43 results found
Charalampous G, Hadjiyiannis C, Hardalupas I, 2019, Proper orthogonal decomposition of primary breakup and spray in co-axial airblast atomizers, Physics of Fluids, Vol: 31, ISSN: 1070-6631
The primary atomisation of a liquid jet by a coaxial stream of high speed gas is analysed by means of Proper Orthogonal Decomposition (POD) for gas to liquid momentum ratios, MR, from 182 to 727 and Weber numbers, We, from 22 to 88. The continuous liquid core is visualised by the optical connectivity technique. The full spray in the near nozzle region is visualised using shadowgraphy. It is found that universal POD modes exist for the continuous liquid core and the near nozzle full spray across all considered flow conditions. The universal POD modes are related to physical structures of the flow. The complexity of the flow, as determined by the energy of the POD modes, is found to be constant for the liquid core across the examined range of flow MR. On the contrary, the complexity of the full spray is inversely proportional to the flow MR. Correlations are established between the spatial and temporal scales of primary atomisation. In addition, a novel method to extrapolate the spatial and temporal scales of the atomisation process beyond the limits of the current measurement resolution is described and demonstrated. Estimates are provided on the number of samples and the sampling rate that are required to fully resolve the flow to specific temporal and spatial scales.
Chen C, Charalampous G, Hardalupas Y, 2018, Laser ignition of methane jets in homogenous and isotropic turbulence, 2018 AIAA Aerospace Sciences Meeting, Publisher: American Institute of Aeronautics and Astronautics
The ignition of a pulsed methane jet by a laser-induced spark was investigated in air under the influence of homogenous and isotropic turbulence with zero mean flow. In this way, the contribution of the turbulent velocity fluctuations of the environment can be isolated from the mean flow component. The fuel jet Reynolds number (Re jet ) was fixed at 160, while the turbulent Reynolds numbers (Re λ ) of the environment was in the range of 0-220. The influence of the surrounding turbulence on the spatial distribution of the flame luminosity was evaluated and an increasingly random distribution of the flame was obtained under higher turbulent intensity. The Minimum Ignition Energy (MIE) was determined by measuring ignition probability for three different ignition locations. The MIE was found to increase with the surrounding turbulent intensity and had a significant dependence on ignition location. A disagreement between the spatial distribution of MIE and mixture fraction was observed. This is probably due to the finite size of the plasma, which has a more significant influence on the ignition probability at locations near the nozzle tip. The increase rate of the MIE with the level of turbulence was different at different ignition locations; this might be caused by the difference in the extent to which turbulence influences the mixture fraction at these locations. This effect is likely to be larger at positions near the methane/jet interface and at a distance from the nozzle exit, while it is smaller at positions near the centreline and the nozzle exit.
Charalampous G, Hardalupas I, 2017, Collisions of droplets on spherical particles, Physics of Fluids, Vol: 29, Pages: 103305-1-103305-15, ISSN: 1070-6631
Head-on collisions between droplets and spherical particles are examined for water droplets in the diameter range between 170 μm and 280 μm and spherical particles in the diameter range between 500 μm and 2000 μm. The droplet velocities range between 6 m/s and 11 m/s, while the spherical particles are fixed in space. The Weber and Ohnesorge numbers and ratio of droplet to particle diameter were between 92 < We < 1015, 0.0070 < Oh < 0.0089, and 0.09 < Ω < 0.55, respectively. The droplet-particle collisions are first quantified in terms of the outcome. In addition to the conventional deposition and splashing regimes, a regime is observed in the intermediate region, where the droplet forms a stable crown, which does not breakup but propagates along the particle surface and passes around the particle. This regime is prevalent when the droplets collide on small particles. The characteristics of the collision at the onset of rim instability are also described in terms of the location of the film on the particle surface and the orientation and length of the ejected crown. Proper orthogonal decomposition identified that the first 2 modes are enough to capture the overall morphology of the crown at the splashing threshold.
