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  • 1.
    Akkerman, V'yacheslav
    et al.
    Umeå University, Faculty of Science and Technology, Physics.
    Bychkov, Vitaly
    Umeå University, Faculty of Science and Technology, Physics.
    Petchenko, Arkady
    Umeå University, Faculty of Science and Technology, Physics.
    Eriksson, Lars-Erik
    Accelerating flames in cylindrical tubes with nonslip at the walls2006In: Combustion and Flame, ISSN 0010-2180, E-ISSN 1556-2921, Vol. 145, no 1-2, p. 206-219Article in journal (Refereed)
    Abstract [en]

    An analytical theory of flame acceleration in cylindrical tubes with one end closed is developed. It is shown that all realistic flames with a large density drop at the front accelerate exponentially because of the nonslip at the tube walls. Such acceleration mechanism is not limited in time and, eventually, it may lead to detonation triggering. It is found that the acceleration rate decreases with the Reynolds number of the flow. On the contrary, the acceleration rate grows with the thermal expansion of the burning matter. It is shown that the flame shape and the velocity profile remain self-similar during the flame acceleration. The theory is validated by extensive direct numerical simulations. The simulations are performed for the complete set of combustion and hydrodynamic equations including thermal conduction, diffusion, viscosity, and chemical kinetics. The simulation results are in very good agreement with the analytical theory.

  • 2.
    Akkerman, V'yacheslav
    et al.
    Umeå University, Faculty of Science and Technology, Physics.
    Bychkov, Vitaly
    Umeå University, Faculty of Science and Technology, Physics.
    Petchenko, Arkady
    Umeå University, Faculty of Science and Technology, Physics.
    Eriksson, Lars-Erik
    Flame oscillations in tubes with nonslip at the walls2006In: Combustion and Flame, ISSN 0010-2180, E-ISSN 1556-2921, Vol. 145, no 4, p. 675-687Article in journal (Refereed)
    Abstract [en]

    A laminar premixed flame front propagating in a two-dimensional tube is considered with nonslip at the walls and with both ends open. The problem of flame propagation is solved using direct numerical simulations of the complete set of hydrodynamic equations including thermal conduction, diffusion, viscosity, and chemical kinetics. As a result, it is shown that flame interaction with the walls leads to the oscillating regime of burning. The oscillations involve variations of the curved flame shape and the velocity of flame propagation. The oscillation parameters depend on the characteristic tube width, which controls the Reynolds number of the flow. In narrow tubes the oscillations are rather weak, while in wider tubes they become stronger with well-pronounced nonlinear effects. The period of oscillations increases for wider tubes, while the average flame length scaled by the tube diameter decreases only slightly with increasing tube width. The average flame length calculated in the present work is in agreement with that obtained in the experiments. Numerical results reduce the gap between the theory of turbulent flames and the experiments on turbulent combustion in tubes.

  • 3.
    Bychkov, Vitaly
    et al.
    Umeå University, Faculty of Science and Technology, Physics.
    Akkerman, V'yacheslav
    Umeå University, Faculty of Science and Technology, Physics.
    Fru, Gordon
    Umeå University, Faculty of Science and Technology, Physics.
    Eriksson, Lars-Erik
    Petchenko, Arkady
    Umeå University, Faculty of Science and Technology, Physics.
    Flame Acceleration in the Early Stages of Burning in Tubes2007In: Combustion and Flame, ISSN 0010-2180, E-ISSN 1556-2921, Vol. 150, no 4, p. 263-276Article in journal (Refereed)
    Abstract [en]

    Acceleration of premixed laminar flames in the early stages of burning in long tubes is considered. The acceleration mechanism was suggested earlier by Clanet and Searby [Combust. Flame 105 (1996) 225]. Acceleration happens due to the initial ignition geometry at the tube axis when a flame develops to a finger-shaped front, with surface area growing exponentially in time. Flame surface area grows quite fast but only for a short time. The analytical theory of flame acceleration is developed, which determines the growth rate, the total acceleration time, and the maximal increase of the flame surface area. Direct numerical simulations of the process are performed for the complete set of combustion equations. The simulations results and the theory are in good agreement with the previous experiments. The numerical simulations also demonstrate flame deceleration, which follows acceleration, and the so-called “tulip flames.”

