Imperial College London

ProfessorChristosMarkides

Faculty of EngineeringDepartment of Chemical Engineering

Professor of Clean Energy Technologies
 
 
 
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Contact

 

+44 (0)20 7594 1601c.markides Website

 
 
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Location

 

404ACE ExtensionSouth Kensington Campus

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Summary

 

Publications

Publication Type
Year
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264 results found

Sapin P, Taleb A, White AJ, Fabris D, Markides CNet al., 2016, EXPERIMENTAL ANALYSIS OF LOSS MECHANISMS IN A GAS SPRING, ASME Power and Energy Conference, Publisher: ASME, Pages: ES2016-59631-ES2016-59631

Reciprocating-piston compressors and expanders arepromising solutions to achieve higher overall efficiencies invarious energy storage solutions. This article presents anexperimental study of the exergetic losses in a gas spring. Consideringa valveless piston-cylinder system allows us to focuson the thermodynamic losses due to thermal-energy exchangeprocesses in reciprocating components. To differentiate this latterloss mechanism from mass leakages or frictional dissipation,three bulk parameters are measured. Pressure and volume arerespectively measured with a pressure transducer and a rotarysensor. The gas temperature is estimated by measuring theTime-Of-Flight (TOF) of an ultrasonic pulse signal across thegas chamber. This technique has the advantage of being fast andnon-invasive. The measurement of three bulk parameters allowsus to calculate the work as well as the heat losses throughouta cycle. The thermodynamic loss is also measured for differentrotational speeds. The results are in good agreement withprevious experimental studies and can be employed to validateCFD or analytical studies currently under development

Conference paper

Kirmse CJW, Oyewunmi, Taleb A, Haslam A, Markides Cet al., 2016, A two-phase single-reciprocating-piston heat conversion engine: Non-linear dynamic modelling, Applied Energy, Vol: 186, Pages: 359-375, ISSN: 0306-2619

A non-linear dynamic framework is presented for the modelling of a novel two-phase heat engine termed ‘Up-THERM’, which features a single solid moving-part (piston). When applied across the device, a constant temperature difference between an external (low- to medium-grade) heat source and an external heat sink is converted into sustained and persistent oscillations of pressure and volumetric fluid displacement. These oscillations are transformed in a load arrangement into a unidirectional flow from which power is extracted by a hydraulic motor. The Up-THERM engine is modelled using a system of first-order differential equations that describe the dominant thermal/fluid processes in each component of the device. For certain components where the deviations from a linear approximation are non-negligible (gas spring in the displacer cylinder, check valves and piston valve, and heat exchangers), a non-linear description is employed. A comparison between the linear and non-linear descriptions of the gas spring at the top of the displacer cylinder reveals that the non-linear description results in more realistic predictions of the oscillation frequency compared to experimental data from a similar device. Furthermore, the shape of the temperature profile over the heat-exchanger surfaces is modelled as following a hyperbolic tangent function, based on findings from an experimental investigation. Following the validation of these important device components, a parametric study is performed on the Up-THERM engine model with the aforementioned non-linear component descriptions, aimed at investigating the effects of important geometric parameters and of the heat-source temperature on key performance indicators, namely the oscillation frequency, power output and exergy efficiency of the engine. The results indicate that the geometric design of the displacer cylinder, including the height of the gas spring at the top of the cylinder, and the heat-source temperature hav

Journal article

Kirmse CJW, Oyewunmi OA, Haslam AJ, Markides Cet al., 2016, Comparison of a novel organic-fluid thermofluidic heat converter and an organic Rankine cycle heat engine, Energies, Vol: 9, ISSN: 1996-1073

The Up-THERM heat converter is an unsteady, two-phase thermofluidic oscillator that employs an organic working fluid, which is currently being considered as a prime-mover in small- to medium-scale combined heat and power (CHP) applications. In this paper, the Up-THERM heat converter is compared to a basic (sub-critical, non-regenerative) organic Rankine cycle (ORC) heat engine with respect to their power outputs, thermal efficiencies and exergy efficiencies, as well as their capital and specific costs. The study focuses on a pre-specified Up-THERM design in a selected application, a heat-source temperature range from 210 °C to 500 °C and five different working fluids (three n-alkanes and two refrigerants). A modeling methodology is developed that allows the above thermo-economic performance indicators to be estimated for the two power-generation systems. For the chosen applications, the power output of the ORC engine is generally higher than that of the Up-THERM heat converter. However, the capital costs of the Up-THERM heat converter are lower than those of the ORC engine. Although the specific costs (£/kW) of the ORC engine are lower than those of the Up-THERM converter at low heat-source temperatures, the two systems become progressively comparable at higher temperatures, with the Up-THERM heat converter attaining a considerably lower specific cost at the highest heat-source temperatures considered.

