327 results found
An JS, Morgan RG, Hale CP, et 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.
Acha S, Le Brun N, Lambert R, et 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
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.
Markides CN, 2015, Low-Concentration Solar-Power Systems Based on Organic Rankine Cycles for Distributed-Scale Applications: Overview and Further Developments, Frontiers in Energy Research, Vol: 3, ISSN: 2296-598X
This paper is concerned with the emergence and development of low-to-medium-grade thermal-energy-conversion systems for distributed power generation based on thermo- dynamic vapor-phase heat-engine cycles undergone by organic working uids, namely organic Rankine cycles (ORCs). ORC power systems are, to some extent, a relatively established and mature technology that is well-suited to converting low/medium-grade heat (at temperatures up to ~300–400°C) to useful work, at an output power scale from a few kilowatts to 10s of megawatts. Thermal ef ciencies in excess of 25% are achievable at higher temperatures and larger scales, and efforts are currently in progress to improve the overall economic viability and thus uptake of ORC power systems, by focusing on advanced architectures, working- uid selection, heat exchangers and expansion machines. Solar-power systems based on ORC technology have a signi cant potential to be used for distributed power generation, by converting thermal energy from simple and low-cost non-concentrated or low-concentration collectors to mechanical, hydrau- lic, or electrical energy. Current elds of use include mainly geothermal and biomass/ biogas, as well as the recovery and conversion of waste heat, leading to improved energy ef ciency, primary energy (i.e., fuel) use and emission minimization, yet the technology is highly transferable to solar-power generation as an affordable alternative to small-to- medium-scale photovoltaic systems. Solar-ORC systems offer naturally the advantages of providing a simultaneous thermal-energy output for hot water provision and/or space heating, and the particularly interesting possibility of relatively straightforward onsite (thermal) energy storage. Key performance characteristics are presented, and important heat transfer effects that act to limit performance are identi ed as noteworthy directions of future research for the further development of this technology.
Oyewunmi OA, Taleb A, Haslam A, et al., 2015, On the use of SAFT-VR Mie for assessing large-glide fluorocarbon working-fluid mixtures in organic rankine cycles, Applied Energy, Vol: 163, Pages: 263-282, ISSN: 1872-9118
By employing the SAFT-VR Mie equation of state, molecular-based models are developed from which the thermodynamic properties of pure (i.e., single-component) organic fluids and their mixtures are calculated. This approach can enable the selection of optimal working fluids in organic Rankine cycle (ORC) applications, even in cases for which experimental data relating to mixture properties are not available. After developing models for perfluoroalkane (n-C4F10 + n-C10F22) mixtures, and validating these against available experimental data, SAFT-VR Mie is shown to predict accurately both the single-phase and saturation properties of these fluids. In particular, second-derivative properties (e.g., specific heat capacities), which are less reliably calculated by cubic equations of state (EoS), are accurately described using SAFT-VR Mie, thereby enabling an accurate prediction of important working-fluid properties such as the specific entropy. The property data are then used in thermodynamic cycle analyses for the evaluation of ORC performance and cost. The approach is applied to a specific case study in which a sub-critical, non-regenerative ORC system recovers and converts waste-heat from a refinery flue-gas stream with fixed, predefined conditions. Results are compared with those obtained when employing analogue alkane mixtures (n-C4H10 + n-C10H22) for which sufficient thermodynamic property data exist. When unlimited quantities of cooling water are utilized, pure perfluorobutane (and pure butane) cycles exhibit higher power outputs and higher thermal efficiencies compared to mixtures with perfluorodecane (or decane), respectively. The effect of the composition of a working-fluid mixture in the aforementioned performance indicators is non-trivial. Only at low evaporator pressures (< 10 bar) do the investigated mixtures perform better than the pure fluids. A basic cost analysis reveals that systems with pure perfluorobutane (and butane) fluids are associated with rela
Markides CN, Mathie R, Charogiannis A, 2015, An experimental study of spatiotemporally resolved heat transfer in thin liquid-film flows falling over an inclined heated foil, International Journal of Heat and Mass Transfer, Vol: 93, Pages: 872-888, ISSN: 0017-9310
This paper describes the development of an experimental technique that combines simultaneous planar laser-induced fluorescence (PLIF) and infrared (IR) thermography imaging, and its application to the measurement of unsteady and conjugate heat-transfer in harmonically forced, thin liquid-film flows falling under the action of gravity over an inclined electrically heated-foil substrate. Quantitative, spatiotemporally resolved and simultaneously conducted measurements are reported of the film thickness, film free-surface temperature, solid–liquid substrate interface temperature, and local/instantaneous heat flux exchanged with the heated substrate. Based on this information, local and instantaneous heat-transfer coefficients (HTCs) are recovered. Results concerning the local and instantaneous HTC and how this is correlated with the local and instantaneous film thickness suggest considerable heat-transfer enhancement relative to steady-flow predictions in the thinner film regions. This behaviour is attributed to a number of unsteady/mixing transport processes within the wavy films that are not captured by laminar, steady-flow analysis. The Nusselt number Nu increases with the Reynolds number Re; at low Re values the mean Nu number corresponds to 2.5, in agreement with the steady-flow theory, while at higher Re, both the Nu number and the HTC exhibit significantly enhanced values. Evidence that the HTC becomes decoupled from the film thickness for the upper range of observed film thicknesses is also presented. Finally, smaller film thickness fluctuation intensities were associated with higher HTC fluctuation intensities, while the amplitude of the wall temperature fluctuations was almost proportional to the amplitude of the HTC fluctuations.
