Publications
349 results found
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.
Moran HR, Magnini M, Markides CN, et 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
Williams AGL, Karapetsas G, Mamalis D, et 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.
Batchvarov A, Kahouadji L, Constante-Amores CR, et 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.
Ajaev V, Gambaryan-Roisman T, Davalos-Orozco LA, et al., 2021, Preface to special issue: heat transfer, waves, and vortext phenomena in two-phase flows, Interfacial phenomena and heat transfer, Vol: 9, Pages: V-VI, ISSN: 2169-2785
PAUL S, HSU W-L, MAGNINI M, et 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.
Padrino JC, Srinil N, Kurushin V, et al., 2021, A ONE-DIMENSIONAL MECHANISTIC MODEL FOR TRACKING UNSTEADY SLUG FLOW, ASME International Mechanical Engineering Congress and Exposition (IMECE), Publisher: AMER SOC MECHANICAL ENGINEERS
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Paul S, Hsu W-L, Magnini M, et 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.
Inguva P, Mason LR, Pan I, et al., 2020, Numerical simulation, clustering, and prediction of multicomponent polymer precipitation, Data-Centric Engineering, Vol: 1
<jats:title>Abstract</jats:title> <jats:p>Multicomponent polymer systems are of interest in organic photovoltaic and drug delivery applications, among others where diverse morphologies influence performance. An improved understanding of morphology classification, driven by composition-informed prediction tools, will aid polymer engineering practice. We use a modified Cahn–Hilliard model to simulate polymer precipitation. Such physics-based models require high-performance computations that prevent rapid prototyping and iteration in engineering settings. To reduce the required computational costs, we apply machine learning (ML) techniques for clustering and consequent prediction of the simulated polymer-blend images in conjunction with simulations. Integrating ML and simulations in such a manner reduces the number of simulations needed to map out the morphology of polymer blends as a function of input parameters and also generates a data set which can be used by others to this end. We explore dimensionality reduction, via principal component analysis and autoencoder techniques, and analyze the resulting morphology clusters. Supervised ML using Gaussian process classification was subsequently used to predict morphology clusters according to species molar fraction and interaction parameter inputs. Manual pattern clustering yielded the best results, but ML techniques were able to predict the morphology of polymer blends with ≥90% accuracy.</jats:p>
Constante-Amores CR, Kahouadji L, Batchvarov A, et 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.
Nazareth RK, Karapetsas G, Sefiane K, et 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.
Farooq U, Stafford J, Petit C, et 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.
Batchvarov A, Kahouadji L, Magnini M, et 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.
Gonçalves GFN, Batchvarov A, Liu Y, et al., 2020, Data-driven surrogate modeling and benchmarking for process equipment, Data-Centric Engineering, Vol: 1, ISSN: 2632-6736
In chemical process engineering, surrogate models of complex systems are often necessary for tasks of domain exploration, sensitivity analysis of the design parameters, and optimization. A suite of computational fluid dynamics (CFD) simulations geared toward chemical process equipment modeling has been developed and validated with experimental results from the literature. Various regression-based active learning strategies are explored with these CFD simulators in-the-loop under the constraints of a limited function evaluation budget. Specifically, five different sampling strategies and five regression techniques are compared, considering a set of four test cases of industrial significance and varying complexity. Gaussian process regression was observed to have a consistently good performance for these applications. The present quantitative study outlines the pros and cons of the different available techniques and highlights the best practices for their adoption. The test cases and tools are available with an open-source license to ensure reproducibility and engage the wider research community in contributing to both the CFD models and developing and benchmarking new improved algorithms tailored to this field.
Constante-Amores CR, Kahouadji L, Batchvarov A, et 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.
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.
Mahmoud K, Harris I, Yassin H, et 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.
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.
Xie Z, Pavlidis D, Salinas P, et 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.
Lei Q, Jackson MD, Muggeridge AH, et 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.
