327 results found
Zadrazil I, Markides CN, 2014, An experimental characterization of liquid films in downwards co-current gas-liquid annular flow by particle image and tracking velocimetry, International Journal of Multiphase Flow, Vol: 67, Pages: 42-53, ISSN: 0301-9322
Neophytou MK-A, Markides CN, Fokaides PA, 2014, An experimental study of the flow through and over two dimensional rectangular roughness elements: Deductions for urban boundary layer parameterizations and exchange processes, Physics of Fluids, Vol: 26, Pages: 086603-1-086603-21, ISSN: 1089-7666
This paper investigates the flow through and over two-dimensional rectangular roughnesselements, arranged in a building-street canyon geometry through a series ofexperiments. Geometries of different packing densities of the roughness elements(λp) were examined and the packing density values ranged from λp = 0.30 to 0.67.The purpose of the work is: (i) to investigate the flow physics observed both at theboundary layer scale as well as at the scale within the roughness elements for a rangeof packing densities, (ii) to deduce parameterizations of the adjusted rough boundarylayer and their variation with a change in the packing density, and (iii) givena particular interest in and application to the urban atmosphere, a final aim at theroughness-element scale is to deduce the variation of the breathability with the packingdensity variation. Particle image velocimetery measurements of the velocity flowfield as well as the turbulent kinetic energy and the Reynolds Stress (within and upto well-above the street canyons) were conducted. The results reveal qualitative flowfeatures as well as features of the adjusted boundary layer structure—in particular theroughness and inertial sublayers, which can be associated with the surface roughnesslength, zero-plane displacement thickness, and the friction velocity. The lowest frictionvelocities are exhibited in the geometries with the highest- and lowest packingdensities while the maximum friction velocities are observed in the medium-packedgeometries. The exchange processes and breathability at the level of the roughnesselements top were characterized and quantified by a mean exchange velocity. Theresults show that unlike friction velocity, the normalized exchange velocity (overthe mean bulk velocity) for the most dense and sparse geometries differ by morethan 80%, with the denser-packed geometries exhibiting lower exchange velocities;this is shown to be related with the thickness of the developed roughness sublay
Morgan R, Markides CN, Zadrazil I, et al., 2014, Investigation into Liquid-Liquid Flow Instability Mechanisms using Laser-Based Optical Diagnostic Techniques, The Geoff Hewitt Celebration Conference – Multiphase Flow: Theory, Modelling, Simulation and Experimentation
Charogiannis A, Markides CN, Mathie R, 2014, Combined PLIF-IR thermal measurements of wavy film flows undergoing forced harmonic excitation, 10th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics (HEFAT2014)
A combined PLIF/IR thermography technique wasdeveloped and employed towards the measurement of unsteadyand conjugate heat transfer in thin, gravity-driven falling liquidfilm flows (with and without flow pulsation) over an inclinedheated metal foil. Simultaneous, local film thickness, film andsubstrate temperature, heat flux exchanged with a heated foiland heat transfer coefficient results are reported for a range ofelectrically applied heat input values, flow Reynolds (Re)numbers and flow pulsation frequencies. Moreover, interfacialwave velocities were calculated from cross-correlations acrosssuccessive thickness profiles. Results concerning theinstantaneous and local heat transfer coefficient variation andhow this is correlated with the instantaneous and local filmthickness variation (waves) suggest that the heat transfercoefficient experiences an enhancement in thinner films. Theparticular observation is most probably attributed to a numberof unsteady flow phenomena within the wavy fluid films thatare not captured by the steady analysis. At low flow Re numbervalues the mean Nusselt (Nu) was around 2.5, in agreementwith laminar flow theory, while at higher Re values, higher Nuwere observed. Finally, lower wave amplitude intensities wereassociated with higher heat transfer coefficient fluctuationintensities.
Morgan RG, Ibarra R, Zadrazil I, et al., 2014, On the role of inlet flow instabilities on horizontal initially stratified liquid-liquid flow development, 10th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics
For a given pair of fluid phases, liquid-liquid flows aregenerally described in terms of regimes (e.g. stratified, wavy ordispersed), which are a function of the Reynolds numbers of theindividual phases, the geometry of the flow, as well as the inletconditions and the distance from the inlet. Typically, injectingthe heavier phase at the bottom of the channel and the lighterphase at the top is the common inlet configuration whenestablishing a liquid-liquid flow for study in a laboratoryenvironment. This configuration corresponds to that expectedin a naturally separated flow orientation, on the assumption thatat long lengths the density difference between the two phaseswill lead to this arrangement of the two phases. In this study, aseries of experiments were designed to investigate the influenceof injecting the heavier phase at the top of the pipe rather thanat the bottom. This modification introduces the possibility ofphase breakup near the inlet by an additional instabilitymechanism (due to the density difference between the twoliquids), which would not appear had the phases beenintroduced in the conventional inlet flow arrangement. Weperform detailed flow measurements and observe that this flowarrangement gives rise to altered flow structures downstream.Moreover, our results suggest that the effects of this instabilitynear the inlet may persist along the pipe and influence theobserved flow behaviour even at long lengths.
