Imperial College London

Professor Omar K. Matar, FREng

Faculty of EngineeringDepartment of Chemical Engineering

Vice-Dean (Education), Faculty of Engineering
 
 
 
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Contact

 

+44 (0)20 7594 9618o.matar Website

 
 
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Assistant

 

Mrs Raluca Reynolds +44 (0)20 7594 5557

 
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Location

 

506ACE ExtensionSouth Kensington Campus

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Summary

 

Publications

Publication Type
Year
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299 results found

Voulgaropoulos V, Kadivar M, Moghimi MA, Maher M, Alawadi H, Matar OK, Markides CNet al., 2021, A combined experimental and computational study of phase-change dynamics and flow inside a sessile water droplet freezing due to interfacial heat transfer, International Journal of Heat and Mass Transfer, Vol: 180, Pages: 1-15, ISSN: 0017-9310

This study experimentally and numerically investigates the freezing characteristics and fluid dynamics of millimetre-sized sessile water droplets submerged in silicone oil at sub-zero temperatures under free convection. Individual water droplets were cooled to sub-zero temperatures (260-270 K) via interfacial heat transfer between the two liquid phases, in an approach different to studies in the literature where the cooling is done either from the solid substrate or from a low-temperature gas phase (such as air) surrounding the droplets. Laser-induced fluorescence was employed to perform spatiotemporally-resolved measurements of the phase distribution (from which interface distributions, freezing fronts, and rates were extracted). The particle image velocimetry was used to generate information on the velocity fields inside the liquid droplets. The experimental data are complemented by computational fluid dynamics (CFD) simulations, which showed acceptable qualitative and quantitative agreement with the experimental results. The experimental and simulation results indicated that prior to the initiation of freezing, two counteracting recirculation zones are generated in the central plane of the droplets, one on either side of the centreline, leading to a net upward flow at the edges and a downward flow in the centre due to the natural convection driven by internal temperature gradients. The nucleation sites appear on the external regions of the recirculation structures (which are locations with higher shear). Once freezing starts, the natural circulation patterns are suppressed, and instead, a sole downwards flow dominates, which is the result of the freezing layer suppressing the water phase. CFD results demonstrated a relatively wide temperature and pressure distribution in the water droplet at the beginning of the freezing stage, which gradually diminishes as the freezing process proceeds. The effect of droplet size and oil temperature on the freezing rates were in

Journal article

Constante-Amores CR, Batchvarov A, Kahouadji L, Shin S, Chergui J, Juric D, Matar OKet al., 2021, Role of surfactant-induced Marangoni stresses in drop-interface coalescence, Journal of Fluid Mechanics, Vol: 925, Pages: 1-21, ISSN: 0022-1120

We study the effect of surfactants on the dynamics of a drop-interface coalescence using full three-dimensional direct numerical simulations. We employ a hybrid interface-tracking/level-set method, which takes into account Marangoni stresses that arise from surface-tension gradients, interfacial and bulk diffusion and sorption kinetic effects. We validate our predictions against the experimental data of Blanchette and Bigioni (Nat. Phys., vol. 2, issue 4, 2006, pp. 254–257) and perform a parametric study that demonstrates the delicate interplay between the flow fields and those associated with the surfactant bulk and interfacial concentrations. The results of this work unravel the crucial role of the Marangoni stresses in the flow physics of coalescence, with particular attention paid to their influence on the neck reopening dynamics in terms of stagnation-point inhibition, and near-neck vorticity generation. We demonstrate that surfactant-laden cases feature a rigidifying effect on the interface compared with the surfactant-free case, a mechanism that underpins the observed surfactant-induced phenomena.

Journal article

Obeysekara A, Salinas P, Heaney CE, Kahouadji L, Via-Estrem L, Xiang J, Srinil N, Nicolle A, Matar OK, Pain CCet al., 2021, Prediction of multiphase flows with sharp interfaces using anisotropic mesh optimisation, Advances in Engineering Software, Vol: 160, Pages: 1-16, ISSN: 0965-9978

We propose an integrated, parallelised modelling approach to solve complex multiphase flow problems with sharp interfaces. This approach is based on a finite-element, double control-volume methodology, and employs highly-anisotropic mesh optimisation within a framework of high-order numerical methods and algorithms, which include adaptive time-stepping, metric advection, flux limiting, compressive advection of interfaces, multi-grid solvers and preconditioners. Each method is integral to increasing the fidelity of representing the underlying physics while maximising computational efficiency, and, only in combination, do these methods result in the accurate, reliable, and efficient simulation of complex multiphase flows and associated regime transitions. These methods are applied simultaneously for the first time in this paper, although some of the individual methods have been presented previously. We validate our numerical predictions against standard benchmark results from the literature and demonstrate capabilities of our modelling framework through the simulation of laminar and turbulent two-phase pipe flows. These complex interfacial flows involve the creation of bubbles and slugs, which involve multi-scale physics and arise due to a delicate interplay amongst inertia, viscous, gravitational, and capillary forces. We also comment on the potential use of our integrated approach to simulate large, industrial-scale multiphase pipe flow problems that feature complex topological transitions.