Charalampous G, Hardalupas I, 2016, How do liquid fuel physical properties affect liquid jet development in atomisers?, Physics of Fluids, Vol: 28, ISSN: 1070-6631
The influence of liquid fuel properties on atomisation remains an open question. Thedroplet sizes in sprays from atomisers operated with different fuels may be modifieddespite the small changes of the liquid properties. This paper examines experimentallythe development of a liquid jet injected from a plain orifice in order to evaluatechanges in its behaviour due to modifications of the liquid properties, which mayinfluence the final atomisation characteristics. Two aviation kerosenes with similar,but not identical physical properties are considered, namely standard JP8 kerosene asthe reference fuel and bio-derived Hydro-processed Renewable Jet (HRJ) fuel as analternative biofuel. The corresponding density, dynamic viscosity, kinematic viscosityand surface tension change by about +5%, -5%, -10% and +5% respectively, whichare typical for ‘drop-in’ fuel substitution. Three aspects of the liquid jet behaviour areexperimentally considered. The pressure losses of the liquid jet through the nozzle areexamined in terms of the discharge coefficient for different flowrates. Themorphology of the liquid jet is visualised using high magnification Laser InducedFluorescence (LIF) imaging. Finally, the temporal development of the liquid jetinterfacial velocity as a function of distance from the nozzle exit is measured fromtime-dependent motion analysis of dual-frame LIF imaging measurements of the jet.The results show that for the small changes in the physical properties between theconsidered liquid fuels, the direct substitution of fuel did not result in a drastic changeof the external morphology of the fuel jets. However, the small changes in thephysical properties modify the interfacial velocities of the liquid and consequently theinternal jet velocity profile. These changes can modify the interaction of the liquid jetwith the surroundings, including air flows in coaxial or cross flow atomisation, andinfluence the atomisation characteristics during changes of liquid
Sung Y, Charalampous G, Hardalupas I, et al., 2016, Laser ignition and flame characteristics of pulsed methane jets in homogeneous isotropic turbulence without mean flow, Proceedings of the Combustion Institute, Vol: 36, Pages: 1653-1660, ISSN: 1873-2704
The influence of turbulence on the minimum ignition energy (MIE) and ignited flame characteristics is investigated for pulsed methane diffusion jets ignited by laser-induced plasma. The methane jet is injected in a volume of homogeneous and isotropic air turbulence without mean flow, with the level of turbulence being controlled independently. The study is carried out for a range of fuel jet (Rejet) Reynolds number, namely 1000, 2000, and 3000, and a range of turbulent (Reλ) Reynolds number, namely 0–207. The results show that the position of the maximum intensity of flame emission was randomly scattered due to the fact that the ignited flame is deflected from the nozzle axis by the turbulent velocity fluctuations. The effect is more profound at higher Rejet. The value of the MIE, determined according to 50% ignitibility of mixture, increases by a factor of 2 for an increase of Reλ from 0 to 207 and by a factor of 5 for an increase of Rejet from 1000 to 3000. Two trends are observed on MIE with Reλ. For low Reλ, MIE is independent of Reλ. Past a critical value of Reλ, MIE increases as a linear function of Reλ. This transition occurs at critical values of Reλ,c = 158, 197 and 202 for Rejet= 1000, 2000 and 3000, respectively. The mean value of MIE for ignition before and after transition is a linear function of Rejet. The difference between the mean value of MIE before transition and after transition is around 5 mJ for all considered Rejet.
Bergeles K, Charalampous G, Hardalupas I, et al., 2016, Breakup of non-spherical droplets, 27th Annual Conference on Liquid Atomization and Spray Systems
Charalampous G, Hardalupas I, 2016, Droplet-particle collisions and breakup in homogenous and isotropic turbulence, 27th Annual Conference on Liquid Atomization and Spray Systems
Andrade P, Charalampous G, Hardalupas Y, 2016, Spray Impingement on Spherical Targets in Homogenous Isotropic Turbulence, 27th Annual Conference on Liquid Atomization and Spray Systems
Charalampous G, Hadjiyiannis C, Hardalupas Y, 2016, Comparative measurement of the breakup length of liquid jets in airblast atomisers using optical connectivity, electrical connectivity and shadowgraphy, Measurement, Vol: 89, Pages: 288-299, ISSN: 1873-412X
A comparative examination of the optical connectivity, electrical connectivity and shadowgraphytechniques for the measurement of the break-up length of atomising liquid jets from a co-axial airblastatomiser is presented. The atomiser was operated over air-to-liquid Momentum Ratios between 27 and335 and Momentum Flux Ratios between 0.67 and 8.27. Shadowgraphy records instantaneous imagesof the shadow of the atomising liquid jet when it is back-illuminated by a light source. The electricalconnectivity uses the continuity of an electrically conducting atomising liquid jet to measure thepotential during the presence of an electrical connection between the spray nozzle and a probe furtherdownstream. The optical connectivity visualises the atomising liquid jet, doped with a fluorescing dye,as it is illuminated from within the nozzle using a laser beam. Comparison of the measured breakuplengths with time resolved shadowgraphy, optical connectivity and electrical connectivity, followingthe proposed novel processing of the time-dependent potential, showed that the mean values are allwithin ±15% of each other. The advantages and limitations of each technique are discussed.