Zadrazil I, Markides CN, 2014, A Mechanism of Polymer Induced Drag Reduction in Turbulent Pipe, 10th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics
Polymer induced drag reduction in turbulent pipe flow wasinvestigated using a non-intrusive laser based diagnostictechnique, namely Particle Image Velocimetry (PIV). The dragreduction was measured in a pressure-driven flow facility, in ahorizontal pipe of inner diameter 25.3 mm at Reynoldsnumbers ranging from 35 000 to 210 000. Three high-molecular-weight polymers (polyethylene oxide 2x10^6 –8x10^6 Da) at concentrations in the range of 5 – 250 wppm wereused. The results, obtained from the PIV measurements, showthat the drag reduction scales with the magnitude of thenormalized streamwise and spanwise rms velocity fluctuationsin the flow. This scaling seems to universal, and is independentof the Reynolds number and in some cases also independent ofthe distance from the wall where the velocity fluctuations areconsidered. Furthermore, the instantaneous PIV observationsindicate that as the level of drag reduction increases, the flow inthe pipe is separated into a low-momentum flow region near thepipe wall and a high-momentum flow region in the turbulentcore. Based on these findings a new mechanism of polymericdrag reduction is proposed in this paper.
Charogiannis A, Markides CN, 2014, Experimental study of falling films by simultaneous laser-induced fluorescence, particle image velocimetry and particle tracking velocimetry, 17th International Symposium on Applications of Laser Techniques to Fluid Mechanics
measurement technique based on the simultaneous implementation of Laser-Induced Fluorescence (LIF),Particle Image Velocimetry (PIV) and Particle Tracking Velocimetry (PTV) has been applied to the study of wavyliquid falling film flows characterized by low Reynolds (Re) and Kapitza (Ka) numbers. The presently examined Renumber range was 2.2 – 8.2, while the Ka number range was 28.6 – 41.4. The experimental methodology wasdeveloped with the ultimate aim of allowing for the evaluation of the local and instantaneous film thickness, interfacialvelocity and velocity field from within the illuminated liquid volume underneath the wavy interface. The majorchallenges associated with the simultaneous implementation of the two optical diagnostic techniques were, firstly, thedevelopment of a refractive index correction approach allowing for liquids of different properties (surface tension andviscosity) to be tested, secondly, the identification of the location of the two liquid boundaries (solid-liquid and gasliquid)in the LIF images, and lastly, the isolation of out-of-plane reflections from primary scattering regions in the rawPIV images. Following a detailed account of the novel practices formulated and utilized in tackling the aforementionedchallenges, the efficacy of the proposed methodology is demonstrated through comparisons between laser-basedmeasurements conducted in flat films, film thickness measurements performed with a micrometer, and the solution tothe Navier-Stokes equation based on the assumptions of one-dimensional (1-D), steady and fully developed flow. Inaddition, sample film topology results are presented for a range of flow pulsation frequencies (1 – 8 Hz), while filmthickness and interfacial velocity time traces were reconstructed and are presented along with film thickness andinterfacial velocity statistical results for select flow conditions.