Magnini M, Matar OK, 2020, Numerical study of the impact of the channel shape on microchannel boiling heat transfer, International Journal of Heat and Mass Transfer, Vol: 150, Pages: 1-16, ISSN: 0017-9310
Flow boiling in multi-microchannel evaporators is recognised as one of the most efficient cooling solutions for high-performance electronics, and has therefore received increasing attention during the recent years. Despite the extensive literature, there is no general agreement yet about the effect of the channel cross-sectional shape on the boiling heat transfer performance, which results on a limited availability of thermal design guidelines and tools. This article presents the results of a systematic analysis of the impact of the channel shape on the bubble dynamics and heat transfer, under flow boiling conditions. Simulations are carried out using a customised version of OpenFOAM, and the Volume-Of-Fluid method is chosen to capture the liquid-vapour interface dynamics. A benchmark flow model is utilised, where a single isolated bubble is seeded at the channel upstream and transported by a liquid flow across the diabatic section, which is heated by a constant and uniform heat flux. Flow conditions that apply well to the flow boiling of water or refrigerant fluids in sub-millimetre channels at low heat flux ( ~ 10 kW/m2) are investigated, with cross-section width-to-height aspect-ratios ranging from 1 to 8, while the hydraulic diameter of the channel is fixed. This study emphasises that the heat transfer performances for different channel shapes are closely related to the perimetral distribution of the liquid film surrounding the very long bubbles. Square channels exhibit the highest heat transfer coefficients at low flow rates, due to a very thin liquid film that forms at the centre of the wall, but are more at risk of film dryout. High aspect-ratio rectangular channels may be beneficial at larger flow rates, as they promote the formation of an extended liquid film that covers up to 80 % of the cross-section perimeter. At larger aspect-ratios, the average heat transfer coefficient along the shorter wall becomes orders of magnitude smaller than the v
Zanganeh H, Kurushina V, Srinil N, et al., 2020, Influence of combined empirical functions on slug flow predictions of pipelines with variable inclinations
Prediction of internal multiphase flows in subsea pipelines is an integral part of the oil and gas production system design. High mass and pressure fluctuations are often encountered during the operation with a liquid-gas slug flow regime exhibiting a sequence of long gas bubbles and aerated liquid slugs. It is important for industry to realistically identify the slug flow occurrence and predict slug flow characteristics, depending on several multiphase flow-pipe parameters. These may be achieved using a one-dimensional, steady-state, mechanistic model accounting for a mass and momentum balance of the two liquid-gas fluids within a controlled volume often referred to as a slug unit. By reducing a 3-D flow problem to a 1-D one, several empirical or closure correlations and associated empirical coefficients have been introduced in the literature and used in commercial software predicting slug flows in subsea jumpers, pipelines and risers with variable inclinations. This study aims to investigate the influence of combined 25 closure functions on the predictions of slug flows in horizontal and inclined pipes based on a steady-state mechanistic model for a wide range of superficial liquid and gas velocities. The model with studied closures is implemented by the authors of this study as the numerical tool iSLUG. The model performance is verified with respect to the estimated film liquid holdup, film length and pressure drop per length of a slug unit for an empirically specified translational velocity, slug liquid holdup, slug liquid length and pipe wall wettability. Closure combinations are analyzed using the relative performance factors and compared against available experimental data in order to identify a set of functions suitable for upward, downward and horizontal flows, and the effect of diameter and inclination on the model prediction is considered. The present method and analysis outcomes may further contribute to the improvement of transient liquid-gas flow model