Journal article

Voulgaropoulos V, Patapas A, Lecompte S, Charogiannis A, Matar OK, De Paepe M, Markides CNet al., 2021, Simultaneous laser-induced fluorescence and capacitance probe measurement of downwards annular gas-liquid flows, International Journal of Multiphase Flow, Vol: 142, Pages: 103665-103665, ISSN: 0301-9322

This study focuses on the characterisation of downwards annular gas-liquid (air-water) flows, by employing a combi-nation of advanced laser-based and capacitance-based measurement methods. A variant of laser-induced fluorescence(LIF), referred to as structured-planar laser-induced fluorescence (S-PLIF), eliminates biases commonly encounteredduring film-thickness measurements of gas-liquid flows, due to refraction and reflection of the light at the interface. Abespoke capacitance probe is also assembled to enable temporally resolved film-thickness measurements with high tem-poral resolution along the circumferential perimeter of the pipe. We compare the film mean thickness, roughness, andprobability density functions obtained with each method. We find that both methods are able to measure time-averagedfilm thickness to within<20% deviations from each other and from results obtained from the available literature. Theresulting probe data suggest a biased (suppressed) standard deviation of the film thickness, which can be attributed toits working principle, i.e., measuring the film thickness averaged along the circumferential perimeter of the pipe. Theauto-correlation functions of the time-traces provide an insight into the characteristic time-scales of the flows, whichspan a range from∼10 ms for highly gas-sheared flows and increase to about 30 ms for the less turbulent falling films.The power spectral densities reveal modal frequencies that start from 2.5 Hz for falling films, and increase with the gasReynolds number by almost an order of magnitude. The turbulent wave activity (slope in the power spectrum) reduceswith a decrease in gas shear, and shows similarities to the decay of homogeneous and isotropic turbulence. The sizes ofthe bubbles entrained in the liquid film are measured from the S-PLIF images, and exhibit log-normal distribution thatbecome flatter with a decrease in the gas Reynolds number. The normalised location of the bubbl

Journal article

Moran HR, Zogg D, Voulgaropoulos V, Van den Bergh WJ, Dirker J, Meyer JP, Matar OK, Markides CNet al., 2021, An experimental study of the thermohydraulic characteristics of flow boiling in horizontal pipes: Linking spatiotemporally resolved and integral measurements, Applied Thermal Engineering, Vol: 194, Pages: 1-17, ISSN: 1359-4311

Data are presented from experiments of flow boiling in a horizontal pipe. Specifically, refrigerant R245fa was evaporated in a 12.6 mm stainless steel pipe to which a uniform heat flux of up to 38 kW/m was applied. The bespoke facility operated at mass fluxes in the range 30–700 kg/m s and a saturation pressure of 1.7 bar. Flow patterns were identified through high-speed imaging and the resulting flow pattern map is compared to existing maps in the literature. Predictive methods for the pressure drop and heat transfer coefficient from common correlations are also compared to the present experimental data, acting as verification of the facility and methods used for the macroscale boiling flows investigated in this work. Laser-induced fluorescence (for the identification of the liquid phase) and particle image velocimetry (for the provision of velocity-field information) were also developed and successfully applied, providing detailed spatially- and temporally-resolved interfacial property, phase distribution and liquid-phase velocity-field data, alongside traditional integral pressure drop and overall heat transfer measurements. The laser-based methods provide new insight into the hydrodynamic and thermal characteristics of boiling flows at this scale, which are linked to the integral thermohydraulic data on flow regimes, pressure drops and heat transfer. This enhanced understanding can improve the design and operation of flow-boiling applications such as organic Rankine cycles and concentrating solar power facilities operating in the direct steam generation mode.

Journal article

Constante-Amores CR, Kahouadji L, Batchvarov A, Shin S, Chergui J, Juric D, Matar OKet al., 2021, Direct numerical simulations of transient turbulent jets: vortex-interface interactions, Journal of Fluid Mechanics, Vol: 922, Pages: 1-28, ISSN: 0022-1120

The breakup of an interface into a cascade of droplets and their subsequent coalescence is a generic problem of central importance to a large number of industrial settings such as mixing, separations and combustion. We study the breakup of a liquid jet introduced through a cylindrical nozzle into a stagnant viscous phase via a hybrid interface-tracking/level-set method to account for the surface tension forces in a three-dimensional Cartesian domain. Numerical solutions are obtained for a range of Reynolds (Re) and Weber (We) numbers. We find that the interplay between the azimuthal and streamwise vorticity components leads to different interfacial features and flow regimes in Re–We space. We show that the streamwise vorticity plays a critical role in the development of the three-dimensional instabilities on the jet surface. In the inertia-controlled regime at high Re and We, we expose the details of the spatio-temporal development of the vortical structures affecting the interfacial dynamics. A mushroom-like structure is formed at the leading edge of the jet inducing the generation of a liquid sheet in its interior that undergoes rupture to form droplets. These droplets rotate inside the mushroom structure due to their interaction with the prevailing vortical structures. Additionally, Kelvin–Helmholtz vortices that form near the injection point deform in the streamwise direction to form hairpin vortices, which, in turn, trigger the formation of interfacial lobes in the jet core. The thinning of the lobes induces the creation of holes which expand to form liquid threads that undergo capillary breakup to form droplets.

