Change search
Refine search result
175176177178179 8851 - 8900 of 8921
CiteExportLink to result list
Permanent link
Cite
Citation style
  • apa
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf
Rows per page
  • 5
  • 10
  • 20
  • 50
  • 100
  • 250
Sort
  • Standard (Relevance)
  • Author A-Ö
  • Author Ö-A
  • Title A-Ö
  • Title Ö-A
  • Publication type A-Ö
  • Publication type Ö-A
  • Issued (Oldest first)
  • Issued (Newest first)
  • Created (Oldest first)
  • Created (Newest first)
  • Last updated (Oldest first)
  • Last updated (Newest first)
  • Disputation date (earliest first)
  • Disputation date (latest first)
  • Standard (Relevance)
  • Author A-Ö
  • Author Ö-A
  • Title A-Ö
  • Title Ö-A
  • Publication type A-Ö
  • Publication type Ö-A
  • Issued (Oldest first)
  • Issued (Newest first)
  • Created (Oldest first)
  • Created (Newest first)
  • Last updated (Oldest first)
  • Last updated (Newest first)
  • Disputation date (earliest first)
  • Disputation date (latest first)
Select
The maximal number of hits you can export is 250. When you want to export more records please use the Create feeds function.
  • 8851. Åkerström, Paul
    et al.
    Oldenburg, Mats
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Austenite decomposition during press hardening of a boron steel: computer simulation and test2006In: Journal of Materials Processing Technology, ISSN 0924-0136, E-ISSN 1873-4774, Vol. 174, no 1-3, p. 399-406Article in journal (Refereed)
    Abstract [en]

    In this work a model to predict the austenite decomposition into ferrite, pearlite, bainite and martensite during arbitrary cooling paths for thin sheet boron steel is used. The model is based on Kirkaldy's rate equations. The basic rate equations has been modified to account for the austenite stabilization effect from the added boron. The model is implemented as part of a material subroutine in the Finite Element Program LS-DYNA 970. Both the obtained simulated volume fractions microconstituents and hardness profiles shows promising agreement to the corresponding experimental observations.

  • 8852.
    Åkerström, Paul
    et al.
    Gestamp Hardtech AB, Luleå.
    Oldenburg, Mats
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Numerical simulation of a thermo-mechanical sheet metal forming experiment2008In: Numisheet 2008: Proceedings of the 7th international conference and workshop on numerical simulation of 3D sheet metal forming processes, September 1-5, 2008, Interlaken, Switzerland / [ed] Pavel Hora, Zurich: Institute of Virtual manufacturing, ETH Zurich , 2008, p. 569-574Conference paper (Refereed)
  • 8853. Åkerström, Paul
    et al.
    Oldenburg, Mats
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Studies of the thermo-mechanical material response of a boron steel by inverse modelling2004In: Proceedings: 2nd International Conference on Thermal Process Modelling and Computer Simulation : Nancy, France, March 31 - April 2, 2003 / [ed] S. Denis, Les Ulis: EDP Sciences, 2004Conference paper (Refereed)
  • 8854. Åkerström, Paul
    et al.
    Wikman, Bengt
    Oldenburg, Mats
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Material parameter estimation for boron steel from simultaneous cooling and compression experiments2005In: Modelling and Simulation in Materials Science and Engineering, ISSN 0965-0393, E-ISSN 1361-651X, Vol. 13, no 8, p. 1291-1308Article in journal (Refereed)
    Abstract [en]

    In order to increase the accuracy of numerical simulations of the hot stamping process, reliable material data is crucial. Traditionally, the material is characterized by several isothermal compression or tension tests performed at elevated temperatures and different strain rates. The drawback of the traditional methods is the appearance of unwanted phases for some test temperatures and durations. Such an approach is also both time consuming and expensive. In the present work an alternative approach is proposed, which reduces unwanted phase changes and the number of experiments. The isothermal mechanical response is established through inverse modelling of simultaneous cooling and compression experiments. The estimated material parameters are validated by comparison with data from a separate forming experiment. The computed global response is shown to be in good agreement with the experiments.

  • 8855.
    Århammar, Cecilia
    et al.
    AB Sandvik Coromant, Sandviken.
    Silvearv, Fredrik
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Bergman, A.
    Division of Materials Theory, Department of Physics and Astronomy, Uppsala University.
    Norgren, Susanne M
    AB Sandvik Coromant, Sandviken.
    Pedersen, Henrik
    Department of Physics, Chemistry and Biology, Linköping University.
    Ahuja, Rajeev B.
    Division of Materials Theory, Department of Physics and Astronomy, Uppsala University.
    A theoretical study of possible point defects incorporated into α-alumina deposited by chemical vapor deposition2014In: Theoretical Chemistry accounts, ISSN 1432-881X, E-ISSN 1432-2234, Vol. 133, no 2, p. 1-11Article in journal (Refereed)
    Abstract [en]

    The energetics and electronic structure of carbon, chlorine, hydrogen, and sulfur in α-Al2O3 was investigated by first principles and thermodynamical calculations. These species are present in the gas phase during the synthesis of α-Al2O3 by chemical vapor deposition (CVD) but little is known of their solubility in this compound. The heat of formation from standard reference states of the elements varying the chemical potential of each element was calculated. An attempt to model the actual conditions in the CVD process was made, using the species and solid compounds present in a common CVD process as reference states. Our calculations suggest that sulfur from the catalyzing agent H2S will not solve in α-Al2O3 during deposition by CVD. It is found that the neutral chlorine and hydrogen interstitial defects display the lowest heat of formation, 281 and 280 kJ/mol, respectively, at the modeled CVD conditions. This energy is too high in order for neutral defects to form during CVD of α-Al2O3 at any significant amounts. The charged defects and their compensation were studied. Carbon substituting oxygen is found to be energetically favored under the modeled CVD conditions, considering carbon dioxide as competing species to solid solubility in α-Al2O3 at an energy of -128 kJ/mol. However, care needs to be taken when choosing the possible competing carbon-containing phases. Compensation of carbon substituting for oxygen by oxygen vacancies takes place at 110 kJ/mol from standard reference states, graphite, fcc-Al and O2. The carbon solubility in Al2O3 is difficult to measure with standard analysis techniques such as X-ray diffraction and energy dispersive X-ray spectroscopy, but several stable compounds in the Al-C-O are available in the literature

  • 8856.
    Åstrand, Anders P.
    et al.
    Umeå University. Department of Applied Physics and Electronics.
    Jalkanen, Ville
    Umeå University. Department of Applied Physics and Electronics.
    Andersson, Britt M.
    Umeå University. Department of Applied Physics and Electronics.
    Lindahl, Olof
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Detection of stiff nodules embedded in soft tissue phantoms, mimicking cancer tumours, using a tactile resonance sensor2014In: Journal of Biomedical Science and Engineering, ISSN 1937-6871, E-ISSN 1937-688X, Vol. 7, no 4, p. 181-193Article in journal (Refereed)
    Abstract [en]

    Background: Prostate cancer (PCa) is the most common form of cancer among males in Europe and in the USA and the most common curative treatment is removal of the prostate, i.e. prostatectomy. After the removal, the prostate is histopathologically analysed. One area of interest is to examine the perifery of the prostate, as tumours on and near the surface can indicate that the PCa has spread to other parts of the body. There are no current methods to examine the surface of the prostate at the time of surgery. Tactile resonance sensors can be used for detecting areas of different stiffness in soft tissue. Human prostate tissue affected by cancer is usually stiffer than healthy tissue, and for this purpose, a tactile resonance sensor was developed. The aim of this study was to investigate the depth at which embedded stiffer volumes could be detected, using soft tissue phantoms. Methods: With the tactile resonance sensor used in this study, the shift of the resonance frequency and the force at contact with tissue can be measured, and combined into a tissue stiffness parameter. The detection sensitivity of the sensor at impression depths, 0.4 and 0.8 mm, was measured for detection of inserted nodules of stiff silicone in softer silicone and in chicken muscle tissue, mimicking prostate tissue with cancer tumours. Results: Measurements on the silicone samples detected the hidden stiffer object at a depth of 1 - 4 mm with a difference in the stiffness parameter of 80 - 900 mN/kHz (p < 0.028, n = 48). At the depth 5 - 6 mm the difference was smaller but still significant < 30 mN/kHz (p < 0.05, n = 24). For the measurements on chicken muscle, the detectable depth was 4 mm (p < 0.05, n = 24). Conclusion: This model study suggests that, with only a small impression depth of ≤1 mm, the resonance sensor system described here can detect stiffness variations located at least 4 mm in silicone and chicken muscle, mimicking tumours in prostate tissue.