Markides CN, Herrando M, 2015, Hybrid PV and solar-thermal systems for domestic heat and power provision in the UK: Techno-economic considerations, Applied Energy, Vol: 161, Pages: 512-532, ISSN: 0306-2619
A techno-economic analysis is undertaken to assess hybrid PV/solar-thermal (PVT) systems for distributedelectricity and hot-water provision in a typical house in London, UK. In earlier work (Herrando et al., 2014), asystem model based on a PVT collector with water as the cooling medium (PVT/w) was used to estimateaverage year-long system performance. The results showed that for low solar irradiance levels and lowambient temperatures, such as those associated with the UK climate, a higher coverage of total householdenergy demands and higher CO2 emission savings can be achieved by the complete coverage of the solar collectorwith PV and a relatively low collector cooling flow-rate. Such a PVT/w system demonstrated an annualelectricity generation of 2.3 MW h, or a 51% coverage of the household’s electrical demand (compared to anequivalent PV-only value of 49%), plus a significant annual water heating potential of to 1.0 MW h, or a 36%coverage of the hot-water demand. In addition, this system allowed for a reduction in CO2 emissionsamounting to 16.0 tonnes over a life-time of 20 years due to the reduction in electrical power drawn fromthe grid and gas taken from the mains for water heating, and a 14-tonne corresponding displacement of primaryfossil-fuel consumption. Both the emissions and fossil-fuel consumption reductions are significantlylarger (by 36% and 18%, respectively) than those achieved by an equivalent PV-only system with the samepeak rating/installed capacity. The present paper proceeds further, by considering the economic aspects ofPVT technology, based on which invaluable policy-related conclusions can be drawn concerning the incentivesthat would need to be in place to accelerate the widespread uptake of such systems. It is found that,with an electricity-only Feed-In Tariff (FIT) support rate at 43.3 p/kW h over 20 years, the system cost estimatesof optimised PVT/w systems have an 11.2-year discounted payback period (PV-only: 6.8 years). Therole and i
Le Brun N, Markides CNM, 2015, A Galinstan-Filled Capillary Probe for Thermal Conductivity Measurements and its Application to Molten Eutectic KNO3-NaNO3-NO2 (HTS) up to 700 K, International Journal of Thermophysics, Vol: 36, Pages: 3222-3238, ISSN: 1572-9567
The successful measurement of the thermal conductivity of molten salts isa challenging undertaking due to the electrically conducting and possibly alsoaggressive nature of the materials, as well as the elevated temperatures atwhich these data are required. For accurate and reproducible measurementsit is important to develop a suitable experimental apparatus and methodology.In this study we explore a modified version of the transient hot-wiremethod, which employs a molten-metal-filled capillary in order to circumventsome of the issues encountered in previous studies. Specifically, by using anovel flexible U-shaped quartz-capillary, filled with a eutectic mixture of gallium,indium and tin, commercially known as Galinstan, we proceed to measurethe thermal conductivity of molten eutectic KNO3−NaNO3−NaNO2.The new probe is demonstrated as being able to measure the thermal conductivityof this molten salt, which is found to range from 0.48 W/m K at500 K to 0.47 W/m K at close to 700 K, with an overall expanded uncertainty(95% confidence) of 3.1%. The quartz is found to retain its electricallyinsulating properties and no current leakage is detected in the sample overthe investigated temperature range. The thermal conductivity data reportedin the present study are also used to elucidate a partial disagreement foundin the literature for this material.