Journal article

Berrahil F, Filali A, Abid C, Benissaad S, Bessaih R, Matar Oet al., 2021, Numerical investigation on natural convection of Al2O3/water nanofluid with variable properties in an annular enclosure under magnetic field, International Communications in Heat and Mass Transfer, Vol: 126, Pages: 1-20, ISSN: 0735-1933

Numerical investigation of the natural convection of Al2O3-water nanofluid is carried out in a differentially heated vertical annulus under a uniform magnetic field. An in-house Fortran code has been developed to solve the system of equations governing the magneto-hydrodynamic flow. Computations are carried out for different Rayleigh numbers (104 ≤ Ra ≤ 106), nanoparticle diameter (dp = 13 and 47 nm), nanoparticle volume fraction (0 ≤ φ ≤ 0.09), radius ratio (2 ≤ λ ≤ 10), and different Hartmann numbers (0 ≤ Ha ≤ 100). According to the simulation data, nanoparticle size is crucial for evaluating nanofluid properties, such as viscosity and thermal conductivity. The computational results reveal that, for nanoparticles with a diameter dp = 47 nm, the average Nusselt number Nu¯i on the inner cylinder wall decreases as the nanofluid volume fraction increases. This decrease in Nu¯i number is observed up to a volume fraction φ = 0.05, after which it increases again. For the full range of volumetric fractions, it is shown that increasing Ra number causes Nu¯i to increase, while increasing Ha number and increasing the magnetic field causes Nu¯i to decrease. Furthermore, as the Ha number increases, the heat transfer enhancement ratio En increases mainly when the magnetic field is oriented radially. Finally, new correlations of Nu¯i versus Ra, φ, Ha, and λ are derived for the axial and radial magnetic fields cases.

Journal article

Moran H, Voulgaropoulos V, Zogg D, Matar O, Markides Cet al., 2021, Experimental observations of flow boiling in horizontal tubes for direct steam generation in concentrating solar power plants, 16th UK Heat Transfer Conference (UKHTC2019), Publisher: Springer Singapore

Conference paper

Inguva PK, Walker PJ, Yew HW, Zhu K, Haslam AJ, Matar OKet al., 2021, Continuum-scale modelling of polymer blends using the Cahn-Hilliard equation: transport and thermodynamics, Soft Matter, Vol: 17, Pages: 5645-5665, ISSN: 1744-683X

The Cahn–Hilliard equation is commonly used to study multi-component soft systems such as polymer blends at continuum scales. We first systematically explore various features of the equation system, which give rise to a deep connection between transport and thermodynamics-specifically that the Gibbs free energy of mixing function is central to formulating a well-posed model. Accordingly, we explore how thermodynamic models from three broad classes of approach (lattice-based, activity-based and perturbation methods) can be incorporated within the Cahn–Hilliard equation and examine how they impact the numerical solution for two model polymer blends, noting that although the analysis presented here is focused on binary mixtures, it is readily extensible to multi-component mixtures. It is observed that, although the predicted liquid–liquid interfacial tension is quite strongly affected, the choice of thermodynamic model has little influence on the development of the morphology.

Journal article

Lavino AD, Smith E, Magnini M, Matar OKet al., 2021, Surface topography effects on pool boiling via non-equilibrium molecular dynamics simulations., Langmuir: the ACS journal of surfaces and colloids, Vol: 37, Pages: 5731-5744, ISSN: 0743-7463

In this work, we investigate nucleate pool boiling via non-equilibrium molecular dynamics simulations. The effect of nano-structured surface topography on nucleation and transition to a film-like boiling regime is studied at the molecular scale, by varying the cavity aspect ratio, wall superheat, and wettability through a systematic parametric analysis conducted on a Lennard-Jones (LJ) system. The interplay of the aforementioned factors is rationalized by means of a classical nucleation theory-based model. The solid surface is heated uniformly from the bottom in order to induce the nanobubble nucleation. Insight into the cavity behavior in heat transfer problems is achieved by looking at temperature and heat flux profiles inside the cavity itself, as well as at the time of nucleation, for different operating conditions. The role of the cavity size in controlling the vapor embryo formation is highlighted, and its dependence on the other investigated parameters is summarized in a phase diagram. Our results show that heterogeneity at the nanoscale plays a key role in determining pool boiling heat transfer performance, suggesting a promising approach to optimize nanostructured surfaces for energy and thermal management applications.

Journal article

Balla M, Tripathi MK, Matar OK, Sahu KCet al., 2021, Interaction of two non-coalescing bubbles rising in a non-isothermal self-rewetting fluid, European Journal of Mechanics - B/Fluids, Vol: 87, Pages: 103-112, ISSN: 0997-7546

The attractive and repulsive behaviours of a pair of initially spherical gas bubbles rising side-by-side in a channel with non-uniformly heated walls containing a self-rewetting liquid are investigated numerically. The surface tension of a self-rewetting fluid exhibits a parabolic temperature dependence with a well-defined minimum, as opposed to linear (common) fluids whose surface tension decreases almost linearly with the increasing temperature. It is found that, for low Reynolds numbers, while in an isothermal medium, two gas bubbles display a repulsive behaviour, they attract in non-isothermal systems. The bubbles in the self-rewetting fluid undergo a plastic collision and show a ‘squeezing and relaxing’ behaviour, whereas they attract and then bounce in the linear fluid. A regime map demarcating the repulsive and attractive behaviours for a self-rewetting fluid is plotted in the Weber number (We) and the dimensionless linear component of the surface tension gradient (M1) space. It isfoundthatthebubblesintheself-rewettingfluidremainsphericalevenforhighWebernumberswhilethey deform considerably in the case of the linear fluid indicating that the attractive behaviour of the bubbles in the self-rewetting fluid is due to the lift force generated by the thermocapillary stresses and not due to the deformation. The mechanism underlying the observed phenomenon is elucidated by studying the drag and lift forces acting on the bubbles, their orientations, and the flow field around them.