Journal article

Oyewunmi OA, Kirmse CJW, Haslam AJ, Muller EA, Markides CNet al., 2016, Working-fluid selection and performance investigation of a two-phase single-reciprocating-piston heat-conversion engine, Applied Energy, Vol: 186, Pages: 376-395, ISSN: 0306-2619

We employ a validated first-order lumped dynamic model of the Up-THERM converter, a two-phase unsteadyheat-engine that belongs to a class of innovative devices known as thermofluidic oscillators, which containfewer moving parts than conventional engines and represent an attractive alternative for remote or off-gridpower generation as well as waste-heat recovery. We investigate the performance the Up-THERM withrespect to working-fluid selection for its prospective applications. An examination of relevant working-fluidthermodynamic properties reveals that the saturation pressure and vapour-phase density of the fluid play importantroles in determining the performance of the Up-THERM – the device delivers a higher power outputat high saturation pressures and has higher exergy efficiencies at low vapour-phase densities. Furthermore,working fluids with low critical temperatures, high critical pressures and exhibiting high values of reducedpressures and temperatures result in designs with high power outputs. For a nominal Up-THERM designcorresponding to a target application with a heat-source temperature of 360 ◦C, water is compared withforty-five other pure working fluids. When maximizing the power output, R113 is identified as the optimalfluid, followed by i-hexane. Fluids such as siloxanes and heavier hydrocarbons are found to maximize theexergy and thermal efficiencies. The ability of the Up-THERM to convert heat over a range of heat-sourcetemperatures is also investigated, and it is found that the device can deliver in excess of 10 kW when utilizingthermal energy at temperatures above 200 ◦C. Of all the working fluids considered here, ammonia, R245ca,R32, propene and butane feature prominently as optimal and versatile fluids delivering high power over awide range of heat-source temperatures.

Journal article

Oyewunmi OA, Markides C, 2016, Thermo-Economic and Heat Transfer Optimization of Working-Fluid Mixtures in a Low-Temperature Organic Rankine Cycle System, Energies, Vol: 9, ISSN: 1996-1073

In the present paper, we consider the employment of working-fluid mixtures in organicRankine cycle (ORC) systems with respect to thermodynamic and heat-transfer performance,component sizing and capital costs. The selected working-fluid mixtures promise reduced exergylosses due to their non-isothermal phase-change behaviour, and thus improved cycle efficienciesand power outputs over their respective pure-fluid components. A multi-objective cost-poweroptimization of a specific low-temperature ORC system (operating with geothermal water at 98 ◦C)reveals that the use of working-fluid-mixtures does indeed show a thermodynamic improvementover the pure-fluids. At the same time, heat transfer and cost analyses, however, suggest that it alsorequires larger evaporators, condensers and expanders; thus, the resulting ORC systems are alsoassociated with higher costs. In particular, 50% n-pentane + 50% n-hexane and 60% R-245fa + 40%R-227ea mixtures lead to the thermodynamically optimal cycles, whereas pure n-pentane and pureR-245fa have lower plant costs, both estimated as having ∼14% lower costs per unit power outputcompared to the thermodynamically optimal mixtures. These conclusions highlight the importanceof using system cost minimization as a design objective for ORC plants.

Journal article

Kirmse CJW, Oyewunmi OA, Haslam AJ, Markides CNet al., A thermo-economic assessment and comparison of the Up-THERM heat converter and an organic Rankine cycle engine, Heat Powered Cycles Conference 2016

In this paper we present a thermodynamic and economic comparison of a recently proposed two-phasethermofluidic oscillator known as the Up-THERM heat converter and the more established organic Rankine cycle(ORC) engine, when converting heat at temperatures below 150 °C using the refrigerant R-227ea as the workingfluid. The Up-THERM heat converter is being considered as a possible prime mover for small- to medium-scalecombined heat and power (CHP) applications. Using suitable thermodynamic models of both systems, it is foundthat the power output and thermal efficiencies of a pre-specified Up-THERM design are generally lower thanthose of an equivalent ORC engine. The Up-THERM, however, also demonstrates higher exergy efficiencies andis associated with lower capital costs, as expected owing to its simple construction and use of fewer and morebasic components. Interestingly, the specific costs (per rated kW) of the ORC engine are lower than those of theUp-THERM converter at lower heat source temperatures, specifically below 130 °C, whereas the Up-THERMbecomes a more cost effective alternative (in terms of the specific cost) to the ORC engine at higher temperatures.

