Imperial College London

ProfessorAlexTaylor

Faculty of EngineeringDepartment of Mechanical Engineering

Professor
 
 
 
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Contact

 

+44 (0)20 7594 7042a.m.taylor

 
 
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Location

 

618City and Guilds BuildingSouth Kensington Campus

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Summary

 

Publications

Publication Type
Year
to

219 results found

McGinn P, Pearce D, Hardalupas I, Taylor AMKP, Vogiatzaki Ket al., 2021, Cavitation bubble cloud break-off mechanisms at micro-channels, Fluids, Vol: 6, ISSN: 2311-5521

This paper provides new physical insight into the coupling between flow dynamics and cavitation bubble cloud behaviour at conditions relevant to both cavitation inception and the more complex phenomenon of flow “choking” using a multiphase compressible framework. Understanding the cavitation bubble cloud process and the parameters that determine its break-off frequency is important for control of phenomena such as structure vibration and erosion. Initially, the role of the pressure waves in the flow development is investigated. We highlight the differences between “physical” and “artificial” numerical waves by comparing cases with different boundary and differencing schemes. We analyse in detail the prediction of the coupling of flow and cavitation dynamics in a micro-channel 20 μm high containing Diesel at pressure differences 7 MPa and 8.5 MPa, corresponding to cavitation inception and "choking" conditions respectively. The results have a very good agreement with experimental data and demonstrate that pressure wave dynamics, rather than the “re-entrant jet dynamics” suggested by previous studies, determine the characteristics of the bubble cloud dynamics under “choking” conditions.

Journal article

Hua X, Liu Y, Chen C, Hardalupas I, Taylor AMKPet al., 2021, Mixing and scalar dissipation rate in a decaying jet, Proceedings of the Combustion Institute, Vol: 38, Pages: 3251-3259, ISSN: 0082-0784

The temporal development of the mixing field in a decaying jet (Re = 50,000) was quantified by measuring mole fraction and scalar dissipation rate (SDR) in a decaying, isothermal,turbulent gaseous jet. The 2D scalar field was measured by using planar laser induced fluorescence of acetone and, with appropriate image processing, this allowed estimation of the SDR using the two in-plane components within 16%error. The instantaneous and averaged distributions of the mole fractionare reported for downstream dimensionless distances up to 7 nozzle exit diameters and 35 exit flow time scales after end of injection. With advection of the last uniform exit concentration(UEC) profile coreaway from the nozzle exit, a region of weak concentration arises at the decaying jet’s trailing edge.Estimates made in a Lagrangian frame of reference show that the trailing edge of the jet becomes leaner,after the end of injection (AEI), faster than in the steady state, confirming the existence of an ‘entrainment wave’. The normalised probability density functions of the 2D SDR at various stations and times AEI differ from a lognormal distribution at both low and high SDR values with negative skewness and positive excess kurtos is. A pseudo 3DSDR, made by including an estimate for the out of plane component, showed reduced departure from log normal.The departure may be attributed to the disappearance of the strong shear layer associated with the absence of nozzle momentum AEI. To the authors’ knowledge, this study provides the first measurements of the SDR in a decaying, isothermal turbulent jet.

Journal article

Karlis E, Hardalupas Y, Taylor AMKP, 2020, Effects of inert fuel diluents on thermoacoustic instabilities in gas turbine combustion, AIAA Journal: devoted to aerospace research and development, Vol: 58, Pages: 2643-2657, ISSN: 0001-1452

The effects of inert diluents in the fuel mixture of a model swirl stabilized gas turbine combustor, under thermoacoustically unstable limit cycle operation, were studied experimentally. The measurements included particle image velocimetry (PIV), high-speed CH* chemiluminescent imaging, and dynamic pressure signals. The paper focuses on the dynamic phenomenon of the period doubling bifurcation, which came about when the equivalence ratio ϕ of undiluted flames was enriched from 0.55 to 0.60 under constant Reynolds number (Re=18,000). The bifurcation featured an emergence of an aerodynamically related timescale in addition to the fundamental timescale that was induced from an unstable acoustic eigenmode. The aerodynamic timescale is introduced by azimuthal convection of a high-heat-release-rate region and is linked in the literature with a precessing motion of the recirculation zone. It was found that on increasing the diluent molar fraction up to 50%, the amplitude and the frequency of the limit cycle fundamental acoustic mode decreased. A mechanism to interpret this suppression is that increasing the diluent molar fraction of the fuel makes the flame more susceptible to quenching because the extinction strain rate of the mixture is decreased. The paper argues that the existence of inert diluents in the fuel can significantly alter the dynamic state of the combustor, because the anchoring locations of the flame greatly depend on the composition-sensitive extinction strain rate of the mixture.