Zadrazil I, Markides CN, 2014, On Disturbance and Ripple Waves in Downwards Annular Flow: Observations by Simultaneous PLIF and PIV/PTV, 17th International Symposium on Applications of Laser Techniques to Fluid Mechanics
Markides CN, Solanki R, Galindo A, 2014, Working fluid selection for a two-phase thermofluidic oscillator: Effect of thermodynamic properties, APPLIED ENERGY, Vol: 124, Pages: 167-185, ISSN: 0306-2619
Zadrazil I, Markides CN, 2014, On Disturbance and Ripple Waves in Downwards Annular Flow: Observations by Simultaneous PLIF and PIV/PTV, 52. European Two-Phase Flow Group Meeting
Zadrazil I, Matar OK, Markides CN, 2014, An experimental characterization of downwards gas-liquid annular flow by laser-induced fluorescence: Flow regimes and film statistics, INTERNATIONAL JOURNAL OF MULTIPHASE FLOW, Vol: 60, Pages: 87-102, ISSN: 0301-9322
White A, McTigue J, Markides C, 2014, Wave propagation and thermodynamic losses in packed-bed thermal reservoirs for energy storage, Applied Energy, Vol: 130, Pages: 648-657, ISSN: 0306-2619
Herrando M, Markides CN, Hellgardt K, 2014, A UK-based assessment of hybrid PV and solar-thermal systems for domestic heating and power: System performance, Applied Energy, Vol: 122, Pages: 288-309, ISSN: 1872-9118
The goal of this paper is to assess the suitability of hybrid PVT systems for the provision of electricity andhot water (space heating is not considered) in the UK domestic sector, with particular focus on a typicalterraced house in London. A model is developed to estimate the performance of such a system. The modelallows various design parameters of the PVT unit to be varied, so that their influence in the overall systemperformance can be studied. Two key parameters, specifically the covering factor of the solar collectorwith PV and the collector flow-rate, are considered. The emissions of the PVT system are compared withthose incurred by a household that utilises a conventional energy provision arrangement. The resultsshow that for the case of the UK (low solar irradiance and low ambient temperatures) a complete coverageof the solar collector with PV together with a low collector flow-rate are beneficial in allowing thesystem to achieve a high coverage of the total annual energy (heat and power) demand, while maximisingthe CO2 emissions savings. It is found that with a completely covered collector and a flow-rate of 20 L/h,51% of the total electricity demand and 36% of the total hot water demand over a year can be coveredby a hybrid PVT system. The electricity demand coverage value is slightly higher than the PV-only systemequivalent (49%). In addition, our emissions assessment indicates that a PVT system can save up to16.0 tonnes of CO2 over a lifetime of 20 years, which is significantly (36%) higher than the 11.8 tonnesof CO2 saved with a PV-only system. All investigated PVT configurations outperformed the PV-only systemin terms of emissions. Therefore, it is concluded that hybrid PVT systems offer a notably improvedproposition over PV-only systems.
Mathie R, Markides CN, White AJ, 2014, A Framework for the Analysis of Thermal Losses in Reciprocating Compressors and Expanders, Heat Transfer Engineering, Vol: 35, Pages: 1435-1449, ISSN: 1521-0537
This article presents a framework that describes formally the underlying unsteady and conjugate heat transfer processes that are undergone in thermodynamic systems, along with results from its application to the characterisation of the reciprocating compression and expansion processes in a gas spring. Specifically, a heat transfer model is proposed that solves the one-dimensional unsteady heat conduction equation in the solid simultaneously with the first law in the gas phase, with an imposed heat transfer coefficient. Even at low compression ratios (of 2.5), notable effects of the solid walls are revealed, with thermodynamic cycle losses of up to 20% (relative to equivalent adiabatic and reversible processes) when unfavourable solid and gas materials are selected, and closer to 10-12% for more common material choices. The contribution of the solid towards these values, through the variations attributed to the thickness of the cylinder wall, is about 10% and 2-4%, respectively; showing a maximum at intermediate thicknesses. At higher compression ratios (of 6) a 19% worst case loss is reported for common materials. These results suggest strongly that, in designing high-efficiency reciprocating machines, the full conjugate and unsteady problem must be considered and that the role of the solid in determining performance cannot, in general, be neglected.
Markides CN, Heyes AL, Childs PRN, 2014, Proceedings of the 13th UK Heat Transfer Conference UKHTC2013, Publisher: DEG Imperial College London, ISBN: 9780957229853
The UK Heat Transfer Conference is now in its 29th year and it remains the premier UK conference for the local and international heat transfer community to meet and present their work. Heat transfer is a critical process in innumerable industrial and commercial processes and despite decades of high quality research there remain many challenges. However, with the desire to move to a more sustainable and low carbon energy future greater emphasis is being placed on the provision of and effective use of heat. The organisers decided to reflect this by enlarging the scope of the conference to explicitly include heat as well as heat transfer. The response was excellent as can be discerned from the list of abstracts presented herein. On the pages that follow we present approximately 100 papers from 24 nations and 5 continents. We are delighted by the level of interest shown in the conference and we hope to make this the best UK Heat Transfer Conference to date.