Palanisamy K, Taleb AI, Markides CN, 2015, Optimizing the Non-Inertive-Feedback Thermofluidic Engine for the Conversion of Low-Grade Heat to Pumping Work, HEAT TRANSFER ENGINEERING, Vol: 36, Pages: 1303-1320, ISSN: 0145-7632
Markides CN, Heyes AL, 2015, Selected Papers from the Thirteenth UK Heat Transfer Conference, HEAT TRANSFER ENGINEERING, Vol: 36, Pages: 1163-1164, ISSN: 0145-7632
Ibarra R, Zadrazil I, Markides CN, et al., 2015, Towards a Universal Dimensionless Map of Flow Regime Transitions in Horizontal Liquid-Liquid Flows, 11th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics
Charogiannis A, Markides CN, Denner F, et al., 2015, A simultaneous application of PLIF-PIV-PTV for the detailed experimental study of the hydrodynamic characteristics of thin film flows, 11th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics (HEFAT2015)
Kirmse C, Oyewunmi OA, Haslam AJ, et al., 2015, A two-phase single-reciprocating-piston heat conversion engine, 11th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics (HEFAT2015)
This paper considers an energy-conversion heat-engineconcept termed ‘Up-THERM’. This machine is capable ofconverting low- to medium-grade heat to useful positivedisplacementwork through the periodic evaporation andcondensation of a working fluid in an enclosed space. Thesealternating phase-change processes drive sustained oscillations ofthermodynamic properties (pressure, temperature, volume) as theworking fluid undergoes an unsteady thermodynamic heatenginecycle. The resulting oscillatory flow of the working fluidis converted into a unidirectional flow in a hydraulic loadarrangement where power can be extracted from the machine.The engine is described with lumped dynamic modelsconstructed using electrical analogies founded on previouslydeveloped thermoacoustic and thermofluidic principles, whichare extended here to include a description of the phase-changeheat-transfer processes. For some sub-components of the engine,such as the gas spring, valves and the temperature profile in theheat exchangers, deviations from the linear theory are nonnegligible.These are modelled using non-linear descriptions. Inparticular, the results of linear and non-linear descriptions of thegas spring are compared using three important performanceindicators — efficiency, power output and frequency.The non-linear description of the gas spring results in morerealisticpredictions of the oscillation frequency compared todirect measurements on an experimental prototype of a similarengine. Owing to its mode of operation and lack of moving parts,the Up-THERM engine does offer a much simpler and morecost-efficient solution than alternative engines for heat recoveryand solar applications. The results from this work suggest thatthis technology can be a competitive alternative in terms of costper unit power in low-power, small-scale applications, especiallyin remote, off-grid settings, for example in developing countrieswhere minimising upfront costs is crucial.
Oyewunmi OA, Haslam AJ, Markides CN, 2015, Towards the computer-aided molecular design of organic rankine cycle systems with advanced fluid theories, SusTEM 2015 International Conference, Pages: 180-189
Organic Rankine cycle (ORC) power-generation systems are increasingly being deployed for heat recovery and conversion from geothermal reservoirs and in several industrial settings. Using a case study of an exhaust flue-gas stream, an ORC power output in excess of 20 MW is predicted at thermal efficiencies ranging between 5% and 15%. The considerable influence on cycle performance of the choice of the working fluid is illustrated with alkane and perfluoroalkane systems modelled using the SAFT-VR Mie equation of state (EoS); in general, the more-volatile pure components (n-butane or n-perfluorobutane) are preferred although some mixtures perform better at restricted cycle conditions.The development of computer-aided molecular design (CAMD) platforms for ORC systems requires both cycle and working-fluid models to be incorporated into a single framework, for the purposes of whole-system design and optimization. Using pure alkanes and their mixtures as a case study, we test the suitability of the recent group-contribution SAFT- Mie EoS method for describing the thermodynamic properties of working fluids relevant to the analysis of ORC systems. The theory is shown to predict accurately the relevant properties of these fluids, thereby suggesting that this SAFT-based CAMD approach is a promising approach towards working-fluid design of ORC power systems.