Journal article

Karlis E, Hardalupas Y, Taylor A, 2020, An experimental study of subcritical transition into thermoacoustic oscillations in a swirl stabilized model gas turbine combustor, AIAA Scitech 2020 Forum, Publisher: American Institute of Aeronautics and Astronautics, Pages: 1-19

Development of gas turbine combustors operating on the lean premixed mode has been hindered by the establishment of thermoacoustic instabilities that may prove challenging to the structural integrity of the engine. Transitions between a quiescent state of thermoacoustic operation towards an oscillatory one occur via a Hopf bifurcation, namely via the emergence of periodic oscillations amidst broadband fluctuations. Hopf bifurcations can be distinguished between supercritical and subcritical, with the later type introducing especially undesirable effects due the abrupt and intermittent nature of their demonstration. Subcritical Hopf bifurcations in a model swirl stabilized gas turbine combustor are investigated in the current work. The system is driven towards the intermittently unstable regime namely close to the subcritical Hopf bifurcation point via gradual hydrogen enrichment of close to blow-off methane mixtures, at a constant equivalence ratio of 0.55. High speed (\SI{3}{kHz}) particle image velocimetry (PIV) measurements were employed to capture the flowfield structure during the transition from the stable to the unstable state. Phase conditioned low speed (\SI{10}{Hz}) OH planar laser induced fluorescence (PLIF) measurements provided information with regards to the flame structure. The transition from the low amplitude state towards the high amplitude one is always preceded by a precessing motion of the inner recirculation zone (IRZ) that is followed by a complete collapse of the IRZ. The collapse is caused by a disturbance of high positive axial velocity, convected downstream by the mean flow acting as the triggering event of the subcritical Hopf bifurcation. The PIV measurements show that the mean and standard deviation of the spatial distribution of the turbulent intensity increase, thus the flame interacts with a greater range of length scales during the intermittent bursts. This is also exemplified by the PDFs of the curvature distribution of the flame f

Conference paper

Karlis E, Liu Y, Hardalupas I, Taylor AMKet al., 2020, Extinction strain rate suppression of the precessing vortex core in a swirl stabilised combustor and consequences for thermoacoustic oscillations, Combustion and Flame, Vol: 211, Pages: 229-252, ISSN: 0010-2180

In the current paper, time resolved high speed optical Particle Image Velocimetry and CH* chemiluminescence measurements were performed, to study self-excited limit cycle combustion instabilities in a swirl stabilized model gas turbine combustor operating at atmospheric pressure with choked propane and air flow supplies. The combustor was operated under a constant Reynolds number (Re=22,000) and four equivalence ratios, namely for operation susceptible to extinction, and ϕ=0.55, ϕ=0.60 and ϕ=0.65 for operation under a thermoacoustically unstable combustion regime, to encounter two limit cycle dynamic states. The period-1 limit cycle was driven by thermoacoustic coupling between the acoustic and the thermal field at a fundamental timescale dictated by an acoustic eigenmode of the combustor. The period-2 limit cycle, further to the fundamental acoustic timescale featured a subharmonic aerodynamic signature in the heat release rate and dynamic pressure spectra caused by the helical coherent structure of a Precessing Vortex Core (PVC). Previous studies have shown that the PVC in the limit cycle regime may be suppressed by the temperature stratification at the inlet of the combustor. A mechanism is suggested to interpret the flame anchoring locations which effectively regulated whether PVC was excited or suppressed. It is showed that the conditions under which the flame attached to the centerbody and suppressed the PVC can be explained by the spatial distribution of the relative ratio of the flow imposed to the mixture extinction strain rate. The PVC was excited due to local extinction by aerodynamic straining at the inlet of the combustor, at the phase angle of maximum dynamic pressure. On increasing the equivalence ratio, the flame became robust to aerodynamic straining and flashed back at the phase angle of maximum dynamic pressure. The PVC was then suppressed due to the relative ratio of the flow imposed to the extinction strain rate, which allowed the establishment

Journal article

Karlis E, Liu Y, Hardalupas Y, Taylor AMKPet al., 2019, H2 enrichment of CH4 blends in lean premixed gas turbine combustion: An experimental study on effects on flame shape and thermoacoustic oscillation dynamics, Fuel, Vol: 254, Pages: 1-16, ISSN: 0016-2361

Thermoacoustic instabilities under lean operation in gas turbine burners hinder the development of lean premixed combustion mode of operation, thus limiting the potential to decrease NOx emissions. A method to improve stability of lean combustion while maintaining low thermal NOx formation is to add hydrogen in typical gas turbine fuels such as natural gas. The present work examines the thermoacoustic dynamic characteristics of hydrogen-enriched methane blends in a swirl stabilized model gas turbine combustor. We blend CH4 with increasing H2 molar content (from 0% to 40%) at a global equivalence ratio of ϕ=0.55 on a constant Reynolds number Re = 19000. The reference case of pure methane is susceptible to blow off at the same equivalence ratio. On increasing the hydrogen content at 10% H2, the flammability limits are extended. However, further increase of the H2 content leads to manifestation of random short bursts of dynamic pressure and heat release. Thermoacoustic dynamics are intermittently injected between a quiescent state and a regime of high amplitude oscillations. On further increasing H2 content, the dynamics are attracted towards a limit cycle; a fully established high amplitude regime, where no requiescence is observed. In this regime, heat release rate and dynamic pressure oscillate in phase and at the same frequency. Based on the observed dynamics, we seek a mixture property to characterize the dynamic state, the combustor operates in. We show that the extinction strain rate associated with each mixture can collapse the dynamic transitions from quiescent to intermittent instabilities and finally to fully established thermoacoustic oscillations. In each dynamic state, we examine the mechanism that affiliates coherent structures of the underlying thermodynamic flow field with the flame through the relation of the spatial distribution of the flow imposed strain rate over the extinction strain rate of each mixture. It is shown that the flame anchoring locat

