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  • 1.
    Adebiyi, Abdulafeez
    et al.
    Department of Mechanical and Aerospace Engineering, West Virginia University, Morgantown, West Virginia 26506, USA.
    Alkandari, Rawan
    Department of Mechanical and Aerospace Engineering, West Virginia University, Morgantown, West Virginia 26506, USA.
    Valiev, Damir
    Umeå University, Faculty of Science and Technology, Department of Applied Physics and Electronics. Center for Combustion Energy, Key Laboratory for Thermal Science and Power Engineering of the Ministry of Education of China, Department of Energy and Power Engineering, Tsinghua University, Beijing, China.
    Akkerman, V’yacheslav
    Department of Mechanical and Aerospace Engineering, West Virginia University, Morgantown, West Virginia, USA.
    Effect of surface friction on ultrafast flame acceleration in obstructed cylindrical pipes2019In: AIP Advances, ISSN 2158-3226, E-ISSN 2158-3226, Vol. 9, no 3, article id 035249Article in journal (Refereed)
    Abstract [en]

    The Bychkov model of ultrafast flame acceleration in obstructed tubes [Valiev et al., “Flame Acceleration in Channels with Obstacles in the Deflagration-to-Detonation Transition,” Combust. Flame 157, 1012 (2010)] employed a number of simplifying assumptions, including those of free-slip and adiabatic surfaces of the obstacles and of the tube wall. In the present work, the influence of free-slip/non-slip surface conditions on the flame dynamics in a cylindrical tube of radius R, involving an array of parallel, tightly-spaced obstacles of size αR, is scrutinized by means of the computational simulations of the axisymmetric fully-compressible gasdynamics and combustion equations with an Arrhenius chemical kinetics. Specifically, non-slip and free-slip surfaces are compared for the blockage ratio, α, and the spacing between the obstacles, ΔZ, in the ranges 1/3 ≤ α ≤ 2/3 and 0.25 ≤ ΔZ/R ≤ 2.0, respectively. 

    For these parameters, an impact of surface friction on flameacceleration is shown to be minor, only 1-4%, slightly facilitating acceleration in a tube with ΔZ/R = 0.5 and moderating acceleration in thecase of ΔZ/R = 0.25. Given the fact that the physical boundary conditions are non-slip as far as the continuum assumption is valid, the presentwork thereby justifies the Bychkov model, employing the free-slip conditions, and makes its wider applicable to the practical reality. Whilethis result can be anticipated and explained by a fact that flame propagation is mainly driven by its spreading in the unobstructed portion ofan obstructed tube (i.e. far from the tube wall), the situation is, however, qualitatively different from that in the unobstructed tubes, wheresurface friction modifies the flame dynamics conceptually.

  • 2.
    Adebiyi, Abdulafeez
    et al.
    Department of Mechanical and Aerospace Engineering, West Virginia University, Morgantown, WV 26506-6106, United States.
    Idowu, Gbolahan
    Department of Mechanical and Aerospace Engineering, West Virginia University, Morgantown, WV 26506-6106, United States.
    Valiev, Damir
    Center for Combustion Energy, Tsinghua University, Beijing 100084, China.
    Akkerman, V'yacheslav
    Department of Mechanical and Aerospace Engineering, West Virginia University, Morgantown, WV 26506-6106, United States.
    Computational simulations of nonequidiffusive premixed flames in obstructed pipes2018Conference paper (Refereed)
    Abstract [en]

    The impact of the Lewis number, Le, on the dynamics and morphology of a premixed flame front, spreading through a toothbrush-like array of obstacles in a semi-open channel, is studied by means of the computational simulations of the reacting flow equations with fully-compressible hydrodynamics and Arrhenius chemical kinetics. The computational approach employs a cell-centered, finite-volume numerical scheme, which is of the 2nd-order accuracy in time, 4th-order in space for the convective terms, and of the 2nd-order in space for the diffusive terms. The channels of blockage ratios 0.33∼0.67 are considered, with the Lewis numbers in the range 0.2≤Le≤2.0 employed. It is shown that the Lewis number influences the flame evolution substantially. Specifically, flame acceleration weakens for Le>1 (inherent to fuel-lean hydrogen or fuel-rich hydrocarbon burning), presumably, due to a thickening of the flame front. In contrast, Le<1 flames (such as that of rich hydrogen or lean hydrocarbon) acquire an extra strong folding of the front and thereby accelerate even much faster. The later effect can be devoted to the onset of the diffusional-Thermal combustion instability. © 2018 Eastern States Section of the Combustion Institute. All rights reserved.