Charogiannis A, An JS, Markides CN, 2015, A Simultaneous Planar Laser-Induced Fluorescence, Particle Image Velocimetry and Particle Tracking Velocimetry Technique for the Investigation of Thin Liquid-Film Flows, Experimental Thermal and Fluid Science, Vol: 68, Pages: 516-536, ISSN: 0894-1777
Ibarra R, Matar OK, Markides CN, et al., 2015, An experimental study of oil-water flows in horizontal pipes, Multiphase 2015, Publisher: BHR Group
This paper reports an effort to investigate the effect of flow velocities and inlet configurations on horizontal oil-water flows in a 32 mm ID acrylic pipe using water and an aliphatic oil (Exxsol D140) as test fluids. The flows of interest were analysed using pressure drop measurements and high-speed photography in an effort to obtain a flow pattern map, pressure gradient profiles and measures of the in situ phase fractions. The experiments reveal a particular effect of the inlet configuration on the observed flow pattern. A horizontal plate, installed at the inlet, generates a transition to stratified flow when the plate height closely matched the in situ water height at high input oil fractions.
Kirmse C, Taleb AJ, Oyewunmi OA, et al., 2015, Performance comparison of a novel thermofluidic organic-fluid heat converter and an organic rankine cycle heat engine, 3rd International Seminar on ORC Power Systems (ASME ORC 2015)
The Up-THERM engine is a novel two-phase heat engine with a single moving part–a vertical solidpiston–that relies on the phase change of a suitable working fluid to produce a reciprocating displacementand sustained thermodynamic oscillations of pressure and flow rate that can be converted to useful work.A model of the Up-THERM engine is developed via lumped dynamic descriptions of the various enginesub-components and electrical analogies founded on previously developed thermoacoustic principles.These are extended here to include a description of phase change and non-linear descriptions of selectedprocesses. The predicted first and second law efficiencies and the power output of a particular Up-THERM engine design aimed for operation in a specified CHP application with heat source and sinktemperatures of 360 ○C and 10 ○C, are compared theoretically to those of equivalent sub-critical, nonregenerativeorganic Rankine cycle (ORC) engines. Five alkanes (from n-pentane to n-nonane) are beingconsidered as possible working fluids for the aforementioned Up-THERM application, and these arealso used for the accompanying ORC thermodynamic analyses. Owing to its mode of operation, lackof moving parts and dynamic seals, the Up-THERM engine promises a simpler and more cost-effectivesolution than an ORC engine, although the Up-THERM is expected to be less efficient than its ORCcounterpart. These expectations are confirmed in the present work, with the Up-THERM engine showinglower efficiencies and power outputs than equivalent ORC engines, but which actually approach ORCperformance at low temperatures. Therefore, it is suggested that the Up-THERM can be a competitivealternative in terms of cost per unit power in low-power/temperature applications, especially in remote,off-grid settings, such as in developing countries where minimising upfront costs is crucial.
Freeman J, Hellgardt K, Markides CN, 2015, An Assessment of Solar-Thermal Collector Designs for Small-Scale Combined Heating and Power Applications in the UK, Heat Transfer Engineering, Vol: 36, Pages: 1332-1347, ISSN: 1521-0537
Freeman J, Hellgardt K, Markides CN, 2015, An assessment of solar-powered organic Rankine cycle systems for combined heating and power in UK domestic applications, Applied Energy, Vol: 138, Pages: 605-620, ISSN: 1872-9118
Markides CN, Smith TCB, 2015, A Dynamic Model for the Optimization of Oscillatory Low Grade Heat Engines, International Conference of Computational Methods in Sciences and Engineering (ICCMSE 2010), Publisher: American Institute of Physics (AIP), Pages: 417-420, ISSN: 1551-7616
The efficiency of a thermodynamic system is a key quantity on which its usefulness and wider applicationrelies. This is especially true for a device that operates with marginal energy sources and close to ambient temperatures.Various definitions of efficiency are available, each of which reveals a certain performance characteristic of a device. Ofthese, some consider only the thermodynamic cycle undergone by the working fluid, whereas others contain additionalinformation, including relevant internal components of the device that are not part of the thermodynamic cycle. Yet othersattempt to factor out the conditions of the surroundings with which the device is interfacing thermally during operation. Inthis paper we present a simple approach for the modeling of complex oscillatory thermal-fluid systems capable ofconverting low grade heat into useful work. We apply the approach to the NIFTE, a novel low temperature difference heatutilization technology currently under development. We use the results from the model to calculate various efficienciesand comment on the usefulness of the different definitions in revealing performance characteristics. We show that theapproach can be applied to make design optimization decisions, and suggest features for optimal efficiency of the NIFTE.
