Imperial College London

Professor Omar K. Matar, FREng

Faculty of EngineeringDepartment of Chemical Engineering

Head of Department of Chemical Engineering



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




Miss Nazma Mojid +44 (0)20 7594 3918




305 ACEACE ExtensionSouth Kensington Campus





Publication Type

319 results found

Municchi F, El Mellas I, Matar OK, Magnini Met al., 2022, Conjugate heat transfer effects on flow boiling in microchannels, International Journal of Heat and Mass Transfer, Vol: 195, Pages: 123166-123166, ISSN: 0017-9310

This article presents a computational study of saturated flow boiling in non-circular microchannels. The unit channel of a multi-microchannel evaporator, consisting of the fluidic channel and surrounding evaporator walls, is simulated and the conjugate heat transfer problem is solved. Simulations are performed using OpenFOAM v2106 and the built-in geometric Volume Of Fluid method, augmented with self-developed libraries to include liquid-vapour phase-change and improve the surface tension force calculation. A systematic study is conducted by employing water at atmospheric pressure, a channel hydraulic diameter of µm, a uniform base heat flux of , and by varying the channel width-to-height aspect-ratio and channel fin thickness in the range –4 and , respectively. The effects of conjugate heat transfer and channel aspect-ratio on the bubble and evaporative film dynamics, heat transfer, and evaporator temperature are investigated in detail. This study reveals that, when the flow is single-phase, higher Nusselt numbers and lower evaporator base temperatures are achieved for smaller channel aspect-ratios, from and when , to and when , for same fin thickness . In the two-phase flow regime, Nusselt numbers in the range are achieved. The trends of the Nusselt number versus the aspect-ratio are non-monotonic and exhibit a marked dependence on the channel fin thickness. For small fin thicknesses, and , an overall ascending trend of for increasing aspect-ratios is apparent, although in the narrower range –2 the Nusselt number appears weakly dependent on . For thicker fins, and , the Nusselt number decreases slightly when increasing the aspect-ratio in the range –2, although this trend is not monotonic when considering the entire range of aspect-ratios investigated. Nonetheless, due to conjugate heat transfer, Nusselt numbers and evaporator base temperatures follow different trends when varying the aspect-ratio, and channels with seem to promo

Journal article

Gonsalves GFN, Matar OK, 2022, Mechanistic modelling of two-phase slug flows with deposition, Chemical Engineering Science, Vol: 259, ISSN: 0009-2509

Despite the large quantity of works dedicated to the analysis and modelling of deposition in single-phase flows, very few models have been proposed for deposition of solids in two-phase pipe flows of gas and liquid. A comprehensive mechanistic model for transient, multiphase pipe flow with phase change is proposed, which takes into account effects of deposition by mass diffusion, ageing, and shearing of the deposit layer. Validation is performed with an exhaustive database of experimental data from the literature, for steady and transient flows without deposition, and deposit thickness measurements for single-phase, slug and stratified flows. Most of the predictions are within a 20% error band from the experimental data, with slightly worse performance in the case of deposition in slug flows. A surrogate model is also developed based on active-learning sampling of the simulator results, demonstrating a technique that could be used for optimization or design of engineering devices.

Journal article

Zhuang Y, Cheng S, Kovalchuk N, Simmons M, Matar OK, Guo Y-K, Arcucci Ret al., 2022, Ensemble latent assimilation with deep learning surrogate model: application to drop interaction in a microfluidics device, Lab on a Chip: miniaturisation for chemistry, physics, biology, materials science and bioengineering, Vol: 22, Pages: 3187-3202, ISSN: 1473-0189

A major challenge in the field of microfluidics is to predict and control drop interactions. This work develops an image-based data-driven model to forecast drop dynamics based on experiments performed on a microfluidics device. Reduced-order modelling techniques are applied to compress the recorded images into low-dimensional spaces and alleviate the computational cost. Recurrent neural networks are then employed to build a surrogate model of drop interactions by learning the dynamics of compressed variables in the reduced-order space. The surrogate model is integrated with real-time observations using data assimilation. In this paper we developed an ensemble-based latent assimilation algorithm scheme which shows an improvement in terms of accuracy with respect to the previous approaches. This work demonstrates the possibility to create a reliable data-driven model enabling a high fidelity prediction of drop interactions in microfluidics device. The performance of the developed system is evaluated against experimental data (i.e., recorded videos), which are excluded from the training of the surrogate model. The developed scheme is general and can be applied to other dynamical systems.