  • 4.
    Bychkov, Vitaly
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Akkerman, Vyacheslav
    Princeton University.
    Valiev, Damir
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Law, Chung K.
    Princeton University.
    Influence of gas compression on flame acceleration in channels with obstacles2010In: Combustion and Flame, ISSN 0010-2180, E-ISSN 1556-2921, Vol. 157, no 10, p. 2008-2011Article in journal (Refereed)
  • 5. Fatehi, Hesameddin
    et al.
    Schmidt, Florian M.
    Umeå University, Faculty of Science and Technology, Department of Applied Physics and Electronics.
    Bai, Xue-Song
    Gas phase combustion in the vicinity of a biomass particle during devolatilization: model development and experimental verification2018In: Combustion and Flame, ISSN 0010-2180, E-ISSN 1556-2921, Vol. 196, p. 351-363Article in journal (Refereed)
    Abstract [en]

    A numerical and experimental study on the devolatilization of a large biomass particle is carried out to quantify the effect of homogeneous volatile combustion on the conversion of the particle and on the temperature and species distribution at the particle vicinity. A global chemical kinetic mechanism and a detailed reaction mechanism are considered in a one dimensional numerical model that takes into account preferential diffusivity and a detailed composition of tar species. An adaptive moving mesh is employed to capture the changes in the domain due to particle shrinkage. The effect of gas phase reactions on pyrolysis time, temperature and species distribution close to the particle is studied and compared to experiments. Online in situ measurements of average H2O mole fraction and gas temperature above a softwood pellet are conducted in a reactor using tunable diode laser absorption spectroscopy (TDLAS) while recording the particle mass loss. The results show that the volatile combustion plays an important role in the prediction of biomass conversion during the devolatilization stage. It is shown that the global reaction mechanism predicts a thin flame front in the vicinity of the particle deviating from the measured temperature and H2O distribution over different heights above the particle. A better agreement between numerical and experimental results is obtained using the detailed reaction mechanism, which predicts a wider reaction zone.

  • 6.
    Petchenko, Arkady
    et al.
    Umeå University, Faculty of Science and Technology, Physics.
    Bychkov, Vitaly
    Umeå University, Faculty of Science and Technology, Physics.
    Axisymmetric versus non-axisymmetric flames in cylindrical tubes2004In: Combustion and Flame, ISSN 0010-2180, E-ISSN 1556-2921, Vol. 136, no 4, p. 429-439Article in journal (Refereed)
  • 7.
    Petchenko, Arkady
    et al.
    Umeå University, Faculty of Science and Technology, Physics.
    Bychkov, Vitaly
    Umeå University, Faculty of Science and Technology, Physics.
    Akkerman, V'yacheslav
    Umeå University, Faculty of Science and Technology, Physics.
    Eriksson, Lars-Erik
    Flame–sound interaction in tubes with nonslip walls2007In: Combustion and Flame, ISSN 0010-2180, E-ISSN 1556-2921, Vol. 149, no 4, p. 418-434Article in journal (Refereed)
    Abstract [en]

    Flame interaction with sound is studied for a premixed flame propagating to the closed end of a tube with nonslip walls. The flow geometry is similar to that in the classical Searby experiments on flame–acoustic interaction [Combust. Sci. Technol. 81 (1992) 221]. The problem is solved by direct numerical simulations of the combustion equations. The flame–sound interaction strongly influences oscillations of the flame front. Particularly, sound noticeably increases the oscillation amplitude in comparison with that in an open tube with nonreflecting boundary conditions at the ends studied previously. Oscillations become especially strong in the second part of the tube, where flame pulsations are in resonance with the acoustic wave. Parameters of the flame oscillations are investigated for different values of the tube width and length. It is demonstrated that the oscillations are stronger in wider tubes, though the investigated tube width is limited by the computational facilities. In sufficiently wide tubes, violent folding of a flame front is observed because of the flame–acoustic resonance. By increasing the Lewis number, one also increases the oscillation amplitude.

  • 8.
    Qu, Zhechao
    et al.
    Umeå University, Faculty of Science and Technology, Department of Applied Physics and Electronics.
    Holmgren, Per
    Umeå University, Faculty of Science and Technology, Department of Applied Physics and Electronics.
    Skoglund, Nils
    Umeå University, Faculty of Science and Technology, Department of Applied Physics and Electronics. Energy Engineering, Department of Engineering Sciences & Mathematics, Luleå University of Technology, SE-971 87 Luleå, Sweden.
    Wagner, David R.
    Umeå University, Faculty of Science and Technology, Department of Applied Physics and Electronics.
    Broström, Markus
    Umeå University, Faculty of Science and Technology, Department of Applied Physics and Electronics.
    Schmidt, Florian M.
    Umeå University, Faculty of Science and Technology, Department of Applied Physics and Electronics.
    Distribution of temperature, H2O and atomic potassium during entrained flow biomass combustion: coupling in situ TDLAS with modeling approaches and ash chemistry2018In: Combustion and Flame, ISSN 0010-2180, E-ISSN 1556-2921, Vol. 188, p. 488-497Article in journal (Refereed)
    Abstract [en]