  • 8857.
    Åström, Anton
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Bandspänningsdon CV902017Independent thesis Advanced level (professional degree), 20 credits / 30 HE creditsStudent thesis
  • 8858.
    Åström, E.
    et al.
    LKAB R&D, Luleå.
    Bonomi, Germano
    Department of Mechanical and Industrial Engineering, University of Brescia.
    Calliari, Irene
    Department of Industrial Engineering, University of Padova.
    Calvini, Piero
    Department of Physics, University of Genova and Sezione INFN di Genova.
    Checchia, Paolo
    INFN Sezione di Padova.
    Donzella, Antonietta
    Department of Mechanical and Industrial Engineering, University of Brescia.
    Faraci, E.
    Centro Sviluppo Materiali SPA, Rome.
    Forsberg, Fredrik
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Gonella, F.
    INFN Sezione di Padova.
    Hu, X.
    Swerea MEFOS AB, Luleå.
    Klinger, Joel A.
    INFN Sezione di Padova.
    Sundqvist Ökvist, Lena
    Swerea MEFOS AB, Luleå.
    Pagano, Davide
    Department of Mechanical and Industrial Engineering, University of Brescia.
    Rigoni, Andrea
    Department of Physics and Astronomy, University of Padova.
    Ramous, Emilio
    Department of Industrial Engineering, University of Padova.
    Urbani, M.
    Department of Physics and Astronomy, University of Padova.
    Vanini, Sara
    Department of Physics and Astronomy, University of Padova.
    Zenoni, Aldo
    Department of Mechanical and Industrial Engineering, University of Brescia.
    Zumerle, Gianni
    Department of Physics and Astronomy, University of Padova.
    Precision measurements of linear scattering density using muon tomography2016In: Journal of Instrumentation, ISSN 1748-0221, E-ISSN 1748-0221, Vol. 11, no P7010Article in journal (Refereed)
    Abstract [en]

    We demonstrate that muon tomography can be used to precisely measure the properties of various materials. The materials which have been considered have been extracted from an experimental blast furnace, including carbon (coke) and iron oxides, for which measurements of the linear scattering density relative to the mass density have been performed with an absolute precision of 10%. We report the procedures that are used in order to obtain such precision, and a discussion is presented to address the expected performance of the technique when applied to heavier materials. The results we obtain do not depend on the specific type of material considered and therefore they can be extended to any application

  • 8859.
    Åström, H.
    et al.
    Luleå tekniska universitet.
    Höglund, Erik
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Rheological properties of lithium, lithium complex, and sodium greases1993In: Journal of Synthetic Lubrication, ISSN 0265-6582, Vol. 10, no 3, p. 225-236Article in journal (Refereed)
    Abstract [en]

    The rheological properties of two lithium, two lithium complex, and two sodium greases, all with the most common NLGI grade two, have been investigated. The greases are based on a mineral and a synthetic oil. The apparatus used was a cone-and-plate rheometer and an impacting-ball apparatus. The impacting-ball apparatus used a steel ball, which impacted a lubricated sintered carbide plate, to measure the shear stress-pressure coefficient, γ, of the lubricant. At pressures found in elastohydrodynamic contacts this coefficient determined the limiting shear stress. The γ-value thus affects the coefficient of friction and consequently, lower γ-value means less friction. The results from the impacting-ball apparatus showed that γ-value was lower for the greases with a synthetic base oil and that the lithium greases gave a lower γ-value than the corresponding base oils. Results from the cone-and-plate rheometer showed the characteristic shear thinning behaviour of the greases and the influence of shear history and temperature. The results from the cond-and-plate rheometer have also been fitted to a four parameter rheological model.

  • 8860. Åström, H.
    et al.
    Höglund, Erik
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Rheological properties of lthium, lithium complex and sodium greases1990In: Nordtrib'90: proceedings of the 4th Nordic Symposium on Tribology, Lubrication, Friction and Wear : 10th-13th June 1990, Hirtshals, Denmark / [ed] Jørgen Jakobsen; Mads Klarskov; Michael Eis, Lyngby: NORDTRIB , 1990Conference paper (Refereed)
  • 8861.
    Åström, Henrik
    et al.
    Luleå tekniska universitet.
    Höglund, Erik
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Inverkan av smörjfetters reologi och smörjförmåga på energiförluster i maskiner1991Report (Other academic)
  • 8862.
    Åström, Henrik
    et al.
    Luleå tekniska universitet.
    Höglund, Erik
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Rheological properties of six greases and their two base oils1990Report (Other academic)
  • 8863.
    Åström, Henrik
    et al.
    Luleå tekniska universitet.
    Höglund, Erik
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Viewpoints on noise in grease lubricated bearings1994In: N L G I Spokesman, ISSN 0027-6782, Vol. 58, no 4Article in journal (Other academic)
    Abstract [en]

    The noise level of a rolling element bearing is normally low, compared to the noise from the machine in which the bearing operates. Under certain circumstances, though, the noise caused by the bearing can be crucial and might influence the operation, for example, of very sensitive equipment. When lubricating rolling element bearings with grease, the noise level will depend upon the type of grease used. The relationship between structure and composition of lithium greases, and the noise they cause in rolling element bearings, is the subject of this paper. Different lithium greases have been studied in a model of the contact between ball and race in a bearing, using a ball and disc machine, as well as optical interferometry. The bearing noise levels have also been measured, using the same greases, and their microscopic structure studied, with the bearing noise level. Normal lithium greases cause disturbances in the model contact and also noise in a ball bearing. The noise level and the amount of disturbance is connected, with formations found in the microscopic grease structure. This technical paper can be obtained from the ELGI Office; see page 38 for ELGI address

  • 8864.
    Åström, Henrik
    et al.
    Luleå tekniska universitet.
    Isaksson, Ove
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Höglund, Erik
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Video recordings of an EHD point contact lubricated with grease1991In: Tribology International, ISSN 0301-679X, E-ISSN 1879-2464, Vol. 24, no 3, p. 179-184Article in journal (Refereed)
    Abstract [en]

    Optical interferometry has been used to study the film thickness of an EHD point contact lubricated with grease. The greases used were two lithium, two lithium complex and two sodium greases, all with NLGI grade two, the most common grade. They were based on one mineral oil of naphthenic type and one synthetic polyalphaolefin. The contact was lubricated without a continuous supply of grease and thus the film thickness decreased with time as the grease was gradually squeezed away from the contact area. The film thickness fluctuated during the measurement. To overcome this problem, the contact point was recorded with a video camera and the results evaluated after the tests. The video camera proved to be a powerful help in this evaluation. The tests were run at 20°C and under pure rolling conditions at a rolling velocity of 0.055 m s-1. The results from all greases showed a decreasing film thickness that tended to stabilize on about 0.2 μm after 40 cycles. The lithium and lithium complex greases showed strong local film thickness variations inside the lubricated contact, deviating from the normally horseshoe-shaped contact, during the first 20 to 40 cycles.