Charalampous G, Hardalupas Y, 2016, Impingement and splashing of droplets on spherical targets, 54th AIAA Aerospace Sciences Meeting, AIAA SciTech
Bergeles K, Charalampous G, Hardalupas Y, 2015, Aerodynamic breakup of non-spherical droplets, 8th International Conference on Multiphase Flow ICMF 2016
Charalampous G, Hardalupas Y, 2015, Droplet-particle collisions in homogeneous and isotropic turbulence, 8th International Conference on Multiphase Flow ICMF 2016
Andrade P, Charalampous G, Hardalupas Y, 2015, Spray Impingement on Spherical Targets in Homogeneous, Isotropic Turbulence, 8th International Conference on Multiphase Flow ICMF 2016
Charalampous G, Chounta P, Hardalupas Y, 2015, Liquid jet breakup in homogeneous and isotropic turbulence without mean flow, 7th European Combustion Meeting
Charalampous G, Soulopoulos N, Hardalupas Y, 2015, Ray tracing analysis of realistic atomizing jet geometries for optical connectivity applications, 53rd AIAA Aerospace Sciences Meeting and Exhibit
Charalampous G, Hardalupas Y, 2014, Application of Proper Orthogonal Decomposition to the morphological analysis of confined co-axial jets of immiscible liquids with comparable densities, Publisher: American Institute of Physics, ISSN: 1089-7666
The development of a round liquid jet under the influence of a confined coaxial flow of an immiscible liquid of comparable density (central to annular flow density ratio of 8:10) was investigated in the vicinity of the nozzle exit. Two flow regimes were considered; one where the annular flow is faster than the central jet, so the central liquid jet is accelerated and one where the annular flow is slower, so the central liquid jet is decelerated. The central jet was visualised by high speed photography. Three modes of jet development were identified and classified in terms of the Reynolds number, Re, of the central jet which was in the range of 525 < Re < 2725, a modified definition of the Weber number, We, which allows the distinction between accelerating and deceleration flows and was in the range of −22 < We < 67 and the annular to central Momentum Ratio, MR, of the two streams which was in the range of 3.6 < MR < 91. By processing the time resolved jet images using Proper Orthogonal Decomposition (POD), it was possible to reduce the description of jet morphology to a small number of spatial modes, which isolated the most significant morphologies of the jet development. In this way, the temporal and spatial characteristics of the instabilities on the interface were clearly identified which highlights the advantages of POD over direct observation of the images. Relationships between the flow parameters and the interfacial waves were established. The wavelength of the interfacial instability was found to depend on the velocity of the fastest moving stream, which is contrary to findings for fluids with large density differences.
Charalampous G, Hardalupas Y, 2014, Comparison between signal attenuation correction methodology for LIF and scattered light intensity measurements in dense sprays, 17th International Symposium of Laser Techniques to fluids mechanics
Charalampous G, Hardalupas Y, 2014, Evaluation of a laser beam scanning method for correction of multiple scattering and attenuation effects in dense spray measurements, 52nd AIAA Aerospace Sciences Meeting and Exhibit
Charalampous G, Hardalupas Y, 2014, Evaluation of a laser beam scanning method for correction of multiple scattering and attenuation effects in dense spray measurements, 52nd AIAA Aerospace Sciences Meeting - AIAA Science and Technology Forum and Exposition, SciTech 2014
When applying laser imaging to dense sprays, the acquired measurements are hindered by optical noise due to multiple scattering, attenuation of the illuminating laser beam as it interacts with the spray and attenuation of the scattered light by droplets between the probed spray region and the detector. An investigation was performed to assess the possibility of addressing these issues by scanning a laser beam across a spray instead of illuminating with a laser sheet. A high precision opto-mechanical arrangement was assembled for measurements of scattered and fluorescent light intensity across a cross section of a flat spray. These measurements were used for applying a correction procedure, described by , to the attenuation of the incident and collected light intensity by a spray. Application of the correction methodology on the collected scattered light profiles across a plane of a spray provided quantitative measurements of the contributions of laser intensity attenuation along the beam path, attenuation of the scattered light intensity between the probed spray region and the imaging camera and the scattering cross section of the spray droplets. From the above, the full cross section of the spray was reconstructed in terms of the scattered light intensity profiles, which are representative of the droplet surface area and liquid volume densities. The reconstructed spray shape was elliptical and symmetric along the two normal axes, as expected for a flat spray, which demonstrates that the developed approach was appropriate.