Journal article

Karlis E, Hardalupas I, Taylor A, Rogerson J, Sadasivuni S, Stopper U, Stöhr Met al., 2019, Thermoacoustic phenomena in an industrial gas turbine combustor at two different mean pressures, AIAA Scitech 2019 Forum, Publisher: Aerospace Research Central

The current paper studies the thermoacoustically unstable combustion, under elevatedmean pressure, of a commercial swirl stabilized gas turbine burner fitted with optically accessible windows. The considered measurements include particle image velocimetry (PIV),OH∗chemiluminescence imaging, high speed broadband flame imaging and dynamic pressuresignals. We study cases A and B, wherein natural gas flames at mean pressures equal to 3 barand 6 bar delivered thermal loads equal to 335 kW and 685 kW respectively. The flow fielddemonstrated a typical vortex breakdown induced inner recirculation zone and a sudden stepexpansion induced outer recirculation zone. In case A, high amplitude dynamic pressure burstswere observed amidst a quiescent acoustic background. The flame was conical, it anchored onthe shear layers of the recirculation zone and it periodically expanded in the outer recirculation zone (ORZ). In case B, the flame was consistently thermoacoustically unstable with seldomrequiescent events, while at the same time expansion to the ORZ was suppressed. By applyingDynamic Mode Decomposition on high speed images of case A, it was showed that this expansionintroduced an additional time scale, further to the fundamental acoustically related timescale.The superposition of two timescales over a turbulent background established an intermittentregime of thermoacoustic instabilities, wherein the dynamics transitioned between quiescentand fully oscillatory. A physical mechanism is suggested to explain the differences betweenthe flame shapes on adjusting mean pressure. The mechanism considers that the premixtureis characterized by a Lewis number lower than unity, the laminar flame speed increases ondecreasing mean pressure and the flow imposed on the flame strain rate oscillated over a periodof thermoacoustic instability. This combination resulted in oscillatory heat release rate, in theregion of the outer shear layers. The phenomenon was more pronounced in case A than

Conference paper

Noh D, Karlis E, Navarro-Martinez S, Hardalupas Y, Taylor AMKP, Fredrich D, Jones WPet al., 2019, Azimuthally-driven subharmonic thermoacoustic instabilities in a swirl-stabilised combustor, Proceedings of the Combustion Institute, Vol: 37, Pages: 5333-5341, ISSN: 0082-0784

A joint experimental and computational study of thermoacoustic instabilities in a model swirl-stabilised combustor is presented. This paper aims to deliver a better characterisation of such instabilities through the examination of measurements in conjunction with the numerical results obtained via Large Eddy Simulation (LES). The experimental configuration features a cylindrical combustion chamber where the lean premixed methane/air flame experiences self-sustained thermoacoustic oscillations. The nonlinear behaviour of the acoustically excited flame is experimentally investigated by broadband chemiluminescence and dynamic pressure measurements. In LES, the Eulerian stochastic field method is employed for the unknown turbulence/chemistry interaction of the gas-phase. Comparisons of the predicted frequency spectrum and phase-resolved flame structure with measurements are found to be in good agreement, confirming the predictive capabilities of the LES methodology. The presence of Period 2 thermoacoustic oscillations, as a consequence of period-doubling bifurcation, is also confirmed in LES. Through the application of nonlinear analysis both to the experimental and numerical acoustic fluctuations, it is highlighted that the nonlinear behaviour of combustion instabilities in the burner under investigation follows a pattern typical of Period 2 oscillations. Furthermore, the current work demonstrates a useful approach, through the use of dynamic mode decomposition (DMD), for the investigation of unstable flame modes at a specific frequency of interest. The experimental and numerical DMD reconstructions suggest that hot combustion products are convected azimuthally at a rate dictated by the subharmonic frequency.

Journal article

Shi Z, Hardalupas I, Taylor AMKP, 2019, Laser-induced plasma image velocimetry, Experiments in Fluids, Vol: 60, ISSN: 0723-4864

A novel velocimetry method is proposed for point velocity measurement, which is based on tracking a laser-induced plasma in a flow. The plasma’s behaviour is first analysed spatially, temporally and spectrally in quiescent air. The dependence of this technique on the delay time between subsequent plasma images and the processing methods are described. It is found that, for optimized operation of the technique in a turbulent air jet (exit diameter 10.0 mm from a 480 mm long pipe; with an averaged velocity of 50 m/s at the jet exit resulting in Reynolds number of 34,000) with 100 µs time delay between plasma images, the systematic and random components of the velocity uncertainty are − 0.51 m/s and ± 3.6 m/s along the laser beam direction, and 1.25 m/s and ± 0.86 m/s along other directions perpendicular to the laser beam. These uncertainties are mainly caused by the asymmetric laser energy deposition during the formation of plasma, and the associated spatial resolution (in this realisation of the instrument) of 5 mm. The mean velocity measurements in the turbulent air jet flow are consistent with the reported flow behaviour in the literature for mean velocity: the turbulent intensity of axial velocity fluctuations is comparable to those in the literature but difference arises due to the limited spatial resolution. This velocimetry method is an alternative to traditional tracer-based velocimetry methods, because it does not require ‘seeding’ of particles or other substances in the flow. It also has the ability to measure local gas mixture composition, using laser-induced breakdown spectroscopy approach, simultaneously with flow velocity, but this aspect is not explored in the current study.

