## Publications

284 results found

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

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Zadrazil I, Matar OK, Markides CN, 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, 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., 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., 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

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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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- Citations: 18

Weinberg FJ, Dunn-Rankin D, Carleton FB,
et al., 2013, Electrical aspects of flame quenching, *PROCEEDINGS OF THE COMBUSTION INSTITUTE*, Vol: 34, Pages: 3295-3301, ISSN: 1540-7489

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- Citations: 15

Zadrazil I, Matar OK, Markides CN, 2013, On the frequency of large waves in vertical gas-liquid annular flows

Mathie R, Markides CN, 2013, Heat Transfer Augmentation in Unsteady Conjugate Thermal Systems – Part I: Semi-Analytical 1-D Framework, *International Journal of Heat and Mass Transfer*, Vol: 56, Pages: 802-818

This paper is concerned with the phenomenon of heat transfer augmentation in one-dimensional unsteady and conjugate fluid–solid systems. The overriding purpose is to propose a simple framework for the description of the effect of unsteadiness on the overall heat exchange performance of these systems, leading to the improved understanding and prediction of related processes. An analytical model is developed that describes the thermal interaction between the solid and the fluid domains with the use of a time-varying heat transfer coefficient. Augmentation is a non-linear effect that arises from an interaction between fluctuations in the heat transfer coefficient and fluctuations in the fluid–solid temperatures. It describes the difference between the time-averaged heat transfer rate, and the multiple of the time-averaged heat transfer coefficient and the time-averaged temperature difference across the fluid. It is found that the degree of augmentation can be defined in terms of key independent problem variables, including: a time-averaged Biot number, a dimensionless solid thickness (normalised by an unsteady thermal diffusion lengthscale), a heat transfer coefficient fluctuation intensity (amplitude normalised by the mean), and a heat capacity ratio between the fluid and solid domains. The model is used to produce regime maps that describe the range of conditions in which augmentation effects are exhibited. Such maps can be used in the design of improved heat exchangers or thermal insulation, for example through the novel selection of materials that can exploit these augmentation effects. Cases are considered for which the bulk fluid temperature is fixed, and for which the bulk fluid temperature is allowed to respond to the solid, both in thermally developing and fully developed flows. Generally the augmentation effect is found to be negative, reflecting a reduction in the heat exchange capability. However, regions of positive augmentation are uncovered i

Markides CN, Chakraborty N, 2013, Statistics of the Scalar Dissipation Rate using Direct Numerical Simulations and Planar Laser-Induced Fluorescence Data, *Chemical Engineering Science*

The statistics of the scalar dissipation rate (SDR) in gaseous flows with Schmidt numbers close to unity were examined in a joint numerical and experimental effort, in a shearless mixing layer in the presence of decaying turbulence using three-dimensional Direct Numerical Simulations (DNS), and in an axisymmetric plume formed by the continuous low-momentum release of an acetone-laden stream (used as a tracer to measure the passive scalar) along the centreline of a turbulent pipe flow of air downstream of a turbulence generating grid using Planar Laser-Induced Fluorescence (PLIF). For the flows examined good agreement was found between the DNS and the experiment, both of which indicate that: (i) the probability density functions of the unconditional and conditional SDR show small departures from lognormality; (ii) the ratio of the standard deviation of the unconditional SDR to its respective Reynolds-averaged mean, as well as the ratio of the standard deviation of the conditional SDR to its conditional mean (these ratios do not vary strongly with the value of the mixture fraction at which it is evaluated), both increase over a few Kolmogorov time scales from zero (at the injector nozzle in the experiment and initially in the DNS) to some value downstream and at later times; (iii) the long-time values of the ratios of the standard deviation to the mean of the conditional and unconditional SDR increase with the turbulent Reynolds number; (iv) for the same turbulent Reynolds number, the DNS and the experiment showed that the ratio related to the unconditional SDR increases to a long-time value of approximately 2.3 (±20%), while the ratio related to the conditional SDR increases quickly to a value that stays within the range 1.0–1.4 (or, 1.2±0.2) and reaches a maximum value of 1.3–1.4 by the end of the DNS run and at the downstream edge of the experimental domain. The development of the conditional SDR fluctuations is discussed by comparing the

Markides CN, Osuolale A, Solanki R,
et al., 2013, Nonlinear Heat Transfer Processes in a Two-Phase Thermofluidic Oscillator, *Applied Energy*

