Imperial College London

Dr Victor Voulgaropoulos

Faculty of EngineeringDepartment of Chemical Engineering

Research Associate







ACE ExtensionSouth Kensington Campus





Publication Type

14 results found

Charogiannis A, Sik An J, Voulgaropoulos V, Markides CNet al., 2019, Structured planar laser-induced fluorescence (S-PLIF) for the accurate identification of interfaces in multiphase flows, International Journal of Multiphase Flow, Vol: 118, Pages: 193-204, ISSN: 0301-9322

Annular flows are employed in numerous engineering and industrial processes relating to the chemical, oil and gas, solar and nuclear energy industries. Yet, the reliable time- and space-resolved measurement of film thickness in these flows still eludes us, as the moving and wavy interface renders the application of optical diagnostics, such as planar laser-induced fluorescence (PLIF), particularly challenging. In this research article, we present a novel adaptation of PLIF, which we refer to as structured PLIF (S-PLIF), and with which we seek to suppress the errors in PLIF-derived film thickness measurements due to total internal reflection (TIR) of the emitted fluorescence at the phase boundary. The proposed measurement approach relies on a periodic modulation of the laser-light intensity along the examined region of the flow in order to generate fluorescence images with alternating bright and dark regions. An image-processing methodology capable of recovering the location of the true gas-liquid interface from such images is presented, and the application of S-PLIF is demonstrated in liquid films in a vertical pipe over the Reynolds number range . The results from this technique are compared to simultaneously recovered, “conventional” (uncorrected) PLIF data, as well as data from other techniques over the same range of conditions, demonstrating the efficacy of S-PLIF. A comparison amongst S-PLIF data obtained with the observation angle between the laser-sheet plane and the camera’s observation axis set to and 90 ∘ is also performed, showing that the employment of is highly advantageous in avoiding distortions caused by reflections of the emitted fluorescence at the film free-surface. The instantaneous and average film-thickness uncertainties of S-PLIF are estimated to be below 10% and 5%, respectively, when measuring smooth films; an improvement over the other optical measurement techniques considered in this work. Finally, the application of S-

Journal article

Voulgaropoulos V, Zadrazil I, Le Brun N, Bismarck A, Markides CNet al., 2019, On the link between experimentally‐measured turbulence quantities and polymer‐induced drag reduction in pipe flows, AIChE Journal, Vol: 65, ISSN: 0001-1541

Journal article

Voulgaropoulos V, Jamshidi R, Mazzei L, Angeli Pet al., 2019, Experimental and numerical studies on the flow characteristics and separation properties of dispersed liquid-liquid flows, Physics of Fluids, Vol: 31, Pages: 073304-1-073304-16, ISSN: 1070-6631

The local dynamics of spatially developing liquid-liquid dispersed flows at low superficial velocities, ranging from 0.2 to 0.8 m s−1, are investigated. The dispersions are generated with an in-line static mixer. Detailed measurements with laser-based diagnostic tools are conducted at two axial pipe locations downstream of the mixer, namely, at 15 and 135 equivalent pipe diameters. Different flow patterns are recorded, and their development along the streamwise direction is shown to depend on the initial size and concentration of the drops as well as the mixture velocity. The drop size is accurately predicted by an empirical formula. The variations in drop concentration over the pipe cross-section along the pipe result in local changes of the physical properties of the mixture and consequently in asymmetrical velocity profiles, with the maxima of the velocity located in the drop-free region. Computational fluid dynamics simulations based on a mixture approach predict the experimental results close to the experimental uncertainties for the majority of the cases. The simulation results reveal that gravity and lift forces, as well as shear-induced diffusion are the most important mechanisms affecting the drop migration. It is found that the drops behave as suspensions of rigid spheres for the conditions investigated, despite the deformation effects, which are found experimentally to be stronger at the densely packed region.

Journal article

Voulgaropoulos V, Aguiar GM, Matar OK, Bucci M, Markides CNet 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.

Conference paper

Aguiar GM, Voulgaropoulos V, Matar OK, Markides CN, Bucci Met al., 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.

Conference paper

Driker J, Juggurnath D, Kaya A, Osowade EA, Simpson M, Lecompte S, Rahim Abadi SMAN, Voulgaropoulos V, Adelaja AO, Dauhoo MZ, Khoodaruth A, Obayopo SO, Olakoyejo OT, Khalil MK, De Paepe M, Meyer JP, Markides CNet al., 2019, Thermal energy processes in direct steam generation solar systems: Boiling, condensation and energy storage, Frontiers in Energy Research, Vol: 6, ISSN: 2296-598X