  • 8865.
    Åström, Henrik
    et al.
    Luleå tekniska universitet.
    Östensen, Jan Ove
    Luleå tekniska universitet.
    Höglund, Erik
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.
    Lubricating grease replenishment in an elastohydrodynamic point contact1993In: Journal of tribology, ISSN 0742-4787, E-ISSN 1528-8897, Vol. 115, no 3, p. 501-506Article in journal (Refereed)
    Abstract [en]

    A ball and disk apparatus was used to investigate the lubricant replenishment of an elastohydrodynamically lubricated point contact. This replenishment of the contact is crucial for building up a lubricating film. Whereas lubricating oil manages to achieve replenishment, lubricating grease appears not to achieve this, with lubricant starvation and a dramatic decrease in film thickness as a result. The distribution of grease around the contact was studied using normal and high-speed video. The movements of grease in the vicinity of the contact could be seen by adding molybdenum disulfide particles to the grease. A recording was then made, using high-speed video recording. The overall cavitation regions were studied using an ordinary video camera and grease without particles. On the basis of the results, possible lubricating grease replenishment mechanisms are discussed. The resulting film thickness was also compared with theoretical predictions using the Hamrock and Dowson starvation criterion, assuming negligible replenishment. The measured film thickness was larger than the predicted, which indicated that some replenishment occurs. In the case of an ordinary thrust ball bearing, replenishment was found to rely on the spin motion of the balls.

  • 8866.
    Åström, Kalle
    et al.
    Centre for Mathematical Sciences, Lund University.
    Persson, Lars-ErikLuleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.Silvestrov, Sergei D.Division of Applied Mathematics, Mälardalen University.
    Analysis for science, engineering and beyond: the tribute workshop in honour of Gunnar Sparr held in Lund, May 8-9, 20082012Collection (editor) (Other academic)
  • 8867. Åström, Peter
    et al.
    Näsström, Mats
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Friction model parameter optimization for friction welding simulation of Ti-62462006In: Trends in welding research: proceedings of the 7th international conference : May 16-20, 2005, Callaway Gardens Resort, Pine Mountain, Georgia, USA / [ed] Stan A. David, Materials Park, Ohio: ASM International, 2006, p. 849-854Conference paper (Refereed)
    Abstract [en]

    With regard to the rotary friction welding process as used for joining of large rotationally symmetric structures in the aerospace industry, the finite element software "MSC. Marc" coupled with the optimisation code INVSYS was used to simulate the friction welding of two cylindrical titanium alloy Ti 6246 specimens while optimising parameters in the friction model. The friction model took into account the friction coefficient variation as a function of normal pressure in the contact, temperature and relative sliding velocity. The unconstrained subspace searching simplex method was employed for optimisation. An objective function was formulated to minimise the error between calculated and measured torques, and a parameter set was identified that produced better flash geometry agreement with experiment.

  • 8868.
    Öberg, Sven
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Calculations of defect related properties in semiconductors1992Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Many of the most important and useful properties of crystalline materials, such as mechanical strength and electrical resistance, are determined by the presence of lattice defects and impurities. Well known examples are dislocations which control plastic flow and make metals ductile, and dopant atoms which control the extrinsic conduction in semiconductors and insulators. In fact the electronic device industry is based on controlled introduction of specific impurities in semiconductors and insulators and methods to avoid or eliminate destructive defects . Thus improvements of existing solid state devices and the invention of new ones are the driving forces for the large amount of experimental and theoretical investigations performed on defects and solids today. In this thesis dislocations and impurities in the most important semiconductors are analysed with mathematical methods, ranging from a method based on isotropic elasticity theory, interatomic potentials and a semi-empirical quantum-mechanical prescription, here applied on dislocations, to an ab initio quantum mechanical method by which properties of defects in crystalline solids can be obtained from first principles, that is, can be derived from the knowledge of only the atomic numbers and masses of constituent atoms, here applied on impurities. The ab initio method is based on Local Density Functional Theory, which provides a many-electron description. The semi-empirical method has been applied on dislocations in Gallium-Arsenide and Cadmium-Telluride. The ab initio method has been applied on various defects in Silicon and Gallium-Arsenide. For Silicon we have investigated interstitial Oxygen complexes, substitutional Carbon and Boron, and intersitial Carbon-Oxygen complexes. For Gallium- Arsenide de the investigation includes the Carbon impurity and the Carbon-Hydrogen complex, the Boron double acceptors, and the so called DX-centre in GaAs:Si. The interaction of hydrogen with impurities in semiconductors has also been investigated. We have also calculated the diffusion barrier of Oxygen in Silicon with excellent results. Our work shows clearly that Local Density Functional theory can provide useful information about defect structures, dynamical properties and diffusion processes in solids.

  • 8869.
    Öberg, Sven
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Adjizian, Jean-Joseph
    Department of Chemistry, University of Sussex, Falmer, Brighton , Nanoscopic Physics (NAPS), Université catholique de Louvain.
    Erbahar, D.
    Institut des Mat´eriaux Jean Rouxel (IMN), Universit´e de Nantes, Centre national de la recherche scientifique (CNRS).
    Rio, J.
    Institut des Mat´eriaux Jean Rouxel (IMN), Universit´e de Nantes, Centre national de la recherche scientifique (CNRS).
    Humbert, Bernard
    Laboratoire de Chimie Physique et Microbiologie pour l'Environnement, Nancy Université, Institut des Mat´eriaux Jean Rouxel (IMN), Universit´e de Nantes, Centre national de la recherche scientifique (CNRS).
    Dossot, Manuel
    Laboratoire de Chimie Physique et Microbiologie pour l'Environnement, Nancy Université, Laboratoire de Chimie Physique et Microbiologie pour l'Environnement, UMR 7564, CNRS–University of Lorraine.
    Soldatov, Alexander
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Lefrant, S.
    Institut des Mat´eriaux Jean Rouxel (IMN), Universit´e de Nantes, Centre national de la recherche scientifique (CNRS).
    Mevellec, J-Y
    Institut des Mat´eriaux Jean Rouxel (IMN), Universit´e de Nantes, Centre national de la recherche scientifique (CNRS).
    Briddon, Patrick R.
    Department of Physics, University of Newcastle, School of Electrical, Electronic and Computer Engineering, University of Newcastle upon Tyne, School of Natural Science, University of Newcastle upon Tyne, Institut des Mat´eriaux Jean Rouxel (IMN), Universit´e de Nantes, Centre national de la recherche scientifique (CNRS).
    Rayson, Mark
    Department of Chemistry, The University of Surrey, Guildford, School of Electrical and Electronic Engineering, Newcastle University, Newcastle upon Tyne.
    Ewels, Christopher P.
    Department of Physics, University of Exeter, CPES, University of Sussex, Falmer, Brighton, Université de Nantes, Institut des Mat´eriaux Jean Rouxel (IMN), Universit´e de Nantes, Centre national de la recherche scientifique (CNRS).
    Effect of functionalization and charging on resonance energy and radial breathing modes of metallic carbon nanotubes2016In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 93, no 4, article id 45408Article in journal (Refereed)
    Abstract [en]