Journal article

Chagot L, Quilodran-Casas C, Kalli M, Kovalchuk NM, Simmons MJH, Matar OK, Arcucci R, Angeli Pet al., 2022, Surfactant-laden droplet size prediction in a flow-focusing microchannel: a data-driven approach, LAB ON A CHIP, ISSN: 1473-0197

Journal article

Botsas T, Pan I, Mason LR, Matar OKet al., 2022, Multiphase flow applications of nonintrusive reduced-order models with Gaussian process emulation, Data-Centric Engineering, Vol: 3, ISSN: 2632-6736

Reduced-order models (ROMs) are computationally inexpensive simplifications of high-fidelity complex ones. Such models can be found in computational fluid dynamics where they can be used to predict the characteristics of multiphase flows. In previous work, we presented a ROM analysis framework that coupled compression techniques, such as autoencoders, with Gaussian process regression in the latent space. This pairing has significant advantages over the standard encoding–decoding routine, such as the ability to interpolate or extrapolate in the initial conditions’ space, which can provide predictions even when simulation data are not available. In this work, we focus on this major advantage and show its effectiveness by performing the pipeline on three multiphase flow applications. We also extend the methodology by using deep Gaussian processes as the interpolation algorithm and compare the performance of our two variations, as well as another variation from the literature that uses long short-term memory networks, for the interpolation.

Journal article

Hennessy MG, Craster R, Matar OK, 2022, Drying-induced stresses in poroelastic drops on rigid substrates, Physical Review E, Vol: 105, ISSN: 2470-0045

We develop a theory for drying-induced stresses in sessile, poroelastic drops undergoing evaporation on rigid surfaces. Using a lubrication-like approximation, the governing equations of three-dimensional nonlinear poroelasticity are reduced to a single thin-film equation for the drop thickness. We find that thin drops experience compressive elastic stresses but the total in-plane stresses are tensile. The mechanical response of the drop is dictated by the initial profile of the solid skeleton, which controls the in-plane deformation, the dominant components of elastic stress, and sets a limit on the depth of delamination that can potentially occur. Our theory suggests that the alignment of desiccation fractures in colloidal drops is selected by the shape of the drop at the point of gelation. We propose that the emergence of three distinct fracture patterns in dried blood drops is a consequence of a nonmonotonic drop profile at gelation. We also show that depletion fronts, which separate wet and dry solid, can invade the drop from the contact line and localize the generation of mechanical stress during drying. Finally, the finite element method is used to explore the stress profiles in drops with large contact angles.

Journal article

Nathanael K, Pico P, Kovalchuk NM, Lavino AD, Simmons MJH, Matar OKet al., 2022, Computational modelling and microfluidics as emerging approaches to synthesis of silver nanoparticles – A review, Chemical Engineering Journal, Vol: 436, Pages: 135178-135178, ISSN: 1385-8947

This review provides an integrated overview of the current state of knowledge for sustainable production of silver nanoparticles (AgNPs), focussing on recent advances in their synthesis using emerging microfluidic-based methods and computational modelling, their properties and practical applications. Special attention is given to the Finke-Watzky two-step kinetic model, which provides the best fitting for nucleation and growth of AgNPs and the multiple operating parameters that affect their physical and chemical properties. An overview of numerical simulations used to model the synthesis of AgNPs across different length and time scales is presented. Investigations made at the molecular scale via molecular dynamics (MD) simulations, at the meso- and macroscale via population balance modelling (PBM) and computational fluid dynamics (CFD), respectively are discussed, alongside data-driven modelling approaches. The review also identifies both limitations and advantages in exploiting the aforementioned techniques, offering a way forward for further investigations on the topic. A critical analysis of the literature leads to confirm that the combination of microfluidics-based synthesis, which enable reactions to be carried out under highly-controlled conditions, along with physics-driven simulations and data-driven models are a powerful tool to effectively link input information of the process and output data related to the properties of the AgNPs. This combined framework therefore provides an opportunity to improve the prediction accuracy of the whole cycle of synthesis of AgNPs and overcome the environmental impact and limitations of traditional methods.

