279 results found
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, ISSN: 0022-1120
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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
Nazareth RK, Karapetsas G, Sefiane K, et al., 2020, Stability of slowly evaporating thin liquid films of binary mixtures, Physical Review Fluids, Vol: 5
© 2020 American Physical Society. 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, ISSN: 2470-0045
© 2020 American Physical Society. 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.
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.
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
Bhatia N, Müller EA, Matar O, 2020, A GPU Accelerated Lennard-Jones System for Immersive Molecular Dynamics Simulations in Virtual Reality, Pages: 19-34, ISSN: 0302-9743
© 2020, Springer Nature Switzerland AG. 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($$10^4$$) and the force models imply a pairwise calculation which scales, in case of a Lennard-Jones system, to the order of O($$N^2$$) 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.
Mahmoud K, Harris I, Yassin H, et al., 2020, Does immersive vr increase learning gain when compared to a non-immersive vr learning experience?, Pages: 480-498, ISSN: 0302-9743
© Springer Nature Switzerland AG 2020. 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.
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.
Voulgaropoulos V, Aguiar GM, Matar OK, et al., 2019, Temperature and velocity field measurements of pool boiling using two-colour laser-induced fluorescence, infrared thermometry and particle image velocimetry, 10th International Conference on Multiphase Flow
We study nucleate pool boiling in water at saturation temperature and ambient pressure under low heat fluxes. A combinationof high-speed and spatially-resolved diagnostic tools are developed and applied to provide detailed insight into the flow andheat transfer mechanisms during bubble life cycle. Two fluorescent dyes with non-overlapping spectra are seeded into thewater and are excited by a Nd:YLF laser sheet at 527 nm. A two-colour laser-induced fluorescence method is employedto individually track the fluorescence of each dye by connecting two cameras, equipped with separate optical filters, to abeamsplitter and a lens. Tracer particles are also introduced in the water to perform simultaneous particle image velocimetrymeasurements. Finally, synchronised high-speed infrared thermometry is conducted to acquire the surface temperature fieldover the heater. The links between the interfacial/bubble dynamics, flow and heat transfer are investigated. Superheated liquidfrom the thermal boundary layer adjacent to the heater is displaced upwards, due to the growth and departure of the bubbles.Two counteracting vortices form on each side of the bubbles during their departure and rise, which contribute to the scavengingand mixing of the bulk water, resulting in a trail of superheated liquid below them.
Moran H, Magnini M, Markides C, et al., 2019, Inertial and buoyancy effects on horizontal flow of elongated bubbles in circular channels, ICMF 2019, Publisher: ICMF
The effects of gravity and inertia on the liquid film thickness surrounding elongated bubble flow in a horizontal tube of circularcross-section are studied through numerical simulations. At low Reynolds and Bond numbers, the inertial and buoyancy effectsare negligible and the liquid film thickness at the tube wall is a function of the Capillary number only; if tube diameter isincreased to the millimetre scale, however, buoyancy forces become significant. Simulations are performed with OpenFOAM(version 1606) and the built in Volume-of-Fluid method for a range of Reynolds, Bond and Capillary numbers, namelyRe= 1−1000,Bo= 0.05−0.42andCa= 0.02−0.09respectively. Two-dimensional simulations capture asymmetry ofthe liquid film thickness due to gravitational effects, but do not capture bubble inclination relative to the channel centreline,as has been demonstrated experimentally in the literature. Three-dimensional simulations capture the transverse flow of thefilm as it drains from the top to the bottom of the tube, and are thus able to demonstrate bubble inclination. Further simulationsthat introduce phase change to the elongated bubble model are underway, aiming to investigate boiling flows, with experimentsbeing performed for comparison and validation.