    While changes in resonant Raman scattering measurements are commonly used to measure the effect of chemical functionalization on single-walled carbon nanotubes, the precise effects of functionalization on these spectra have yet to be clearly identified. In this density functional theory study, we explore the effects of functionalization on both the nanotube resonance energy and frequency shifts in radial breathing mode. Charge transfer effects cause a shift in the first Van Hove singularity spacings, and hence resonance excitation energy, and lead to a decrease in the radial breathing mode frequency, notably when the Fermi level decreases. By varying stochastically the effective mass of carbon atoms in the tube, we simulate the mass effect of functionalization on breathing mode frequency. Finally, full density functional calculations are performed for different nanotubes with varying functional group distribution and concentration using fluorination and hydrogenation, allowing us to determine overall effect on radial breathing mode and charge transfer. The results concur well with experiment, and we discuss the importance when using Raman spectroscopy to interpret experimental functionalization treatments

  • 8870.
    Öberg, Sven
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Ewels, C P
    University of Exeter.
    Jones, R
    University of Exeter.
    Hallberg, T
    Linköping University.
    Lindström, J L
    Linköping University.
    Murin, L I
    Institute of Solid State and Semiconductor Physics, Minsk.
    Briddon, P R
    University of Newcastle Upon Tyne.
    First stage of oxygen aggregation in silicon: the oxygen dimer1998In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 81, no 14, p. 2930-2933Article in journal (Refereed)
    Abstract [en]

    The structure and dynamic properties of the interstitial oxygen dimer in silicon are found using a combination of infrared spectroscopy and ab initio modeling. We find that the stable dimer consists of a pair of inequivalent weakly coupled interstitial oxygen atoms separated by a Si-Si bond. Two high frequency modes are decoupled in one 16O-18O combination but are strongly mixed in the other combination. A third lower lying mode involves the compression of the Si-Si bond joining the oxygen atoms and gives distinct modes in the mixed 16O-18O case.

  • 8871.
    Öberg, Sven
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Stich, P K
    University of Exeter.
    Jones, R
    University of Exeter.
    Heggie, M I
    University of Exeter.
    First-principles calculations of the energy barrier to dislocation motion in Si and GaAs1995In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 51, no 19, p. 13138-13145Article in journal (Refereed)
    Abstract [en]

    The single kink formation and migration energies Fk and Wm of 90° glide partial dislocations in Si and GaAs are calculated using an ab initio local density-functional cluster method. Kink migration occurs via a concerted exchange of an atom at a dislocation core with one of its glide plane nearest neighbors. By constraining these atoms to sit in high-energy positions and relaxing a surrounding cluster of atoms, sufficient points in configuration space can be sampled for the energy barrier for the first step in kink pair formation to be estimated. By including an estimate of the elastic energy of the interaction of kink pairs, the single kink formation energy is calculated. It is found that Fk and Wm for Si are 0.1 and 1.8 eV, respectively. For the 90° α glide partial in GaAs, these quantities are 0.07 and 0.7 eV, respectively, and 0.3 and 1.1 eV for β partials

  • 8872.
    Ögren, Yngve
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science. RISE ETC.
    Camera and laser based diagnostics relevant for entrained flow gasifiers2018Licentiate thesis, comprehensive summary (Other academic)
  • 8873.
    Ögren, Yngve
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science. RISE ETC AB.
    Improving the efficiency of entrained flow gasifiers by real time in-situ diagnostics and burner design2019Doctoral thesis, comprehensive summary (Other academic)
  • 8874.
    Ögren, Yngve
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science. RISE-Energy Technology Center AB, Piteå, Sweden.
    Gullberg, Marcus
    RISE-Energy Technology Center AB, Piteå, Sweden.
    Wennebro, Jonas
    RISE-Energy Technology Center AB, Piteå, Sweden.
    Sepman, Alexey
    RISE-Energy Technology Center AB, Piteå, Sweden.
    Tóth, Pál
    RISE-Energy Technology Center AB, Piteå, Sweden. University of Miskolc, Department of Combustion and Thermal Energy, Miskolc-Egyetemváros, Miskolc, Hungary.
    Wiinikka, Henrik
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science. RISE-Energy Technology Center AB, Piteå, Sweden.
    Influence of oxidizer injection angle on the entrained flow gasification of torrefied wood powder2018In: Fuel processing technology, ISSN 0378-3820, E-ISSN 1873-7188, Vol. 181, p. 8-17Article in journal (Refereed)
    Abstract [en]

    In the present work, 5 different axisymmetric burners with different directions of the oxidizer inlets were experimentally tested during oxygen blown gasification of torrefied wood powder. The burners were evaluated under two different O2/fuel ratios at a thermal power of 135 kWth, based on the heating value of torrefied wood powder. The evaluation was based on both conventional methods such as gas chromatography measurements and thermocouples and in-situ measurements using Tunable Diode Laser Absorption Spectroscopy. It was shown that changes in the near burner region influence the process efficiency significantly. Changing the injection angle of the oxidizer stream to form a converging oxidizer jet increased process efficiency by 20%. Besides increased process efficiency, it was shown that improvements in burner design also influence carbon conversion and hydrocarbon production. The burner with the best performance also produced less CH4 and achieved the highest carbon conversion. The effect of generating swirl via rotating the oxidizer jet axes was also investigated. Swirl broadened or removed the impingement area between the fuel and oxidizer jets, however resulting in differences in performance within the measurement uncertainty. 

  • 8875.
    Ögren, Yngve
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.
    Sepman, Alexey
    RISE-Energy Technology Center AB .
    Qu, Zhechao
    Umeå University, Thermochemical Energy Conversion Laboratory, Department of Applied Physics and Electronics.
    Schmidt, Florian M.
    Umeå University, Thermochemical Energy Conversion Laboratory, Department of Applied Physics and Electronics.
    Wiinikka, Henrik
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.
    Comparison of Measurement Techniques for Temperature and Soot Concentration in Premixed, Small-Scale Burner Flames2017In: Energy & Fuels, ISSN 0887-0624, E-ISSN 1520-5029, Vol. 31, no 10, p. 11328-11336Article in journal (Refereed)
    Abstract [en]

    Optical and intrusive measurement techniques for temperature and soot concentration in hot reacting flows were tested on a small-scale burner in fuel-rich, oxygen-enriched atmospheric flat flames produced to simulate the environment inside an entrained flow reactor. The optical techniques comprised two-color pyrometry (2C-PYR), laser extinction (LE), and tunable diode laser absorption spectroscopy (TDLAS), and the intrusive methods included fine-wire thermocouple thermometry (TC) and electrical low pressure impactor (ELPI) particle analysis. Vertical profiles of temperature and soot concentration were recorded in flames with different equivalence and O2/N2 ratios. The 2C-PYR and LE data were derived assuming mature soot. Gas temperatures up to 2200 K and soot concentrations up to 3 ppmv were measured. Close to the burner surface, the temperatures obtained with the pyrometer were up to 300 K higher than those measured by TDLAS. Further away from the burner, the difference was within 100 K. The TC-derived temperatures were within 100 K from the TDLAS results for most of the flames. At high signal-to-noise ratio and in flame regions with mature soot, the temperatures measured by 2C-PYR and TDLAS were similar. The soot concentrations determined with 2C-PYR were close to those obtained with LE but lower than the ELPI results. It is concluded that the three optical techniques have good potential for process control applications in combustion and gasification processes. 2C-PYR offers simpler installation and 2D imaging, whereas TDLAS and LE provide better accuracy and dynamic range without calibration procedures.