A two-phase thermofluidic oscillator was recently reported as being capable of undergoing sustained operation when a constant and low temperature difference is applied to the device, which consists of a network of tubes, compartments and two heat exchanger blocks. Within this arrangement a working fluid undergoes thermodynamic property oscillations that describe a heat engine cycle. Previous attempts to model the dynamic behaviour of this thermofluidic engine for performance predictions have been based on linear analyses. These have provided us with useful knowledge of the necessary minimum temperature difference for operation, and the resulting oscillation frequency and efficiency. However, experimental observations suggest a limit cycle operation associated exclusively with nonlinear systems. The present paper presents an effort to devise a nonlinear model for the device. Indicative results from this model are discussed, and the predictions are compared to those from the linear equivalents and experimental observations. The results reveal that although both linear and nonlinear models predict similar oscillation frequencies, the nonlinear model predicts lower exergetic efficiencies. This probably arises from the inability of the linear model to capture the saturation in the rate of heat exchange between the working fluid and the heat exchangers. The present effort aims to provide a better understanding of this device and to suggest improved design guidelines for increased efficiency and power density.

Solanki R, Mathie R, Galindo A,
et al., 2013, Modelling of a Two-Phase Thermofluidic Oscillator for Low-Grade Heat Utilisation: Accounting for Irreversible Thermal Losses, *Applied Energy*

The Non-Inertive-Feedback Thermofluidic Engine (NIFTE) is a two-phase thermofluidic oscillator which, by means of persistent periodic thermal-fluid oscillations when placed across a steady temperature difference, is capable of utilising low-grade (i.e. low temperature) heat to induce a fluid motion. Two linearised models of the NIFTE are presented in this paper, both containing a description of the phase-change convective heat transfer that takes place between the working fluid and the heat exchangers. The first model (LTP) imposes a steady linear temperature profile along the surface of the heat exchangers; and the second (DHX) allows the solid heat exchanger blocks to store and release heat dynamically as they interact thermally with the working fluid. In earlier work [Solanki et al., Applied Thermal Engineering, 2012] it was found that these models predict the oscillation (i.e. operation) frequency of an existing NIFTE prototype pump well, but overestimate its reported efficiency. Specifically, the LTP and DHX models predicted exegetic efficiencies 11 and 30 times higher than those observed experimentally, respectively. In the present paper, a dissipative thermal loss parameter that can account for the exergetic losses due to the parasitic, cyclic phase change and heat exchange within the device is included in both models in an effort to make realistic predictions of the exergetic efficiencies. The LTP and DHX models, including and excluding the thermal loss parameter, are compared to experimental data. It is found that the inclusion of the thermal loss parameter increases the predicted oscillation frequencies in the DHX model, but has a negligible effect on the frequencies predicted by the LTP model. A more significant effect is observed on the exergetic efficiencies, whereby the inclusion of the thermal loss parameter leads to a greatly improved prediction by both the LTP and DHX models, both in trend and approximate magnitude, of the exergetic efficiency of th

Zadrazil I, Markides CN, Naraigh LO, et al., Dynamics of Turbulent Falling Films, American Physical Society - Division of Fluid Dynamics

The dynamics of laminar falling films have received considerable attention over the past several decades. In contrast, turbulent falling films have been the subject of far fewer studies. We seek to redress this balance by studying the stability of falling films which have already undergone a transition from a laminar to a turbulent flow regime. We derive a uniform-film base-state for this flow by assuming the averaged turbulent velocity field to be steady and fully-developed, and by employing a modified version of mixing-length theory. The latter features an interpolation function for the eddy viscosity, and van Driest-type functions for turbulence-damping near the wall and interface regions. The predicted base-state streamwise velocity component is in good agreement with experimental data. A linear stability analysis of this base-state is then carried out by solving a modified version of the Orr-Sommerfeld equation. Our results suggest that the unstable mode is a long-wave one. This provides motivation for the derivation of long-wave equations for the nonlinear evolution of the film.