Conference paper

Freeman J, Hellgardt K, Markides CN, 2016, Working Fluid Selection and Electrical Performance Optimisation of a Domestic Solar-ORC Combined Heat and Power System for Year-Round Operation in the UK, Applied Energy, Vol: 186, Pages: 291-303, ISSN: 0306-2619

In this paper, we examine the electrical power-generation potential of adomestic-scale solar combined heating and power (S-CHP) system featuringan organic Rankine cycle (ORC) engine and a 15-m2solar-thermal collectorarray. The system is simulated with a range of organic working fluids andits performance is optimised for operation in the UK climate. The findingsare applicable to similar geographical locations with significant cloud coverage,a low solar resource and limited installation areas. A key feature of thesystem’s design is the implementation of fixed fluid flow-rates during operationin order to avoid penalties in the performance of components suffered atpart-load. Steady operation under varying solar irradiance conditions is providedby way of a working-fluid buffer vessel at the evaporator outlet, whichis maintained at the evaporation temperature and pressure of the ORC. Byincorporating a two-stage solar collector/evaporator configuration, a maximumnet annual electrical work output of 1070 kWh yr−1(continuous averagepower of 122 W) and a solar-to-electrical efficiency of 6.3% is reported withHFC-245ca as the working fluid at an optimal evaporation temperature of126 ◦C (corresponding to an evaporation pressure of 16.2 bar). This is equivalentto ∼ 32% of the electricity demand of a typical/average UK home, andrepresents an improvement of more than 50% over a recent effort by the sameauthors based on an earlier S-CHP system configuration and HFC-245fa asthe working fluid [1]. A performance and simple cost comparison with standalone,side-by-side PV and solar-thermal heating systems is presented.

Journal article

Guarracino I, Freeman J, Markides CN, Experimental evaluation of a 3-D dynamic solar-thermal collector model under time-varying environmental conditions, ECOS 2016, 29th International conference on Efficiency, Cost, Optimization, Simulation and Environmental impact of Energy Systems

Reliable dynamic models are required for the correct prediction of the performance of solar-thermal collectors under variable solar-irradiance conditions. In this paper we present a 3-dimensional (3-D) dynamic thermal model applied to three different collector geometries: a flat plate collector (FPC), an evacuated tube collector (ETC), and also a hybrid photovoltaic-thermal (PVT) collector. Results from the model are evaluated against real data from a series of dynamic and steady-state experiments performed in Limassol, Cyprus and London, UK. The 3-D model equations are summarised and the test apparatuses and procedures are described. In the transient response tests, the model is found to under-predict the time constant for the ETC and PVT collectors by 35-55%, while for the simpler FPC the time constant is under-predicted by 20-35%. The collector model is also implemented into a wider domestic hot-water system model that includes a hot-water storage tank, in order to assess performance predictions over a diurnal operating period on an intermittently cloudy day. The results are compared to a single-node quasi-steady state model that uses the collector steady-state efficiency coefficients and a single-node dynamic model that uses a lumped collector thermal capacity (determined using experimental and calculation-based methods in the European Standard for solar collector testing). The 3-D model is shown to provide promising results that are within the range predicted by the two single-node dynamic models. For the PVT collector simulated under intermittent conditions, the predicted net daily energy gain to the store is found to be within 2% of experimentally obtained results. By comparison, a quasi-steady state model based on the collector’s steady-state efficiency curve is found to over-predict the thermal energy gain to the store by 8% over the same operating period.

Conference paper

Kirmse CJW, Oyewunmi OA, Haslam A, Markides CNet al., A THERMO-ECONOMIC COMPARISON OF THE UP-THERM HEAT CONVERTER AND AN ORGANIC RANKINE CYCLE HEAT ENGINE, 12th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics

In this paper we compare a recently proposed two-phase thermofluidicoscillator device termed ‘Up-THERM’ to a basic(sub-critical, non-regenerative) equivalent organic Rankine cycle(ORC) engine. In the Up-THERM heat converter, a constanttemperature difference imposed by an external heat source andsink leads to periodic evaporation and condensation of the workingfluid, which gives rise to sustained oscillations of pressureand volumetric displacement. These oscillations are convertedin a load arrangement into a unidirectional flow, which passesthrough a hydraulic motor that extracts useful work from the device.A pre-specified Up-THERM design is being considered in aselected application with two n-alkanes, n-hexane and n-heptane,as potential working fluids. One aim of this work is to evaluatethe potential of this proposed design. The thermodynamic comparisonshows that the ORC engine outperforms the Up-THERMheat converter in terms of power output and thermal efficiency,as expected. An economic comparison, however, reveals that thecapital costs of the Up-THERM are lower than those of the ORCengine. Nevertheless, the specific costs (per unit power) favourthe ORC engine due to its higher power output. Some aspects ofthe proposed Up-THERM design are identified for improvement

Conference paper

Oyewunmi OA, Kirmse CJW, Pantaleo AM, Markides Cet al., Performance of working-fluid mixtures in an ORC-CHP system for different heat demand segments, 29th international conference on Efficiency, Cost, Optimisation, Simulation and Environmental Impact of Energy Systems

