| 1. |
- Alekseev, Vladimir, et al.
(författare)
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Laminar premixed flat non-stretched lean flames of hydrogen in air
- 2015
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Ingår i: Combustion and Flame. - : Elsevier BV. - 0010-2180. ; 162:10, s. 4063-4074
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Tidskriftsartikel (refereegranskat)abstract
- Laminar burning velocity of lean hydrogen + air flames at standard conditions is still a debated topic in combustion. The existing burning velocity measurements possess a large spread due to the use of different measurement techniques and data processing approaches. The biggest uncertainty factor in these measurements comes from the necessity to perform extrapolation to the flat flame conditions, since all of the previously obtained data were recorded in stretched flames. In the present study, laminar burning velocity of lean hydrogen + air flames and its temperature dependence were for the first time studied in stretch-free flat flames on a heat flux burner. The equivalence ratio was varied from 0.375 to 0.5 and the range of the unburned gas temperatures was 278-358 K. The flat flames tended to form cells at adiabatic conditions, therefore special attention was paid to the issue of their appearance. The shape of the flames was monitored by taking OH* images with an EM-CCD camera. In most cases, the burning velocity had to be extrapolated from flat subadiabatic conditions, and the impact of this procedure was quantified by performing measurements in H-2 + air mixtures diluted by N-2. The effect of extrapolation was estimated to be of negligible importance for the flames at standard conditions. The measured burning velocities at 298 K showed an important difference to the previously obtained literature values. The temperature dependence of the burning velocity was extracted from the measured results. It was found to be in agreement with the trends predicted by the detailed kinetic modeling, as opposed to a vast majority of the available literature data. (C) 2015 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
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| 2. |
- Berrocal, Edouard, et al.
(författare)
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Analysis of the SLIPI technique for multiple scattering suppression in planar imaging of fuel sprays
- 2009
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Ingår i: 11th Triennial International Annual Conference on Liquid Atomization and Spray Systems. - 9781617826535
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Konferensbidrag (refereegranskat)abstract
- Structured Laser Illumination Planar Imaging (SLIPI) is a new laser sheet based diagnostic able to significantly increase the contrast of spray images by removing the multiple scattering noise contribution. The technique has been recently developed and applied to the study of a conventional hollow-cone water spray, where the transmission through the near-field spray was 26%. In such condition, it has been shown that 44% of the total optical signal, corresponding to multiply scattered photons, could be removed. In order to now employ the technique to more challenging sprays, such as air-blast atomizer and Diesel sprays, where the transmission can be reduced down to ~0.25%, further investigations and refinements of the approach are required. This article focuses on the analysis, optimization and application of SLIPI for fuel sprays by means of a modern 3-dimensional computational model. The simulation is performed via a validated Monte Carlo code in association with a ray-tracing approach, to simulate the propagation of the incident laser radiation through the spray and the collection optics respectively. This computational work aims to quantify the amount of multiple light scattering detected by both the conventional Mie laser sheet imaging and the SLIPI technique. Results are compared for two hollow-cone fuel sprays of different transmission and droplet size properties. In the first spray the laser transmission, at λ = 532 nm, is 5% in the dense region and 27% in the dilute region, with a droplet size distribution ranging from 8 to 68 µm. The second spray is assumed to be more highly atomized, with a transmission of only 0.17% in the dense region and 7.5% in the dilute region, and with a droplets size distribution ranging from 4 to 34 µm. From these numerical calculations, it is observed that the resultant SLIPI signal tends to be closer from the pure single scattering signal when reducing the spatial period of the incident modulated light. We demonstrate here that the technique should be able to suppress an unwanted light contribution up to 91%, of the light intensity detected in the conventional planar imaging.
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| 3. |
- Berrocal, Edouard, et al.
(författare)
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Application of structured illumination for multiple scattering suppression in planar laser imaging of dense sprays.
- 2008
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Ingår i: Optics Express. - 1094-4087. ; 16:22, s. 17870-17881
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Tidskriftsartikel (refereegranskat)abstract
- A novel approach to reduce the multiple light scattering contribution in planar laser images of atomizing sprays is reported. This new technique, named Structured Laser Illumination Planar Imaging (SLIPI), has been demonstrated in the dense region of a hollow-cone water spray generated in ambient air at 50 bars injection pressure. The idea is based on using an incident laser sheet which is spatially modulated along the vertical direction. By properly shifting the spatial phase of the modulation and using post-processing of the successive recorded images, the blurring effects from multiple light scattering can be mitigated. Since hollow-cone sprays have a known inner structure in the central region, the efficiency of the method could be evaluated. We demonstrate, for the case of averaged images, that an unwanted contribution of 44% of the detected light intensity can be removed. The suppression of this diffuse light enables an increase from 55% to 80% in image contrast. Such an improvement allows a more accurate description of the near-field region and of the spray interior. The possibility of extracting instantaneous flow motion is also shown, here, for a dilute flow of water droplets. These results indicate promising applications of the technique to denser two-phase flows such as air-blast atomizer and diesel sprays.