Journal article

Heaney CE, Wolffs Z, Tómasson JA, Kahouadji L, Salinas P, Nicolle A, Navon IM, Matar OK, Srinil N, Pain CCet al., 2022, An AI-based non-intrusive reduced-order model for extended domains applied to multiphase flow in pipes, Physics of Fluids, Vol: 34, Pages: 1-22, ISSN: 1070-6631

The modeling of multiphase flow in a pipe presents a significant challenge for high-resolution computational fluid dynamics (CFD) models due to the high aspect ratio (length over diameter) of the domain. In subsea applications, the pipe length can be several hundreds of meters vs a pipe diameter of just a few inches. Approximating CFD models in a low-dimensional space, reduced-order models have been shown to produce accurate results with a speed-up of orders of magnitude. In this paper, we present a new AI-based non-intrusive reduced-order model within a domain decomposition framework (AI-DDNIROM), which is capable of making predictions for domains significantly larger than the domain used in training. This is achieved by (i) using a domain decomposition approach; (ii) using dimensionality reduction to obtain a low-dimensional space in which to approximate the CFD model; (iii) training a neural network to make predictions for a single subdomain; and (iv) using an iteration-by-subdomain technique to converge the solution over the whole domain. To find the low-dimensional space, we compare Proper Orthogonal Decomposition with several types of autoencoder networks, known for their ability to compress information accurately and compactly. The comparison is assessed with two advection-dominated problems: flow past a cylinder and slug flow in a pipe. To make predictions in time, we exploit an adversarial network, which aims to learn the distribution of the training data, in addition to learning the mapping between particular inputs and outputs. This type of network has shown the potential to produce visually realistic outputs. The whole framework is applied to multiphase slug flow in a horizontal pipe for which an AI-DDNIROM is trained on high-fidelity CFD simulations of a pipe of length 10 m with an aspect ratio of 13:1 and tested by simulating the flow for a pipe of length 98 m with an aspect ratio of almost 130:1. Inspection of the predicted liquid volume

Journal article

Abubakar HA, Matar OK, 2022, Linear stability analysis of Taylor bubble motion in downward flowing liquids in vertical tubes, Journal of Fluid Mechanics, Vol: 941, ISSN: 0022-1120

Taylor bubbles are a feature of the slug flow regime in gas–liquid flows in vertical pipes. Their dynamics exhibits a number of transitions such as symmetry breaking in the bubble shape and wake when rising in downward flowing and stagnant liquids, respectively, as well as breakup in sufficiently turbulent environments. Motivated by the need to examine the stability of a Taylor bubble in liquids, a systematic numerical study of a steadily moving Taylor bubble in stagnant and flowing liquids is carried out, characterised by the dimensionless inverse viscosity (Nf), Eötvös (Eo) and Froude numbers (Um), the latter being based on the centreline liquid velocity, using a Galerkin finite-element method. A boundary-fitted domain is used to examine the dependence of the steady bubble shape on a wide range of Nf, Eo and Um. Our analysis of the bubble nose and bottom curvatures shows that the intervals Eo=[20,30) and Nf=[60,80) are the limits below which surface tension and viscosity, respectively, have a strong influence on the bubble shape. In the interval Eo=(60,100], all bubble features studied are weakly dependent on surface tension. A linear stability analysis of the axisymmetric base states shows that there exist regions of (Nf,Eo,Um) space within which the bubble is unstable and assumes an asymmetric shape. To elucidate the mechanisms underlying the instability, an energy budget analysis is carried out which reveals that perturbation growth is driven by the bubble pressure for Eo≥100, and by the tangential interfacial stress for Eo<100. Examples of the asymmetric bubble shapes and their associated flow fields are also provided near the onset of instability for a wide range of Nf, Eo and Um.

Journal article

Kahouadji L, Batchvarov A, Adebayo IT, Jenkins Z, Shin S, Chergui J, Juric D, Matar OKet al., 2022, A numerical investigation of three-dimensional falling liquid films, Environmental Fluid Mechanics, Vol: 22, Pages: 367-382, ISSN: 1567-7419

In this article, we present a full three-dimensional numerical study of thin liquid films falling on a vertical surface, by solving the full three-dimensional Navier–Stokes equations with a hybrid front-tracking/level-set method for tracking the interface. General falling film flow applications span across many types of process industries but also occur in a multitude of natural and environmental applications such as ice sheets, glaciology and even volcanic lava flows. In this study, we propose three configurations of falling films. Two of them, with small and moderate Reynolds number, are set to mimic pulsed and forced falling film types inside a minimum periodic domain, able to cover entirely the temporal evolution of a single wave. The latest example, corresponding to a high Reynolds number, is initialised with a flat interface without any specific perturbations. For the first time, this study highlights the natural transition from a non-deformed interface to its first streamwise disturbance (two-dimensional wavy flow), and then a second spanwise wave disturbance (three-dimensional wavy flow).