Matar OK, Angeli P, Kawaji M, 2020, Remembering Geoff Hewitt, International Journal of Multiphase Flow, Vol: 122, Pages: 1-2, ISSN: 0301-9322
Aksan N, Andreani M, Bechta S, et al., 2019, The life and the contribution of B. R. Sehgal, G. Yadigaroglu and G. Hewitt: Remembrance statements Preface, NUCLEAR ENGINEERING AND DESIGN, Vol: 354, ISSN: 0029-5493
Magnini M, Matar OK, 2019, Fundamental study of wax deposition in crude oil flows in a pipeline via interface-resolved numerical simulations, Industrial & Engineering Chemistry Research, Vol: 58, Pages: 21797-21816, ISSN: 0888-5885
This work presents a fundamental analysis of the mechanisms governing wax deposition and removal in crude oil transportation pipelines. We utilize a numerical framework where oil and deposit are treated as two immiscible phases, and the volume-of-fluid (VOF) method is adopted to resolve the unsteady dynamics of the free interface. Deposition is modeled locally at the oil–deposit interface via a chemical equilibria model, here adapted to the VOF method. Deposit ageing is included via a thixotropic rheological model. The results emphasize that the deposit pattern may appear as a uniform axisymmetric film covering the pipe wall or be completely stratified. Although different mechanisms of deposit mobilization may occur, the removal rates correlate well with the Reynolds number of the bulk flow and the viscosity of the deposit layer. The simulation data are used to benchmark closure laws for the velocity and temperature within the film, and a prediction method for the steady-state deposit thickness is proposed.
Russell AW, Kahouadji L, Mirpuri K, et al., 2019, Mixing viscoplastic fluids in stirred vessels over multiple scales: An experimental and CFD approach, Chemical Engineering Science, Vol: 208, ISSN: 1873-4405
Dye visualisation techniques and CFD are used to characterise the flow of viscoplastic CarbopolTM solutions in stirred vessel systems over multiple scales. Centrally-mounted, geometrically-similar Rushton turbine (RT) impellers are used to agitate various Carbopol 980 (C980) fluids. The dimensionless cavern diameters, Dc/D, are scaled against a combination of dimensionless parameters: Rem-0.3Rey0.6n-0.1ks-1, where Rem, Rey, n and ks are the modified power-law Reynolds number, yield stress Reynolds number, flow behaviour index and impeller geometry constant, respectively. Excellent collapse of the data is demonstrated for the fluids and flows investigated. Additional data are collected using a pitched-blade turbine (PBT) with cavern size similarity being shown between the RT and PBT datasets. These results are important in the context of scale-up/scale-down mixing processes in stirred vessels containing complex fluids and can be used to show that flow similarity can be achieved in these systems if the processes are scaled appropriately.
Theodorakis PE, Smith ER, Craster RV, et al., 2019, Molecular dynamics simulation of the super spreading of surfactant-laden droplets. A review, Fluids, Vol: 4, Pages: 1-23, ISSN: 2311-5521
Superspreading is the rapid and complete spreading of surfactant-laden droplets on hydrophobic substrates. This phenomenon has been studied for many decades by experiment, theory, and simulation, but it has been only recently that molecular-level simulation has provided significant insights into the underlying mechanisms of superspreading thanks to the development of accurate force-fields and the increase of computational capabilities. Here, we review the main advances in this area that have surfaced from Molecular Dynamics simulation of all-atom and coarse-grained models highlighting and contrasting the main results and discussing various elements of the proposed mechanisms for superspreading. We anticipate that this review will stimulate further research on the interpretation of experimental results and the design of surfactants for applications requiring efficient spreading, such as coating technology.