Ibarra R, Markides CN, Matar OK, 2014, A review of liquid-liquid flow patterns in horizontal and slightly inclined pipes, Multiphase Science and Technology, Vol: 26, Pages: 171-198, ISSN: 0276-1459
© 2014 by Begell House, Inc. This paper presents a review of the co-current flow of two immiscible liquids in horizontal and slightly inclined pipes. Liquid-liquid flows are present in a wide variety of industrial processes, such as chemicals, pharmaceuticals, and food processing. However, this phenomenon is mainly studied in the oil industry, especially in the analysis of oil-water mixtures encountered in long transportation pipelines from the wellhead to the processing facility. The hydrodynamic behavior of liquid-liquid flows is more complex than that of gas-liquid flows because of density ratio, viscosity ratio, interfacial forces, and pipe wettability. This means that a significant number of different flow patterns can be obtained from different fluid properties and pipe characteristics. Furthermore, the flow pattern classification of liquid-liquid flows is arbitrary and several researchers use their own classification, complicating comparison and analysis of flow pattern maps. In this paper, a unified flow pattern classification for liquid-liquid flow is proposed. This classification enables the direct comparison of flow pattern maps for further analysis.
Mathie R, Markides CN, 2014, HEAT TRANSFER AUGMENTATION IN CONVECTING FILM FLOWS, PROCEEDINGS OF THE ASME SUMMER HEAT TRANSFER CONFERENCE - 2013, VOL 1
Markides CN, 2014, Preface: UK heat transfer conference 2013, Computational Thermal Sciences, Vol: 6, Pages: vi-vii, ISSN: 1940-2503
Purvis JA, Mistry RD, Markides CN, et al., 2013, An experimental investigation of fingering instabilities and growth dynamics in inclined counter-current gas-liquid channel flow, PHYSICS OF FLUIDS, Vol: 25, ISSN: 1070-6631
Zadrazil I, Matar OK, Markides CN, 2013, Slug front gas entrainment in gas-liquid two-phase horizontal flow using hi-speed slug-tracking, American Physical Society - Division of Fluid Dynamics
A gas-liquid flow regime where liquid-continuous regions travel at high speeds (i.e. slugs) through a pipe separated by regions of stratified flow (i.e. elongated bubbles) is referred to as a ``slug flow.'' This regime is characterised by the turbulent entrainment of gas into the slug front body. We use a high-speed camera mounted on a moving robotic linear rail to track the formation of naturally occurring slugs over 150 pipe diameters. We show that the dynamics of the slugs become progressively more complex with increasing liquid and gas Reynolds numbers. Based on the slug- tracking visualization we present, over a range of conditions: (i) phenomenological observations of the formation and development of slugs, and (ii) statistical data on the slug velocity and gas entrainment rate into the slug body.
Zadrazil I, Matar OK, Markides CN, 2013, On the Frequency of Large Waves in Vertical Gas-Liquid Annular Flow, American Institute of Chemical Engineers
Zhao Y, Markides CN, Matar OK, et al., 2013, Disturbance wave development in two-phase gas-liquid upwards vertical annular flow, International Journal of Multiphase Flow, Vol: 55, Pages: 111-129, ISSN: 0301-9322
Disturbance waves are of central importance in annular flows. Such waves are characterised by their large amplitudes relative to the mean film thickness, their high translation velocities relative to the mean film speed, and their circumferential coherence. The present paper is concerned with the existence, development and translation of disturbance waves in upwards, gas–liquid annular flows. Experiments are described, which featured simultaneous high-frequency film thickness measurements from multiple conductance probes positioned circumferentially and axially along a vertical pipe, these measurements were aimed at studying the three-dimensional development of these interfacial structures as a function of distance from the tube inlet. From the results, it is found that disturbance waves begin to appear and to achieve their circumferential coherence from lengths as short as 5–10 pipe diameters downstream of the liquid injection location; this coherence gradually strengthens with increasing distance from the inlet. It is further shown that the spectral content of the entire interfacial wave activity shifts to lower frequencies with increasing axial distance from the inlet, with the peak frequency levelling off after approximately 20 pipe diameters. Interestingly, on the other hand, the frequency of occurrence of the disturbance waves first increases away from the inlet as these waves form, reaches a maximum at a length between 7.5 and 15 pipe diameters that depends on the flow conditions, and then decreases again. This trend becomes increasingly evident at higher gas and/or liquid flow-rates. Both wave frequency measures increase monotonically at higher gas and/or liquid flow-rates.