Aguiar GM, Voulgaropoulos V, Matar OK, et al., 2019, Experimental investigation of bubble nucleation, growth and departure using synchornized IR thermometry, two-colour LIF and PIV, 18th International Topical Meeting on Nuclear Reactor Thermal Hydraulics - NURETH, Publisher: American Nuclear Society (ANS)
Boiling is a very effectiveheat removal process exploited in many applications, from electronic devicesto nuclear reactors. However, the physical mechanisms involved in this process are not fully understood yet, due toits complexity, whicharises from the many interacting sub-processes involved in the nucleation, growth, and detachment of isolated bubbles. Here, we present the methodology and initialresults from an experimental investigation aimed at elucidating and quantifying the mechanisms involved in a bubble life cycle (fromnucleation until departure). Towards this aim, we use synchronized high-speed infrared(IR)thermometry, ratiometric two-color laser-induced fluorescence (2cLIF) and particle image velocimetry (PIV). Infrared thermometry is used to measure the time-dependent temperature and heat flux distributions overthe boilingsurface, which are usefulto quantify the transfer of energy associated with the evaporation of the micro-layer. Two-color laser-induced fluorescence is used to measure the time-dependent temperature distribution in the liquid phase. Particle image velocimetry is employedto measure the velocity field around the bubble, necessary to elucidate the bubble growth and departure mechanisms. The investigation also revealsother fundamental heat transferaspects such as the dynamics of the near-wall superheated liquid layer, the mixing effect produced by bubble growth and departure, as well as convection effects around the bubble.
Conroy DT, Espin L, Matar OK, et al., 2019, Thermocapillary and electrohydrodynamic effects on the stability of dynamic contact lines, Physical Review Fluids, Vol: 4, ISSN: 2469-990X
Motivated by the need to understand how external fields influence the stability of dynamic contact lines, the linear stability of gravity-driven spreading of a thin liquid film in the presence of electric and temperature fields is studied. The film is confined from below by a flat substrate and from above by an air gap and another flat substrate. An electrostatic potential difference or temperature difference can be applied between the two substrates and the liquid is taken to be a perfect dielectric whose surface tension decreases linearly with temperature. Traveling-wave solutions are found for the film profile, and both electric and temperature fields influence the height of the capillary ridge of liquid that forms near the advancing contact line. The linear stability analysis shows that electric fields destabilize the film front to transverse perturbations and that temperature fields can either stabilize or destabilize the front, depending on the direction of the temperature gradient. An energy analysis reveals that the electric field in the capillary ridge is most responsible for the enhancement of the perturbation growth. For the case of temperature fields, the perturbed temperature gradients are the dominant mechanism through which the perturbation in film height is affected.
Magnini M, Khodaparast S, Matar OK, et al., 2019, Dynamics of long gas bubbles rising in a vertical tube in a cocurrent liquid flow, Physical Review Fluids, Vol: 4, ISSN: 2469-990X
When a confined long gas bubble rises in a vertical tube in a cocurrent liquid flow, its translational velocity is the result of both buoyancy and mean motion of the liquid. A thin film of liquid is formed on the tube wall and its thickness is determined by the interplay of viscous, inertial, capillary and buoyancy effects, as defined by the values of the Bond number (Bo≡ρgR2/σ with ρ being the liquid density, gthe gravitational acceleration, R the tube radius, and σ the surface tension), capillary number (Cab≡μUb/σ with Ub being the bubble velocity and μ the liquid dynamic viscosity), and Reynolds number (Reb≡2ρUbR/μ). We perform experiments and numerical simulations to investigate systematically the effect of buoyancy (Bo=0–5) on the shape and velocity of the bubble and on the thickness of the liquid film for Cab=10−3–10−1 and Reb=10−2–103. A theoretical model, based on an extension of Bretherton's