Organic Rankine cycle (ORC) power systems are being increasingly deployed for waste heat recovery andconversion to power in several industrial settings. In the present paper, we investigate the use of working-fluidmixtures in ORC systems operating in combined heat and power mode (ORC-CHP) with shaft power providedby the expander/turbine and heating provided by the cooling-water exiting the condenser. The waste-heatsource is a flue gas stream from a refinery boiler with a mass flow rate of 560 kg/s and an inlet temperature of330 °C. When using working fluids comprising normal alkanes, refrigerants and their subsequent mixtures, theORC-CHP system is demonstrated as being capable of delivering over 20 MW of net shaft power and up to15 MW of heating, leading to a fuel energy savings ratio (FESR) in excess of 20%. Single-component workingfluids such as pentane appear optimal at low hot-water supply temperatures, and fluid mixtures becomeoptimal at higher temperatures, with the combination of octane and pentane giving an ORC-CHP systemdesign with the highest efficiency. The influence of heat demand intensity on the global system conversionefficiency and optimal working fluid selection is also explored.

Conference paper

Charogiannis A, Zadrazil I, Markides C, 2016, Development of a Thermographic Imaging Technique for Simultaneous Interfacial Temperature and Velocity Measurements, 3rd International Conference on Fluid Flow, Heat and Mass Transfer (FFHMT’16), Publisher: International ASET Inc., Pages: 116-116

An experimental technique, hereby referred to as ‘thermographic particle velocimetry’ (TPV) and capable of recovering twodimensional(2-D) surface temperature and velocity measurements at the interface of multiphase flows is presented. The proposedtechnique employs a single infrared (IR) imager and highly reflective, silver-coated particles, which when suspended near or at theinterface, can be distinguished from the surrounding fluid due to their different emissivity. The development of TPV builds upon ourprevious IR imaging studies of heated liquid-film flows; yet, the same measurement principle can be applied for the recovery of 2-Dtemperature- and velocity-field information at the interface of any flow with a significant density gradient between two fluid phases. Theimage processing steps used to recover the temperature and velocity distributions from raw IR frames are demonstrated by applicationof TPV in a heated and stirred flow in an open container, and include the decomposition of each raw frame into separate thermal andparticle frames, the application of perspective distortion corrections and spatial calibration, and the implementation of standard particleimage velocimetry algorithms. Validation experiments dedicated to the measurement of interfacial temperature and velocity were alsoconducted, with deviations between the results generated from TPV and those from accompanying conventional techniques not exceedingthe errors associated with the latter. Finally, the capabilities of the proposed technique are demonstrated by conducting temperature andvelocity measurements at the gas-liquid interface of a wavy film flow downstream of a localised heater.

Conference paper

Guarracino I, Freeman J, Ekins-Daukes N, Markides CNet al., PERFORMANCE ASSESSMENT AND COMPARISON OF SOLAR ORC AND HYBRID PVT SYSTEMS FOR THE COMBINED DISTRIBUTED GENERATION OF DOMESTIC HEAT AND POWER, HEFAT, 12th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics

Solar-thermal collectors and photovoltaic panels are effectivesolutions for the decarbonisation of electricity and hot waterprovision in dwellings. In this work, we provide the first insightfulcomparison of these two competing solar-energy technologies forthe provision of combined heating and power (CHP) in domesticapplications. The first such system is based on an array of hybridPV-Thermal (PVT) modules, while the second is based on a solarthermalcollector array of the same area (based on a constrainedroof-space) that provides a thermal-energy input to an organicRankine cycle (ORC) engine for electricity generation. Simulationresults of the annual operation of these two systems are presentedin two geographical regions: Larnaca, Cyprus (as an example of ahot, high-irradiance southern-European climate) and London, UK(as an example of a cooler, lower-irradiance northern-Europeanclimate). Both systems have a total collector array area of 15 m2,equivalent to the roof area of a single residence, with the solarORCsystem being associated with a lower initial investment cost(capex) that is expected to play a role in the economic comparisonbetween the two systems. The electrical and thermal outputs of thetwo systems are found to be highly dependent on location. ThePVT system is found to provide an annual electricity output of2090 kWhe yr-1in the UK, which increases to 3620 kWhe yr-1inCyprus. This is equivalent to annual averages of 240 and 410 We,respectively, or between 60% and 110% of household demand.The corresponding additional thermal (hot water) output alsoincreases, from 860 kWhth yr-1in the UK, to 1870 kWhth yr-1inCyprus. It is found that the solar-ORC system performance ishighly sensitive to the system configuration chosen; the particularconfiguration studied here is found to be limited by the amount ofrejected thermal energy that can be reclaimed for water heating.The maximum electrical output from the solar-ORC configurationexplored in this study is 450 kWhe yr-1(

Conference paper

Taleb A, Sapin P, Barfuß C, White AJ, Fabris D, Markides CNet al., Wall temperature and system mass effects in a reciprocating gas spring, INTERNATIONAL CONFERENCE ON EFFICIENCY, COST, OPTIMIZATION, SIMULATION AND ENVIRONMENTAL IMPACT OF ENERGY SYSTEMS