Katsikadakos D, Charalampous G, Hardalupas Y, et al., 2013, Numerical and experimental evaluation of equivalence ratio measurement using OH* and CH* chemiluminescence in laminar premixed n-butanol counterflow flames, Proceedings of the European Combustion Meeting
Hadjiyiannis C, Sahu S, Charalampous G, et al., 2013, Correlation Between the Primary Break-up of Liquid Jets and Downstream Spray Characteristics in Air-blast Atomizers, Proceedings of the European Combustion Meeting
Hadjiyiannis C, Sahu S, Charalampous G, et al., 2013, Simultaneous measurement of primary break-up and downstream spray characteristics in air-blast atomizers, 8th International Conference on Multiphase Flow
Lian H, Charalampous G, Hardalupas Y, 2013, Time resolved investigation of clustering of droplets in a "box of turbulence", 8th International Conference on Multiphase Flow
Charalampous G, Hardalupas Y, 2013, Investigation of aviation fuel injection process using dual-frame Laser-Induced Fluorescence imaging, 8th International Conference on Multiphase Flow
Lian H, Charalampous G, Hardalupas Y, 2013, Preferential concentration of poly-dispersed droplets in stationary isotropic turbulence, EXPERIMENTS IN FLUIDS, Vol: 54, ISSN: 0723-4864
The preferential concentration of poly-dispersed water droplets with a range of Sauter mean diameters between 25 and 95 μm has been studied experimentally in stationary homogeneous isotropic turbulence with four different intensities, characterized by turbulent Reynolds numbers based on Taylor microscale, of Re λ = 107, 145, 185 and 213. The image processing method of recorded scattered light intensity images from droplets is described and its ability to identify droplets is assessed in terms of image quality. The influence of image processing parameters on measured characteristics of droplet clustering is evaluated. The radial distribution function (RDF) and 2D Voronoï analysis quantified the magnitude of preferential droplet concentration and the results from both methods agreed well. RDF showed that the characteristic length scale of resulting droplet clusters varies between 20 and 30 times the Kolmogorov length scale over all the experimental conditions. It was found that the preferential concentration is more appropriately described by a Stokes number, based on various representative diameters, namely the arithmetic mean diameter, D10, or the diameter, DN60%, below which 60% of the total droplet number in the spray is present, or the diameter, DV5%, which carries 5% of the total liquid volume in the spray. The magnitude of droplet preferential concentration was maximum when the proposed Stokes number was around unity for all experimental conditions. Little dependence of the magnitude of preferential concentration on turbulent Reynolds numbers was found, in contrast to the recent DNS findings (Tagawa et al. in J Fluid Mech 693:201–215, 2012).
Charalampous G, Hardalupas Y, Brown C, et al., 2013, Investigation of injection characteristics of alternative aviation fuels by Laser-Induced Fluorescence imaging, 51st AIAA Aerospace Sciences Meeting Including the New Horizons Forum and Aerospace Exposition
Charalampous G, Hadjiyiannis C, Hardalupas Y, 2012, Numerical and experimental investigation of the optical connectivity technique in cross flow atomization, 12th International Conference on Liquid Atomization and Spray Systems
Charalampous G, Hardalupas Y, 2012, Estimation of the droplet size spread with the laser induced fluorescence/Mie technique, OPTICS LETTERS, Vol: 37, Pages: 2040-2042, ISSN: 0146-9592
Charalampous G, Hardalupas Y, 2012, Numerical evaluation of the optical connectivity technique for breakup length measurements of liquid columns, LIP 2012 - Lasers and Interactions with Particles
Charalampous G, Hadjiyiannis C, Hardalupas G, 2011, Proper Orthogonal Decomposition analysis of photographic and optical connectivity time-resolved images of an atomising liquid jet, 24th Annual Conference on Liquid Atomization and Spray Systems
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