    Tunable diode laser absorption spectroscopy (TDLAS) is employed for simultaneous detection of gas temperature, water vapor (H2O) and gas-phase atomic potassium, K(g), in an atmospheric, research-scale entrained flow reactor (EFR). In situ measurements are conducted at four different locations in the EFR core to study the progress of thermochemical conversion of softwood and Miscanthus powders with focus on the primary potassium reactions. In an initial validation step during propane flame operation, the measured axial EFR profiles of H2O density-weighted, path-averaged temperature, path-averaged H2O concentration and H2O column density are found in good agreement with 2D CFD simulations and standard flue gas analysis. During biomass conversion, temperature and H2O are significantly higher than for the propane flame, up to 1500 K and 9%, respectively, and K(g) concentrations between 0.2 and 270 ppbv are observed. Despite the large difference in initial potassium content between the fuels, the K(g) concentrations obtained at each EFR location are comparable, which highlights the importance of considering all major ash-forming elements in the fuel matrix. For both fuels, temperature and K(g) decrease with residence time, and in the lower part of the EFR, K(g) is in excellent agreement with thermodynamic equilibrium calculations evaluated at the TDLAS-measured temperatures and H2O concentrations. However, in the upper part of the EFR, where the measured H2O suggested a global equivalence ratio smaller than unity, K(g) is far below the predicted equilibrium values. This indicates that, in contrast to the organic compounds, potassium species rapidly undergo primary ash transformation reactions even if the fuel particles reside in an oxygen-deficient environment.

  • 9.
    Valiev, Damir
    et al.
    Princeton University.
    Akkerman, Vyacheslav
    West Virginia University.
    Kuznetsov, Mikhail
    Karlsruhe Institute of Technology.
    Eriksson, Lars-Erik
    Chalmers University of Technology.
    Law, Chung K.
    Princeton University.
    Bychkov, Vitaly
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Influence of gas compression on flame acceleration in the early stage of burning in tubes2013In: Combustion and Flame, ISSN 0010-2180, E-ISSN 1556-2921, Vol. 160, no 1, p. 97-111Article in journal (Refereed)
    Abstract [en]

    The mechanism of finger flame acceleration at the early stage of burning in tubes was studied experimentally by Clanet and Searby [Combust. Flame 105 (1996) 2251 for slow propane-air flames, and elucidated analytically and computationally by Bychkov et al. [Combust. Flame 150 (2007) 2631 in the limit of incompressible flow. We have now analytically, experimentally and computationally studied the finger flame acceleration for fast burning flames, when the gas compressibility assumes an important role. Specifically, we have first developed a theory through small Mach number expansion up to the first-order terms, demonstrating that gas compression reduces the acceleration rate and the maximum flame tip velocity, and thereby moderates the finger flame acceleration noticeably. This is an important quantitative correction to previous theoretical analysis. We have also conducted experiments for hydrogen-oxygen mixtures with considerable initial values of the Mach number, showing finger flame acceleration with the acceleration rate much smaller than those obtained previously for hydrocarbon flames. Furthermore, we have performed numerical simulations for a wide range of initial laminar flame velocities, with the results substantiating the experiments. It is shown that the theory is in good quantitative agreement with numerical simulations for small gas compression (small initial flame velocities). Similar to previous works, the numerical simulation shows that finger flame acceleration is followed by the formation of the "tulip" flame, which indicates termination of the early acceleration process.