Gupta A, Markides CN, Mathie R, 2013, Investigation of a thermal storage system based on phase change material: heat transfer and performance characterisation, 13th UK Heat Transfer Conference, UKHTC2013, Publisher: Design Engineering Group, Imperial College London, Pages: 182-1-182-8
The integration of latent heat storage solutions into modern domestic heating systems has the potentialto enhance the overall system performance compared to standard hot-water systems (radiators andtanks) due to augmentation of the stored heat by the latent heat of a suitable material. This paperpresents computational prediction and experimental validation of the dynamic behaviour andperformance of an active thermal storage system for domestic applications, based on the use of ahydrated salt PCM. The thermal extraction and heating rates for the PCM tank are compared to a waterfilled tank. Flow and temperature fields are analysed in a customised storage tank design for heattransfer and performance characterisation. Experimental findings show excellent agreement with the3D CFD simulation results. The heat removal performance has been identified as being the limitingfactor when compared to a water-based system. It is also confirmed that the PCM solution has thecapability to store a large amount of heat effectively but design improvements are required toeliminate the cooling limited heat transfer process in the current apparatus.
Oyewunmi OA, Taleb A, Haslam A, et al., 2013, An assessment of working-fluid mixtures in organic Rankine cycles for waste-heat recovery using SAFT-VR., 2nd International Seminar on ORC Power Systems (ASME ORC 2013)
Zadrazil I, Markides CN, Hewitt GF, et al., 2013, Wave Structure and Velocity Profiles in Downwards Gas-Liquid Annular Flow, 8th International Conference on Multiphase Flow
The downwards co-current gas-liquid annular flows inside a vertically oriented pipe have been experimentally investigated.The measurements and characterisation were performed using advanced optical non-intrusive laser-based techniques, namelyLaser Induced Fluorescence, and Particle Image/Tracking Velocimetry. The investigated conditions were in the range of ReL =306 – 1,532 and ReG = 0 – 84,600. Temporal film thickness time traces were constructed using the Laser Induced Fluorescenceimages. Based on these, the wave frequency was evaluated using direct wave counting approach and power spectral densityanalysis. Additionally, qualitative PIV observations revealed the presence of recirculation zones within a wave front ofdisturbance waves.
Markides CN, Gupta A, 2013, Experimental investigation of a thermally powered central heating circulator: Pumping characteristics, Applied Energy, Vol: 110, Pages: 132-146, ISSN: 1872-9118
A thermally powered circulator based on a two-phase thermofluidic oscillator was constructed and operated successfully as a replacement for a central heating hot water circulator coupled to a domestic gas-fired boiler. During regular operation the thermally powered circulator demonstrated a pumped flow-rate that decreased monotonically as the head applied across it increased. A maximum measured flow-rate of 850 L/h was achieved at zero head, and a maximum head of 8.4 mH2O was attained at near-stalling (zero flow-rate) conditions. In agreement with previous modelling studies of the technology, increased inertia in the load line seems to lead to improved circulator performance. Further, the oscillating circulator exhibited an operational frequency between 0.24 and 0.33 Hz, which was mostly determined by the circulator configuration. The pumping capacity was strongly affected by the oscillating liquid amplitudes in the power cylinder that defined the positive displacement amplitudes of the liquid piston into and out of the hot water circuit. The best circulator configuration was associated with lower operation frequencies and relatively large ratios of suction to discharge displacement.
Markides CN, 2013, The role of pumped and waste heat technologies in a high-efficiency sustainable energy future for the UK, Applied Thermal Engineering, Vol: 53, Pages: 197-209, ISSN: 1359-4311
This paper begins with an overview of the current supply and demand characteristics of primary energy for the provision of heat and power in the UK. This is followed by a brief review of a variety of solutions that are being proposed towards the establishment of asustainable energy landscape, including clean coal, wind and solar energy. The discussion extends to the economics and performance of various renewable energy systems in comparison to fossil fuel equivalents. Placed in this context, the study then focuses specifically on the role of pumped heat, combined heat and power (CHP) schemes, and options for the recovery and conversion of waste heat into useful work, all of which have a potential to contribute towards the creation of a ‘high-efficiency sustainable energy future’. It is concluded that although the problem is complex, the relative costs of competing technologies are not prohibitive, but comparable, leading to an inability to make a decisive choice and delaying progress. CHP and pumped heat are found to be similar in terms of overall efficiency, with the load factor (heat-to-power demand ratio) being of critical importance. Various waste heat conversion systems are also found to be similar in terms of the important indicator of power per unit cost.
Solanki R, Galindo A, Markides CN, 2013, The role of heat exchange on the behaviour of an oscillatory two-phase low-grade heat engine, APPLIED THERMAL ENGINEERING, Vol: 53, Pages: 177-187, ISSN: 1359-4311
Kerkemeier SG, Markides CN, Frouzakis CE, et al., 2013, Direct numerical simulation of the autoignition of a hydrogen plume in a turbulent coflow of hot air, JOURNAL OF FLUID MECHANICS, Vol: 720, Pages: 424-456, ISSN: 0022-1120
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