## Publications

295 results found

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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- Citations: 31

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.

Zadrazil I, Bismarck A, Hewitt GF,
et al., 2012, Shear Layers in the Turbulent Pipe Flow of Drag Reducing Polymer Solutions, *Chemical Engineering Science*, Vol: 72, Pages: 142-154

A range of high molecular weight polymers (polyethylene oxide) was dissolved at very low concentrations – in the order of few wppm – in a solvent (water). The Newtonian character of the polymer solutions was confirmed by rheological measurements. The polymer solutions were then pumped through a long horizontal pipe section in fully developed turbulent conditions. The flow experienced a reduction in frictional drag when compared to the drag experienced by the equivalent flow of the pure solvent. Specifically, drag reduction was measured at Reynolds numbers ranging from 3.5×10^4 to 2.1×10^5 in a pressure driven flow facility with a circular tube section of internal diameter 25.3 mm. The turbulent flow was visualized by Particle Image Velocimetry and the resulting data were used to investigate the effect of the drag reducing additives on the turbulent pipe flow. Close attention was paid to the mean and instantaneous velocity fields, as well as the two-dimensional vorticity and streamwise shear strain rate. The results indicate that drag reduction is accompanied by the appearance of “shear layers” (i.e. thin filament-like regions of high spatial velocity gradients) that act as interfaces separating low-momentum flow regions near the pipe wall and high-momentum flow regions closer to the centerline. The shear layers are not stationary. They are continuously formed close to the wall at a random frequency and move towards the pipe centerline until they eventually disappear, thus occupying or existing within a “shear layer region”. It is found that the mean thickness of the shear layer region is correlated with the measured level of drag reduction. The shear layer region thickness is increased by the presence of polymer additives when compared to the pure solvent, in a similar way to the thickening of the buffer layer. The results provide valuable insights into the characteristics of the turbulent pipe flow of a solvent contai

Markides CN, Fokaides P, Neophytou MK-A, 2012, An experimental investigation of the flow and transfer processes in homogeneous urban street-canyon geometries using Particle Image Velocimetry, 7th International Symposium on Turbulence Heat and Mass Transfer (THMT), Publisher: BEGELL HOUSE, INC, Pages: 530-539

Solanki R, Galindo A, Markides CN, 2012, Dynamic modelling of a two-phase thermofluidic oscillator for efficient low grade heat utilization: Effect of fluid inertia, Publisher: ELSEVIER SCI LTD, Pages: 156-163, ISSN: 0306-2619

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- Citations: 34

Markides CN, Smith TCB, 2011, A Dynamic Model for the Efficiency Optimization of an Oscillatory Low Grade Heat Engine, *Energy*, Vol: 12, Pages: 6967-6980

A simple approach is presented for the modeling of complex oscillatory thermal-fluid systems capable of converting low grade heat into useful work. This approach is applied to the NIFTE, a novel low temperature difference heat utilization technology currently under development. Starting from a first-order linear dynamic model of the NIFTE that consists of a network of interconnected spatially lumped components, the effects of various device parameters (geometric and other) on the thermodynamic efficiencies of the device are investigated parametrically. Critical components are highlighted that require careful design for the optimization of the device, namely the feedback valve, the power cylinder, the adiabatic volume and the thermal resistance in the heat exchangers. An efficient NIFTE design would feature a lower feedback valve resistance, with a shorter connection length and larger connection diameter; a smaller diameter but taller power cylinder; a larger (time-mean) combined vapor volume at the top part of the device; as well as improved heat transfer behavior (i.e. reduced thermal resistance) in the hot and cold heat exchanger blocks. These modifications have the potential of increasing the relevant form of the second law efficiency of the device by 50% points, corresponding to a 3.8% point increase in thermal efficiency.