Conference paper

Taleb AI, Timmer MAG, El-Shazly MY, Samoilov A, Kirillov VA, Markides CNet al., 2016, A single-reciprocating-piston two-phase thermofluidic prime-mover, Energy, Vol: 104, Pages: 250-265, ISSN: 0360-5442

We explore theoretically a thermodynamic heat-engine concept that has the potential of attaining a high efficiency and power density relative to competing solutions, while having a simple construction with few moving parts and dynamic seals, allowing low capital and operating costs, and long lifetimes. Specifically, an unsteady heat-engine device within which a working uid undergoes a power cycle featuring phase-change, termed the `Evaporative Reciprocating-Piston Engine' (ERPE), is considered as a potential prime mover foruse in combined heat and power (CHP) applications. Based on thermal/uid-electrical analogies, a theoretical ERPE device is conceptualized initially in the electrical-analogy domain as a linearized, closed-loop active electronic circuit model. The circuit-model representation is designed to potentially exhibit high efficiencies compared to similar, existing two-phase unsteady heat engines. From the simplified circuit model in the electrical domain, and using the thermal/uid-electrical analogies, one possible configuration of a correspondingphysical ERPE device is derived, based on an early prototype of a device currently under development that exhibits some similarities with the ERPE, and used as a physical manifestation of the proposed concept. The corresponding physical ERPE device relies on the alternating phase change of a suitable working-fluid (here, water) to drive a reciprocating displacement of a single vertical piston and to produce sustained oscillations of thermodynamic properties within an enclosed space. Four performance indicators are considered: the operational frequency, the power output, the exergy efficiency, and the heat input/temperature difference imposed externally on the device's heat exchangers that is necessary to sustain oscillations. The effects of liquidinertia, viscous drag, hydrostatic pressure, vapour compressibility and two-phase heat transfer in the various engine components/compartments a

Journal article

Cherdantsev AV, An J, Zadrazil I, Markides CNet al., An investigation of film wavy structure in annular flow using two simultaneous LIF approaches, 12th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics

The paper is devoted to development and validation of film thickness measurement techniques in interfacial gas-liquid flows. The specific flow investigated here is that of downwards (co-flowing) annular flow in a vertical pipe, however, many of the observations and findings are transferable to similar flow geometries. Two advanced spatially resolved techniques, namely planar laser-induced fluorescence and brightness-based laser-induced fluorescence , are used simultaneously in the same area of interrogation. A single laser sheet is used to excite fluorescence along one longitudinal section of the pipe, and two cameras (one for each method) are placed at different angles to the plane of the laser sheet in order to independently recover the shape of the interface along this section. This allows us to perform a cross-validation of the two techniques and to analyse their respective characteristics, advantages and shortcomings.

Conference paper

Acha Izquierdo S, Van Dam KH, Markides C, Shah N, Bustos-Turu Get al., 2016, Simulating residential electricity and heat demand in urban areas using an agent-based modelling approach, Energycon 2016, Publisher: IEEE

Cities account for around 75% of the global energy demand and are responsible for 60-70% of the global greenhouse gasses emissions. To reduce this environmental impact it is important to design efficient energy infrastructures able to deal with high level of renewable energy resources. A crucial element in this design is the quantitative understanding of the dynamics behind energy demands such as transport, electricity and heat. In this paper an agent-based simulation model is developed to generate residential energy demand profiles in urban areas, influenced by factors such as land use, energy infrastructure and user behaviour. Within this framework, impact assessment of low carbon technologies such as plug-in electric vehicles and heat pumps is performed using London as a case study. The results show that the model can generate important insights as a decision support tool for the design and planning of sustainable urban energy systems.

Conference paper

Barfuss C, 2016, Numerical Simulation of Heat Transfer Effects during Compression and Expansion in Gas Springs

Thesis dissertation

Sapin P, Taleb A, Barfuß C, White AJ, Fabris D, Markides CNet al., Thermodynamic Losses in a Gas Spring: Comparison of Experimental and Numerical Results, International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics

Reciprocating-piston devices can be used as high-efficiencycompressors and/or expanders. With an optimal valve design andby carefully adjusting valve timing, pressure losses during intakeand exhaust can be largely reduced. The main loss mechanismin reciprocating devices is then the thermal irreversibility dueto the unsteady heat transfer between the compressed/expandedgas and the surrounding cylinder walls. In this paper, pressure,volume and temperature measurements in a piston-cylindercrankshaft driven gas spring are compared to numerical results.The experimental apparatus experiences mass leakage while theCFD code predicts heat transfer in an ideal closed gas spring.Comparison of experimental and numerical results allows one tobetter understand the loss mechanisms in play. Heat and masslosses in the experiment are decoupled and the system lossesare calculated over a range of frequencies. As expected, compressionand expansion approach adiabatic processes for higherfrequencies, resulting in higher efficiency. The objective of thisstudy is to observe and explain the discrepancies obtained betweenthe computational and experimental results and to proposefurther steps to improve the analysis of the loss mechanisms.