Journal article

Constante-Amores CR, Kahouadji L, Batchvarov A, Shin S, Chergui J, Juric D, Matar OKet al., 2021, Dynamics of a surfactant-laden bubble bursting through an interface, Journal of Fluid Mechanics, Vol: 911, Pages: 1-17, ISSN: 0022-1120

We study the effect of surfactant on the dynamics of a bubble bursting through an interface. We perform fully three-dimensional direct numerical simulations using a hybrid interface-tracking/level-set method accounting for surfactant-induced Marangoni stresses, sorption kinetics and diffusive effects. We select an initial bubble shape corresponding to a large Laplace number and a vanishingly small Bond number in order to neglect gravity, and isolate the effects of surfactant on the flow. Our results demonstrate that the presence of surfactant affects the dynamics of the system through Marangoni-induced flow, driving motion from high to low concentration regions, which is responsible for the onset of a recirculation zone close to the free surface. These Marangoni stresses rigidify the interface, delay the cavity collapse and influence the jet breakup process.

Journal article

van Rooij S, Magnini M, Matar OK, Haussener Set al., 2021, Numerical optimization of evaporative cooling in artificial gas diffusion layers, Applied Thermal Engineering, Vol: 186, Pages: 1-10, ISSN: 1359-4311

The utilization of evaporative cooling in the gas diffusion layers (GDLs) of fuel cells or electrolyzers can effectively dissipate the heat produced by high power density operation, thus leading to economically more competitive electrochemical cells. The highly porous GDLs offer a large surface area, allowing to cope with larger heat fluxes and leading to larger evaporation rates. The understanding of the best GDL structure and cell operating conditions for optimized cooling is difficult to determine, given the complexity of the multi-physical processes involved. A direct pore-level numerical modeling framework was developed to analyze the heat and mass transport phenomena occurring within GDLs with integrated evaporative cooling. A three-dimensional model was developed that solves the Navier-Stokes equations, species transport and energy conservation equations in the gas domain, and energy conservation equations in the stagnant fluid phase and solid phase. Evaporation at the liquid-vapor interface was modeled using kinetic theory. The GDL geometry was approximated by an artificial lattice so as to enable the analysis of the effect of a systematic change in the geometry on the transport and evaporation characteristics. A parametric study indicated that increasing the GDL’s porosity from 0.8 to 0.9 and the operating temperature from 60 to 80 led to an increase of the evaporation rate of 19.9% and 197%, respectively. Changing the thermophysical properties of the carrier gas (air to hydrogen) enhanced the evaporation rate, and therefore the cooling of the GDL, by a factor 2.7. The decrease of the amount of vapor in the carrier gas at the water-gas interface impacted positively the evaporative cooling in the GDL.

Journal article

Stafford J, Uzo N, Farooq U, Favero S, Wang S, Chen H-H, L'Hermitte A, Petit C, Matar Oet al., 2021, Real-time monitoring and hydrodynamic scaling of shear exfoliated graphene, 2D Materials, Vol: 8, Pages: 1-17, ISSN: 2053-1583

Shear-assisted liquid exfoliation is a primary candidate for producing defect-free two-dimensional (2D) materials. A range of approaches that delaminate nanosheets from layered precursors in solution have emerged in recent years. Diverse hydrodynamic conditions exist across these methods, and combined with low-throughput, high-cost characterization techniques, strongly contribute to the wide variability in performance and material quality. Nanosheet concentration and production rate are usually correlated against operating parameters unique to each production method, making it difficult to compare, optimize and predict scale-up performance. Here, we reveal the shear exfoliation mechanism from precursor to 2D material and extract the derived hydrodynamic parameters and scaling relationship that are key to nanomaterial output and common to all shear exfoliation processes. Our investigations use conditions created from two different hydrodynamic instabilities—Taylor vortices and interfacial waves—and combine materials characterization, fluid dynamics experiments and numerical simulations. Using graphene as the prototypical 2D material, we find that scaling of concentration of few-layer nanosheets depends on local strain rate distribution, relationship to the critical exfoliation criterion, and precursor residence time. We report a transmission-reflectance method to measure concentration profiles in real-time, using low-cost optoelectronics and without the need to remove the layered precursor material from the dispersion. We show that our high-throughput, in situ approach has broad uses by controlling the number of atomic layers on-the-fly, rapidly optimizing green solvent design to maximize yield, and viewing live production rates. Combining the findings on the hydrodynamics of exfoliation with this monitoring technique, we unlock targeted process intensification, quality control, batch traceability and individually customizable 2D materials on-demand.