  • 8876.
    Ögren, Yngve
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science. RISE-Energy Technology Center AB.
    Tóth, Pál
    RISE-Energy Technology Center AB.
    Garami, Attila
    University of Miskolc, Faculty of Material Sciences and Engineering, Institute of Energy and Quality Affairs, Department of Combustion Technology and Thermal Energy.
    Sepman, Alexey
    RISE-Energy Technology Center AB.
    Wiinikka, Henrik
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science. RISE-Energy Technology Center AB.
    Development of a vision-based soft sensor for estimating equivalence ratio and major species concentration in entrained flow biomass gasification reactors2018In: Applied Energy, ISSN 0306-2619, E-ISSN 1872-9118, Vol. 226, p. 450-460Article in journal (Refereed)
    Abstract [en]

    A combination of image processing techniques and regression models was evaluated for predicting equivalence ratio and major species concentration (H2, CO, CO2 and CH4) based on real-time image data from the luminous reaction zone in conditions and reactors relevant to biomass gasification. Two simple image pre-processing routines were tested: reduction to statistical moments and pixel binning (subsampling). Image features obtained by using these two pre-processing methods were then used as inputs for two regression algorithms: Gaussian Process Regression and Artificial Neural Networks. The methods were evaluated by using a laboratory-scale flat-flame burner and a pilot-scale entrained flow biomass gasifier. For the flat-flame burner, the root mean square error (RMSE) were on the order of the uncertainty of the experimental measurements. For the gasifier, the RMSE was approximately three times higher than the experimental uncertainty – however, the main source of the error was the quantization of the training dataset. The accuracy of the predictions was found to be sufficient for process monitoring purposes. As a feature extraction step, reduction to statistical moments proved to be superior compared to pixel binning.

  • 8877.
    Öhman, Eric
    Luleå University of Technology, Department of Engineering Sciences and Mathematics.
    Undersökning av verktygsparametrar för dimensionering av tunnare sågverktyg2019Independent thesis Basic level (professional degree), 10 credits / 15 HE creditsStudent thesis
    Abstract [en]

    This degree thesis has been carried out in commision by LSAB and Luleå University of Technology (LTU) to study cutting forces for wide bandsaw blades. Large-scale bandsaws are typically used in industrial sawmills and one way to increase yield for timber is to reduce the width of cut. The aim of this study was to investigate which tooth design parameters had the biggest effect on cutting effectiveness when the cutting width was reduced.

    A series of tests were carried out to measure cutting forces using various tooth design parameters. The workpieces used for testing cutting forces were fresh spruce and high-density polyethylene (HDPE) thermoplastic. The density of spruce wood blocks was measured using computed tomography (CT) scanning. The cutting force and density data were plotted against each other to present cutting force characteristics for the different teeth – these diagrams could aid the design of narrow width cutting tools.

    The results have shown that it is important to maintain tooth symmetry. When the tangential or radial clearance angle differs between left and right side of the tooth, the lateral or side-to-side force increases. Also, the normal force decreased, meaning that the tooth further engaged the wood for assymetrical tooth design. Notably, the ratio between the two cut widths (measured in mm) being tested is 2.5/2.9 or 0.86:1 and when sawing HDPE plastic the main cutting force of the narrower cut width measured 86 % of the cutting force from the wider cut width.

  • 8878.
    Öhman, Johan
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    3D localization in digital holography from scattered light from micrometer-sized particles2018Licentiate thesis, comprehensive summary (Other academic)
    Abstract [en]

    When a particle is illuminated by a beam of light it will scatter and redistribute the light in all directions. How it scatters depends on the size, shape and refractive index of the particle. Additionally, it depends on the wavelength and polarization of the illuminating beam. The direction and distance to the observer relative the particle also needs to be considered.  A digital holographic imaging system is used to collect parts of the scattered light from micrometer-sized particles. By utilizing digital holography a three-dimensional reconstruction of the imaged scene is possible. Traditionally, particles are localized based on the intensity in the holographic reconstructions. In this licentiate thesis, the phase response of the scattered light is investigated and utilized. An alternative method for locating spherical particles is presented. The method locate particles based on a simple feature of a propagating wave, namely the fact that the wavefront curvature changes from converging to diverging at the axial location of the particle. The wavefront curvature is estimated using two different methods. The first method estimates the lateral phase-gradients using a finite-difference method. The second method uses a three-dimensional parametric model based on a Chebyshev polynomial expansion. The methods are demonstrated using both simulations and experimental measurements. The simulations are based on the Lorenz-Mie scattering theory for spherical particles and are combined with an imaging system model. Experiments are performed using an off-axis polarization sensitive digital holographic system with a coherent Nd:YAG laser. Measurements of stationary particles are made to validate and evaluate the proposed method. It is found that these methods estimate the true axial position and does not have the offset that is associated with intensity-based methods. Additionally, it is possible to exclude noise that shows up as false particles since noise does not have the same phase response as a real particle. The second method, that uses a parametric model, also improves the standard deviation in the positioning.

  • 8879.
    Öhman, Johan
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Gren, Per
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Sjödahl, Mikael
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Polarization-resolved dual-view holographic system for 3D inspection of scattering particles2019In: Applied Optics, ISSN 1559-128X, E-ISSN 2155-3165, Vol. 58, no 34, p. G31-G40Article in journal (Refereed)
    Abstract [en]

    A novel dual-view polarization-resolved pulsed holographic system for particle measurements is presented. Both dual-view configuration and polarization-resolved registration are well suited for particle holography. Dual-view registration improves the accuracy in the detection of 3D position and velocities, and polarization-resolved registration provides polarization information about individual particles. The necessary calibrations are presented, and aberrations are compensated for by mapping the positions in the two views to positions in a global coordinate system. The system is demonstrated on a sample consisting of 7 μm spherical polystyrene particles dissolved in water in a cuvette. The system is tested with different polarizations of the illumination. It is found that the dual view improves the accuracy significantly in particle tracking. It is also found that by having polarization-resolved holograms, it is possible to separate naturally occurring sub-micrometer particles from the larger, 7 μm seeding particles.

  • 8880.
    Öhman, Johan
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Sjödahl, Mikael
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Axial Particle Positioning by Wavefront Parameterization using Chebyshev Polynomials and Off-axis Digital Holography2017In: Digital Holography and Three-Dimensional Imaging, Washington: The Optical Society , 2017, article id M4A.3Conference paper (Refereed)
    Abstract [en]

    A particle can be axially positioned where its scattered light has a plane wavefront. The phase anomaly compared to a plane wave is fitted to 3D Chebyshev polynomial, where coefficients correspond to the axial position.

  • 8881.
    Öhman, Johan
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Sjödahl, Mikael
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Improved particle position accuracy from off-axis holograms using a Chebyshev model2018In: Applied Optics, ISSN 1559-128X, E-ISSN 2155-3165, Vol. 57, no 1, p. A157-A163Article in journal (Refereed)
    Abstract [en]

    Side scattered light from micrometer-sized particles is recorded using an off-axis digital holographic setup. From holograms, a volume is reconstructed with information about both intensity and phase. Finding particle positions is non-trivial, since poor axial resolution elongates particles in the reconstruction. To overcome this problem, the reconstructed wavefront around a particle is used to find the axial position. The method is based on the change in the sign of the curvature around the true particle position plane. The wavefront curvature is directly linked to the phase response in the reconstruction. In this paper we propose a new method of estimating the curvature based on a parametric model. The model is based on Chebyshev polynomials and is fit to the phase anomaly and compared to a plane wave in the reconstructed volume. From the model coefficients, it is possible to find particle locations. Simulated results show increased performance in the presence of noise, compared to the use of finite difference methods. The standard deviation is decreased from 3–39 μm to 6–10 μm for varying noise levels. Experimental results show a corresponding improvement where the standard deviation is decreased from 18 μm to 13 μm.