Journal article

Karlis E, Hardalupas I, Taylor AMKP, 2019, Effects of inert fuel diluents on the dynamic state of a thermoacoustically unstable gas turbine combustor, AIAA SciTech 2019, Publisher: American Institute of Aeronautics and Astronautics

The effects of inert diluents in the fuel mixture of a model swirl stabilized gas turbine com-bustor, under thermoacoustically unstable limit cycle operation, were studied experimentally.The measurements included Particle Image Velocimetry (PIV), high speed CH∗chemilumines-cent imaging and dynamic pressure signals. The paper focuses on the dynamic phenomenon ofthe period doubling bifurcation, which came about when the equivalence ratio (φ) of undilutedflames was enriched from 0.55 to 0.60 under constant Reynolds number (Re). The bifurcationfeatured an emergence of an aerodynamically related timescale in addition to the fundamentaltimescale which was induced from an unstable acoustic eigenmode. The aerodynamic timescaleis introduced by azimuthal convection of a high heat release rate region, and is linked in theliterature with a precessing motion of the recirculation zone. Prior to the bifurcation, theflame anchored between the wall and the outer shear layers of the recirculation zone assuminga V-shape. The dynamics were attracted to a Period-1 limit cycle. Post the bifurcation the flameexpanded in the inner shear layers of the recirculation zone assuming an M-shape, while thedynamics were attracted to a Period-2 limit cycle. The latter operational condition was subjectto nitrogen dilution in order to parametrically increase the molar fraction of inert diluent inthe fuel stream. It was found that on increasing the diluent molar fraction the amplitude andthe frequency of the limit cycle fundamental acoustic mode decreased. Also, inert dilution sup-pressed the aerodynamic timescale and the flame assumed a V-shape again. A mechanism tointerpret this mechanism is suggested. Increasing the diluent molar fraction of the fuel makesthe flame susceptible to quenching because the extinction strain rate of the mixture decreased.Thus, the flow imposed strain rates quenched the flame when it attempted to anchor on theinner shear layers of the vortex breakdown induced r

Conference paper

Tsekenis S-A, Ramaswamy KG, Tait N, Hardalupas Y, Taylor A, McCann Het al., 2018, Chemical species tomographic imaging of the vapour fuel distribution in a compression-ignition engine, International Journal of Engine Research, Vol: 19, Pages: 718-731, ISSN: 1468-0874

This article reports the first application of chemical species tomography to visualise the in-cylinder fuel vapour concentration distribution during the mixing process in a compression-ignition engine. The engine was operated in motored conditions using nitrogen aspiration and fired conditions using a gasoline-like blend of 50% iso-dodecane and 50% n-dodecane. The tomography system comprises 31 laser beams arranged in a co-planar grid located below the injector. A novel, robust data referencing scheme was employed to condition the acquired data for image reconstruction using the iterative Landweber algorithm. Tomographic images were acquired during the compression stroke at a rate of 13 frames per crank angle degree within the same engine cycle at 1200 r min−1. The temperature-dependent fuel evaporation rate and mixing evolution were observed at different injection timings and intake pressure and temperature conditions. An initial cross-validation of the tomographic images was performed with planar laser-induced fluorescence images, showing good agreement in feature localisation and identification. This is the first time chemical species tomography using near-infrared spectroscopic absorption has been validated under engine conditions, and the first application of chemical species tomography to a compression-ignition engine.

Journal article

Sahu S, Hardalupas I, Taylor AMKP, 2018, Interaction of droplet dispersion and evaporation in a polydispersed spray, Journal of Fluid Mechanics, Vol: 846, Pages: 37-81, ISSN: 0022-1120

The interaction between droplet dispersion and evaporation in an acetone spray evaporating under ambient conditions is experimentally studied with an aim to understand the physics behind the spatial correlation between the local vapour mass fraction and droplets. The influence of gas-phase turbulence and droplet–gas slip velocity of such correlations is examined, while the focus is on the consequence of droplet clustering on collective evaporation of droplet clouds. Simultaneous and planar measurements of droplet size, velocity and number density, and vapour mass fraction around the droplets, were obtained by combining the interferometric laser imaging for droplet sizing and planar laser induced fluorescence techniques (Sahu et al., Exp. Fluids, vol. 55, 1673, 2014b, pp. 1–21). Comparison with droplet measurements in a non-evaporating water spray under the same flow conditions showed that droplet evaporation leads to higher fluctuations of droplet number density and velocity relative to the respective mean values. While the mean droplet–gas slip velocity was found to be negligibly small, the vaporization Damköhler number ( ) was approximately ‘one’, which means the droplet evaporation time and the characteristic time scale of large eddies are of the same order. Thus, the influence of the convective effect on droplet evaporation is not expected to be significant in comparison to the instantaneous fluctuations of slip velocity, which refers to the direct effect of turbulence. An overall linearly increasing trend was observed in the scatter plot of the instantaneous values of droplet number density ( ) and vapour mass fraction ( ). Accordingly, the correlation coefficient of fluctuations of vapour mass fraction and droplet number density ( ) was relatively high ( ) implying moderately high correlation. However, considerable spread of the versus scatter plot along both coordinates demonstrated the influence on droplet evaporation due t

