simulation of two-phase flow within a porous medium. The numerical model is based on the resolution of the Navier–Stokes equations as well as a phase fraction equation incorporating a dynamic contact angle model with wetting hysteresis effect. After the model is validated for a single phase, a two-phase flow simulation is performed on both a Newtonian and a non-Newtonian fluid; the latter consists of a polymer solution displaying a shear-thinning power law viscosity. To investigate the effects of contact angle hysteresis and the non-Newtonian nature of the fluid, simulations of both drainage and imbibition are carried out in order to analyze water and oil saturation—particularly critical parameters such as initial water saturation (Swi) and residual oil saturation (Sor) are assessed in terms of wettability. Additionally, the model sensitivities to the consistency factor (χ), the flow behavior index (n), and the advancing and receding contact angles are tested. Interestingly, the model correctly retrieves the variation in Sor and wettability and predicts behavior over a wide range of contact angles that are difficult to probe experimentally.

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