Markides CN, Smith TCB, A dynamic model for the optimization of oscillatory low grade heat engines, AIP Conference Proceedings, Vol: 1642, ISSN: 1551-7616
The efficiency of a thermodynamic system is a key quantity on which its usefulness and wider application relies. This is especially true for a device that operates with marginal energy sources and close to ambient temperatures. Various definitions of efficiency are available, each of which reveals a certain performance characteristic of a device. Of these, some consider only the thermodynamic cycle undergone by the working fluid, whereas others contain additional information, including relevant internal components of the device that are not part of the thermodynamic cycle. Yet others attempt to factor out the conditions of the surroundings with which the device is interfacing thermally during operation. In this paper we present a simple approach for the modeling of complex oscillatory thermal-fluid systems capable of converting low grade heat into useful work. We apply the approach to the NIFTE, a novel low temperature difference heat utilization technology currently under development. We use the results from the model to calculate various efficiencies and comment on the usefulness of the different definitions in revealing performance characteristics. We show that the approach can be applied to make design optimization decisions, and suggest features for optimal efficiency of the NIFTE.
Hussain T, Markides CN, Balachandran R, 2015, Flame dynamics in a micro-channeled combustor, Publisher: American Institute of Physics (AIP), ISSN: 1551-7616
The increasing use of Micro-Electro-Mechanical Systems (MEMS) has generated a significant interest in combustion-based power generation technologies, as a replacement of traditional electrochemical batteries which are plagued by low energy densities, short operational lives and low power-to-size and power-to-weight ratios. Moreover, the versatility of integrated combustion-based systems provides added scope for combined heat and power generation. This paper describes a study into the dynamics of premixed flames in a micro-channeled combustor. The details of the design and the geometry of the combustor are presented in the work by Kariuki and Balachandran . This work showed that there were different modes of operation (periodic, a-periodic and stable), and that in the periodic mode the flame accelerated towards the injection manifold after entering the channels. The current study investigates these flames further. We will show that the flame enters the channel and propagates towards the injection manifold as a planar flame for a short distance, after which the flame shape and propagation is found to be chaotic in the middle section of the channel. Finally, the flame quenches when it reaches the injector slots. The glow plug position in the exhaust side ignites another flame, and the process repeats. It is found that an increase in air flow rate results in a considerable increase in the length (and associated time) over which the planar flame travels once it has entered a micro-channel, and a significant decrease in the time between its conversion into a chaotic flame and its extinction. It is well known from the literature that inside small channels the flame propagation is strongly influenced by the flow conditions and thermal management. An increase of the combustor block temperature at high flow rates has little effect on the flame lengths and times, whereas at low flow rates the time over which the planar flame front can be observed decreases and the time of e
Markides CN, 2015, Preface of the “Symposium on processes and systems for efficient clean energy generation, utilisation and thermal management”, AIP Conference Proceedings, Vol: 1642, ISSN: 1551-7616
Zadrazil I, Matar OK, Markides CN, 2014, Phase-locked measurements of gas-liquid horizontal flows, American Physical Society - Division of Fluid Dynamics
A flow of gas and liquid in a horizontal pipe can be described in terms of various flow regimes, e.g. wavy stratified, annular or slug flow. These flow regimes appear at characteristic gas and liquid Reynolds numbers and feature unique wave phenomena. Wavy stratified flow is populated by low amplitude waves whereas annular flow contains high amplitude and long lived waves, so called disturbance waves, that play a key role in a liquid entrainment into the gas phase (droplets). In a slug flow regime, liquid-continuous regions travel at high speeds through a pipe separated by regions of stratified flow. We use a refractive index matched dynamic shadowgraphy technique using a high-speed camera mounted on a moving robotic linear rail to track the formation and development of features characteristic for the aforementioned flow regimes. We show that the wave dynamics become progressively more complex with increasing liquid and gas Reynolds numbers. Based on the shadowgraphy measurements we present, over a range of conditions: (i) phenomenological observations of the formation, and (ii) statistical data on the downstream velocity distribution of different classes of waves.