  • 8882.
    Öhman, Johan
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Sjödahl, Mikael
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Off-axis digital holographic particle positioning based on polarization-sensitive wavefront curvature estimation2016In: Applied Optics, ISSN 1559-128X, E-ISSN 2155-3165, Vol. 55, no 27, p. 7503-7510Article in journal (Refereed)
    Abstract [en]

    Poor axial resolution in holographic particle imaging applications makes particle positioning in 3D space morecomplex since the positions are not directly obtained. In this paper we estimate the axial position of micrometerparticles by finding the location where the wavefront curvature from the scattered light becomes zero. By record-ing scattered light at 90°using off-axis holography, the complex amplitude of the light is obtained. Byreconstruction of the imaged scene, a complex valued volume is produced. From this volume, phase gradientsare calculated for each particle and used to estimate the wavefront curvature. From simulations it is found that thewavefront curvature became zero at the true axial position of the particle. We applied this metric to track an axialtranslation experimentally using a telecentric off-axis holographic imaging system with a lateral magnification ofM1.33. A silicon cube with molded particles inside was used as sample. Holographic recordings are performedboth before and after a 100μm axial translation. From the estimated positions, it was found that the mean dis-placement of particles between recordings was 105.0μm with a standard deviation of 25.3μm.

  • 8883.
    Öhman, Johan
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Sjödahl, Mikael
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Gren, Per
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Polarization Resolved Dual-View Holographic System for Investigation of Microparticles2019In: OSA Technical Digest (Optical Society of America, 2019), 2019, article id Th2A.5Conference paper (Refereed)
    Abstract [en]

    A dual-view polarization resolved digital-holographic system is presented. The necessary calibration for both polarization and spatial coordinates are outlined. As an example the system is is used to track spherical microparticles in a cuvette.

  • 8884.
    Öhman, L. O.
    et al.
    Umeå universitet.
    Ganemi, B.
    Kungliga tekniska högskolan, KTH.
    Björnbom, E.
    Kungliga tekniska högskolan, KTH.
    Rahkamaa, K.
    Åbo Akademi.
    Keiski, R. L.
    Uleåborg universitet.
    Paul, Jan
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Catalyst preparation through ion-exchange of zeolite Cu-, Ni-, Pd-, CuNi- and CuPd-ZSM-52002In: Materials Chemistry and Physics, ISSN 0254-0584, E-ISSN 1879-3312, Vol. 73, no 2-3, p. 263-267Article in journal (Refereed)
    Abstract [en]

    Ion-exchanged zeolite ZSM-5 is the best known catalyst for direct NOx decomposition and a viable candidate for NOx reduction with methane. The preparation is crucial for the efficiency of the conversion and this paper describes the ion-exchange, with Cu2+, Ni2+ or Pd2+, and the dual exchange, with Cu2+/Ni2+ or Cu2+/Pd2+, under appropriate pH and ion concentrations for maximum dispersion.

  • 8885.
    Öhman, L.O.
    et al.
    Umeå universitet.
    Paul, Jan
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Materials aspects of titanium-doped aluminas: 14%Ti/γ-Al2O3/Cu and sulfided Al2O3-TiO2/NiMo2002In: Materials Chemistry and Physics, ISSN 0254-0584, E-ISSN 1879-3312, Vol. 73, no 2-3, p. 242-251Article in journal (Refereed)
    Abstract [en]

    Materials aspects of titanium-doped alumina catalysts are discussed in light of a partly new synthesis route, ion exchange reactions and sulfidation. Starting with a solution of the isopropoxides of aluminum and titanium, we aimed at an Al:Ti ratio of 6:1, which corresponds to the maximum acidity of the final product. Transmission electron micrographs showed that the synthesis gave a product of nearly monodisperse, rectangular particles and X-ray diffractograms revealed an amorphous to nano-crystalline structure reminiscent of γ-Al2O3. The Brunauer-Emmet-Teller area was 340 m2 g-1 and the H+ titration found 3 acidic sites nm-2. These numbers indicate that the titanium modification may fourfold increase the ion exchange capacity of a regular alumina. The importance of titanium modification for catalysis is discussed in connection with selective catalytic reduction with hydrocarbon reactions over Cu/14%Ti-Al2O3 and hydrogenation of 1-Me-naphthalene over sulfided NiMo/Al2O3-TiO2. The Ti4+ ions in the alumina lattice are almost irreducible, but infrared spectra suggested that these ions act as dispersion agents not only for adsorbed Cu2+ ions, but also for a supported phase of sulfided NiMo. Finally, we show that a simple weight control can be used to monitor the sulfidation and to indicate if the synthesis has produced a separate, reducible titania phase.

  • 8886.
    Öhman, Marcus
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.
    Boström, Dan
    Umeå universitet.
    Skoglund, Nils
    Umeå universitet.
    Grimm, Alejandro
    Boman, Christoffer
    Umeå universitet.
    Kofod-Hansen, Marie
    Torvforsk.
    Minskade askrelaterade driftsproblem genom inblandning av torv i åkerbränslen2010Report (Other academic)
    Abstract [sv]

    Torvbränslen används ofta i kombination med trädbränslen i värme- och kraftvärmeverk. Den kombinationen har visat sig ge förbränningstekniska fördelar kanske främst minskad risk för askrelaterade driftsproblem. Erfarenheter saknas dock vad gäller eventuella positiva sameldningseffekter/erfoderliga inblandningsgrader av torv i många kommande åkerbränslen såsom salix, rörflen och halm. Projektets mål har därför varit att: i) bestämma vilka typiska inblandningsgrader av torv i olika åkerbränslen (halm, salix och rörflen) som krävs för att uppnå positiva effekter m a p slaggning, beläggningsbildning/(högtemperaturkorrosion) och bäddagglomerering, samt ii) demonstrera möjligheterna att minska uppkomsten av askrelaterade driftsproblem i förbränningsanläggningar genom inblandning av torv till intressanta åkerbränslen. Termokemiska modellberäkningar utfördes för att bestämma effekter av torvinblandning till halm, salix och rörflen med hög och låg askhalt på reduktionen av slaggnings-, beläggningsbildnings-/(korrosion)- och bäddagglomereringsrisken vid förbränning. Dessa resultat och tidigare utförda försök i bänkskala låg sedan till grund för valda bränslekombinationer, torvinblandningsgrader och torvprov i de sedan utförda demonstrationsförsöken. Dessa demonstrationsförsök utfördes i en rosteranläggning på 4 MW (rörflen/rörflensmixar och salix/salixmixar) och i en pelletsbrännare/-panna i bänkskala (20 kW) (halm/halmmixar). Resultaten visar att inblandning av typisk starrbaserad bränntorv i salix och rörflen med låg askhalt ger positiva effekter vad avser bäddagglomerering och beläggningsbildning/(korrosion) i pannors konvektionsdelar redan vid relativt låga inblandningsgrader (15 vikts-% på TS basis). En starrbaserad bränntorv med relativt högt Ca/Si förhållande bör väljas för sameldning med salix i rosteranläggningar för att inte öka slaggningsrisken. Samma torvtyp kan också i rosteranläggningar nyttjas i sameldning med rörflen med låg askhalt (relativt låga inblandningsgrader räcker d v s 15 vikts-% på TS basis) och vetehalm (höga inblandningsgrader krävs d v s upp mot 40 vikts-% på TS basis) för att reducera slaggningsrisken. Vid val av torvslag för att maximera de ovanstående positiva effekterna vid förbränning kan därför en allmän rekommendation göras att torvar med hög askhalt (starrinnehållande torv), och gärna med högt inslag av svavel, ger de bästa sameldningsegenskaperna med det tilläget att vid rostereldning bör en torv med relativt högt Ca/Si förhållande väljas (gärna upp mot 1 på vikts-% basis). Rörflen med hög askhalt förväntas inte ge upphov till några större problem med beläggnings- och slaggnings-/bäddagglomereringsproblem och är därför i första hand inte intressant att samelda med torv utifrån ett askrelaterat driftsproblemperspektiv. För att reducera beläggningstendensen till låga nivåer vid sameldning med vetehalm krävs troligen så höga nivåer att detta inte är praktiskt intressant.