Journal article

Ibarra R, Matar OK, Markides CN, 2021, Experimental investigations of upward-inclined stratified oil-water flows using simultaneous two-line planar laser-induced fluorescence and particle velocimetry, International Journal of Multiphase Flow, Vol: 135, Pages: 1-16, ISSN: 0301-9322

Experiments are performed in low-inclination (≤ 5°) upward stratified oil (Exxsol D140) and water flows. The flows are investigated using a novel two-line laser-based diagnostic measurement technique that combines planar laser-induced fluorescence and particle image/tracking velocimetry to obtain two-dimensional (2-D) space- and time-resolved phase and velocity information. The technique enables direct measurements in the non-refractive-index-matched fluids of interest, as opposed to substitute fluids which are matched optically but whose properties may be less representative of those in real field applications. Flow conditions span in situ Reynolds numbers in the range 1300-3630 in the oil phase and 1810-11540 in the water phase, and water cuts of 10% and 20%. Instantaneous velocity vector-fields reveal the presence of complex flow structures in the water phase at low mixture velocities, which become less coherent with increasing pipe inclinations. These structures contribute to the generation of interfacial waves, increase the unsteadiness of the flow and the rate of momentum transfer to the oil phase. Statistical information on the interface heights, mean axial and wall-normal velocity profiles and fluctuations, Reynolds stresses, and mixing lengths is obtained from the analysis of the spatiotemporally resolved phase and velocity data. The normalised mean and rms velocity characteristics (velocity fluctuations and Reynolds stress) are shown to be weakly-dependent on the pipe inclination as the mixture velocity increases. Finally, predictions from a linear mixing-length model agree reasonably well with measurements for the water layer and near-interface regions.

Journal article

Moran HR, Magnini M, Markides CN, Matar OKet al., 2021, Inertial and buoyancy effects on the flow of elongated bubbles in horizontal channels, International Journal of Multiphase Flow, Vol: 135, Pages: 1-13, ISSN: 0301-9322

When a long gas bubble travels in a horizontal liquid-filled channel of circular cross-section, a liquid film is formed between the bubble and the channel wall. At low Reynoldsand Bond numbers, inertial and buoyancy effects are negligible, and the liquid film thicknessis a function of the capillary number only. However, as the tube diameter is increased to themillimetre scale, both buoyancy and inertial forces may become significant. We present theresults of a systematic analysis of the bubble shape, inclination, and liquid film thicknessfor a wide range of capillary, Bond, and Reynolds numbers, namely 0.024≤Cal≤0.051,0.11≤Bo≤3.5, and 1≤Rel≤750. Three-dimensional numerical simulations of the floware performed by employing the Volume-Of-Fluid method implemented in OpenFOAM. Inagreement with previous studies, we observe that buoyancy lifts the bubble above the chan-nel axis, making the top liquid film thinner, and thickening the bottom film. As the Bondnumber approaches unity, the cross-sectional shape of the bubble deviates significantly froma circular shape, due to flattening of the bottom meniscus. The simulations demonstratethe existence of a cross-stream film flow that drains liquid out of the top film and drives ittowards the bottom film region. This drainage flow causes inclination of the bubble, witha larger inclination angle along the bottom plane of the bubble than the top. As buoyancybecomes even more significant, draining flows become less effective and the bubble inclina-tion reduces. A theoretical model for the liquid film thickness and bubble speed is proposedembedding dependencies on both capillary and Bond numbers, which shows good agreementwith the reported numerical results. Inertial forces tend to shrink the bubble cross-sectionand further lift the bubble above the channel centreline, so that the bottom film thicknessincreases significantly with the Reynolds number, whereas the top film thickness is less

Journal article

Williams AGL, Karapetsas G, Mamalis D, Sefiane K, Matar OK, Valluri Pet al., 2021, Spreading and retraction dynamics of sessile evaporating droplets comprising volatile binary mixtures, Journal of Fluid Mechanics, Vol: 907, Pages: 1-46, ISSN: 0022-1120

The dynamics of thin volatile droplets comprising of binary mixtures deposited on a heated substrate are investigated. Using lubrication theory, we develop a novel one-sided model to predict the spreading and retraction of an evaporating sessile axisymmetric droplet formed of a volatile binary mixture on a substrate with high wettability. A thin droplet with a moving contact line is considered, taking into account the variation of liquid properties with concentration as well as the effects of inertia. The parameter space is explored and the resultant effects on wetting and evaporation are evaluated. Increasing solutal Marangoni stress enhances spreading rates in all cases, approaching those of superspreading liquids. To validate our model, experiments are conducted with binary ethanol–water droplets spreading on hydrophilic glass slides heated from below. The spreading rate is quantified, revealing that preferential evaporation of the more volatile component (ethanol) at the contact line drives superspreading, leading in some cases to a contact line instability. Good qualitative agreement is found between our model and experiments, with quantitative agreement being achieved in terms of spreading rate.

Journal article

Batchvarov A, Kahouadji L, Constante-Amores CR, Norões Gonçalves GF, Shin S, Chergui J, Juric D, Craster RV, Matar OKet al., 2021, Three-dimensional dynamics of falling films in the presence of insoluble surfactants, Journal of Fluid Mechanics, Vol: 906, Pages: A16-1-A16-13, ISSN: 0022-1120

We study the effect of insoluble surfactants on the wave dynamics of vertically falling liquid films. We use three-dimensional numerical simulations and employ a hybrid interface-tracking/level-set method, taking into account Marangoni stresses induced by gradients of interfacial surfactant concentration. Our numerical predictions for the evolution of the surfactant-free, three-dimensional wave topology are validated against the experimental work of Park & Nosoko (AIChE J., vol. 49, 2003, pp. 2715–2727). The addition of surfactants is found to influence significantly the development of horseshoe-shaped waves. At low Marangoni numbers, we show that the wave fronts exhibit spanwise oscillations before eventually acquiring a quasi-two-dimensional shape. In addition, the presence of Marangoni stresses is found to suppress the peaks of the travelling waves and preceding capillary wave structures. At high Marangoni numbers, a near-complete rigidification of the interface is observed.