Charogiannis A, Zadrazil I, Markides CN, 2014, Wave and flow field phenomena in planar falling films by simultaneous Laser-Induced Fluorescence and Particle Image/Tracking Velocimetry, American Physical Society - Division of Fluid Dynamics
Zadrazil I, Markides CN, 2014, Turbulence Scaling in Pipe Flows Exhibiting Polymer-Induced Drag Reduction, American Physical Society - Division of Fluid Dynamics
Non-intrusive laser based diagnostics technique, namely Particle Image Velocimetry, was used to in detail characterise polymer induced drag reduction in a turbulent pipe flow. The effect of polymer additives was investigated in a pneumatically-driven flow facility featuring a horizontal pipe test section of inner diameter 25.3 mm and length 8 m. Three high molecular weight polymers (2, 4 and 8 MDa) at concentrations of 5 -- 250 wppm were used at Reynolds numbers from 35000 to 210000. The PIV derived results show that the level of drag reduction scales with different normalised turbulence parameters, e.g. streamwise and spanwise velocity fluctuations, vorticity or Reynolds stresses. These scalings are dependent of the distance from the wall, however, are independent of the Reynolds numbers range investigated.
Gupta A, Mathie R, Markides CN, 2014, An experimental and computational investigation of a thermal storage system based on a phase change material: Heat transfer and performance characterization, Computational Thermal Sciences, Vol: 6, Pages: 341-359, ISSN: 1940-2554
The integration of latent heat storage solutions into modern heating and cooling systems has the potential to enhance overall system performance compared to standard hot water systems (radiators and tanks) due to an augmentation of the stored heat by the latent heat of a suitable material. This paper presents computational predictions complemented by experimental measurements of the dynamic behavior and performance of an active thermal storage system for domestic applications, based on the use of a hydrated salt phase change material (PCM) and a conventional cylindrical storage tank. The thermal storage (heating) and extraction (cooling) rates for this PCM-filled tank are compared to a water-filled tank. Flow and temperature fields are analyzed in a customized storage tank design for heat transfer and performance characterization. Experimental findings show good agreement with full 3-D simulation results. The heat removal characteristic is identified as being the main factor limiting the arrangement’s performance when compared to a water-based system, due to the solidification of the PCM onto the pipes, and a significant consequent decrease in heat flux. It is confirmed that the PCM thermal storage solution has the capability to store a large amount of heat effectively, but design improvements are required to eliminate the cooling-limited heat transfer process in the investigated arrangement.
Oyewunmi OA, Taleb AI, Haslam AJ, et al., 2014, An assessment of working-fluid mixtures using SAFT-VR Mie for use in organic Rankine cycle systems for waste-heat recovery, Computational Thermal Sciences, Vol: 6, Pages: 301-316, ISSN: 1940-2503
© 2014 by Begell House, Inc. Working-fluid mixtures offer an improved thermal match to heat source streams in organic Rankine cycles (ORCs) over pure (single) fluids. In the present work we investigate the selection of working-fluid mixtures and component mixing ratios for an ORC system from a thermodynamic and economic point of view. A mathematical model of a subcritical, nonregenerative ORC is constructed. We employ the SAFT-VR Mie equation of state, a state-of-the-art version of the statistical associating fluid theory (SAFT), to predict the thermodynamic state properties and phase behavior of the fluid mixtures. The effect of the working-fluid mixture selection on the efficiency and power output from the cycle is investigated, as is its effect on the sizes of the various components of the ORC engine. This is done in order to appreciate the role that the fluid mixtures have on the investment/capital costs attributed to the installation of such a unit, intended for waste-heat recovery and conversion to power. Results of an ORC using a binary decane–butane mixture as the working fluid demonstrate a significant improvement in the cost per unit power output compared to the two pure fluid components. Specifically, the added costs of the four main ORC system components (pump, expander, and two heat exchangers) were found to be as low as 120–130 £/kW, 20–30% lower compared to the pure fluids.
McTigue JD, White AJ, Markides CN, 2014, Parametric studies and optimisation of pumped thermal electricity storage, Applied Energy, Vol: 137, Pages: 800-811, ISSN: 0306-2619
Guarracino I, Freeman J, Markides CN, 2014, Dynamic testing and modelling of solar collectors, 14th UK Heat Transfer Conference 2015
Solar-thermal collectors operating under real conditions rarely reach steady-state operation due to temporal fluctuations inthe climate/environmental conditions and thermal loads. Figure 1 shows typical UK weather map. The incident irradiance,the ambient temperature and the wind speed can vary during the day as shown in Figures 2-4. These figures show the datacollected in London at a temporal resolution of 1-minute. As a consequence of the time-varying inputs, collector models thatdescribe dynamic behaviour are required for the accurate prediction of the thermal output and for optimising the controlstrategy of such systems. We develop detailed 3-D thermal sub-models that can be adapted to various geometries orcollector configurations, including vacuum-tube thermal collectors, sheet-and-tube thermal and PV/T collectors.
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