Zhao Y, Zadrazil I, Markides CN, et al., Wave structure in Upwards Gas-Liquid Annular Flows, American Physical Society - Division of Fluid Dynamics

A two-phase flow system in a vertical pipe in which the liquid around the pipe periphery is lifted by the gas core is referred to as an ``upwards annular flow'' (UAF). UAFs have a complex interfacial structure, which consists of short-lived, small-amplitude ``ripple'' waves, and large amplitude, high-speed ``disturbances'' waves. Two sets of flush-mounted electrically conducting probes together with axial view photography were used to study UAFs. The overall wave frequency decreased with increasing distance from the inlet until saturation. Disturbance waves were observed over a wide range (both low and high) of liquid Reynolds numbers, ReL, while ripples were observed at lower ReL. Disturbance ``bursts,'' which are a source of liquid entrainment into the gas core, were also observed, with increasing frequency at progressively higher ReL. The waves appeared more chaotic near the inlet, which hindered the formation of the correlated waves. As the small (ripple) waves coalesced into bigger waves with increasing distance from the inlet, the waves became more coherent around the pipe periphery. The results that will be presented comprise: (i) statistical film thickness data, and (ii) wave, frequency, velocity, and wavelength.

Zadrazil I, Hewitt GF, Matar OK, et al., Wave Structure and Velocity Profiles in Downwards Gas-Liquid Annular Flows, American Physical Society, American Physical Society - Division of Fluid Dynamics

A downwards flow of gas in the core of a vertical pipe, and of liquid in the annulus between the pipe wall and the gas phase is referred to as a ``downwards annular flow'' (DAF). DAFs are conventionally described in terms of short-lived, small-amplitude ``ripples,'' and large-amplitude, high-speed ``disturbances.'' We use a combination of Laser Induced Fluorescence (LIF), Particle Image and Tracking Velocimetry (PIV, PTV) to study DAFs. We demonstrate through these techniques that the liquid films become progressively more complex with increasing liquid Reynolds number (ReL), while a similar increase of complexity is observed for increasing gas Reynolds number (ReG). Disturbance waves are observed for low and high ReL, and ripples for intermediate ReL. Additionally, a high degree of rolling breakdown of disturbance waves is observed in falling films at the highest ReL, which is a source of bubble entrainment into the film body. Our results will comprise: (i) statistical data on film thickness, and (ii) wave frequency, velocity, wavelength. In addition, a qualitative (e.g. re-circulation zones) and quantitative (e.g. mean/rms velocity profiles) velocity characterisation of the film flows will be presented.

Mathie R, Nakamura H, Markides CN, 2012, Heat Transfer Augmentation in Unsteady Conjugate Thermal Systems – Part II: Applications, *International Journal of Heat and Mass Transfer*, Vol: 56, Pages: 819-833

Many energy conversion and other thermal-fluid systems exhibit unsteady convective heat exchange. In such systems, generic spatiotemporal variations in the flow give rise to variations in the heat flux for a given fluid–solid temperature difference, which can be interpreted as spatiotemporal fluctuations of the instantaneous heat transfer coefficient. These variations can lead to unsteady conjugate heat transfer, in which the exchanged heat flux arises from an interaction between the bulk fluid temperature and the temperature in the solid. Further, the nonlinear coupling between the fluctuating temperature differences and the heat transfer coefficient can lead to an effect we refer to as augmentation, which quantitatively describes the ability of a particular arrangement to have a different time-mean heat flux from the product between the mean heat transfer coefficient and the mean temperature difference across the fluid. It is important to be able to understand and to model in a simple framework the effects of the material properties, the geometry and the character of the heat transfer coefficient on the thermal response of the fluid–solid system, and consequently to predict the overall heat transfer performance of these systems.This paper, which follows on from its predecessor [1], is concerned with the phenomenon of augmentation in simple, one-dimensional, unsteady and conjugate fluid–solid systems. A simple semi-analytical one-dimensional model of heat transfer with a time-varying heat transfer coefficient, which was presented in Mathie and Markides [1], is applied herein to two different paradigm problems. Such a model can be an important tool in the design of improved heat exchangers and thermal insulation, through for example, the novel selection of materials to exploit these augmentation effects. The first flow considered is a thin, wavy fluid film flowing over a heated plate. This film flow exhibits a periodic fluctuation in the heat trans

Morgan RG, Markides CN, Zadrazil I,
et al., 2012, Characteristics of horizontal liquid-liquid flows in a circular pipe using simultaneous high-speed laser-induced fluorescence and particle velocimetry, *International Journal of Multiphase Flow*, Vol: 49, Pages: 99-118, ISSN: 1879-3533