Journal article

Joshi M, Gosala D, Allen C, Srinivasan S, Ramesh A, Vanvoorhis M, Taylor A, Vos K, Shaver G, McCarthy J, Farrell L, Koeberlein EDet al., 2018, Diesel Engine Cylinder Deactivation for Improved System Performance over Transient Real-World Drive Cycles, SAE Technical Papers, Vol: 2018-April

© 2018 SAE International; Eaton Corporation. Effective control of exhaust emissions from modern diesel engines requires the use of aftertreatment systems. Elevated aftertreatment component temperatures are required for engine-out emissions reductions to acceptable tailpipe limits. Maintaining elevated aftertreatment components temperatures is particularly problematic during prolonged low speed, low load operation of the engine (i.e. idle, creep, stop and go traffic), on account of low engine-outlet temperatures during these operating conditions. Conventional techniques to achieve elevated aftertreatment component temperatures include delayed fuel injections and over-squeezing the turbocharger, both of which result in a significant fuel consumption penalty. Cylinder deactivation (CDA) has been studied as a candidate strategy to maintain favorable aftertreatment temperatures, in a fuel efficient manner, via reduced airflow through the engine. This work focuses on prediction and demonstration of fuel economy benefits of CDA when implemented at idle and low load portions of the emission certification cycles, such as the heavy duty federal test procedure (HD-FTP), and other real-world drive cycles, including the Orange County bus and port drayage creep cycles. A 3.4% benefit in fuel economy has been demonstrated over the HD-FTP, while maintaining tailpipe-out NOx emissions. Greater improvements in fuel economy have been predicted over the real world cycles, with a 5.6% reduction predicted over the Orange County bus cycle and 35% reduction predicted over the port drayage creep cycle.

Journal article

Bergeles K, Hardalupas I, Taylor AMKP, 2018, On the transient flow inside and around a deforming millimetre class oil droplet falling under the action of gravity in stagnant air, Physics of Fluids, Vol: 30, ISSN: 1070-6631

The liquid flow inside, and the induced air flow around, a falling droplet in stagnant air was numerically investigated using the volume of fluid method to describe the droplet interface. The droplet consisted of oil with the same surface tension and with viscosity as parameter. It was injected into stagnant air with an initial velocity of 1 m/s; therefore, the initial Weber (We = 0.14), Reynolds (Re = 141), and Bond (Bo = 2.4) numbers remained constant during the parametric study whilst the initial Capillary (Ca) and Ohnesorge (Oh) numbers varied by an order of magnitude from 0.46 to 4.6 and from 0.044 to 0.44, respectively. We examined the effect of viscosity on the flow inside, and around, the droplet as well as on the droplet deformation and its natural frequency. This investigation showed a strong dependence of the deformation with liquid viscosity. Specifically, the droplets achieved their final deformation in under-damped, for low viscosity, and in over-damped, for high viscosity, oscillation modes. After a critical time tcrit (or Recrit), the instantaneous air flow symmetry was disturbed, initially in the wake and soon after in the interior of the droplet and in the vortex shedding downstream of the droplet. The air flow in the wake region detached from the droplet surface and resulted in a wake which was approximately 1.5 times longer and wider than the wake behind a solid sphere at the same Re number at steady state conditions. A roller-vortex structure (called rollex) was established upon injection in the immediate wake of the droplet, forming the necessary kinematic link between the directions of the internal circulation in the droplet (Hill vortex) and of the external recirculating air flow in the droplet’s wake. The droplet drag coefficients were compared with corresponding values used in droplet breakup models: although, ultimately, the droplet drag coefficient converged to the values given by the models, the initial magnitudes after injection w

Journal article

Charalambides AG, Sahu S, Hardalupas I, Taylor AMKP, Urata Yet al., 2017, Evaluation of Homogeneous Charge Compression Ignition (HCCI) autoignition development through chemiluminescence imaging and Proper Orthogonal Decomposition, Applied Energy, Vol: 210, Pages: 288-302, ISSN: 0306-2619

Homogeneous Charge Compression Ignition (HCCI) engines deliver high thermal efficiency and, therefore, low CO2 emissions, combined with low NOX and particulate emissions. However, HCCI operation is not possible at all conditions due to the inability to control the autoignition process and new understanding is required. A high-swirl low-compression-ratio, optically accessed engine that can produce overall fuel lean, axially stratified charge (richer fuel mixture close to the cylinder head was achieved using port injection against open valve and homogeneous mixture during injection against closed valve timing) was operated in HCCI mode without and with spark-assist mixture ignition. The present study investigates the differences in the HCCI autoignition process and the propagation of the autoignition front with homogeneous mixture or fuel charge stratification, internal Exhaust Gas Recirculation (iEGR) (introduced by utilizing different camshafts) and spark-assisted iEGR lean combustion. In order to visualize the HCCI process, chemiluminescence flame images, phase-locked to a specific crank angle, were acquired. In addition, time-resolved images of the developing autoignition flame front were captured. Proper Orthogonal Decomposition (POD) was applied to the acquired images to investigate the temporal and spatial repeatability of the autoignition front and compare these characteristics to the considered scenarios. The eigenvalues of the POD modes provided quantitative measure of the probability of the corresponding flame structures. The first POD mode showed higher probability of single autoignition sites originating from a particular location (depending on the scenario). However, the contribution from other modes cannot be neglected, which signified multiple locations of the single autoignition and also, multiple sites of self-ignition of the fuel-air mixture. It was found that increasing iEGR resulted in random combustion (multiple autoignition sites and fronts), wh

