| 1. |
- Feuk, Henrik, et al.
(författare)
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Sources of error for single-shot PMT-based phosphor thermometry in harsh conditions
- 2021
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Ingår i: Measurement Science and Technology. - : IOP Publishing. - 0957-0233 .- 1361-6501. ; 32:8
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Tidskriftsartikel (refereegranskat)abstract
- This study investigates photomultiplier tube (PMT) nonlinearities, relevant for lifetime phosphor thermometry, at various decay times to assess and minimize the impact on temperature measurement accuracy. The focus is single-shot measurements performed in harsh environments where phosphor signal attenuation often is a concern. The sensitivity of decay time measurements to changing phosphorescence intensity is therefore investigated. The experimental results show that for the studied phosphors and detectors, shorter decay times between 20 ns and 6 µs, saturation effects in the PMTs decreased the measured decay time with increasing signal attenuation. For longer phosphorescence decay times, in the millisecond regime, nonlinearity effects led to an increase in the measured decay time with increasing signal attenuation. The specific detector nonlinearity response will vary among detectors, but the introduced methodology for detector analysis is a useful resource for assessing and improving accuracy in lifetime phosphor thermometry measurements.
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| 2. |
- Huang, Jianqing, et al.
(författare)
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A detailed study on the micro-explosion of burning iron particles in hot oxidizing environments
- 2022
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Ingår i: Combustion and Flame. - : Elsevier BV. - 0010-2180. ; 238
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Tidskriftsartikel (refereegranskat)abstract
- As a promising carbon-free fuel, iron powder can directly combust with air and has great potential to provide clean and high-grad heat for various applications. The combustion characteristics of iron particles are of great significance for developing iron combustion model, designing efficient combustor, and optimizing combustion technologies. In this work, the micro-explosion behavior of burning iron particles was experimentally investigated based on optical diagnostics. With two high-speed cameras operating at 10,000 frames per second, the three-dimensional (3D) motion and mean surface temperature of burning iron particles during the micro-explosion process were measured using the stereo imaging technique and two-color pyrometry, respectively. The probability of micro-explosions in different oxidizing environments were statistically studied. Three distinct micro-explosion modes have been observed. The results showed that the micro-explosion of burning iron particles heavily depended on oxygen concentration. The micro-explosion would slightly reduce the particle surface temperature by 30–70 K within 0.5 ms, since a lot of smaller fragments were produced. In addition, the 3D velocity of most fragments would sharply increase to 2–6 times within 0.2 ms after the micro-explosion occurred. Regarding the mechanism of the micro-explosion, three types of potential gas sources inside the particle were discussed. The sharp gradients of gas temperature and oxygen concentration may facilitate the rapid increase of the internal pressure in the particle, which eventually causes the micro-explosion.
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| 3. |
- LI, SHEN, et al.
(författare)
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Stereoscopic high-speed imaging of iron microexplosions and nanoparticle-release
- 2021
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Ingår i: Optics Express. - 1094-4087. ; 29:21, s. 34465-34476
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Tidskriftsartikel (refereegranskat)abstract
- In this work, the combustion behavior of seeded iron particles (d50 = 70 μm) in a laminar diffusion flame was studied in a modified Mckenna flat-flame burner. Two high speed cameras in stereo configuration allowed 3D position and 3D velocity measurements of burning iron particles as well as 3D evaluation of particle microexplosions. Microexplosive processes are important since it can affect both combustion stability and formation of product components. The observed microexplosions happened before particle extinction resulting in change of trajectories, velocities, radiation intensities and fragmentation into smaller particles. It was observed for the first time that fragments of these microexplosions tend to produce planar structures. A frequent release phenomenon was observed during the iron particle combustion using magnified thermal radiation imaging and high-speed shadowgraphy. This release phenomenon was indirectly confirmed with scanning electron microscopy of combust products, revealing multiple cracked particle shells and hollow structures. Black body radiation characteristics was observed indicating the release being in condensed phase and emission spectroscopy identified FeO as intermediate species during combustion. The observed release is believed to mainly consist of iron-oxide nanoparticles formed in the homogenous reaction between vapor iron and oxidizers.
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| 4. |
- Pignatelli, Francesco, et al.
(författare)
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Effect of CO2 dilution on structures of premixed syngas/air flames in a gas turbine model combustor
- 2023
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Ingår i: Combustion and Flame. - : Elsevier Inc.. - 0010-2180 .- 1556-2921. ; 255
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Tidskriftsartikel (refereegranskat)abstract
- The impact of CO2 dilution on combustion of syngas (a mixture of H2, CO, and CH4) was investigated in a lab-scale gas turbine model combustor at atmospheric pressure conditions. Two mild dilution levels of CO2, corresponding to 15% and 34% of CO2 mole fraction in the syngas/CO2 mixtures, were experimentally investigated to evaluate the effects of CO2 dilution on the flame structures and the emissions of CO and NOx. All experiments were performed at a constant Reynolds number (Re = 10000). High-speed flame luminescence, simultaneous planar laser-induced fluorescence (PLIF) measurements of the OH radicals and particle image velocimetry (PIV) were employed for qualitative and quantitative assessment of the resulting flame and flow structures. The main findings are: (a) the operability range of the syngas flames is significantly affected by the CO2 dilution, with both the lean blowoff (LBO) limit and the flashback limit shifting towards fuel-richer conditions as the CO2 dilution increases; (b) syngas flames exhibit flame-pocket structures with chemical reactions taking place in isolated pockets surrounded by non-reacting fuel/air mixture; (c) the inner recirculation zone tends to move closer to the burner axis at high CO2 dilution, and (d) the NOx emission becomes significantly lower with increasing CO2 dilution while the CO emission exhibits the opposite trend. The flame-pocket structure is more significant with increased CO2 dilution level. The low NOx emissions and high CO emissions are the results of the flame-pocket structures. © 2023 The Author(s)
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| 5. |
- Sanned, David, et al.