This paper describes a set of experiments on liquid–liquid flows in a horizontal circular tube. The liquids used in the experiments were an aliphatic hydrocarbon oil (Exxsol D80) and an aqueous solution of glycerol. The concentration of glycerol in the solution was adjusted so that the two liquids had the same refractive index, and optical distortions due to the curvature of the (transparent) circular tube test section were corrected for with the use of a graticule technique. The test section was far downstream of an inlet section that established an initially stratified co-current flow of the two immiscible liquids, with the Exxsol D80 oil flowing over the glycerol/water solution. The flows were investigated at the test section with the application of laser-based optical diagnostic methods, which included high-speed simultaneous Planar Laser Induced Fluorescence (PLIF), Particle Image Velocimetry (PIV) and Particle Tracking Velocimetry (PTV). These techniques allowed the reliable evaluation of the nature of the investigated horizontal liquid–liquid flows (i.e., the flow patterns from phase distribution information), together with the detailed spatiotemporally resolved measurement of key flow characteristics such as phase and velocity distributions, and also of important parameters such as droplet size. The resulting PLIF images provide a clear indication of the distribution of the phases within a plane that passed through the channel centreline, and are used to obtain qualitative information about the arising flow patterns. The images were also used quantitatively to generate data on phase distribution, in situ phase fraction, interface level and droplet size distribution. Much of the PLIF data on in situ phase fraction and interface level agrees well with predictions from a simple stratified laminar–laminar flow model. The particle velocimetry methods (PIV and PTV) provide data on the velocity profiles in the investigated flows. Over the range of

Zadrazil I, Markides CN, Matar OK, et al., Characterisation of Downwards Co-Current Gas-Liquid Annular Flows, Turbulence, Heat and Mass Transfer 7, Publisher: Begell House

The hydrodynamic characteristics of downwards co-current two-phase (gas-liquid) flows inside avertical tube (ID = 32 mm) have been investigated experimentally. Advanced optical techniques, namely LaserInduced Fluorescence and Particle Tracking Velocimetry, were utilised for the characterisation of these flowsover a wide range of gas and liquid superficial velocities (U_G = 0 – 34 m/s and U_L = 0.034 – 0.182 m/s),corresponding to Reynolds numbers Re_G = 0 – 84,600 and Re_L = 1,230 – 6,130. A flow regime map, whichcontains a previously unreported flow regime, is constructed based on the flow observations. The quantitativeanalysis of the liquid films allows the generation of film thickness, wave frequency, bubble size, bubblefrequency and velocity profile data. It was found that the different observed flow regimes posses a characteristiccombination of the investigated quantitative parameters. A model, based on modified mixing-length theory, wasused to predict the liquid film velocity profiles and good agreement was found with the experimental results.

Morgan RG, Markides CN, Hale CP,
et al., 2012, Horizontal liquid-liquid flow characteristics at low superficial velocities using laser-induced fluorescence, *INTERNATIONAL JOURNAL OF MULTIPHASE FLOW*, Vol: 43, Pages: 101-117, ISSN: 0301-9322

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White A, Parks G, Markides CN, 2012, Thermodynamic Analysis of Pumped Thermal Electricity Storage, *Applied Thermal Engineering*

The increasing use of renewable energy technologies for electricity generation, many of which have an unpredictably intermittent nature, will inevitably lead to a greater need for electricity storage. Although there are many existing and emerging storage technologies, most have limitations in terms of geographical constraints, high capital cost or low cycle life, and few are of sufficient scale (in terms of both power and storage capacity) for integration at the transmission and distribution levels. This paper is concerned with a relatively new concept which will be referred to here as Pumped Thermal Electricity Storage (PTES), and which may be able to make a significant contribution towards future storage needs. During charge, PTES makes use of a high temperature ratio heat pump to convert electrical energy into thermal energy which is stored as ‘sensible heat’ in two thermal reservoirs, one hot and one cold. When required, the thermal energy is then converted back to electricity by effectively running the heat pump backwards as a heat engine. The paper focuses on thermodynamic aspects of PTES, including energy and power density, and the various sources of irreversibility and their impact on round-trip efficiency. It is shown that, for given compression and expansion efficiencies, the cycle performance is controlled chiefly by the ratio between the highest and lowest temperatures in each reservoir rather than by the cycle pressure ratio. The sensitivity of round-trip efficiency to various loss parameters has been analysed and indicates particular susceptibility to compression and expansion irreversibility.

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