Journal article

Hardalupas Y, Hong C, Keramiotis C, Ramaswamy KG, Soulopoulos N, Taylor AMKP, Touloupis D, Vourliotakis G, Founti MAet al., 2017, An investigation of the effect of post-injection schemes on soot reduction potential using optical diagnostics in a single-cylinder optical diesel engine, International Journal of Engine Research, Vol: 18, Pages: 400-411, ISSN: 1468-0874

This work employs a combination of pressure trace analysis, high-speed optical measurements and laser-based techniques for the assessment of the effects of various post-injection schemes on the soot reduction potential in an optical single-cylinder light-duty diesel engine. The engine was operated under a multiple injection scheme of two pilot and one main injection, typical of a partially premixed combustion mode, at the lower end of the load and engine speed range (ca 2.0 bar IMEP at 1200 r/min). Experiments considering the influence of the post-injection fuel amount (up to 15% of the total fuel quantity per cycle) and the post-injection timing within the expansion stroke (5, 10 and 15 CAD aTDC), under a constant total fuel mass per cycle, have been conducted. Findings were analysed via means of pressure trace and apparent rate of heat transfer analyses, as well as a series of optical diagnostic techniques, namely, high-speed flame natural luminosity imaging, CH*, C∗2 and OH* line-of-sight chemiluminescence, as well as planar laser-induced incandescence measurements at 31 and 50 CAD aTDC. The combination of post-injection fuel amount and timing has substantial effects on charge reactivity and soot oxidation potential. The analysis reveals that an amount of fuel (7% of the total fuel mass per cycle) injected more than 10 CAD after the main combustion event leads to higher levels of soot emissions, while a larger amount of fuel (15% of the total fuel mass) injected 5 CAD after the main combustion event appears to have a beneficial effect on the soot oxidation processes. Overall, results indicate that a post-injection scheme close to the main combustion phasing could reduce soot levels and improve engine performance, that is, higher IMEP levels at the same fuel consumption rates, although it could increase engine noise.

Journal article

Shi Z, Hardalupas Y, Taylor AMKP, 2017, Laser-induced breakdown spectroscopy measurements of local equivalence ratio measurement in opposed jet methane and propane flames, 8th European Combustion Meeting (ECM) 2017, Publisher: The Combustion Institute

The current research reports gas composition measurements, based on Laser Induced Breakdown Spectroscopy (LIBS) in non-reacting flows and flames of premixed streams in an opposed-jet burnerusing both atomic and molecular emissions. The dependence of spectral intensity ratios of H/O and C2/CN on methane-air and propane-air mixture composition was quantified over a wide range of conditions extending from pure air to pure fuel. The presence of a flame within the laser beam led to significant deteriorationof the signal-to-noise ratio of the instantaneous LIBS signal, caused by the variability of the induced plasma, which led tothe increased uncertainty of the instantaneous gas composition measurements. The increase of the measurement uncertaintyin flames was quantified and corrected by increasing the laser pulse energy, which maintained the measurement uncertainty of instantaneous gas composition below 20%. LIBS was successfully used to measure gas composition in mixtures of methane, propane and air and the results demonstrated its feasibility for instantaneous measurements of local air -fuel ratio.

Conference paper

Karlis E, Hardalupas Y, Taylor AMKP, 2017, Experimental characterization of intermittency of thermoacoustic instability in a swirl stabilized combustor, 8th European Combustion Meeting (ECM) 2017, Publisher: The Combustion Institute

In the context of this work we attempt to examine the intermittency characteristics of thermoacoustic instabilities in an experimental swirl stabilized combustor operating with lean premixed air methanemixture. The instabilities are examined employing nonlinear dynamics tools, which can reveal the structure of the oscillations and allow for the development of methods that can forecast the onset of oscillations.