Direct steam generation coupled with solar energy is a promising technology which can reduce the dependency on fossil fuels. It has the potential to impact the power-generation sector as well as industrial sectors where significant quantities of process steam are required. Compared to conventional concentrated solar power systems, which use synthetic oils or molten salts as the heat transfer fluid, direct steam generation offers an opportunity to achieve higher steam temperatures in the Rankine power cycle and to reduce parasitic losses, thereby enabling improved thermal efficiencies. However, this is associated with non-trivial challenges, which need to be addressed before such systems can become more economically competitive. Specifically, important thermal-energy processes take place during flow boiling, flow condensation and thermal-energy storage, which are highly complex, multi-scale and are multi-physics in nature that involve phase-change, unsteady and turbulent multiphase flows in the presence of conjugate heat transfer. This paper reviews our current understanding and ability to predict these processes, and knowledge that has been gained from experimental and computational efforts in the literature. In addition to Rankine cycles, organic Rankine cycle applications, which are relevant to lower operating temperature conditions, are also considered. This expands the focus to beyond water as the working fluid and includes refrigerants also. In general, significant progress has been achieved, yet there remain challenges in our capability to design and to operate effectively high-performance and low-cost systems with confidence. Of interest are the flow regimes, heat transfer coefficients and pressure drops during the thermal processes present in direct steam generation systems including those occurring in the solar collectors, condensers and relevant energy storage schemes during thermal charging and thermal discharging. A brief overview of some energy storage

Journal article

Wright SF, Charogiannis A, Voulgaropoulos V, Matar OK, Markides CNet al., 2018, Laser-based measurements of stratified liquid-liquid pipe flows interacting with jets in cross-flow, 19th International Symposium on the Application of Laser and Imaging Techniques to Fluid Mechanics

At low velocities, horizontal liquid-liquid flows uder go gravitation ally-induced stratification, which in many practical applications complicatessignificantly the direct measurement of the average properties of theflow. The extent of flow stratification, however, can be limited through in line mixing leading to the formation of liquid-liquid dispersions withmore homogenous properties. In this work, we focus on the use of‘active’ mixing methods using jets in cross flows (JICFs). In this paper,a dedicated experimental flow facility for the investigation of such flowsis presented, along with the accompanying laser-based optical measurement techniques and associated algorithms that have beendeveloped for this investigation. The facility allows simultaneous,space-and time-resolved phase and velocity information to be generatedvia plan ar laser-induced fluorescence (PLIF) and particle velocimetry(PIV/ PTV), with stereo-PIV used to provide information on the third (out-of-plane) velocity component. Preliminary experimental results arepresented which demonstrate the capabilities of this arrangementfor optically examining stratified liquid-liquid flows interacting withJICFs, leading to new insights into these complex flows. The key resultsinclude phenomena of jets interacting with the liquid-liquid inter face,recirculation zones that lead to further mixing, the presence of complexcompound droplets, droplet size distributions, and water concentrationprofiles.

Conference paper

Moran H, Gupta A, Voulgaropoulos V, Markides Cet al., Autoignition of a liquid n-heptane jet injected into a confined turbulent hot co-flow, 3rd SEE SDEWES 2018, Publisher: SDEWES

Alternatives to conventional combustion engines, such as gasoline direct injection engines, homogeneous charge compression injection engines and dual-fuel turbines, promise improved fuel efficiency and reduced emissions. The present study of liquid-fuel autoignition in turbulent flows explores the underlying phenomena in these applications towards next-generation combustors. Experiments have been performed on the autoignition of continuous liquid n-heptane jets injected axisymmetrically into confined turbulent coflows of preheated air. Jet atomisation was characterised using high-speed imaging, and autoignition locations and corresponding delay times were recorded for various bulk air temperatures and velocities. Two turbulence-generating plates with different perforation sizes were used to investigate the role of turbulence in affecting the phenomena under investigation. Smaller droplets formed in flows with lower turbulence intensities and larger integral lengthscales. The autoignition length increased and delay time decreased with increasing bulk air velocity, the latter being contrary to results from pre-vaporized n-heptane autoignition in an identical apparatus.

Conference paper

Voulgaropoulos V, 2018, Dynamics of spatially evolving dispersed flows

This dissertation provides a unique insight into the flow dynamics of evolving dispersed pipe flows. Kinetically unstable liquid-liquid dispersions are actuated in two horizontal flow loop systems. Novel conductivity and optical laser-based experimental methods are developed and applied at several axial locations capturing the flow characteristics and separation properties of the dispersions downstream the pipe with combined measurements of drop sizes, phase fractions and velocities. Flow pattern transitions are recorded for low mixture velocities as the dispersions flow. Drops segregate and coalesce forming a second continuous layer. Drop size measurements exhibit growth of the drops along the streamwise direction independent of the flow pattern, with larger drops recorded closer to the direction of buoyancy. A phenomenological model based on batch vessel settlers is modified and is found to predict well the axial evolution of the dispersions. Holdup and velocity measurements acquired from laser diagnostics are compared with CFD predictions obtained using a mixture approach implementing an effective viscosity model. Good comparisons are obtained by considering sedimentation, shear-induced diffusion and lift. The dispersions behave as suspensions of solid rigid spheres for the conditions investigated. Asymmetry in the velocity profiles is found for both experiments and simulations as the dispersions separate, with the maxima of the velocity located in the drop-free layer. Due to the prominent role of coalescence in the system, its dynamics are studied both during pipe flow and in a Hele-Shaw cell. For the former, high resolution velocity field measurements illustrate the vortices generated from the rupture point of the film inside a coalescing drop and its expanding neck until it fully merges with the bulk, being in agreement with scaling laws for immobile systems. The latter cases are used to investigate the effect of surface active agents and complex fluids. Surfa