Conference paper

Taleb A, Barfuß C, Sapin P, Willich C, White AJ, Fabris D, Markides CNet al., The Influence of Real Gases Effects on Thermally Induced Losses in Reciprocating Piston-Cylinder Systems, International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics

The efficiency of expanders is of prime importance for variousclean energy technologies. Once mechanical losses (e.g. throughvalves) are minimized, losses due to unsteady heat exchange betweenthe working fluid and the solid walls of the containingdevice can become the dominant loss mechanism. In this device,gas spring devices are investigated numerically in order to focusexplicitly on the thermodynamic losses that arise due to thisunsteady heat transfer. The specific aim of this study is to investigatethe behaviour of real gases in gas springs and comparethis to that of ideal gases in order to attain a better understandingof the impact of real gas effects on the thermally losses inreciprocating piston expanders and compressors. A CFD-modelof a gas spring is developed in OpenFOAM. Three different gasmodels are compared: an ideal gas model with constant thermodynamicand transport properties; an ideal gas model withtemperature-dependent properties; and a real gas model using thePeng-Robinson equation of state with temperature and pressuredependentproperties. Results indicate that, for simple, monoanddiatomic gases like helium or nitrogen, there is a negligibledifference in the pressure and temperature oscillations over a cyclebetween the ideal and real gas models. However, when lookingat a heavier (organic) molecule such as propane, the ideal gasmodel tends to overestimate the temperature and pressure comparedto the real gas model, especially if no temperature dependencyof thermodynamic properties is taken into account. Additionally,the ideal gas model (both alternatives) underestimatesthe thermally induced loss compared to the real gas model forheavier gases. Real gas effects must be taken into account in orderto predict accurately the thermally induced loss when usingheavy molecules in such devices.

Conference paper

Denner F, Pradas M, Charogiannis A, Markides C, van Wachem B, Kalliadasis Set al., 2016, Self-similarity of solitary waves on inertia-dominated falling liquid films, Physical Review E, Vol: 93, ISSN: 1539-3755

We propose consistent scaling of solitary waves on inertia-dominated falling liquid films, which accurately accounts for the driving physical mechanisms and leads to a self-similar characterization of solitary waves. Direct numerical simulations of the entire two-phase system are conducted using a state-of-the-art finite volume framework for interfacial flows in an open domain that was previously validated against experimental film-flow data with excellent agreement. We present a detailed analysis of the wave shape and the dispersion of solitary waves on 34 different water films with Reynolds numbers Re=20–120 and surface tension coefficients σ=0.0512–0.072Nm−1 on substrates with inclination angles β=19∘–90∘. Following a detailed analysis of these cases we formulate a consistent characterization of the shape and dispersion of solitary waves, based on a newly proposed scaling derived from the Nusselt flat film solution, that unveils a self-similarity as well as the driving mechanism of solitary waves on gravity-driven liquid films. Our results demonstrate that the shape of solitary waves, i.e., height and asymmetry of the wave, is predominantly influenced by the balance of inertia and surface tension. Furthermore, we find that the dispersion of solitary waves on the inertia-dominated falling liquid films considered in this study is governed by nonlinear effects and only driven by inertia, with surface tension and gravity having a negligible influence.

Journal article

Guarracino I, Mellor A, Ekins-Daukes N, Markides CNet al., 2016, Dynamic coupled thermal-and-electrical modelling of sheet-and-tube hybrid photovoltaic/thermal (PVT) collectors, Applied Thermal Engineering, Vol: 101, Pages: 778-795, ISSN: 1873-5606

In this paper we present a dynamic model of a hybrid photovoltaic/thermal (PVT) collector with a sheet-and-tube thermal absorber. The model is used in order to evaluate the annual generation of electrical energy along with the provision of domestic hot-water (DHW) from the thermal energy output, by using real climate-data at high temporal resolution. The model considers the effect of a non-uniform temperature distribution on the surface of the solar cell on its electrical power output. An unsteady 3-dimensional numerical model is developed to estimate the performance of such a collector. The model allows key design parameters of the PVT collector to vary so that the influence of each parameter on the system performance can be studied at steady state and at varying operating and atmospheric conditions. A key parameter considered in this paper is the number of glass covers used in the PVT collector. The results show that while the thermal efficiency increases with the additional glazing, the electrical efficiency deteriorates due to the higher temperature of the fluid and increased optical losses, as expected. This paper also shows that the use of a dynamic model and of real climate-data at high resolution is of fundamental importance when evaluating the yearly performance of the system. The results of the dynamic simulation with 1-min input data show that the thermal output of the system is highly dependent on the choice of the control parameters (pump operation, differential thermostat controller, choice of flow rate etc.) in response to the varying weather conditions. The effect of the control parameters on the system's annual performance can be captured and understood only if a dynamic modelling approach is used. The paper also discusses the use of solar cells with modified optical properties (reduced absorptivity/emissivity) in the infrared spectrum, which would reduce the thermal losses of the PVT collector at the cost of only a small loss in electrical output