Journal article

Li W, Zhou H, Zhao K, Jin J, Chen X, Wakil Shahzad M, Jiang Y, Matar OK, Dai S, Bin Xu B, Sun Let al., 2022, Conformally anodizing hierarchical structure in a deformed tube towards energy-saving liquid transportation, Chemical Engineering Journal, Vol: 431, Pages: 1-7, ISSN: 1385-8947

The creation of drag-reducing surfaces in deformed tubes is of vital importance to thermal management, energy, and environmental applications. However, it remains a great challenge to tailor the surface structure and wettability inside the deformed tubes of slim and complicated feature. Here, we describe an electrochemical anodization strategy to achieve uniform and superhydrophobic coating of TiO2 nanotube arrays throughout the inner surface in deformed/bend titanium tubes. Guided by a hybrid carbon fibre cathode, conformal electric field can be generated to adaptatively fit the complex geometries in the deformed tube, where the structural design with rigid insulating beads can self-stabilize the hybrid cathode at the coaxial position of the tube with the electrolyte flow. As a result, we obtain a superhydrophobic coating with a water contact angle of 157° and contact angle hysteresis of less than 10°. Substantial drag reduction can be realised with an overall reduction up to 25.8 % for the anodized U-shaped tube. Furthermore, we demonstrate to spatially coat tubes with complex geometries, to achieve energy-saving liquid transportation. This facile coating strategy has great implications in liquid transport processes with the user-friendly approach to engineer surface regardless of the deformation of tube/pipe.

Journal article

Pan I, Mason LR, Matar OK, 2022, Data-centric engineering: integrating simulation, machine learning and statistics. challenges and opportunities, Chemical Engineering Science, Vol: 249, ISSN: 0009-2509

Recent advances in machine learning, coupled with low-cost computation, availability of cheap streaming sensors, data storage and cloud technologies, has led to widespread multi-disciplinary research activity with significant interest and investment from commercial stakeholders. Mechanistic models, based on physical equations, and purely data-driven statistical approaches represent two ends of the modelling spectrum. New hybrid, data-centric engineering approaches, leveraging the best of both worlds and integrating both simulations and data, are emerging as a powerful tool with a transformative impact on the physical disciplines. We review the key research trends and application scenarios in the emerging field of integrating simulations, machine learning, and statistics. We highlight the opportunities that such an integrated vision can unlock and outline the key challenges holding back its realisation. We also discuss the bottlenecks in the translational aspects of the field and the long-term upskilling requirements for the existing workforce and future university graduates.

Journal article

Lew JH, Matar OK, Müller EA, Maung MTM, Luckham PFet al., 2022, Adsorption of hydrolysed polyacrylamide onto calcium carbonate, Polymers, Vol: 14, Pages: 405-405, ISSN: 2073-4360

Carbonate rock strengthening using chemical techniques is a strategy to prevent excessive fines migration during oil and gas production. We provide herein a study of the adsorption of three types of hydrolysed polyacrylamide (HPAM) of different molecular weight (F3330S, 11–13 MDa; F3530 S, 15–17 MDa; F3630S, 18–20 MDa) onto calcium carbonate (CaCO3) particles via spectrophotometry using a Shimadzu UV-2600 spectrometer. The results are compared to different adsorption isotherms and kinetic models. The Langmuir isotherm shows the highest correlation coefficient (R2 &gt; 0.97) with equilibrium parameters (RL) ranging between 0 and 1 for all three HPAMs, suggesting a favorable monolayer adsorption of HPAM onto CaCO3. The adsorption follows pseudo-second order kinetics, indicating that the interaction of HPAM with CaCO3 is largely dependent on the adsorbate concentration. An adsorption plot reveals that the amount of HPAM adsorbed onto CaCO3 at equilibrium increases with higher polymer molecular weight; the equilibrium adsorbed values for F3330S, F3530S and F3630S are approximately 0.24 mg/m2, 0.31 mg/m2, and 0.43 mg/m2, respectively. Zeta potential analysis shows that CaCO3 has a zeta potential of +12.32 mV, which transitions into negative values upon introducing HPAM. The point of zero charge (PZC) is observed at HPAM dosage between 40 to 50 ppm, in which the pH here lies between 9–10.