Journal article

PAUL S, HSU W-L, MAGNINI M, MASON LR, ITO Y, HO Y-L, MATAR OK, DAIGUJI Het al., 2021, Analysis and control of vapor bubble growth inside solid-state nanopores, Journal of Thermal Science and Technology, Vol: 16, Pages: 1-20, ISSN: 1880-5566

The increasing demands of computational power have accelerated the development of 3D circuits in the semiconductor industry. To resolve the accompanying thermal issues, two-phase microchannel heat exchangers using have emerged as one of the promising solutions for cooling purposes. However, the direct boiling in microchannels and rapid bubble growth give rise to highly unstable heat flux on the channel walls. In this regard, it is hence desired to control the supply of vapor bubbles for the elimination of the instability. In this research, we investigate a controllable bubble generation technique, which is capable of periodically producing bubble seeds at the sub-micron scale. These nanobubbles were generated in a solid-state nanopore filled with a highly concentrated electrolyte solution. As an external electric field was applied, the localized Joule heating inside the nanopore initiated the homogeneous bubble nucleation. The bubble dynamics was analyzed by measuring the ionic current variation through the nanopore during the bubble nucleation and growth. Meanwhile, we theoretically examined the bubble growth and collapse inside the nanopore by a moving boundary model. In both approaches, we demonstrated that by altering the pore size, the available sensible heat for the bubble growth can be manipulated, thereby offering the controllability of the bubble size. This unique characteristic renders nanopores suitable as a nanobubble emitter for microchannel heat exchangers, paving the way for the next generation microelectronic cooling applications.

Journal article

Paul S, Hsu W-L, Magnini M, Mason LR, Ho Y-L, Matar OK, Daiguji Het al., 2020, Single-bubble dynamics in nanopores: Transition between homogeneous and heterogeneous nucleation, Physical Review Research, Vol: 2, Pages: 1-14, ISSN: 2643-1564

When applying a voltage bias across a thin nanopore, localized Joule heating can lead to single-bubble nucleation, offering a unique platform for studying nanoscale bubble behavior, which is still poorly understood. Accordingly, we investigate bubble nucleation and collapse inside solid-state nanopores filled with electrolyte solutions and find that there exists a clear correlation between homo/heterogeneous bubble nucleation and the pore diameter. As the pore diameter is increased from 280 to 525 nm, the nucleation regime transitions from predominantly periodic homogeneous nucleation to a nonperiodic mixture of homogeneous and heterogeneous nucleation. A transition barrier between the homogeneous and heterogeneous nucleation regimes is defined by considering the relative free-energy costs of cluster formation. A thermodynamic model considering the transition barrier and contact-line pinning on curved surfaces is constructed, which determines the possibility of heterogeneous nucleation. It is shown that the experimental bubble generation behavior is closely captured by our thermodynamic analysis, providing important information for controlling the periodic homogeneous nucleation of bubbles in nanopores.

Journal article

Constante-Amores CR, Kahouadji L, Batchvarov A, Shin S, Chergui J, Juric D, Matar OKet al., 2020, Rico and the jets: Direct numerical simulations of turbulent liquid jets, Physical Review Fluids, Vol: 5, Pages: 110501-1-110501-4, ISSN: 2469-990X

This paper is associated with a poster winner of a 2019 American Physical Society's Division of Fluid Dynamics (DFD) Milton van Dyke Award for work presented at the DFD Gallery of Fluid Motion. The original poster is available online at the Gallery of Fluid Motion, https://doi.org/10.1103/APS.DFD.2019.GFM.P0020.

Journal article

Nazareth RK, Karapetsas G, Sefiane K, Matar OK, Valluri Pet al., 2020, Stability of slowly evaporating thin liquid films of binary mixtures, Physical Review Fluids, Vol: 5, Pages: 104007 – 1-104007 – 32, ISSN: 2469-990X

We consider the evaporation of a thin liquid layer which consists of a binary mixture of volatile liquids. The mixture is on top of a heated substrate and in contact with the gas phase that consists of the same vapor as the binary mixture. The effects of thermocapillarity, solutocapillarity, and the van der Waals interactions are considered. We derive the long-wave evolution equations for the free interface and the volume fraction that govern the two-dimensional stability of the layer subject to the above coupled mechanisms and perform a linear stability analysis. Our results demonstrate two modes of instabilities, a monotonic instability mode and an oscillatory instability mode. We supplement our results from stability analysis with transient simulations to examine the dynamics in the nonlinear regime and analyze how these instabilities evolve with time. More precisely we discuss how the effect of relative volatility along with the competition between thermal and solutal Marangoni effect define the mode of instability that develops during the evaporation of the liquid layer due to preferential evaporation of one of the components.