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| 4. |
- Berrocal, Edouard, et al.
(författare)
-
Detailed visualization of spray dynamics using Light Sheet Fluorescence Microscopic imaging
- 2016
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Ingår i: ILASS – Europe 2016, 27th Annual Conference on Liquid Atomization and Spray Systems, 4-7 September 2016.
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Konferensbidrag (refereegranskat)abstract
- We demonstrate the use of Light Sheet Fluorescence Microscopic (LSFM) imaging for viewing the dynamic of atomizing sprays with high contrast and resolution. The technique presents several advantages: first liquid fluorescence gives a more faithful representation of the structure of liquid bodies, droplets and ligaments than Mie scattering does. The reason for this is that the signal is emitted by the fluorescing dye molecules inside the liquid itself and not generated at the air-liquid interfaces. Second, despite the short depth of field (~0.2 mm) obtained when using the long range microscope, the contribution of out-of-focus light is much smaller on a light sheet than on a line-of-sight configuration providing more clearly sectioned images. Finally by positioning the light sheet on the spray periphery, toward the camera objective, the effects due to multiple light scattering phenomena can be reduced to some extent. All those features provide, for many spray situations, images with high fidelity of the liquid fluid, allowing the extraction of the velocity vectors at the liquid boundaries. Here, double frame images were recorded with a sCMOS camera with a time delay of 5 μs between exposures. A typical pressure-swirl atomizer is used here producing a water hollow-cone spray which was imaged between 20 bars and 100 bars in liquid injection pressure. Such data are important for the validation of CFD models simulating liquid breakups in the near-field spray region.
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| 5. |
- Berrocal, Edouard
(creator_code:cre_t)
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DEVICE FOR GENERATING TEMPORALLY OFFSET , SPATIALLY MODULATED ILLUMINATION REGIONS
- 2019
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Patent (övrigt vetenskapligt/konstnärligt)abstract
- A device is provided for generating temporally offset , spa tially modulated illumination regions (22 , 22 ') having peri odic modulation patterns that are phase - shifted with respect to one another . The device has two pulsed laser sources (121 , 122) that are triggerable in a manner temporally offset with respect to one another and that generate two laser beams pulsed in a temporally offset manner . Intensity modulators (16) are provided for spatially periodic intensity modulation of the laser beams perpendicular to the direction of propa gation thereof . A beam superimposing device (126) com bines the beam paths of the laser beams in a common beam path section and a beam shaper (20 , 20 ') shapes the illumi nation region shaping. The common beam path section is configured so that the laser beams combined by the beam superimposing device (126) are polarized differently and the intensity modulators are upstream of an optically anisotropic beam splitter (18).
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| 6. |
- Berrocal, Edouard, et al.
(författare)
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High-contrast imaging through scattering media using structured illumination and Fourier filtering
- 2016
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Ingår i: Optics Letters. - 0146-9592. ; 41:23, s. 5612-5615
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Tidskriftsartikel (refereegranskat)abstract
- We show in this Letter a novel approach for high-contrast imaging through scattering media by combining structured illumination and Fourier filtering (SIF). To assess the image contrast enhancement at different image spatial frequencies, the modulation transfer function is calculated for four detection schemes: (1) no filtering, (2) Fourier filtering, (3) structured illumination, and (4) SIF filtering. A scattering solution consisting of D = 7.3 μm polystyrene spheres immersed in distilled water and illuminated at λ = 671 nm is used here. We demonstrate the possibility of obtaining, with SIF, an image contrast up to 60% at an optical depth of OD = 10, improving the contrast by a factor of 40 over conventional transmission imaging.
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| 7. |
- Berrocal, Edouard, et al.
(författare)
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Laser light scattering in turbid media part I: Experimental and simulated results for the spatial intensity distribution
- 2007
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Ingår i: Optics Express. - 1094-4087. ; 15:17, s. 10649-10665
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Tidskriftsartikel (refereegranskat)abstract
- We investigate the scattering and multiple scattering of a typical laser beam (lambda= 800 nm) in the intermediate scattering regime. The turbid media used in this work are homogeneous solutions of monodisperse polystyrene spheres in distilled water. The two-dimensional distribution of light intensity is recorded experimentally, and calculated via Monte Carlo simulation for both forward and side scattering. The contribution of each scattering order to the total detected light intensity is quantified for a range of different scattering phase functions, optical depths, and detection acceptance angles. The Lorentz-Mie scattering phase function for individual particles is varied by using different sphere diameters (D = 1 and 5 mu m). The optical depth of the turbid medium is varied (OD = 2, 5, and 10) by employing different concentrations of polystyrene spheres. Detection angles of theta(a) = 1.5 and 8.5 are considered. A novel approach which realistically models the experimental laser source is employed in this paper, and very good agreement between the experimental and simulated results is demonstrated. The data presented here can be of use to validate other modern Monte Carlo models, which generate high resolution light intensity distributions. Finally, an extrapolation of the Beer-Lambert law to multiple scattering is proposed based on the Monte Carlo calculation of the ballistic photon contribution to the total detected light intensity.