Thesis dissertation

Voulgaropoulos V, Angeli P, 2017, Optical measurements in evolving dispersed pipe flows, Experiments in Fluids, Vol: 58, ISSN: 0723-4864

Optical laser-based techniques and an extensive data analysis methodology have been developed to acquire flow and separation characteristics of concentrated liquid–liquid dispersions. A helical static mixer was used at the inlet of an acrylic 4 m long horizontal pipe to actuate the dispersed flows at low mixture velocities. The organic (913 kg m−3, 0.0046 Pa s) and aqueous phases (1146 kg m−3, 0.0084 Pa s) were chosen to have matched refractive indices. Measurements were conducted at 15 and 135 equivalent pipe diameters downstream the inlet. Planar laser induced fluorescence (PLIF) measurements illustrated the flow structures and provided the local in-situ holdup profiles. It was found that along the pipe the drops segregate and in some cases coalesce either with other drops or with the corresponding continuous phase. A multi-level threshold algorithm was developed to measure the drop sizes from the PLIF images. The velocity profiles in the aqueous phase were measured with particle image velocimetry (PIV), while the settling velocities of the organic dispersed drops were acquired with particle tracking velocimetry (PTV). It was also possible to capture coalescence events of a drop with an interface over time and to acquire the instantaneous velocity and vorticity fields in the coalescing drop.

Journal article

Chinaud M, Voulgaropoulos V, Angeli P, 2016, Surfactant effects on the coalescence of a drop in a Hele-Shaw cell, Physical Review E, Vol: 94, ISSN: 1539-3755

In this work the coalescence of an aqueous drop with a flat aqueous-organic interface was investigated in a thingap Hele-Shaw cell. Different concentrations of a nonionic surfactant (Span 80) dissolved in the organic phasewere studied. We present experimental results on the velocity field inside a coalescing droplet in the presence ofsurfactants. The evolution of the neck between the drop and the interface was studied with high-speed imaging.It was found that the time evolution of the neck at the initial stages of coalescence follows a linear trend, whichsuggests that the local surfactant concentration at the neck region for this stage of coalescence can be consideredquasiconstant in time. This neck expansion can be described by the linear law developed for pure systems when thesurfactant concentration at the neck is assumed higher than in the bulk solution. In addition, velocity and vorticityfields were computed inside the coalescing droplet and the bulk homophase using a high-speed shadowgraphytechnique. The significant wall effects in the Hele-Shaw cell in the transverse axis cause the two vertical velocitycomponents towards the singularity rupture point, from the drop and from the bulk homophase, to be of thesame order of magnitude. This movement together with the neck expansion creates two pairs of counteractingvortices in the drop and in the bulk phase. The neck velocity is the average of the advection velocities of the twocounteracting vortex pairs on each side of the neck. The presence of the surfactant slows down the dynamics ofthe coalescence, affects the propagation direction of the pair of vortices in the bulk phase, and reduces their sizefaster compared to the system without surfactant.

Journal article

Dong T, Weheliye WH, Voulgaropoulos V, Angeli Pet al., 2016, On the effect of surfactants on drop coalescence with liquid/liquid interfaces, Pages: 833-835

Conference paper

Voulgaropoulos V, Zhai L, Loannou K, Angeli Pet al., 2016, Evolution of unstable liquid-liquid dispersions in horizontal pipes, Pages: 305-318

© BHR Group 2016. In this work, the flow pattern and drop size development of kinetically unstable oil-water dispersions is studied along a horizontal test section. Experiments with tap water and a low viscosity kerosene oil (5.5 mPa s) are conducted in a 7 m long acry lic pipe with 37 mm ID. Dispersed oil (lows are actuated for a wide range of phase fractions and relatively low mixture velocities by using a multi-nozzle inlet with 1056 nozzles. Highspeed visualisations show that the flow remains fully dispersed downstream the inlet only at high mixture velocities. At low velocities a continuous oil layer forms, while there is an accumulation of dispersed drops at the upper part of the pipe. The drop size evolution is tracked with a conductivity probe and is shown to depend on the spatial configuration of the flow.

Conference paper

Passos AD, Voulgaropoulos VP, Paras SV, Mouza AAet al., 2015, The effect of surfactant addition on the performance of a bubble column containing a non-Newtonian liquid, CHEMICAL ENGINEERING RESEARCH & DESIGN, Vol: 95, Pages: 93-104, ISSN: 0263-8762

Journal article

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