lubrication theory, is developed and utilized for parametric analyses; its predictions compare well with the experimental and numerical data. This study shows that buoyancy effects on bubbles rising in a cocurrent liquid flow make the liquid film thicker and the bubble rise faster, when compared to the negligible gravity case. In particular, gravitational forces impact considerably the bubble dynamics already when Bo<0.842, with Bocr=0.842 being the critical value below which a bubble does not rise in a stagnant liquid in a circular tube. The liquid film thickness and bubble velocity in a liquid coflow may vary by orders of magnitude as a result of small changes of Bo around this critical value. The reduction of the liquid film thickness for increasing values of the Reynolds numbers, usually observed for Reb≲102 when Bo≪1, becomes more evident at larger Bond numbers. Buoyancy effects also have a significant influence on the features of the undulation appearing near the rear m
Shahruddin S, Jimenez-Serratos G, Britovsek G, et al., 2019, Fluid-solid phase transition of n-alkane mixtures: Coarse-grained molecular dynamics simulations and diffusion-ordered spectroscopy nuclear magnetic resonance, Scientific Reports, Vol: 9, ISSN: 2045-2322
Wax appearance temperature (WAT), defined as the temperature at which the first solid paraffin crystal appears in a crude oil, is one of the key flow assurance indicators in the oil industry. Although there are several commonly-used experimental techniques to determine WAT, none provides unambiguous molecular-level information to characterize the phase transition between the homogeneous fluid and the underlying solid phase. Molecular Dynamics (MD) simulations employing the statistical associating fluid theory (SAFT) force field are used to interrogate the incipient solidification states of models for long-chain alkanes cooled from a melt to an arrested state. We monitor the phase change of pure long chain n-alkanes: tetracosane (C24H50) and triacontane (C30H62), and an 8-component surrogate n-alkane mixture (C12-C33) built upon the compositional information of a waxy crude. Comparison to Diffusion Ordered Spectroscopy Nuclear Magnetic Resonance (DOSY NMR) results allows the assessment of the limitations of the coarse-grained models proposed. We show that upon approach to freezing, the heavier components restrict their motion first while the lighter ones retain their mobility and help fluidize the mixture. We further demonstrate that upon sub-cooling of long n-alkane fluids and mixtures, a discontinuity arises in the slope of the self-diffusion coefficient with decreasing temperature, which can be employed as a marker for the appearance of an arrested state commensurate with conventional WAT measurements.
Xiao D, Heaney CE, Mottet L, et al., 2019, A reduced order model for turbulent flows in the urban environment using machine learning, Building and Environment, Vol: 148, Pages: 323-337, ISSN: 0360-1323
To help create a comfortable and healthy indoor and outdoor environment in which to live, there is a need to understand turbulent air flows within the urban environment. To this end, building on a previously reported method , we develop a fast-running Non-Intrusive Reduced Order Model (NIROM) for predicting the turbulent air flows found within an urban environment. To resolve larger scale turbulent fluctuations, we employ a Large Eddy Simulation (LES) model and solve the resulting computational model on unstructured meshes. The objective is to construct a rapid-running NIROM from these results that will have ‘similar’ dynamics to the original LES model. Based on Proper Orthogonal Decomposition (POD) and machine learning techniques, this Reduced Order Model (ROM) is six orders of magnitude faster than the high-fidelity LES model and we demonstrate how ‘similar’ it can be to the high-fidelity model by comparing statistical quantities such as the mean flows, Reynolds stresses and probability densities of the velocities. We also include validation of the high-fidelity model against data from wind tunnel experiments.This paper represents a key step towards the use of reduced order modelling for operational purposes with the tantalising possibility of it being used in place of Gaussian plume models, and the potential for greatly improved model fidelity and confidence.