  • 3.
    Akkerman, Vyacheslav
    et al.
    West Virginia University, Morgantown, WV, United States.
    Bilgili, Serdar
    West Virginia University, Morgantown, WV, United States.
    Bychkov, Vitaly
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Valiev, Damir
    Umeå University, Faculty of Science and Technology, Department of Applied Physics and Electronics.
    Demir, Sinan
    West Virginia University, Morgantown, WV, United States.
    Morella, Haley
    West Virginia University, Morgantown, WV, United States.
    Impacts of the Lewis and Markstein numbers effects on the flame acceleration in channels2016Conference paper (Other academic)
    Abstract [en]

    The effects of flame stretch and thermal/molecular diffusion on the flame acceleration in channels are quantified by means of the analytical and computational endeavours. The internal transport flame properties are accounted in the theory by means of the Markstein number, Mk. Being a positive or negative function of the thermal-chemical combustion parameters, such as the thermal expansion ratio and the Lewis and Zeldovich numbers, the Markstein number either moderates or promotes the flame acceleration. While Mk may provide a substantial impact on the flame acceleration rate in narrow channels, this effects diminishes with the increase of the channel width. The analysis is accompanied by extensive computational simulations of the Navier-Stokes combustion equations, which clarify the impact of the Lewis number on the flame acceleration. It is obtained that, for Le below a certain critical value, at the initial stage of flame acceleration, a globally-convex flame front is splits into two or more "fingers", accompanied by a drastic increase in the flame surface area and associated enhancement of the flame acceleration. Overall, the thermal-diffusive effects substantially facilitate the flame acceleration scenario, thereby advancing a potential deflagration-to-detonation transition. 

  • 4.
    Akkerman, Vyacheslav
    et al.
    Department of Mechanical and Aerospace Engineering, West Virginia University, Morgantown, WV, United States .
    Valiev, Damir
    Umeå University, Faculty of Science and Technology, Department of Applied Physics and Electronics.
    Effect of gas compression on flame acceleration in obstructed cylindrical tubes2016Conference paper (Other academic)
    Abstract [en]

    The role of gas compression on the process of extremely fast flame acceleration in obstructed cylindrical tubes is studied analytically and validated by computational simulations. The acceleration leading to a deflagration-to-detonation transition is associated with a powerful jet-flow produced by delayed combustion in spaces between the obstacles. This acceleration mechanism is Reynolds-independent and conceptually laminar, with turbulence playing only a supplementary role. In this particular work, the incompressible formulation [Combust. Flame 157 (2010) 1012], Ref. 15 is extended to account for small but finite initial Mach number up to the first-order terms. While flames accelerate exponentially during the initial stage of propagation, when the compressibility is negligible, with continuous increase in the flame velocity with respect to the tube wall, the flame-generated compression waves subsequently moderate the acceleration process by affecting the flame shape and velocity, as well as the flow driven by the flame. It is demonstrated that the moderation effect is substantial, and as soon as gas compression is relatively small, the present theory is in good quantitative agreement with the computational simulations. The limitations of the incompressible theory are thereby underlined, and a critical blockage ratio for with this acceleration mechanism can be evaluated.

  • 5.
    Alkhabbaz, Mohammed
    et al.
    Department of Mechanical and Aerospace Engineering, West Virginia University, Morgantown, West Virginia 26506, USA.
    Abidakun, Olatunde
    Department of Mechanical and Aerospace Engineering, West Virginia University, Morgantown, West Virginia 26506, USA.
    Valiev, Damir
    Umeå University, Faculty of Science and Technology, Department of Applied Physics and Electronics. Center for Combustion Energy, Key Laboratory for Thermal Science and Power Engineering of Ministry of Education of China, Department of Energy and Power Engineering, Tsinghua University, Beijing, China.
    Akkerman, V’yacheslav
    Department of Mechanical and Aerospace Engineering, West Virginia University, Morgantown, West Virginia 26506, USA.
    Impact of the Lewis number on finger flame acceleration at the early stage of burning in channels and tubes2019In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 31, no 8, article id 083606Article in journal (Refereed)
    Abstract [en]