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| 8. |
- Berrocal, Edouard, et al.
(författare)
-
Laser light scattering in turbid media Part II: Spatial and temporal analysis of individual scattering orders via Monte Carlo simulation.
- 2009
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Ingår i: Optics Express. - 1094-4087. ; 17:16, s. 13792-13809
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Tidskriftsartikel (refereegranskat)abstract
- In Part I of this study [1], good agreement between experimental measurements and results from Monte Carlo simulations were obtained for the spatial intensity distribution of a laser beam propagating within a turbid environment. In this second part, the validated Monte Carlo model is used to investigate spatial and temporal effects from distinct scattering orders on image formation. The contribution of ballistic photons and the first twelve scattering orders are analyzed individually by filtering the appropriate data from simulation results. Side-scattering and forward-scattering detection geometries are investigated and compared. We demonstrate that the distribution of positions for the final scattering events is independent of particle concentration when considering a given scattering order in forward detection. From this observation, it follows that the normalized intensity distribution of each order, in both space and time, is independent of the number density of particles. As a result, the amount of transmitted information is constant for a given scattering order and is directly related to the phase function in association with the detection acceptance angle. Finally, a contrast analysis is performed in order to quantify this information at the image plane.
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| 9. |
- Berrocal, Edouard, et al.
(författare)
-
Light sheet fluorescence microscopic imaging for high-resolution visualization of spray dynamics
- 2018
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Ingår i: International Journal of Spray and Combustion Dynamics. - : SAGE Publications. - 1756-8277 .- 1756-8285. ; 10:1, s. 86-98
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Tidskriftsartikel (refereegranskat)abstract
- In this study, the use of light sheet fluorescence microscopic imaging is demonstrated for viewing the dynamic of atomizing sprays with high contrast and resolution. The technique presents several advantages. First, liquid fluorescence gives a more faithful representation of the structure of liquid bodies, droplets, and ligaments than Mie scattering does. The reason for this is that the signal is emitted by the fluorescing dye molecules inside the liquid itself and not generated at the air–liquid interfaces. Second, despite the short depth of field (∼200 µm) obtained when using the long range microscope, the contribution of out-of-focus light is much smaller on a light sheet configuration than for line-of-sight detection, thus providing more clearly sectioned images. Finally, by positioning the light sheet on the spray periphery, toward the camera objective, the effects due to multiple light scattering phenomena can be reduced to some extent. All these features provide, for many spray situations, good fidelity images of the liquid fluid, allowing the extraction of the velocity vectors at the liquid boundaries. Here, double frame images were recorded with a sCMOS camera with a time delay of 5 µs between exposures. A typical pressure-swirl atomizer is used producing a water hollow-cone spray, which was imaged in the near-nozzle region and further downstream for injection pressures between 20 bar and 100 bar. Furthermore, near-nozzle spray shape visualization of a direct-injection spark ignition injector was conducted, describing the disintegration of the liquid fuel and droplet formation. Such data are important for the validation of computational fluid dynamics models simulating liquid breakups in the near-field spray region.
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| 10. |
- Berrocal, Edouard, et al.
(författare)
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Multiple Scattering Suppression In Planar Laser Imaging Of Dense Sprays By Means Of Structured Illumination
- 2010
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Ingår i: Atomization and Sprays. - 1044-5110 .- 1936-2684. ; 20:2, s. 133-139
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Tidskriftsartikel (refereegranskat)abstract
- A novel method to reduce the multiply scattered light contribution to images recorded with planar laser imaging is demonstrated. The technique, structured laser illumination planar imaging (SLIPI), is based on spatially modulated excitation light and is tested here within the dense region of a hollow-cone spray. The main idea is to use a laser sheet that is spatially modulated along the vertical direction. By both shifting the spatial phase of the modulation and using adequate image post processing of the successive recorded images, it is possible to remove a significant amount of the multiply scattered light detected. In this paper, SLIPI is applied for imaging within a typical hollow-cone water spray generated in ambient air at 50 bars injection pressure from a pressure-swirl nozzle. Because this type of spray has a known inner structure, the method can be evaluated, demonstrating that 47% of the detected light arising from multiple scattering can be suppressed, resulting in an increase from 61% to 89 % in image contrast. Such an improvement allows more accurate interpretation and analysis of the near-field region of atomizing sprays. The possibility of extracting instantaneous flow motion is also demonstrated for the case of a dilute nebulizer. All these results indicate promising applications of the technique in denser turbid media, such as air-blast atomizer or diesel sprays.
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