González-Garay A, Pozo C, Galán-Martín Á, et al., 2019, Assessing the performance of UK universities in the field of chemical engineering using data envelopment analysis, Education for Chemical Engineers, Vol: 29, Pages: 29-41, ISSN: 1749-7728
University rankings have become an important tool to compare academic institutions within and across countries. Yet, they rely on aggregated scores based on subjective weights which render them sensitive to experts’ preferences and not fully transparent to final users. To overcome this limitation, we apply Data Envelopment Analysis (DEA) to evaluate UK universities in the field of chemical engineering as a case study, using data retrieved from two national rankings. DEA is a non-parametric approach developed for the multi-criteria assessment of entities that avoids the use of subjective weightings and aggregated scores; this is accomplished by calculating an efficiency index, on the basis of which universities can be classified as either ‘efficient’ or ‘inefficient’. Our analysis shows that the Higher Education Institutions (HEI) occupying the highest positions in the chemical engineering rankings might not be the most efficient ones, and vice versa, which highlights the need to complement the use of rankings with other analytical tools. Overall, DEA provides further insight into the assessment of HEIs, allowing institutions to better understand their weaknesses and strengths, while pinpointing sources of inefficiencies where improvement efforts must be directed.
Shaffee SNA, Luckham PF, Matar OK, et al., 2019, Numerical investigation of sand-screen performance in the presence of adhesive effects for enhanced sand control, SPE Journal, Vol: 24, Pages: 2195-2208, ISSN: 1086-055X
In many industrial processes, an effective particle-filtration system is essential for removing unwanted solids. The oil and gas industry has explored various technologies to control and manage excessive sand production, such as by installing sand screens or injecting consolidation chemicals in sand-prone wells as part of sand-management practices. However, for an unconsolidated sandstone formation, the selection and design of effective sand control remains a challenge. In recent years, the use of a computational technique known as the discrete-element method (DEM) has been explored to gain insight into the various parameters affecting sand-screen-retention behavior and the optimization of various types of sand screens (Mondal et al. 2011, 2012, 2016; Feng et al. 2012; Wu et al. 2016).In this paper, we investigate the effectiveness of particle filtration using a fully coupled computational-fluid-dynamics (CFD)/DEM approach featuring polydispersed, adhesive solid particles. We found that an increase in particle adhesion reduces the amount of solid in the liquid filtrate that passes through the opening of a wire-wrapped screen, and that a solid pack of particle agglomerates is formed over the screen with time. We also determined that increasing particle adhesion gives rise to a decrease in packing density and a diminished pressure drop across the solid pack covering the screen. This finding is further supported by a Voronoi tessellation analysis, which reveals an increase in porosity of the solid pack with elevated particle adhesion. The results of this study demonstrate that increasing the level of particle agglomeration, such as by using an adhesion-promoting chemical additive, has beneficial effects on particle filtration. An important application of these findings is the design and optimization of sand-control processes for a hydrocarbon well with excessive sand production, which is a major challenge in the oil and gas industry.
Lapidot T, Matar OK, Heng JYY, 2019, Calcium sulphate crystallisation in the presence of mesoporous silica particles: experiments and population balance modelling, Chemical Engineering Science, Vol: 202, Pages: 238-249, ISSN: 0009-2509
A population balance model is used to investigate the effect of mesoporous silica particles on calcium sulphate crystallisation in a stirred batch crystalliser. The model accounts for nucleation, growth, agglomeration, breakage, and particle-assisted nucleation, and the model equations are solved numerically using the method of classes over a logarithmic, non-uniform mesh. The crystallisation process is characterized experimentally using electrical conductivity to track the ion concentration and laser diffraction to measure the steady-state crystal size distribution obtained at the end of the experiments. The experiments are carried out over a range of temperatures, initial supersaturations, particle pore diameters, and particle loadings. The model is first fitted to experimental data obtained in the absence of particles to determine kinetic parameters of the nucleation, growth, agglomeration, and breakage for pure calcium sulphate crystallisation. Varying pore diameter did not influence the catalytic effect of the particles, however, particle loading was found to significantly decrease the nucleation induction time. The model was extended to account for the presence of particles by fitting two additional mechanisms. The first proposed a particle-assisted nucleation where nuclei are produced via heterogeneous crystallisation, then detach by particle-particle collision that is second-order with respect to particle loading. The second proposed that the crystal breakage frequency increases linearly with particle loading. Good agreement with the experimental data is demonstrated over the range of conditions examined.
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