  • 8887.
    Öhman, Marcus
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.
    Gilbe, Carl
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.
    Näzelius, Ida-Linn
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.
    Hedman, Henry
    Energy Technology Centre, Piteå.
    Boström, Dan
    Umeå universitet.
    Boman, Christoffer
    Umeå universitet.
    Backman, Rainer
    Umeå universitet.
    Slag formation during combustion of biomass fuels with low phosphorus content2011In: From research to industry and markets: proceedings ; 19th European Biomass Conference and Exhibition ; ICC Berlin, International Congress Center Berlin, Germany, conference 6 - 10 June 2011, exhibition 6 - 9 June 2011 / [ed] Martin Faulstich, Florence: ETA - Renewable Energies , 2011, p. 1267-1270Conference paper (Refereed)
    Abstract [en]

    Slag and ash deposition problems have more than occasionally been observed in biomass fueled plants in general and especially in pellet fired plants. These problems can lead to reduced accessibility as well as performance of the combustion appliances. Slag formation including ash transformation mechanisms, measures for prevention and prediction during combustion of biomass fuels has been studied by the research groups at Luleå University of Technology, Umeå University and Energy Technology Center for over one decade now. Several (about 40) different biomass fuels/fuel mixtures have been studied and many different fuel additives/co-combustion fuels have been used to combat slag formation. About 20 scientific papers and 10 technical reports have been published by the research groups within the topic of slag formation during combustion of biomass fuels with low phosphorus content. The objective of the present work was to summarize the general experiences and conclusions of our research within the area of slag formation during combustion of fuels with low phosphorus and varying silicon content. Silicon rich fuels, i.e. fuel ash dominated by silicate-alkali chemistry (e.g. straw fuels), generally shows relatively high slagging tendencies. Exceptions to these general trends exists i.e. fuels with very high Si/K-ratios. Wood derived fuels with a relatively low inherent silicon content shows low or relatively moderate slagging tendencies. However, severe contamination of sand material to woody biomass fuels may greatly enhance the slagging tendencies. Fuel additive addition to problematic woody biomass and straw significantly reduces the slagging tendencies. When adding limestone and kaolin to the problematic wood and straw fuels the composition of the formed slag is changed from relatively low temperature melting silicates to high temperature melting silicates and oxides. The standard ash fusion tests and chemical equilibrium calculations gives useful qualitatively information of the slagging tendencies. However, both methods must be further improved before quantitatively results can be used. The use of different fuel indexes to predict the slagging tendencies could be an interesting option. More research within this field is, however, needed.

  • 8888.
    Öhman, Marcus
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.
    Gilbe, R
    Boman, C
    Erhardsson, T
    Pommer, L
    Boström, D
    Nordin, A
    Samuelsson, R
    Burvall, J
    Minskade askrelaterade driftsproblem via torv i biobränslen2007In: Nordisk Energi, ISSN 1654-0328, Vol. 6, p. 62-63Article in journal (Other (popular science, discussion, etc.))
  • 8889.
    Öhman, Marcus
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.
    Nyström, Ida-Linn
    Gilbe, Carl
    Boström, Dan
    Umeå university.
    Lindström, Erica
    Umeå university.
    Boman, Christoffer
    Umeå university.
    Backman, Rainer
    Umeå university.
    Hedman, Henry
    Energy Technology Centre, Piteå.
    Samuelsson, Robert
    Swedish University of Agricultural Sciences.
    Burvall, Jan
    Swedish University of Agricultural Sciences.
    Shaojun, X
    Swedish University of Agricultural Sciences.
    Slag formation during combustion of biomass fuels2009Conference paper (Other academic)
    Abstract [en]

    Slag and ash deposition problems have more than occasionally been observed in biomass fueled plants in general and especially in pellet fired plants. These problems could lead to reduced accessibility as well as performance of the combustion appliances. Slag formation including measures for prevention and prediction during combustion of biomass fuels has been studied by the research groups at Luleå University of Technology, Umeå University, Energy Technology Center and Swedish University of Agricultural Sciences for about one decade know. Several (about 40) different biomass fuels/fuel mixtures have been studies and many different fuel additives/co-combustion fuels have been used to combat slag formation. About 25 scientific papers and 10 technical reports have been published by the research groups within the topic of slag formation during combustion of biomass fuels. The general experiences and conclusions of our research within the area will be presented.

  • 8890.
    Öhman, Micael
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Wood Science and Engineering.
    Correspondences between manually estimated compression wood in Norway spruce and the warp of the sawn timber1999In: European Journal of Wood and Wood Products, ISSN 0018-3768, E-ISSN 1436-736X, Vol. 57, no 5, p. 391-396Article in journal (Refereed)
    Abstract [en]

    Compression wood is regarded as a serious defect which affects the warp and machinability of sawn timber. To handle these problems, different regulations have been developed regarding grading of sawlogs and of sawn timber. This study is an attempt to clarify the relation between the amount of visible compression wood in Norway spruce (Picea abies L.) and the warping of the sawn timber in terms of bow, spring and twist as well as further deformation after ripping of the dried products. The amount of compression wood was defined and measured on logs according to the methods of the Swedish Timber Measurement Council (Regulations for measuring of round wood) and on the sawn timber according to the Nordic Timber. The impact of two different drying schedules was also investigated. The study shows that visible compression wood in both the butt end of the log and within the sawn timber was a rather poor indicator of the warp of the dried sawn timber. In no comparison did the correlation coefficient, r, exceed 0.3. In contrast to this, the correlation between the amount of compression wood and the warp of secondary products was fair, r = 0.79. This means that it should be possible to identify sawn timber less suitable for secondary processing by the amount of compression wood. The corresponding correlation between compression wood in the butt end of the log and the warp of the secondary products was r = 0.46. No significant differences could be shown in the degree of warp, as related to compression wood, between sawn timber or secondary products, dried at a wet-bulb temperature of 55 °C/117 h, LT-schedule, and a dry-bulb temperature of 110 °C/24 h, HT-schedule, respectively.

  • 8891.
    Öhman, Micael
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Wood Science and Engineering.
    Grade prediction of Pinus sylvestris logs with the aid of a radiograph image log1998In: Scandinavian Journal of Forest Research, ISSN 0282-7581, E-ISSN 1651-1891, Vol. 13, p. 83-89Article in journal (Refereed)
  • 8892.
    Öhman, Micael
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Wood Science and Engineering.
    Methods for avoiding the negative effects of compression wood2001Report (Other academic)
    Abstract [en]

    This is a study of how the selection of raw material and sawing methods affects the magnitude of bow, spring and twist as well as the yield of accepted wall studs when graded with respect to straightness. The focus was on the relation between compression wood in butt logs of Norway spruce (Picea abies (L.) Karst) and the straightness of wall studs, in order to improve the sawmills ability to satisfy the customer demands by straighter sawn products. The following aspects of compression wood in the production process were examined:

  • 8893.
    Öhman, Micael
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Wood Science and Engineering.
    Modelling compression wood in sawn timber of Scot pine and Norway spruce1999In: Connection between silviculture and wood quality through modelling approaches and simulation softwares: third workshop : La Londe-Les-Maures, France, 1999 : proceedings / [ed] Gérard Nepveu, Nancy: INRA Editions, 1999Conference paper (Refereed)
  • 8894.
    Öhman, Micael
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Wood Science and Engineering.
    Plank grade indicators in radiograph images of Scots pine logs1999In: European Journal of Wood and Wood Products, ISSN 0018-3768, E-ISSN 1436-736X, Vol. 57, no 5, p. 359-363Article in journal (Refereed)
    Abstract [en]

    It is possible today to detect the internal structure of a log by using X-ray scanning technology. By visual inspection of a longitudinal radiograph image (LRI) of a log, it is possible to manually predict the coming grade of the centre planks. The objective of this study was to identify the features of Scots pine logs visible in the LRI's that were of importance in a manual grading process. The identification of useful features was determined by a survey among respondents connected to the wood and sawmill industry. The test logs originate from the Swedish stem bank and the evaluation of the results was based on the statistical method of partial least square regression (PLS). This study shows that useful indicators of the true grade of the centre planks were the knots and knot-related features as well as the butt swell and the heart wood fraction of the log.