Journal article

Valdés JP, Kahouadji L, Matar OK, 2022, Current advances in liquid–liquid mixing in static mixers: A review, Chemical Engineering Research and Design, Vol: 177, Pages: 694-731, ISSN: 0263-8762

This review article revisits the role of static mixers in the process industry nowadays and summarizes the most relevant developments and literature available on this type of mixers handling liquid-phase systems. In particular, this review seeks to discuss in depth the progress that has been made on the hydrodynamic understanding of immiscible liquid–liquid dispersions and emulsion formation using motionless types of mixers, both through experimental and computational approaches. Models and correlations on key process parameters, such as mean droplet size and pressure drop, proposed over the last couple of decades, are compiled and discussed. The latest progress on computational modelling through numerous frameworks is also thoroughly covered. In addition, this paper includes a brief review of the fundamental concepts in liquid static mixing and emulsion formation to further enrich the discussion on the innovations made on this field.

Journal article

Moran H, Pervunin K, Matar O, Markides Cet al., 2021, Laser-based diagnostics of slug flow boiling in a horizontal pipe, Interfacial Phenomena and Heat Transfer, Vol: 9, Pages: 27-41, ISSN: 2169-2785

We present results from an experimental investigation on flow boiling, in the slug flow regime, of refrigerant R245fa through a 12.6-mm inner diameter horizontal plain pipe using particle image velocimetry (PIV) and an interface detection method. The study is supplemented by an overview of the state-of-the-art in experimental research of two-phase dispersed pipe flows and the development of modern optical and laser-based full-field non-intrusive measurement techniques as applied to these flows. We consider different flow conditions, with heat fluxes over the range 5.3 to 7.9 kW/m2 and mass fluxes from 300 to 460 kg/m2•s. Significant disturbances in the instantaneous velocity fields are revealed in both the noses and tails of slugs, with their values being two times higher behind vapour bubbles. The slug passage frequency is determined based on the results of the interface detection method. The vapour bubble velocity is found to increase linearly with the interfacial velocity of the two-phase mixture, while its gradient grows with the heat flux. Moreover, at increased heat fluxes the bubbles may move even faster than the mixture itself, which implies that they must significantly enhance local turbulence, thereby additionally intensifying heat transfer. In addition to the conclusions, we provide practical recommendations for possible future research in this particular field of fluid mechanics and the further development of sophisticated laser-based measurement techniques for boiling, and similar, flows.

Journal article

Filali A, Khezzar L, Semmari H, Matar Oet al., 2021, Application of artificial neural network for mixed convection in a square lid-driven cavity with double vertical or horizontal oriented rectangular blocks, International Communications in Heat and Mass Transfer, Vol: 129, ISSN: 0735-1933

The practicability of using Artificial Neural Network (ANN) to predict the thermal behaviour due to mixed convection is established. Numerical simulations are conducted first for a laminar mixed convection problem in a lid-driven square cavity with two internal rectangular blocks, oriented vertically or horizontally. CFD results are used for training and testing the ANN to predict new cases; thus, saving effort and computation time and validate the obtained numerical results of Nusselt number. A wide range of Reynolds (100 ≤ Re ≤ 1500), Grashof numbers (1.5 × 104 ≤ Gr ≤ 105), Richardson number (0.00667 ≤ Ri ≤ 10) and the distance between the two blocks (0.2 ≤ W/L ≤ 0.8) are considered. Results indicated that varying the distance W/L has an important influence on the Nusselt number. It was observed that increasing Re and Gr numbers magnitudes leads to an increased Nusselt number and that Nusselt number obtained for the case of vertical blocks is higher than the case of horizontal blocks. Furthermore, the maximum Nu number obtained for the vertical blocks was at W/L = 0.5 and Ri = 0.044 and for the horizontal blocks case was at W/L = 0.2 and Ri = 0.044. Finally, new correlations of Nu number versus Re, Gr and the spacing ratio between the two blocks W/L are derived for possible utilization in engineering design.

Journal article

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, Pages: 175-178

Direct steam generation (DSG) for concentrating solar power (CSP) is an emerging technology that can unlock new avenues for efficient and affordable energy utilisation and expand the current capabilities of CSP. However, the direct evaporation of steam in parabolic trough solar collectors presents control and operational challenges due to the inherently complex two-phase nature and inherent unsteadiness of the boiling flow inside the tubes. Thus, a fundamental understanding of the hydrodynamic and heat transfer characteristics of this two-phase flow is required. A bespoke flow boiling facility has been constructed for the investigation of boiling flows using laser diagnostic techniques to gain accurate spatio-temporally resolved information on the flow characteristics. The facility has been validated and a proof of concept of the application of particle image velocimetry to boiling flows performed.

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

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