(författare)
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3D-tomographic reconstruction of gliding arc plasma
- 2023
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Ingår i: Applied Physics Letters. - 0003-6951. ; 123:7
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Tidskriftsartikel (refereegranskat)abstract
- his study presents three-dimensional (3D) emission tomography on gliding arc discharge for volumetric measurements of plasma luminosity fields. The 3D tomography of the plasma luminosity field enables quantification and characterization of 3D plasma features, which are not easily accessible in two-dimensional measurements. Simultaneous projections of the plasma discharge were imaged using multiple CMOS cameras, and an in-house developed tomographic method was used for the 3D reconstruction of the luminosity fields. Results show good field reconstruction quality and expected gliding arc topologies. Comparisons between arc 3D length and 2D projected length displayed that 2D measurements underestimated length by around 15% at the highest tested flow case. The mean 3D length initially increased with increasing air flow, while later decreasing at even higher flows. The standard deviation of 3D length increased with increasing flow. Arc curvature and overlap were generally seen to increase with higher flows in contrast to arc volume that was seen to decrease with increasing flow rates. This study aims to facilitate instantaneous 3D tomographic measurements of plasma luminosity fields to provide a detailed quantification of 3D characteristics and correlations of typical plasma features, thereby providing paths to remove line-of-sight effects and compensate for loss of information that may occur during two-dimensional measurements. The presented technique is applicable not only to gliding arcs but also to various other plasma systems.
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| 6. |
- Sanned, David, et al.
(författare)
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Arbitrary position 3D tomography for practical application in combustion diagnostics
- 2022
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Ingår i: Measurement Science and Technology. - : IOP Publishing. - 0957-0233 .- 1361-6501. ; 33:12
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Tidskriftsartikel (refereegranskat)abstract
- This work aims to make three-dimensional (3D) tomographic techniques more flexible and accessible to in-situ measurements in practical apparatus by allowing arbitrary camera placements that benefit applications with more restrictive optical access. A highly customizable, in-house developed tomographic method is presented, applying smoothness priors through Laplacian matrices and hull constraints based on 3D space carving. The goal of this paper is to showcase a reconstruction method with full user control that can be adopted to various 3D field reconstructions. Simulations and experimental measurements of unsteady premixed CH4/air and ethanol (C2H5OH) diffusion pool flames were evaluated, comparing arbitrarily placed cameras around the probed domain to the more commonly used in-plane-half-circle camera arrangement. Reconstructions reproduced expected topological field features for both flame types. Results showed slight decrease in reconstruction quality for arbitrarily placed cameras compared to in-plane-half-circle arrangement. However, at lower numbers of camera views (N q ⩽ 6) arbitrary placement showed better results. The introduced methodology will be useful for optically limited setups in terms of handling a priori information, camera placement and 3D field evaluation.
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| 7. |
- Sanned, David
(författare)
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Optical Diagnostics of Reactive Flows : Application of 3D emission tomography and laser-based methods
- 2023
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Nowadays, numerous specialized systems, ranging from electronics to energy production processes can be found both in science and industry. However due to their specialized nature they often exhibit high sensitivity to intrusive measurements, which can potentially perturb the sought-after characteristics or quantity and consequently compromise overall accuracy. In these scenarios, optical measurements can offer a potential solution owing to their often-non-intrusive nature based on the photon-in-photon-out concept. The application of different optical techniques does not only enable the achievement of non-invasiveness but can also offer the capability to measure and quantify observable phenomena that may not always be accessible through conventional probing methods.The work within this thesis centers on three-dimensional (3D) emission tomography and its application in the context of reactive flows. Many reactive flows, such as those found in combustion processes, inherently exhibit three-dimensional characteristics, benefitting from measurement techniques that can perform measurements in all three dimensions. The work covers the fundamentals of emission tomography and showcases the techniques application in both combustion and plasma research. The application in combustion diagnostics yielded volumetric flame reconstructions and investigated the use of arbitrary sensor positions to overcome limitations in optical access. Similarly, plasma diagnostic application allowed for volumetric gliding arc reconstructions facilitating quantification of 3D characteristics such as 3D arc length and arc volume. This is followed by stereoscopic 3D particle tracking applied in iron combustion, looking at particle micro explosions and 3D velocities.Subsequently, various optical laser-based techniques applied in the work within this thesis are presented, each accompanied by an experimental application. These techniques include particle image velocimetry (PIV), for flow field analysis, and laser-induced fluorescence (LIF), for hydroxide (OH) analysis, both applied in a model lab-scale gas turbine swirl combustor. Additionally, fluorescence lifetime imaging (FLI) was utilized in gliding arc plasma investigations in combination with 3D emission tomography to investigate OH fluorescence lifetimes.Investigation of potential nanoparticle release during iron combustion was carried out using shadowgraphy, enabling the visualization of release trails produced by individual iron particles. Moreover, work within phosphor thermometry was performed, investigating the impact of PMT non-linearity effects on measured phosphorescence lifetimes and applied for in-situ surface temperature measurements on heat exchanger pipes in a multi-fuel Stirling engine.
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