  • 10.
    Valiev, Damir
    et al.
    Umeå University, Faculty of Science and Technology, Physics.
    Bychkov, Vitaly
    Umeå University, Faculty of Science and Technology, Physics.
    Akkerman, Vyacheslav
    Princeton University.
    Law, Chung K.
    Princeton University.
    Eriksson, Lars-Erik
    CTH.
    Flame acceleration in channels with obstacles in the deflagration-to-detonation transition2010In: Combustion and Flame, ISSN 0010-2180, E-ISSN 1556-2921, Vol. 157, no 5, p. 1012-1021Article in journal (Refereed)
    Abstract [en]

    It was demonstrated recently in Bychkov et al. [Bychkov et al., Phys. Rev. Lett. 101 (2008) 164501], that the physical mechanism of flame acceleration in channels with obstacles is qualitatively different from the classical Shelkin mechanism. The new mechanism is much stronger, and is independent of the Reynolds number. The present study provides details of the theory and numerical modeling of the flame acceleration. It is shown theoretically and computationally that flame acceleration progresses noticeably faster in the axisymmetric cylindrical geometry as compared to the planar one, and that the acceleration rate reduces with increasing Mach number and thereby the gas compressibility. Furthermore, the velocity of the accelerating flame saturates to a constant value that is supersonic with respect to the wall. The saturation state can be correlated to the Chapman–Jouguet deflagration as well as the fast flames observed in experiments. The possibility of transition from deflagration-to-detonation in the obstructed channels is demonstrated.

  • 11.
    Valiev, Damir
    et al.
    Department of Mechanical and Aerospace Engineering, Princeton University, Princeton, NJ 08544-5263, USA.
    Zhu, Manqi
    Computational Fluid Dynamics Team, CERFACS, Toulouse 31057, France.
    Bansal, Gaurav
    Intel Corporation, Hillsboro, OR 97124, USA.
    Kolla, Hemanth
    Combustion Research Facility, Sandia National Laboratories, Livermore, CA 94550, USA.
    Law, Chung K.
    Department of Mechanical and Aerospace Engineering, Princeton University, Princeton, NJ 08544-5263, USA.
    Chen, Jacqueline H.
    Combustion Research Facility, Sandia National Laboratories, Livermore, CA 94550, USA.
    Pulsating instability of externally forced premixed counterflow flame2013In: Combustion and Flame, ISSN 0010-2180, E-ISSN 1556-2921, Vol. 160, no 2, p. 285-294Article in journal (Refereed)
    Abstract [en]

    The diffusive-thermal pulsating instability of Le > 1 flames can considerably alter global quantities such as the flammability limit and mass burning rate, making its study practically relevant. In the present study we investigate the behavior of pulsating flames in unsteady flow fields using one-dimensional and two-dimensional flame simulations of laminar premixed rich hydrogen/air flame in a counterflow configuration, focusing on the response of the flame to imposed fluctuations in strain rate and equivalence ratio. These effects become important when the flame propagates in an unsteady flow field, for example, in turbulent flows. In the case of strain rate forcing, the flame is found to undergo oscillatory extinction if the forcing frequency is less than the pulsation frequency. For strain rate forcing frequencies higher than the pulsation frequency, the flame is found to be largely unresponsive to the upstream flow velocity fluctuations. The parametric study for equivalence ratio forcing shows that the pulsating instability is promoted with increasing inlet velocity, increasing amplitude and mean value of the imposed composition fluctuation. At the same time, it is observed that increasing the frequency of the imposed oscillations may attenuate the pulsating instability. Moreover, it is found that a flame subjected to pulsating extinction may be able to sustain pulsating combustion if forced with high-frequency inlet composition variation. Based on the insights gained from one-dimensional simulations, two-dimensional simulations of these pulsating flames are performed to provide additional insights on the shape and location of cells and cusp formation in these flames.

  • 12.
    Wiinikka, Henrik
    et al.
    Energy Technology Centre, Box 726, S-941 28 Piteå, Sweden.
    Gebart, Rikard
    Energy Technology Centre, Box 726, S-941 28 Piteå, Sweden.
    Boman, Christoffer
    Umeå University, Faculty of Science and Technology, Department of Applied Physics and Electronics, Energy Technology and Thermal Process Chemistry.
    Boström, Dan
    Umeå University, Faculty of Science and Technology, Department of Applied Physics and Electronics, Energy Technology and Thermal Process Chemistry.
    Nordin, Anders
    Umeå University, Faculty of Science and Technology, Department of Applied Physics and Electronics, Energy Technology and Thermal Process Chemistry.
    Öhman, Marcus
    Umeå University, Faculty of Science and Technology, Department of Applied Physics and Electronics, Energy Technology and Thermal Process Chemistry.
    High-temperature aerosol formation in wood pellets flames: Spatially resolved measurements2006In: Combustion and Flame, ISSN 0010-2180, E-ISSN 1556-2921, Vol. 147, no 4, p. 278-293Article in journal (Refereed)
    Abstract [en]