Conference paper

Liu Y, Vourliotakis G, Hardalupas Y, Taylor AMKPet al., 2017, A numerical study on the ability to predict the heat release rate, equivalence ratio and NO emission using chemiluminescence in counterflow premixed methane flames, 8th European Combustion Meeting (ECM) 2017, Publisher: The Combustion Institute

Chemiluminescenceemission from flames hasbeen implemented to monitor and control heat release rate(HRR), local equivalence ratio(ER)and key pollutant emissions ingas turbine combustors and automotive engines. In the present study, in order to simultaneously simulate the chemiluminescence of OH*, CH*(A), C2*and CO2*(where *denotes the excited state) and to obtain insighton the relation between chemiluminescence, heat release, equivalence ratio and NO emission,numerical studies on 1-D counterflow premixed methane flames were conducted. A new detailed reaction mechanism, incorporating sub-reaction modelsfor excited state OH*, CH*(A), C2*and CO2*radicals was assembledin this study.Threedetailed reaction mechanismsavailable in the literature for C1–C3 hydrocarbonswereemployed in the current work. Results show that OH*, CH*(A) and CO2*chemiluminescencecan accurately reproducethe heatrelease rate trend, while the OH*/CH*(A) chemiluminescent intensity ratio variesnon-monotonically with the equivalence ratio.Further, it is shown that the CO2*and C2*chemiluminescence can be utilized to indicate the levels ofNO emissions. However, the choice of thefuel oxidantchemical mechanism can highly influence the model’s ability to predict the behavior of the aforementioned combustion parametersthrough chemiluminescence simulations.

Conference paper

Henkel S, Hardalupas Y, Taylor AMKP, Conifer C, Cracknell R, Goh TK, Reinicke PB, Sens M, Rieß Met al., 2017, Injector fouling and its impact on engine emissions and spraycharacteristics in gasoline direct injection engines, SAE International Journal of Fuels and Lubricants, Vol: 10, ISSN: 1946-3960

In Gasoline Direct Injection engines, direct exposure of the injector to the flame can cause combustion products to accumulate on the nozzle, which can result in increased particulate emissions. This research observes the impact of injector fouling on particulate emissions and the associated injector spray pattern and shows how both can be reversed by utilising fuel detergency. For this purpose multi-hole injectors were deliberately fouled in a four-cylinder test engine with two different base fuels. During a four hour injector fouling cycle particulate numbers (PN) increased by up to two orders of magnitude. The drift could be reversed by switching to a fuel blend that contained a detergent additive. In addition, it was possible to completely avoid any PN increase, when the detergent containing fuel was used from the beginning of the test. Microscopy showed that increased injector fouling coincided with increased particulate emissions. Based on these results a selection of the injectors was installed in a laboratory injection chamber and the spray patterns were investigated with a high speed camera. Injectors corresponding to the largest PN drift produced the thinnest spray jets with the deepest penetration. These factors amplify the risk of wall wetting and provide an explanation for the increase of PN. The positive effect of the detergent was also reflected in the spray pattern analysis, which illustrates the potential benefits of such fuel additives.

Journal article

Tanaka D, Uchida R, Noda T, Kolbeck A, Henkel S, Hardalupas Y, Taylor AMKP, Aradi Aet al., 2017, Effects of Fuel Properties Associated with In-Cylinder Behavior onParticulate Number from a Direct Injection Gasoline Engine, SAE World Congress Experience

Conference paper

Touloupis D, Hardalupas Y, Hong C, Keramiotis C, Ramaswamy KG, Soulopoulos N, Taylor AMKP, Vourliotakis Get al., 2017, An experimental investigation on the effect of pilot injection dwell time on combustion characteristics and soot emissions in a single-cylinder optical diesel engine, 8th European Combustion Meeting (ECM)

Conference paper

Liu Y, Vourliotakis G, Hardalupas Y, Taylor AMKPet al., 2017, Experimental and numerical study of chemiluminescence characteristics in premixed counterflow flames of methane based fuel blends, 55th AIAA Aerospace Sciences Meeting, Publisher: AIAA

Non-intrusive chemiluminescence measurements have been used as heat release rate and equivalence ratio indicators for gas turbine combustor active control. In the present study, measurements and modelling of OH*, CH(A)*, C 2 *, and CO 2 * chemiluminescence are used to examine chemiluminescence sensing of heat release rate and equivalence ratio in premixed counterflow methane – air flames with equivalence ratio from 0.6 to 1.3 and strain rate from 80 to 400 s -1 . Two spectrally resolved detecting optical systems were used to detect spatially-averaged (global) and spatially resolved (local) chemiluminescence characteristics in the reaction zone. A recently published reaction mechanism 1 for the chemiluminescence of the OH*, CH*, and C 2 * species is incorporated to GRI-Mech 3.0. The augmented mechanism is further validated against the experimental results of the present study and is used to predict the chemiluminescence characteristics of premixed counterflow methane – air flames. The mechanism includes OH* chemiluminescence formation paths from hydrogen reaction, which have not been evaluated before in premixed counterflow flames. The CHEMKIN based counterflow flame code, OPPDIF is employed to simulate the experiments. The calculated OH* and CH(A)* chemiluminescence agrees well with the experimental results measured by both optical methods. Both the experimental and numerical results demonstrate the ability of OH* and CH(A)* intensities to mark heat release rate in methane – air flames. Overall, CH* may be preferable for heat release rate sensing applications at elevated equivalence ratio and strain rate. For equivalence ratio sensing in methane combustion, the measured and simulated OH*/CH(A)* chemiluminescent intensity ratio is highly dependent on equivalence ratio and nearly independent of strain rate. Thus, this ratio can be used to monitor equivalence ratio. However, a non-monotonic behavior of the OH*/CH* ratio for very lean combustion (E