Journal article

Charogiannis A, Heiles B, Mathie R, Matar O, Markides Cet al., Spatiotemporally resolved heat transfer measurements in falling-film flows over an inclined heated foil, International Symposium and School of Young Scientists INTERFACIAL PHENOMENA AND HEAT TRANSFER

Conference paper

Charogiannis A, Markides C, Zadrazil I, 2016, Thermographic Particle Velocimetry (TPV) for Simultaneous Interfacial Temperature and Velocity Measurements, International Journal of Heat and Mass Transfer, Vol: 97, Pages: 589-595, ISSN: 0017-9310

We present an experimental technique, that we refer to as ‘thermographic particle velocimetry’ (TPV),which is capable of the simultaneous measurement of two-dimensional (2-D) surface temperature andvelocity at the interface of multiphase flows. The development of the technique has been motivated bythe need to study gravity-driven liquid-film flows over inclined heated substrates, however, the samemeasurement principle can be applied for the recovery of 2-D temperature- and velocity-field informationat the interface of any flow with a sufficient density gradient between two fluid phases. The proposedtechnique relies on a single infrared (IR) imager and is based on the employment of highly reflective(here, silver-coated) particles which, when suspended near or at the interface, can be distinguished fromthe surrounding fluid domain due to their different emissivity. Image processing steps used to recover thetemperature and velocity distributions include the decomposition of each original raw IR image into separatethermal and particle images, the application of perspective distortion corrections and spatial calibration,and finally the implementation of standard particle velocimetry algorithms. This procedure isdemonstrated by application of the technique to a heated and stirred flow in an open container. In addition,two validation experiments are presented, one dedicated to the measurement of interfacial temperatureand one to the measurement of interfacial velocity. The deviations between the results generatedfrom TPV and those from accompanying conventional techniques do not exceed the errors associatedwith the latter.

Journal article

Chatzopoulou MA, Keirstead J, Fisk D, Markides CNet al., Characterising the impact of HVAC design variables on buildings energy performance, using a Global Sensitivity Analysis framework, CLIMA 2016 - 12th REHVA World Congress

Conference paper

Chatzopoulou MA, Keirstead J, Fisk D, Markides CNet al., Informing low carbon HVAC systems modelling and design, using a Global Sensitivity analysis framework, ASME 2016 Power and Energy

Conference paper

Le Brun N, Markides, Bismarck, Zadrazil, Lewis Normanet al., 2016, On the drag reduction effect and shear stability of improved acrylamide copolymers for enhanced hydraulic fracturing, Chemical Engineering Science, Vol: 146, Pages: 135-143, ISSN: 0009-2509

Polymeric drag reducers, such as partially hydrolysed polyacrylamide (PHPAAm), are important chemical additives in hydraulic fracturing fluids as they can significantly decrease the frictional pressure drop in the casing (by up to 80%),resulting in an increase of the injection rate that can be delivered to the fracturing point. The incorporation of sodium 2-acrylamido-2-methylpropane sulfonic acid (NaAMPS) moieties in to polyacrylamide (PAAm) can further improve the performance of fracturing fluids by addressing some compatibility issues related to the use of PHPA Am, e.g., the sensitivity to water salinity . In this study, three types of poly(acrylamide-co-NaAMPS) and pure PHPAAm were investigated with respect to polymer induced drag reduction and mechanical polymer degradationin turbulent pipe flow in a pressure-driven pipe flow facility. The test section comprised a horizontal 1” bore circular cross-section pipe. The facility was modified in order to allow, long time/length experiments by automatically recirculating the polymer solution in a closed-loop through the test section.The presence of NaAMPS groups in the copolymer backbone is found to increase the ability of PHPAAm to reduce frictional drag while the vulnerability to mechanical degradation remains unaffected. The drag reduction of NaAMPS copolymer solutions can be described by a modified version of Virk’s correlation (1967), extended to include the effect of Reynolds number. Polymer mechanical degradation is found to proceed until the friction reducer is almost ineffective in reducing drag. This phenomenon is in contrast with the most common correlationfor polymer degradation, which predicts the existence of a n asymptotic(but finite) limit to the reduced drag reduction.