    The formation and evolution of high-temperature aerosols during fixed bed combustion of wood pellets in a realistic combustion environment were investigated through spatially resolved experiments. The purpose of this work was to investigate the various stages of aerosol formation from the hot flame zone to the flue gas channel. The investigation is important both for elucidation of the formation mechanisms and as a basis for development and validation of particle formation models that can be used for design optimization. Experiments were conducted in an 8-kW-updraft fired-wood-pellets combustor. Particle samples were withdrawn from the centerline of the combustor through 10 sampling ports by a rapid dilution sampling probe. The corresponding temperatures at the sampling positions were in the range 200-1450 degrees C. The particle sample was size-segregated in a low-pressure impactor, allowing physical and chemical resolution of the fine particles. The chemical composition of the particles was investigated by SEM/EDS and XRD analysis. Furthermore, the experimental results were compared to theoretical models for aerosol formation processes. The experimental data show that the particle size distribution has two peaks, both of which are below an aerodynamic diameter of 2.5 mu m (PM2.5). The mode diameters of the fine and coarse modes in the PM2.5 region were similar to 0.1 and similar to 0.8 mu m, respectively. The shape of the particle size distribution function continuously changes with position in the reactor due to several mechanisms. Early, in the flame zone, both the fine mode and the coarse mode in the PM2.5 region were dominated by particles from incomplete combustion, indicated by a significant amount of carbon in the particles. The particle concentrations of both the fine and the coarse mode decrease rapidly in the hot oxygen-rich flame due to oxidation of the carbon-rich particles. After the hot flame, the fine mode concentration and particle diameter increase gradually when the temperature of the flue gas drops. The main contribution to this comes from condensation on preexisting particles in the gas of alkali sulfates, alkali chlorides, and Zn species formed from constituents vaporized in the fuel bed. The alkali sulfates were found to condense at a temperature of similar to 950 degrees C and alkali chlorides condensed later at similar to 600 degrees C. This agrees well with results of chemical equilibrium calculation of the gas-to-particle conversion temperature. After the hot flame the coarse mode concentration decreased very little when the flue gas was cooled. In addition to carbon, the coarse mode consists of refractory metals and also considerable amounts of alkali. (c) 2006 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

  • 13. Wiinikka, Henrik
    et al.
    Toth, Pal
    Jansson, Kjell
    Molinder, Roger
    Broström, Markus
    Umeå University, Faculty of Science and Technology, Department of Applied Physics and Electronics.
    Sandström, Linda
    Lighty, JoAnn S.
    Weiland, Fredrik
    Particle formation during pressurized entrained flow gasification of wood powder: effects of process conditions on chemical composition, nanostructure, and reactivity2018In: Combustion and Flame, ISSN 0010-2180, E-ISSN 1556-2921, Vol. 189, p. 240-256Article in journal (Refereed)
    Abstract [en]

    The influence of operating condition on particle formation during pressurized, oxygen blown gasification of wood powder with an ash content of 0.4 wt% was investigated. The investigation was performed with a pilot scale gasifier operated at 7 bar(a). Two loads, 400 and 600 kW were tested, with the oxygen equivalence ratio (λ) varied between 0.25 and 0.50. Particle concentration and mass size distribution was analyzed with a low pressure cascade impactor and the collected particles were characterized for morphology, elemental composition, nanostructure, and reactivity using scanning electron microscopy/high resolution transmission electron microscopy/energy dispersive spectroscopy, and thermogravimetric analysis. In order to quantify the nanostructure of the particles and identify prevalent sub-structures, a novel image analysis framework was used. It was found that the process temperature, affected both by λ and the load of the gasifier, had a significant influence on the particle formation processes. At low temperature (1060 °C), the formed soot particles seemed to be resistant to the oxidation process; however, when the oxidation process started at 1119 °C, the internal burning of the more reactive particle core began. A further increase in temperature ( > 1313 °C) lead to the oxidation of the less reactive particle shell. When the shell finally collapsed due to severe oxidation, the original soot particle shape and nanostructure also disappeared and the resulting particle could not be considered as a soot anymore. Instead, the particle shape and nanostructure at the highest temperatures ( > 1430 °C) were a function of the inorganic content and of the inorganic elements the individual particle consisted of. All of these effects together lead to the soot particles in the real gasifier environment having less and less ordered nanostructure and higher and higher reactivity as the temperature increased; i.e., they followed the opposite trend of what is observed during laboratory-scale studies with fuels not containing any ash-forming elements and where the temperature was not controlled by λ.

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