  • 8895.
    Öhman, Micael
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Wood Science and Engineering.
    Prediction of the properties of sawn timber by visual inspection of sawlogs1998Licentiate thesis, comprehensive summary (Other academic)
  • 8896.
    Öhman, Micael
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Wood Science and Engineering.
    The measurement of compression wood and other wood features and the prediction of their impact on wood products2001Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Wood is a complex and highly variable biological material formed to give the tree best possible conditions for sustaining life. Thus every piece of wood is possessed of unique qualities. The great challenge in the process of manufacturing wood products has always been to select pieces of wood with properties that fulfil requirements for the product. The importance of selecting the right piece of wood will increase along with demands from customers for products with specific properties, such as moisture content, warp, strength, biological and aesthetic features. In order to supply customers with the products they request, a considerably improved selection of the raw material is needed. The earlier an accurate selection can be done, the better. To improve this selection or pregrading process, knowledge of the relationships between different features and different aspects of quality, as well as methods for measuring external and internal features, must be developed. The main objective of this work was to contribute to improved predictability of the quality of dried sawn products using the features both of logs and of sawn green products as input. This work was divided into two parts: Part one focused on the possibility of learning how to predict the quality grades of centre planks by manually inspecting longitudinal radiograph images (LRI) that depict the density variation within a log. In a survey respondents were interviewed regarding their interpretations of the density related features visible in the LRIs of Scots pine logs (Pinus silvestris L). The purpose was to be able to use these interpretations in predicting the final quality of planks sawn from the logs. The LRIs were reconstructed with the aid of an X-ray CT scanner. Part two focused on the relationship between compression wood (CW) in foremost butt logs of Norway spruce (Picea abies (L.) Karst) and the warp of the sawn products, on how to detect CW and on how to predict warp. The logs used in the study were chosen among logs delivered to and sawn at commercial sawmills located in the northern part of Sweden in order to assure that conditions in the study match those extant in commercial sawmills. The most important findings in this thesis are: Longitudinal radiograph images of the density variation within a log can be a powerful aid in manual grading of logs with respect to the quality of the resulting sawn products. The shape of the sawn, but not dried, centre planks is an indicator of both the amount and distribution of compression wood. Basing the cutting of planks on their shapes while still green can considerably increase the total length of acceptably straight dried products. This improvement is achieved through the elimination of compression wood.

  • 8897.
    Öhman, Micael
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Wood Science and Engineering.
    Ekevad, Mats
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Wood Science and Engineering.
    Grubii, Victor
    Remoistering of the wood before planing: a method for improved quality2015In: Proceedings of the 22nd International Wood Machining Seminar / [ed] Roger Hernández; Claudia B. Cáceres, Quebec city, Kanada: Universite Laval , 2015, p. 245-251, article id 5Conference paper (Refereed)
    Abstract [en]

    It is beneficial if the machining of wooden products is done at a moisture content equal to the climate the product is meant to be used in. For indoor products in central heated houses such moisture content is about 5-10%. For planing this is often a too low moisture content showing an increased risk of poor surface quality due to severe torn grain. Contrary to this too high moisture content will result in a fuzzy grain surface and problems with swelling and shrinkage of the product. The roughness of a machined wooden surface is affected by a number of different parameters like cutting tool geometry, machine settings and wood structure. The latter is the hardest to control since the surface quality is a result of the local combination of density, grain direction and moisture content. The larger the variation in wood features the more difficult it is to find a combination of tools and machine settings that will give a high surface quality.This study showed that by wetting the surface before machining, in this case planing, the average surface quality could be increased. No time dependences could be shown, wetting short before planing did show as good improvements as wetting treatment for 30 minutes or more.The study was based on a total of 120 test surfaces of Scots pine (Pinus silvestris L.). In order to maximize the variation in grain angle and density variations the test surfaces contained both clear wood as well as green knots.

  • 8898.
    Öhman, Micael
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Wood Science and Engineering.
    Grubii, Victor
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Wood Science and Engineering.
    Sandberg, Dick
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Wood Science and Engineering.
    Ekevad, Mats
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Wood Science and Engineering.
    Moistening of the wood surface before planing for improved surface quality2016In: Wood Material Science & Engineering, ISSN 1748-0272, E-ISSN 1748-0280, Vol. 11, no 3, p. 156-163Article in journal (Refereed)
    Abstract [en]

    The roughness of a machined wooden surface is affected by a number of factors such as cutting tool geometry, machine settings and wood structure. The influence of wood structure on wood surface quality is difficult to control since the surface roughness is dependent on the local combination of density, grain direction and moisture content (MC). The greater the variation in wood features, the more difficult it is to find a combination of tools and machine settings that will give a high surface quality. The purpose was to study the impact of a surface wetting treatment before planing in order to reduce torn grain in the wood surface near knots in sawn timber of low MC. The study was based on a total of 120 specimens of Scots pine (Pinus sylvestris L.). To maximize the variation in grain orientation and density, the specimens contained both clear wood and knots. The results showed that when the surface was moistened before planing, chipped and torn grain in areas of deviating grain close to knots decreased. The response to wetting was rapid, wetting less than 30 s before planing gave as good an improvement as treatment time of 30 min or more.

  • 8899.
    Öhman, Micael
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Wood Science and Engineering.
    Nyström, Jan
    Measurement of green plank shape for prediction and elimination of compression wood2002In: Scandinavian Journal of Forest Research, ISSN 0282-7581, E-ISSN 1651-1891, Vol. 17, no 4, p. 377-384Article in journal (Refereed)
    Abstract [en]

    The objective of this study was to predict the amount and the distribution of compression wood (CW) within a Norway spruce [Picea abies (L.) Karst.] plank based on green plank curvature. The findings indicated a possibility of predicting the longitudinal distribution of CW from the green plank curvature. Areas free from CW showed a typical concave shape in relation to the centre of the log, while CW was present when a convex shape was shown. The larger the magnitude of convex curvature, the higher the concentrations of CW that could be found, and a larger fraction of dried planks was rejected due to excessive warp. This study also determined what information can be used to eliminate areas of high concentrations of CW by cutting and how cutting affects the grading results with respect to warp. Over 50% of the plank length showing a high concentration of CW (> 30% of the cross-cut volume) was successfully cut off. Cutting strategies based on predicted CW concentrations resulted in a 10-40% increase in accepted plank length.

  • 8900.
    Öhman, Micael
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Wood Science and Engineering.
    Nyström, Jan
    Modelling distribution and amount of compression wood by the shape of the green bow within planks of Norway spruce2003In: IWSS 5: Proceedings / Fifth International conference om image processing and scanning of wood, March 23 to 26, 2003, Bad Waltersdorf, Austria, Europe, Joanneum research , 2003, p. 13-24Conference paper (Other academic)
175176177178179 8851 - 8900 of 8921
CiteExportLink to result list
Permanent link
Cite
Citation style
  • apa
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf