| 1. |
- Lehnert, Bastian, et al.
(författare)
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Quantifying extinction imaging of fuel sprays considering scattering errors
- 2023
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Ingår i: International Journal of Engine Research. - 1468-0874. ; 24:10, s. 4413-4420
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Tidskriftsartikel (refereegranskat)abstract
- In this work, we use the measurement technique of high-speed Diffuse Back Illumination Extinction Imaging (DBI-EI) to obtain quantitative information in the form of projected liquid volume (PLV) in a highly transient GDI process. For the DBI-EI setup we use a LED-Panel as the light source, which fulfills diffuse back illumination extinction imaging criteria. Measurements were carried out in a constant volume chamber, allowing easy optical access, and enabling measurements at real world ambient engine conditions. For the experiments, we use an Engine Combustion Network (ECN) Spray G injector and measure the sprays at ECN conditions. Moreover, we mount the injector in a motorized rotational system, enabling measurements of the sprays at precisely defined angles of observation. The DBI-EI technique requires a light source radiating uniformly in a certain range of an angle. Because of the diffuse radiation, an error in the quantification of the liquid phase results from the detection of multiple and forward scattered photons. This leads to an underestimation of the optical depth ((Formula presented.)), which further results in a false calculation of the projected liquid volume. Therefore, we must assume that DBI-EI results are wrong. To enable the use of DBI-EI in all spray regions independent of the measurement setup, we present a simulation-based method, which is correcting the (Formula presented.) for scattering effects. Results show, that the measured (Formula presented.) of the experimental setup, which we used in this work, is underestimated by at least a factor of 2.2. This factor increases with increasing spray densities. We can use the corresponding corrected PLV data to reconstruct three-dimensional data of the liquid volume fraction with the tomographic method filtered back projection. Thus, we obtain time and spatial resolved quantitative spray information, with an approach to correct undesired scattering effects, while keeping the experimental effort low.
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| 2. |
- Lehnert, Bastian, et al.
(författare)
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Tomographic reconstruction of spray evolution considering multiple light scattering effects : Characterization of an ECN Spray G Injector
- 2022
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Ingår i: COMODIA2022 proceedings. ; , s. 337-345
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Konferensbidrag (refereegranskat)abstract
- Meeting stricter legal emission limits and the simultaneous introduction of new synthetic fuels are key challenges for current and future research in the field of engine combustion. A deep knowledge of spray behavior is mandatory to address these problems, as air‐fuel mixture and spray propagation in gasoline direct injection (GDI) are essential processes to achieve a highly efficient and clean combustion. Thus, a lot of effort is put into the identification of air‐fuel spray distributions. Most of them use lasers or X‐ray sources, which are accompanied by a high experimental complexity and further drawbacks. In this work, the measurement technique application of high speed diffuse back illumination extinction imaging (DBIEI) is used to obtain quantitative information in the form of projected liquid volume fraction (PLV). The DBIEI setup is simplified to enable an easier and quicker application for different experimental environments, using a LED-Panel as light source, which fulfills diffuse back illumination (DBI) criteria. Measurements are done in a constant volume chamber, allowing easy optical access with up to 200 mm in diameter and enabling measurements at real world ambient engine conditions. An engine combustion network (ECN) Spray G injector is used. ECN ambient conditions G1 (3.5 kg/m3 ambient density at 300°C), G2 (0.5 kg/m3 ambient density at 60°C) and G3 (1.01 kg/m3 ambient density at 60°C) are chosen. Isooctane is used as fuel. The injector is mounted in a motorized rotational system, enabling measurements of the spray at defined and exact angles. The DBIEI measurement technique suppresses the effect of beam steering at elevated ambient conditions, allowing the measurement of scattering based light attenuation by spray droplets. This requires a light source radiating uniformly over a certain angle‐range. Nevertheless, an inherent error in the quantification of liquid phase results from the detection of multiple scattered photons. The error is even more enhanced when using a non‐collimated light source. This leads to an underestimation of the optical depth (OD), which further results in a false calculation of the projected liquid volume. Therefore, beside very low‐density regions e.g. at spray boundaries, PLV results have to be assumed wrong. To enable the use of DBIEI in more dense spray regions, we present a simulation-based method correcting the ODs. Derived from this the corrected values indicate an underestimation of the OD of a factor greater than 2. The corresponding PLV data at different viewing angles is then used to reconstruct three-dimensional data of the liquid volume fraction (LVF) with filtered back projection (FBP). Thereby we can obtain time and spatial resolved quantitative spray information using an easy experimental setup, with an approach to correct beam steering and multiple scattering, while the experimental effort is kept low by using LED light sources. This data can be used for comparison, calibration and evaluation of simulation data of transient sprays, leading to detailed knowledge of spray behavior and mixture formation in different conditions.
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| 3. |
- Miehe, Georg, et al.
(författare)
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The Kobresia pygmaea ecosystem of the Tibetan highlands – Origin, functioning and degradation of the world's largest pastoral alpine ecosystem: Kobresia pastures of Tibet
- 2019
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Ingår i: Science of the Total Environment. - : Elsevier BV. - 0048-9697. ; 648, s. 754-771
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Tidskriftsartikel (refereegranskat)abstract
- With 450,000 km2 Kobresia (syn. Carex) pygmaea dominated pastures in the eastern Tibetan highlands are the world's largest pastoral alpine ecosystem forming a durable turf cover at 3000–6000 m a.s.l. Kobresia's resilience and competitiveness is based on dwarf habit, predominantly below-ground allocation of photo assimilates, mixture of seed production and clonal growth, and high genetic diversity. Kobresia growth is co-limited by livestock-mediated nutrient withdrawal and, in the drier parts of the plateau, low rainfall during the short and cold growing season. Overstocking has caused pasture degradation and soil deterioration over most parts of the Tibetan highlands and is the basis for this man-made ecosystem. Natural autocyclic processes of turf destruction and soil erosion are initiated through polygonal turf cover cracking, and accelerated by soil-dwelling endemic small mammals in the absence of predators. The major consequences of vegetation cover deterioration include the release of large amounts of C, earlier diurnal formation of clouds, and decreased surface temperatures. These effects decrease the recovery potential of Kobresia pastures and make them more vulnerable to anthropogenic pressure and climate change. Traditional migratory rangeland management was sustainable over millennia, and possibly still offers the best strategy to conserve and possibly increase C stocks in the Kobresia turf.
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