Publications
227 results found
Bättig RJ, Jasper T, Hardalupas Y, et al., 2023, Zero Emission Hydrogen Internal Combustion Engine for a 5 kW Mobile Power Generator: Conversion Strategy for Carburetted SI Engines
A carburetted, spark ignited gasoline fuelled engine of a 5 kW rated power generator was converted to run on hydrogen. As opposed to large parts of current research, the engine conversion's foremost goal was not to maximise efficiency and power output but rather to find a cost-effective and low-complexity conversion approach to introduce clean fuels to existing engines. To allow for the increased volumetric fuel flow, the riser of the original carburettor was enlarged. The hydrogen flow into the venturi was metered with the help of a pressure regulator from a widely available conversion kit. The effects of different hydrogen-fuel-feed pressures on engine performance, operational stability and emission levels were examined experimentally. It was found that the hydrogen-line pressure before startup has to be set precisely (±5 mbar) to allow for stable and emission free operation. While the rated power of the converted engine was lowered to 57% of the original's generator, efficiency was increased by up to 9 percentage points while lowering NOx emissions drastically. In fact, under stable hydrogen supply conditions, it was shown that an average concentration of only 19 ppm NOx is feasible without any exhaust gas aftertreatment. An analytic derivation of lambda values from emission data has been examined. With a fairly constant offset of 4.3% between the measured and calculated lambda value, the stoichiometrically derived formula for lambda proved to yield precise results with little calculation effort. As a main limiting factor for continuous low emission levels, pressure fluctuations in the hydrogen supply line were identified to strongly influence the fuel flow rate leading to inconsistent air-to-fuel ratios. Furthermore, large, abrupt reductions in engine load were identified to be critical regarding running stability and backfiring events.
Kolokotronis D, Sahu S, Hardalupas I, et al., 2023, Bulk cavitation in model gasoline injectors and their correlation with the instantaneous liquid flow field, Fluids, Vol: 8, Pages: 1-24, ISSN: 2311-5521
It is well established that spray characteristics from automotive injectors depend on, among other factors, whether cavitation arises in the injector nozzle. Bulk cavitation, which refers to the cavitation development distant from walls and thus far from the streamline curvature associated with salient points on a wall, has not been thoroughly investigated experimentally in injector nozzles. Consequently, it is not clear what is causing this phenomenon. The research objective of this study was to visualize cavitation in three different injector models (designated as Type A, Type B, and Type C) and quantify the liquid flow field in relation to the bulk cavitation phenomenon. In all models, bulk cavitation was present. We expected this bulk cavitation to be associated with a swirling flow with its axis parallel to that of the nozzle. However, liquid velocity measurements obtained through particle image velocimetry (PIV) demonstrated the absence of a swirling flow structure in the mean flow field just upstream of the nozzle exit, at a plane normal to the hypothetical axis of the injector. Consequently, we applied proper orthogonal decomposition (POD) to analyze the instantaneous liquid velocity data records in order to capture the dominant coherent structures potentially related to cavitation. It was found that the most energetic mode of the liquid flow field corresponded to the expected instantaneous swirling flow structure when bulk cavitation was present in the flow.
Tingas EA, Taylor AM, 2023, Hydrogen: Where it Can Be Used, How Much is Needed, What it May Cost, Green Energy and Technology, Pages: 3-64
Although the subject of this book is hydrogen in Thermal Engines, neither the fuel nor the engines are widely commercially available at the time of publication: nor is it likely that this situation will change before the end of this decade. This notwithstanding, the background to this book is with the applications known colloquially as the ‘hard to decarbonise’, by which is meant primarily long-haul transport by land (farther than 500 miles or multi-shift routes), sea and air, and which need shaft power as might be provided by a thermal engine. This introductory chapter is concerned with how quickly, how widely, and at what cost this fuel might be used in these applications in the years up to 2050, and beyond. This chapter starts by tracing, briefly, how there has developed an understanding of the deleterious effects of anthropogenic emissions of carbon dioxide, an important ‘greenhouse gas’ (GHG), on the climate. To reduce these effects, we next trace out how there came to be a level of global agreement that the emissions of these gases should be curtailed, and over what timescale. The GHG are emitted by the consumption of fossil fuels to power a wide range of industrial activities: consequently, there is a myriad of roadmaps and proposals as to how balance economic development with protection of the environment. This involves, at the very least, eliminating the emission of GHG: preferably, by ceasing to burn fossil fuels. For the ‘hard to decarbonise’ application considered in this book, which is predominantly transportation, this implies some fuel other than a hydrocarbon: hydrogen has been proposed as a candidate fuel in some sectors. The suggestion implies the need to evaluate the engineering science and technological aspects of burning hydrogen, which is the concern of the rest of the book. This chapter investigates which applications might use hydrogen, how hydrogen can be manufacture
Emerique C, Jasper T, Hardalupas I, et al., 2022, Converting a production 6-cylinder Heavy Duty Engine to Dual-fuel mode operation using supervisory calibration and manifold injection, 10th International Conference on Modeling and Diagnostics for Advanced Engine Systems (COMODIA 2022)