Balla M, Tripathi MK, Sahu KC, et al., 2019, Non-isothermal bubble rise dynamics in a self-rewetting fluid: three-dimensional effects, Journal of Fluid Mechanics, Vol: 858, Pages: 689-713, ISSN: 0022-1120
The dynamics of a gas bubble in a square channel with a linearly increasing temperature at the walls in the vertical direction is investigated via three-dimensional numerical simulations. The channel contains a so-called ‘self-rewetting’ fluid whose surface tension exhibits a parabolic dependence on temperature with a well-defined minimum. The main objectives of the present study are to investigate the effect of Marangoni stresses on bubble rise in a self-rewetting fluid using a consistent model fully accounting for the tangential surface tension forces, and to highlight the effects of three-dimensionality on the bubble rise dynamics. In the case of isothermal and non-isothermal systems filled with a ‘linear’ fluid, the bubble moves in the upward direction in an almost vertical path. In contrast, strikingly different behaviours are observed when the channel is filled with a self-rewetting fluid. In this case, as the bubble crosses the location of minimum surface tension, the buoyancy-induced upward motion of the bubble is retarded by a thermocapillary-driven flow acting in the opposite direction, which in some situations, when thermocapillarity outweighs buoyancy, results in the migration of the bubble in the downward direction. In the later stages of this downward motion, as the bubble reaches the position of arrest, its vertical motion decelerates and the bubble encounters regions of horizontal temperature gradients, which ultimately lead to the bubble migration towards one of the channel walls. These phenomena are observed at sufficiently small Bond numbers (high surface tension). For stronger self-rewetting behaviour, the bubble undergoes spiralling motion. The mechanisms underlying these three-dimensional effects are elucidated by considering how the surface tension dependence on temperature affects the thermocapillary stresses in the flow. The effects of other dimensionless numbers, such as Reynolds and Froude numbers, are also investig
Lei Q, Xie Z, Pavlidis D, et al., 2018, The shape and motion of gas bubbles in a liquid flowing through a thin annulus, Journal of Fluid Mechanics, Vol: 285, Pages: 1017-1039, ISSN: 0022-1120
We study the shape and motion of gas bubbles in a liquid flowing through a horizontal or slightly inclined thin annulus. Experimental data show that in the horizontal annulus, bubbles develop a unique ‘tadpole-like’ shape with a semi-circular cap and a highly stretched tail. As the annulus is inclined, the bubble tail tends to vanish, resulting in a significant decrease of bubble length. To model the bubble evolution, the thin annulus is conceptualised as a ‘Hele-Shaw’ cell in a curvilinear space. The three-dimensional flow within the cell is represented by a gap-averaged, two-dimensional model, which achieved a close match to the experimental data. The numerical model is further used to investigate the effects of gap thickness and pipe diameter on the bubble behaviour. The mechanism for the semi-circular cap formation is interpreted based on an analogous irrotational flow field around a circular cylinder, based on which a theoretical solution to the bubble velocity is derived. The bubble motion and cap geometry is mainly controlled by the gravitational component perpendicular to the flow direction. The bubble elongation in the horizontal annulus is caused by the buoyancy that moves the bubble to the top of the annulus. However, as the annulus is inclined, the gravitational component parallel to the flow direction becomes important, causing bubble separation at the tail and reduction in bubble length.
Kahouadji L, Nowak E, Kovalchuk N, et al., 2018, Simulation of immiscible liquid-liquid flows in complex microchannel geometries using a front-tracking scheme, MICROFLUIDICS AND NANOFLUIDICS, Vol: 22, ISSN: 1613-4982
The three-dimensional two-phase flow dynamics inside a microfluidic device of complex geometry is simulated using a parallel, hybrid front-tracking/level-set solver. The numerical framework employed circumvents numerous meshing issues normally associated with constructing complex geometries within typical computational fluid dynamics packages. The device considered in the present work is constructed via a module that defines solid objects by means of a static distance function. The construction combines primitive objects, such as a cylinder, a plane, and a torus, for instance, using simple geometrical operations. The numerical solutions predicted encompass dripping and jetting, and transitions in flow patterns are observed featuring the formation of drops, ‘pancakes’, plugs, and jets, over a wide range of flow rate ratios. We demonstrate the fact that vortex formation accompanies the development of certain flow patterns, and elucidate its role in their underlying mechanisms. Experimental visualisation with a high-speed imaging are also carried out. The numerical predictions are in excellent agreement with the experimental data.
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