Journal article

Acha S, Le Brun N, Lambert R, Bustos-Turu G, Shah N, Markides CNet al., 2016, UK half-hourly regional electricity cost modelling for commercial end users

© 2018 University of Minho. All rights reserved. The rising prices of electricity in the UK risks rendering businesses uncompetitive if these costs are not controlled. This issue has created the need to properly comprehend the tariffs and costing framework that influence the total cost of electricity for non-domestic customers. This paper details an open source method to model UK electricity regional costs (MUKERC) for commercial end-users; allowing users to visualise and calculate the cost of the electricity they consume. The methodology consists in a bottom-up model that defines individually all the tariff components and then aggregates them to quantify the cost of a kWh across each half-hour of the day. The disaggregated structure of MUKERC allows users to conduct specific analysis of tariff components and to understand their rich temporal and spatial features. This granularity facilitates understanding which tariffs influence costs more during different time periods. Emphasis is given to showcasing commodity prices and network charges; known as Transmission Use of System and Distribution Use of System tariffs. ‘Representative day’ electricity price curves for different day types, voltage level connections, and across different UK regions for 2016-17 are presented. Outputs from MUKERC can better inform companies on their energy costs and therefore allows them to perform comprehensive and bespoke energy management and energy efficiency strategies as it is possible to understand when and where the cost of electricity is more expensive. Results show that commercial buildings connected at Low Voltage in North Wales and Merseyside and the South West face the highest average electricity prices, whereas consumers connected to High Voltage in London and the North West have the cheapest electricity in the UK. Other significant findings indicate sites connected at low-voltage pay 7.5% more than high-voltage sites, winter weekday costs are 18% higher than s

Conference paper

An JS, Morgan RG, Hale CP, Zadrazil I, Hewitt GF, Markides CNet al., 2016, A three-phase slug flow investigation by tomographic dual-beam X-ray imaging: Slug frequency measurement and lessons for correlation development and application, Multiphase Science and Technology, Vol: 28, Pages: 71-98, ISSN: 0276-1459

© 2016 by Begell House, Inc. New measurement data on three-phase (air-oil-water) slug flows are reported in a long (37 m), largediameter (3 in nominal, 77.9 mm bore) pipe, generated by using a nonintrusive technique based on a dual-beam X-ray tomography system. Based on this measurement data, the frequency of hydrodynamic slugs is determined at a position 30.6 m (∼400 diameters) downstream of the inlet and over a range of inlet flow conditions with superficial velocities: 2-6 m/s (air), 0-0.5 m/s (oil), 0-0.5 m/s (water), from which slug frequency trends specific to this three-phase flow system are identified and reported in the literature for the first time. The slug frequency data are subsequently used to examine the feasibility and reliability of using slightly modified versions of many of the current two-phase gas-liquid slug frequency correlations in order to predict the measured three-phase slug frequencies observed in the experiments. It is found that, in general, these correlations provide poor slug frequency predictions in the investigated flows; nevertheless, the correlations that tend to perform best are those that include terms that attempt to account for variations in the fluid properties. The approach presented in this paper provides a method for reasonable three-phase slug frequency prediction as a first approximation, although the accuracy of this prediction can be improved if the apparent liquid-liquid mixture-viscosity can be determined more reliably in situ. The data made available in the present paper are to the best knowledge of the authors not presently available in the literature, and can be used to develop and validate advanced multiphase flow models, beyond acting as a benchmark database for correlation checks and improvement, as is done here.

Journal article

Chatzopoulou M-A, Keirstead J, Fisk D, Markides CNet al., 2016, INFORMING LOW CARBON HVAC SYSTEMS MODELLING AND DESIGN, USING A GLOBAL SENSITIVITY ANALYSIS FRAMEWORK, 10th ASME International Conference on Energy Sustainability, Publisher: AMER SOC MECHANICAL ENGINEERS

Conference paper

Oyewunmi OA, Markides CN, 2015, EFFECT OF WORKING-FLUID MIXTURES ON ORGANIC RANKINE CYCLE SYSTEMS: HEAT TRANSFER AND COST ANALYSIS, 3RD International Seminar on ORC Power Systems, Publisher: University of Liège and Ghent University

The present paper considers the employment of working-fluid mixtures in organic Rankine cycle (ORC)systems with respect to heat transfer performance, component sizing and costs, using two sets of fluidmixtures: n-pentane + n-hexane and R-245fa + R-227ea. Due to their non-isothermal phase-change behaviour,these zeotropic working-fluid mixtures promise reduced exergy losses, and thus improved cycleefficiencies and power outputs over their respective pure-fluid components. Although the fluid-mixturecycles do indeed show a thermodynamic improvement over the pure-fluid cycles, the heat transfer andcost analyses reveal that they require larger evaporators, condensers and expanders; thus, the resultingORC systems are also associated with higher costs, leading to possible compromises. In particular,70 mol% n-pentane + 30 mol% n-hexane and equimolar R-245fa + R-227ea mixtures lead to the thermodynamicallyoptimal cycles, whereas pure n-pentane and pure R-227ea have lower costs amounting to14% and 5% per unit power output over the thermodynamically optimal mixtures, respectively.

Conference paper

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