Carbon dioxide emissions from heavy-duty vehicles can be reduced by converting, or retrofitting, conventional diesel engines into “dual fuel” mode operation using hydrogen and diesel fuel. Conversion can involve small and relatively cheap changes to design, thereby allowing a diesel engine to be fuelled by either conventionally, using diesel fuel (directinjection), or in conjunction with hydrogen (indirect injection – "fumigation”) in the intake manifold. We evaluate the conversion of a production, 6-cylinder 12.7L truck engine to run in dual fuel mode. Commercially viable conversions would permit, in the short term, the no-regrets ‘pump priming’ of a hydrogen pricing, production, storage and distribution infrastructure which, in the medium term, could also benefit the development of hydrogen-powered inshore marine vessels and trucks powered by, for example, fuel cells. The aim of this work was to establish the limitations and advantages of pursuing the conversion of an in-production heavy duty engine to dual fuel operation with the least changes to the engine. The conversion used supervisory calibration, which consisted in maximizing the amount of hydrogen injection on top of diesel fuel, without any optimization of the other engine parameters. The strategy was, at a given operating point, to decrease the load seen by the diesel ECU while also increasing hydrogen injection until the torque at this operating point was recovered. As a result, the load ‘detected’ by the diesel ECU was lower when the engine was running on the dual fuelmode than when it was running on diesel fuel only. The displacement of diesel fuel by hydrogen directly resulted in a reduction in carbon dioxide emissions. Nevertheless, the variability of the hydrogen-air premixed mixture combustion was kept under control and, in particular, knock had to be avoided.At low loads, up to 70% of the energy required to drive the truck could be provided by
Liu Y, Hardalupas Y, Taylor AMKP, 2022, A detailed CO2(1B2) chemiluminescence chemical kinetics model for carbon monoxide and hydrocarbon oxidation, Fuel, Vol: 323, Pages: 1-10, ISSN: 0016-2361
The CO2(1B2) - CO2(X1Σ+g) transition is a source of chemiluminescence from CO and hydrocarbon premixed flames and can be used as a diagnostic; however, its chemistry is not well known due to its broadband nature. Although several attempts have been made to model CO2(1B2) chemiluminescence, none performs well in hydrocarbon flames. We propose a new detailed kinetic model for CO2(1B2) chemiluminescence, based on shock tube experiments in the literature and on opposed flame data presented here. The mechanism consists of 26 reactions which describe the formation of the lower excited state molecule CO2(3B2) (R1), the inter-system crossing reaction between CO2(3B2) and CO2(1B2) (R2), CO2(1B2), the formatting reaction path in hydrocarbon flames (R3), CO2(1B2) radiative quenching (R4) and collisional quenching of CO2(3B2) and CO2(1B2) (R5-R26). The reaction rates constants of R1 and R3 within ± 60% and ± 32% uncertainty, respectively, were determined as follows:
Chantriaux F, Quenouille T, Doan NAK, et al., 2022, Multiscale analysis of turbulence-flame interaction based on measurements in premixed flames, Combustion and Flame, Vol: 239, Pages: 1-14, ISSN: 0010-2180
Multi-scale analysis of turbulence–flame interaction is performed using experimental data sets from three methane- and propane-fired premixed, turbulent V-flames, at an approach flow turbulent Reynolds number of 450 and a ratio of r.m.s. fluctuating velocity from the mean to laminar flame speed of between 2.1 and 3.0, straddling the border between corru-gated flamelets and thin reaction zone in the Borghi-Peters diagram. The measurements were made in the plane of a single laser sheet using stereo particle image velocimetry SPIV and planar laser-induced fluorescence to measure three orthogonal components of velocityand flame OH. Methods to approximate the remaining, unmeasured, out of plane derivatives are described. The instantaneous SPIV images were bandpass filtered at user-specified characteristic length scales Lω and Ls (for vorticity and strain rate, respectively) resulting in instantaneous bandpass-filtered velocity fields, uLω b and uLs b , which were further analysedto give the bandpass filtered vorticity field, ωLω = ∇ × uLωb , the strain-rate field, eLs ij , and the tangential strain rate field aLsT .This work quantifies two aspects of turbulence-flame interaction. The first aspect is that of the flame interaction of eddies of size Ls on the turbulence, as found by the statistics of the alignment of vorticity with strain rate. We find that vortical eddies with scale about Lω = 2δth (where δth is the flame thickness) are stretched by Ls structures which are largerthan about 2 Lω, with this factor broadly true also for vortical eddies of scales Lω = 4δth and Lω = 6δth. Within the limitations of the data set, these findings are consistent with those in the literature on reacting and non-reacting flows, suggesting that the premixed flame has had little influence on the vortex stretching mechanism.The second aspect of turbulence-flame interaction examined is that of
McGinn P, Pearce D, Hardalupas I, et 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.
Hua X, Liu Y, Chen C, et 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.
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.
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
Karlis E, Liu Y, Hardalupas I, et 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
Karlis E, Liu Y, Hardalupas Y, et 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
Karlis E, Hardalupas I, Taylor A, et 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
Noh D, Karlis E, Navarro-Martinez S, et 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.
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.
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
Tsekenis S-A, Ramaswamy KG, Tait N, et 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.
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
Joshi M, Gosala D, Allen C, et 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.
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
Charalambides AG, Sahu S, Hardalupas I, et 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
Hardalupas Y, Hong C, Keramiotis C, et 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.
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.
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.
Liu Y, Vourliotakis G, Hardalupas Y, et 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.
Henkel S, Hardalupas Y, Taylor AMKP, et 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.
Tanaka D, Uchida R, Noda T, et al., 2017, Effects of Fuel Properties Associated with In-Cylinder Behavior onParticulate Number from a Direct Injection Gasoline Engine, SAE World Congress Experience
Touloupis D, Hardalupas Y, Hong C, et 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)
Liu Y, Vourliotakis G, Hardalupas Y, et 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
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.
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