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.

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.

About Computations of Hele-Shaw Flow of Non-Newtonian Fluids

1998 ◽  
Vol 543 ◽  
Author(s):  
L. Kondic ◽  
P. Fast ◽  
M. J. Shelley
Keyword(s):  
Solidification Front ◽  
Two Phase Flow ◽  
Newtonian Fluids ◽  
Phase Flow ◽  
Fluid Interface ◽  
Two Phase ◽  
Length Scales ◽  
Close Analogy ◽  
Newtonian Liquids ◽  
Hele Shaw Cell

AbstractThe flow of a fluid confined between two solid plates (Hele-Shaw cell) is of considerable interest in a variety of applications. Further interest in two phase flow in this geometry stems from the close analogy between the dynamics of fluid-fluid interface and the propagation of the solidification front. While the flow of Newtonian fluids is rather well understood, it is much more complicated to compute flows of non-Newtonian fluids. We find that the dense-branching morphology of Newtonian liquids may be replaced by dendritic fingers with stable tips and sidebranches, and discuss resulting length scales.

Flow Distribution Control Between Two Parallel Meso-Scale Evaporators With Electrohydrodynamic Conduction Pumping

2016 ◽  
Author(s):  
Lei Yang ◽  
Michal Talmor ◽  
Jamal Seyed-Yagoobi
Keyword(s):  
Two Phase Flow ◽  
Finite Thickness ◽  
Flow Distribution ◽  
Control Technique ◽  
Working Fluid ◽  
Phase System ◽  
Phase Flow ◽  
Two Phase ◽  
Macro Scale ◽  
Meso Scale

Electrohydrodynamic (EHD) conduction pumps generate pressure to drive dielectric liquids via the electrical Coulomb force exerted within heterocharge layers of finite thickness in the vicinity of the electrodes. By applying an external electric field in a dielectric liquid, the heterocharge layers form due to the net charges as a result of the process of enhanced dissociation of neutral molecules versus the recombination of the generated ions. EHD conduction pumping can be applied to enhance and control mass and heat transfer of both isothermal and nonisothermal liquid and two-phase fluid, with many advantages such as simple design, no moving parts and low power consumption. It also shows its potential as an active control technique for flow distribution for multi-scale systems in both terrestrial and microgravity environment. Flow distribution control based on EHD conduction pumping mechanism was previously investigated in macro-scale. This study experimentally examines its capability in controlling two-phase flow distribution between two parallel meso-scale evaporators. The working fluid was refrigerant HCFC-123. It has been found that an EHD conduction pump could effectively control the two-phase flow distribution via adjusting the flow rate in each branch line, and facilitate the recovery from dry-out condition in two-phase system.

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.

scholarly journals Numerical investigation of a moisture wave-type vane separator

2019 ◽  
Vol 140 ◽  
pp. 06011
Author(s):  
Ivan Kasatkin ◽  
Mikle Egorov ◽  
Nikolay Rakov
Keyword(s):  
Two Phase Flow ◽  
Numerical Study ◽  
Euler Method ◽  
Saturated Steam ◽  
Phase Flow ◽  
Water Droplets ◽  
Wave Type ◽  
Two Phase ◽  
Separation Processes ◽  
Drainage Channels

This study aims to determine the appropriate method for modeling separation processes in wave-type moisture vane separators and to analyze possible design improvements of the typical wave-type vane separator using numerical simulation methods. It discusses conditions of the secondary droplets generation phenomena. The applicability of the particle transport method for modeling the working process in wave-type vane separators is confirmed. The study explores the water droplets distribution pattern in the dispersed two-phase flow of a separator. A numerical study of the workflow in the typical steam-water separator with wave-type vanes was carried out. A dispersed two-phase flow in a separator is modelled as two separate flows. Dry saturated steam is considered as a continuous medium using the Euler method; the liquid phase is represented as a stream of water droplets described by the Lagrange method. Authors propose an option of modernization of the separator design applying three drainage channels on the path of small droplets.

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.

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