Journal article

Farooq U, Stafford J, Petit C, Matar OKet al., 2020, Numerical simulations of a falling film on the inner surface of a rotating cylinder, Physical Review E, Vol: 102, Pages: 043106 – 1-043106 – 13, ISSN: 2470-0045

A flow in which a thin film falls due to gravity on the inner surface of a vertical, rotating cylinder is investigated. This is performed using two-dimensional (2D) and 3D direct numerical simulations, with a volume-of-fluid approach to treat the interface. The problem is parameterized by the Reynolds, Froude, Weber, and Ekman numbers. The variation of the Ekman number (Ek), defined to be proportional to the rotational speed of the cylinder, has a strong effect on the flow characteristics. Simulations are conducted over a wide range of Ek values (0≤Ek≤484) in order to provide detailed insight into how this parameter influences the flow. Our results indicate that increasing Ek, which leads to a rise in the magnitude of centrifugal forces, produces a stabilizing effect, suppressing wave formation. Key flow features, such as the transition from a 2D to a more complex 3D wave regime, are influenced significantly by this stabilization and are investigated in detail. Furthermore, the imposed rotation results in distinct flow characteristics such as the development of angled waves, which arise due to the combination of gravitationally and centrifugally driven motion in the axial and azimuthal directions, respectively. We also use a weighted residuals integral boundary layer method to determine a boundary in the space of Reynolds and Ekman numbers that represents a threshold beyond which waves have recirculation regions.

Journal article

Batchvarov A, Kahouadji L, Magnini M, Constante-Amores CR, Shin S, Chergui J, Juric D, Craster RV, Matar OKet al., 2020, Effect of surfactant on elongated bubbles in capillary tubes at high Reynolds number, Physical Review Fluids, Vol: 5, Pages: 093605 – 1-093605 – 21, ISSN: 2469-990X

The effect of surfactants on the tail and film dynamics of elongated gas bubbles propagating through circular capillary tubes is investigated by means of an extensive three-dimensional numerical study using a hybrid front-tracking/level-set method. The focus is on the visco-inertial regime, which occurs when the Reynolds number of the flow is much larger than unity. Under these conditions, “clean” bubbles exhibit interface undulations in the proximity of the tail, with an amplitude that increases with the Reynolds number. We perform a systematic analysis of the impact of a wide range of surfactant properties, including elasticity, bulk surfactant concentration, solubility, and diffusivity, on the bubble and flow dynamics in the presence of inertial effects. The results show that the introduction of surfactants is effective in suppressing the tail undulations as they tend to accumulate near the bubble tail. Here large Marangoni stresses are generated, which lead to a local “rigidification” of the bubble. This effect becomes more pronounced for larger surfactant elasticities and adsorption depths. At reduced surfactant solubility, a thicker rigid film region forms at the bubble rear, where a Couette film flow is established, while undulations still appear at the trailing edge of the downstream “clean” film region. In such conditions, the bubble length becomes an influential parameter, with short bubbles becoming completely rigid.

Journal article

Constante-Amores CR, Kahouadji L, Batchvarov A, Shin S, Chergui J, Juric D, Matar OKet al., 2020, Dynamics of retracting surfactant-laden ligaments at intermediate Ohnesorge number, Physical Review Fluids, Vol: 5, Pages: 084007 – 1-084007 – 24, ISSN: 2469-990X

The dynamics of ligaments retracting under the action of surface tension occurs in a multitude of natural and industrial applications; these include inkjet printing and atomization. We perform direct, fully three-dimensional, two-phase numerical simulations of the retracting process over a range of system parameters that account for surfactant solubility, sorption kinetics, and Marangoni stresses. Our results indicate that the presence of surfactant inhibits the “end-pinching” mechanism and promotes neck reopening through Marangoni-flow; this is induced by the formation of surfactant concentration gradients that drive flow-reversal toward the neck. The vortical structures associated with this flow are also analyzed in detail. We also show that these Marangoni stresses lead to interfacial rigidification, observed through a reduction of the retraction velocity and ligament kinetic energy.

Journal article

Magnini M, Matar OK, 2020, Morphology of long gas bubbles propagating in square capillaries, International Journal of Multiphase Flow, Vol: 129, Pages: 1-13, ISSN: 0301-9322

We present the results of a systematic analysis of the morphology of the thin lubrication film surround- ing a long gas bubble transported by a liquid flow in a square capillary. Direct numerical simulations of the flow are performed using the Volume-Of-Fluid method implemented in OpenFOAM, for a range of capillary and Reynolds numbers Ca = 0 . 002 −0 . 5 and Re = 1 −20 0 0 , and very long bubbles, up to 20 times the hydraulic diameter of the channel. The lubrication film surrounding the bubbles is always re- solved by the computational mesh, and therefore the results are representative of a fully-wetting liquid. This study shows that when Ca ≥0.05, the long gas bubble exhibits an axisymmetric shape on the chan- nel cross-section, whereas for lower capillary numbers the bubble flattens at the centre of the channel wall and thick liquid lobes are left at the corners. When Ca ≤0.01, the thin film at the centre of the wall assumes a saddle-like shape, which leads to the formation of two constrictions at the sides of the liquid film profile, where minimum cross-sectional values of the film thickness are observed. The result- ing cross-stream capillary pressure gradients drain liquid out of the thin-film, whose thickness decreases indefinitely as a power-law of the distance from the bubble nose. Therefore, the film thickness depends on the length of the bubble, unlike flow in circular channels. We report detailed values of the centre- line, diagonal and minimum film thickness along the bubble, bubble speed, and cross-sectional gas area fraction, at varying Ca and Re. Inertial effects retard the formation of the saddle-shaped thin-film at the channel centre, which may never form if the bubble is not sufficiently long. However, the film thins at a faster rate towards the bubble rear as the Reynolds number of the flow is increased.