Conference paper

Taylor AMKP, Whitelaw JH, 2017, VELOCITY AND TEMPERATURE MEASUREMENTS IN A PREMIXED FLAME WITHIN AN AXISYMMETRIC COMBUSTOR., AGARD, ISSN: 0549-7191

Measurements of velocity, temperature, and noise characteristics are reported for a premixed natural gas, air flame stabilized on a disc baffle located on the axis of a round pipe, and for a corresponding isothermal flow. The stability limits of the flame are identified and measurements of mean axial velocity, the variance of the corresponding fluctuations and noise intensity provided for equivalence ratios in the range 0. 7 to 1. 6. Center-line distributions of mean axial velocity, the variance of the corresponding fluctuations and mean temperature are reported and an analysis is presented of the uncertainties of the laser-anemometer instrumentation and bare-wire thermocouple measurements. It is shown that the range of equivalence ratios which allow stable length of the recirculation region are increased by combustion; and that the center-line distribution of mean temperature is comparatively uniform for more than 3 baffle diameters downstream.

Conference paper

Hardalupas I, Hong CH, Keramiotis CH, Ramaswamy GK, Soulopoulos N, Taylor AMKP, Touloupis D, Vourliotakis G, Founti MAet al., 2016, Towards identifying flame patterns in multiple, late injection schemes on a single cylinder optical diesel engine, Combustion Science and Technology, Vol: 188, Pages: 2217-2235, ISSN: 1563-521X

The work investigates the effect of various post-injection strategies on the flame patterns in a RicardoHydra optical single cylinder light duty diesel engine, operated in a partially premixed combustionmode (PPC), under low load (IMEP: ca. 2.3 bar) low speed (1200 rpm) conditions. The effect of postinjectionfuel amount (12 and 24% of the total fuel quantity per cycle) and post-injection timing (0,5, 10 deg aTDC) are investigated via pressure trace analysis and optical measurements. Flamepropagation is captured by means of high speed flame natural luminosity imaging and of CH*, C2*and OH*line-of-sight chemiluminescence measurements. Results indicate that post-injectionssuppress mixture reactivity but enhances oxidation, and that a larger amount of fuel and/or later postinjection, leads to higher levels of natural luminosity, indicating possible higher soot-out emissions,while post injection close to the main combustion event appears to have a beneficial effect on the sootoxidation processes.

Journal article

Hardalupas I, Ramaswamy KG, Taylor AMKP, Kato A, Urata Y, Matsuura Ket al., 2016, Effect of Dwell Time on Liquid Penetration of Multiple Injection Diesel Sprays in a Constant Volume Chamber, 27th Annual Conference on Liquid Atomization and Spray Systems

Conference paper

Hardalupas I, Nikita C, Taylor AM, Onorati C, Montenegro G, Cerri Tet al., 2016, Experimental Validation of Pressure Loss Models under Pulsating Conditions in three way Junctions, THIESEL 2016 Conference on Thermo- and Fluid Dynamic Processes in Direct Injection Engines

Conference paper

Hardalupas I, Hong C, Taylor AM, 2016, IGNITION DELAY AND BURN DURATION OF REFINERY BASED FUELS IN A CONSTANT VOLUME VESSEL AT DIESEL ENGINE CONDITION, 36th FISITA World Automotive Congress

Conference paper

Hardalupas I, Stetsyuk V, Soulopoulos N, Taylor AMKPet al., 2016, Scalar dissipation rate statistics in turbulent swirling jets, Physics of Fluids, Vol: 28, ISSN: 1089-7666

The scalar dissipation rate statistics were measured in an isothermal flow formed by discharging a central jet in an annular stream of swirling air flow. This is a typical geometry used in swirl-stabilised burners, where the central jet is the fuel. The flow Reynolds number was 29 000, based on the area-averaged velocity of 8.46 m/s at the exit and the diameter of 50.8 mm. The scalar dissipation rate and its statistics were computed from two-dimensional imaging of the mixture fraction fields obtained with planar laser induced fluorescence of acetone. Three swirl numbers, S, of 0.3, 0.58, and 1.07 of the annular swirling stream were considered. The influence of the swirl number on scalar mixing, unconditional, and conditional scalar dissipation rate statistics were quantified. A procedure, based on a Wiener filter approach, was used to de-noise the raw mixture fraction images. The filtering errors on the scalar dissipation rate measurements were up to 15%, depending on downstream positions from the burner exit. The maximum of instantaneous scalar dissipation rate was found to be up to 35 s−1, while the mean dissipation rate was 10 times smaller. The probability density functions of the logarithm of the scalar dissipation rate fluctuations were found to be slightly negatively skewed at low swirl numbers and almost symmetrical when the swirl number increased. The assumption of statistical independence between the scalar and its dissipation rate was valid for higher swirl numbers at locations with low scalar fluctuations and less valid for low swirl numbers. The deviations from the assumption of statistical independence were quantified. The conditional mean of the scalar dissipation rate, the standard deviation of the scalar dissipation rate fluctuations, the weighted probability of occurrence of the mean conditional scalar dissipation rate, and the conditional probability are reported.

Journal article

Bergeles K, Charalampous G, Hardalupas I, Taylor AMKPet al., 2016, Breakup of non-spherical droplets, 27th Annual Conference on Liquid Atomization and Spray Systems

Conference paper

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