Markides CN, Mastorakos E, 2010, Experimental Investigation of the Effects of Turbulence and Mixing on Autoignition Chemistry, *Flow Turbulence and Combustion*, Vol: 86, Pages: 585-608, ISSN: 1573-1987

The autoignition of acetylene, released from a finite-sized circular nozzleinto a turbulent coflow of hot air confined in a pipe, has been the subject of a recentexperimental study to supplement previous work for hydrogen and n-heptane. Aswith hydrogen and n-heptane, autoignition appears in the form of well-defined localizedspots. Quantitative information is presented concerning the effects of turbulenceintensity, turbulent lengthscale and injector diameter on the location of autoignition.The effects of these parameters on inhomogeneous autoignition have not beeninvestigated experimentally before. The present study establishes that increasingthe bulk velocity increases the autoignition length, as was reported for hydrogenand n-heptane. For the same turbulence intensity, the autoignition length increasesas the injector diameter increases and as the turbulent lengthscale decreases. Asimultaneous decrease in turbulence intensity and increase in lengthscale causesa reduction in autoignition length. Further, the frequency of appearance of theautoignition spots has also been measured. It is found to increase when autoignitionoccurs closer to the injector, and also at higher velocities. The observed trends areconsistent with expectations arising from the dependence of the mixture fraction andthe scalar dissipation rate on the geometrical and flow parameters. The data can beused for the validation of turbulent combustion models.

Fokaides PA, Markides CN, Neophytou M, 2009, Ventilation characteristics of the built environment and their effects on the urban microclimate, *SUSTAINABLE DEVELOPMENT AND PLANNING IV, VOLS 1 AND 2*, Vol: 120, Pages: 271-281, ISSN: 1743-3541

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- Citations: 1

Markides CN, Mastorakos E, 2009, Experimental investigation of the effects of turbulence and mixing on autoignition chemistry, 6th International Symposium on Turbulence, Heat and Mass Transfer, Publisher: BEGELL HOUSE, INC, Pages: 605-608

Markides CN, Mastorakos E, 2008, Measurements of the statistical distribution of the scalar dissipation rate in turbulent axisymmetric plumes, *FLOW TURBULENCE AND COMBUSTION*, Vol: 81, Pages: 221-234, ISSN: 1386-6184

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- Citations: 14

Markides C, 2008, Autoignition in Turbulent Flows: Experimental Observations and Investigation of the Effects of Turbulence and Mixture Inhomogeneities, Publisher: VDM Verlag, ISBN: 9783836494342

In this work gaseous fuels were released continuously and concentrically into confined annular co-flows of turbulent hot air. Following injection the fuel and air mixed and at some length downstream of the nozzle the reactive mixture autoignited. Original phenomena are reported of autoignition spots, unsteady flame propagation and extinction or flashback. The frequency of the spots was measured, as were their acoustic and chemiluminescence characteristics. Optical measurements of the autoignition locations were made and used to estimate mean delay times from injection. As would be expected by considerations of simple chemical kinetics and the mean concentration field, higher air temperatures and lower fuel velocities resulted in autoignition closer to the injector. However, as the air velocity and hence also turbulent fluctuations were increased, autoignition shifted downstream and was delayed, while its frequency and sound intensity decreased. Such and other situations are presented that cannot be explained purely in terms of chemical arguments, i.e. homogeneous delay times, highlighting the significance of the mixing field through the mixture fraction and scalar dissipation rate.

Markides CN, Mastorakos E, 2008, Flame propagation following the autoignition of axisymmetric hydrogen, acetylene, and normal-heptane plumes in turbulent coflows of hot air, *JOURNAL OF ENGINEERING FOR GAS TURBINES AND POWER-TRANSACTIONS OF THE ASME*, Vol: 130, ISSN: 0742-4795

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- Citations: 7

Markides CN, De Paola G, Mastorakos E, 2007, Measurements and simulations of mixing and autoignition of an n-heptane plume in a turbulent flow of heated air, *EXPERIMENTAL THERMAL AND FLUID SCIENCE*, Vol: 31, Pages: 393-401, ISSN: 0894-1777

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- Citations: 44

Markides CN, Mastorakos E, 2006, Measurements of scalar dissipation in a turbulent plume with planar laser-induced fluorescence of acetone, *CHEMICAL ENGINEERING SCIENCE*, Vol: 61, Pages: 2835-2842, ISSN: 0009-2509

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- Citations: 23

Markides CN, Mastorakos E, 2006, Flame propagation following the autoignition of axisymmetric hydrogen, acetylene and normal-heptane plumes in turbulent co-flows of hot air, 51st ASME Turbo Expo 2006, Publisher: AMER SOC MECHANICAL ENGINEERS, Pages: 843-852

Markides CN, Mastorakos E, 2005, An experimental study of hydrogen autoignition in a turbulent co-flow of heated air, *PROCEEDINGS OF THE COMBUSTION INSTITUTE*, Vol: 30, Pages: 883-891, ISSN: 1540-7489

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- Citations: 115

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