Journal article

Bhatia N, Müller EA, Matar O, 2020, A GPU accelerated Lennard-Jones system for immersive molecular dynamics simulations in virtual reality, International Conference on Human-Computer Interaction, Publisher: Springer International Publishing, Pages: 19-34, ISSN: 0302-9743

Interactive tools and immersive technologies make teaching more engaging and complex concepts easier to comprehend are designed to benefit training and education. Molecular Dynamics (MD) simulations numerically solve Newton’s equations of motion for a given set of particles (atoms or molecules). Improvements in computational power and advances in virtual reality (VR) technologies and immersive platforms may in principle allow the visualization of the dynamics of molecular systems allowing the observer to experience first-hand elusive physical concepts such as vapour-liquid transitions, nucleation, solidification, diffusion, etc. Typical MD implementations involve a relatively large number of particles N = O( 104 ) and the force models imply a pairwise calculation which scales, in case of a Lennard-Jones system, to the order of O( N2 ) leading to a very large number of integration steps. Hence, modelling such a computational system over CPU along with a GPU intensive virtual reality rendering often limits the system size and also leads to a lower graphical refresh rate. In the model presented in this paper, we have leveraged GPU for both data-parallel MD computation and VR rendering thereby building a robust, fast, accurate and immersive simulation medium. We have generated state-points with respect to the data of real substances such as CO 2 . In this system the phases of matter viz. solid liquid and gas, and their emergent phase transition can be interactively experienced using an intuitive control panel.

Conference paper

Mahmoud K, Harris I, Yassin H, Hurkxkens TJ, Matar OK, Bhatia N, Kalkanis Iet al., 2020, Does immersive VR increase learning gain when compared to a non-immersive VR learning experience?, International Conference on Human-Computer Interaction, Publisher: Springer International Publishing, Pages: 480-498, ISSN: 0302-9743

Currently, computer assisted learning and multimedia form a key part of teaching. Interactivity and feedback are valuable in promoting active as opposed to passive learning. The study is conducted as an assessment of the impact of immersive VR on learning gain compared with a non-immersive video capture of VR, with a primary research question focusing on exploring learning gain and a secondary question exploring user experience, whereby understanding this is paramount to recognizing how to achieve a complete and effective learning experience. The study found immersive VR to significantly increase learning gain whilst two key measures of reported experience; enjoyment and concentration, also appeared significantly higher for the immersive VR learners. The study suggests extensive avenues for further research in this growing field, recognizing the need to appeal to a variety of students’ learning preferences. For educators, the relevance of self-directed and student-centered learning to enable active learning in the immersive tool is highlighted. Findings of such VR-based studies can be applied across several disciplines, including medical education; providing opportunity for users to learn without real-world consequences of error such as in surgical intervention.

Conference paper

Xie Z, Pavlidis D, Salinas P, Matar O, Pain Cet al., 2020, A control volume finite element method for three‐dimensional three‐phase flows, International Journal for Numerical Methods in Fluids, Vol: 92, Pages: 765-784, ISSN: 0271-2091

A novel control volume finite element method with adaptive anisotropic unstructured meshes is presented for three‐dimensional three‐phase flows with interfacial tension. The numerical framework consists of a mixed control volume and finite element formulation with a new P1DG‐P2 elements (linear discontinuous velocity between elements and quadratic continuous pressure between elements). A “volume of fluid” type method is used for the interface capturing, which is based on compressive control volume advection and second‐order finite element methods. A force‐balanced continuum surface force model is employed for the interfacial tension on unstructured meshes. The interfacial tension coefficient decomposition method is also used to deal with interfacial tension pairings between different phases. Numerical examples of benchmark tests and the dynamics of three‐dimensional three‐phase rising bubble, and droplet impact are presented. The results are compared with the analytical solutions and previously published experimental data, demonstrating the capability of the present method.

Journal article

Lei Q, Jackson MD, Muggeridge AH, Salinas P, Pain CC, Matar OK, Årland Ket al., 2020, Modelling the reservoir-to-tubing pressure drop imposed by multiple autonomous inflow control devices installed in a single completion joint in a horizontal well, Journal of Petroleum Science and Engineering, Vol: 189, Pages: 1-16, ISSN: 0920-4105

Autonomous inflow control devices (AICDs) are used to introduce an additional pressure drop between the reservoir and the tubing of a production well that depends on the fluid phase flowing into the device: a larger pressure drop is introduced when unwanted phases such as water or gas enter the AICD. The additional pressure drop is typically represented in reservoir simulation models using empirical relationships fitted to experimental data for a single AICD. This approach may not be correct if each completion joint is equipped with multiple AICDs as the flow at different AICDs may be different. We use high-resolution numerical modelling to determine the total additional pressure drop introduced by two AICDs installed in a single completion joint in a horizontal well. The model captures the multiphase flow of oil and water through the inner annulus into each AICD. We explore a number of relevant oil-water inflow scenarios with different flow rates and water cuts. Our results show that if only one AICD is installed, the additional pressure drop is consistent with the experimentalzly-derived empirical formulation. However, if two AICDs are present, there is a significant discrepancy between the additional pressure drop predicted by the simulator and the empirical relationship. This discrepancy occurs because each AICD has a different total and individual phase flow rate, and the final steady-state flow results from a self-organising mechanism emerging from the system. We report the discrepancy as a water cut-dependent correction to the empirical equation, which can be used in reservoir simulation models to better capture the pressure drop across a single completion containing two AICDs. Our findings highlight the importance of understanding how AICDs modify flow into production wells, and have important consequences for improving the representation of advanced wells in reservoir simulation models.

Journal article

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