    For premixed combustion in channels and tubes with one end open, when a flame is ignited at the centerline at the closed end of the pipe and it propagates toward the open one, significant flame acceleration occurs at an early stage of the combustion process due to formation of a finger-shaped flame front. This scenario is tagged “finger flame acceleration” (FFA), involving an initially hemispherical flame kernel, which subsequently acquires a finger shape with increasing surface area of the flame front. Previous analytical and computational studies of FFA employed a conventional assumption of equidiffusivity when the thermal-to-mass-diffusivity ratio (the Lewis number) is unity (Le = 1). However, combustion is oftentimes nonequidiffusive (Le ≠ 1) in practice such that there has been a need to identify the role of Le in FFA. This demand is addressed in the present work. Specifically, the dynamics and morphology of the Le ≠ 1 flames in two-dimensional (2D) channels and cylindrical tubes are scrutinized by means of the computational simulations of the fully compressible reacting flow equations, and the role of Le is identified. Specifically, the Le > 1 flames accelerate slower as compared with the equidiffusive ones. In contrast, the Le < 1 flames acquire stronger distortion of the front, experience the diffusional-thermal combustion instability, and thereby accelerate much faster than the Le = 1 flames. In addition, combustion in a cylindrical configuration shows stronger FFA than that under the same burning conditions in a 2D planar geometry.

  • 6.
    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)
  • 7.
    Bychkov, Vitaly
    et al.
    Umeå University, Faculty of Science and Technology, Physics.
    Akkerman, Vyacheslav
    Princeton University.
    Valiev, Damir
    Umeå University, Faculty of Science and Technology, Physics.
    Law, Chung K.
    Princeton University.
    Role of Compressibility in Moderating Flame Acceleration in Tubes2010In: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics: Statistical Physics, Plasmas, Fluids, and Related Interdisciplinary Topics, ISSN 1063-651X, E-ISSN 1095-3787, Vol. 81, no 2, p. 026309-Article in journal (Refereed)
    Abstract [en]

    The effect of gas compression on spontaneous flame acceleration leading to deflagration-to-detonation transition is studied theoretically for small Reynolds number flame propagation from the closed end of a tube. The theory assumes weak compressibility through expansion in small Mach number. Results show that the flame front accelerates exponentially during the initial stage of propagation when the Mach number is negligible. With continuous increase in the flame velocity with respect to the tube wall, the flame-generated compression waves subsequently moderate the acceleration process by affecting the flame shape and velocity, as well as the flow driven by the flame.

  • 8.
    Bychkov, Vitaly
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Matyba, Piotr
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Akkerman, Vyacheslav
    Department of Mechanical and Aerospace Engineering, Princeton University, Princeton, New Jersey 08544-5263, USA.
    Modestov, Mikhail
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Valiev, Damir
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Brodin, Gert
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Law, Chung K.
    Department of Mechanical and Aerospace Engineering, Princeton University, Princeton, New Jersey 08544-5263, USA.
    Marklund, Mattias
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Edman, Ludvig
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Speedup of doping fronts in organic semiconductors through plasma instability2011In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 107, no 1, p. 016103-016107Article in journal (Refereed)
    Abstract [en]

    The dynamics of doping transformation fronts in organic semiconductor plasma is studied for application in light-emitting electrochemical cells. We show that new fundamental effects of the plasma dynamics can significantly improve the device performance. We obtain an electrodynamic instability, which distorts the doping fronts and increases the transformation rate considerably. We explain the physical mechanism of the instability, develop theory, provide experimental evidence, perform numerical simulations, and demonstrate how the instability strength may be amplified technologically. The electrodynamic plasma instability obtained also shows interesting similarity to the hydrodynamic Darrieus-Landau instability in combustion, laser ablation, and astrophysics.

  • 9.
    Bychkov, Vitaly
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Valiev, Damir
    Akkerman, V'yacheslav
    Law, Chung K.
    Gas compression moderates flame acceleration in deflagration-to-detonation transition2012In: Combustion Science and Technology, ISSN 0010-2202, E-ISSN 1563-521X, Vol. 184, no 7-8, p. 1066-1079Article in journal (Refereed)
    Abstract [en]

    The effect of gas compression at the developed stages of flame acceleration in smooth-wall and obstructed channels is studied. We demonstrate analytically that gas compression moderates the acceleration rate, and we perform numerical simulations within the problem of flame transition to detonation. It is shown that flame acceleration undergoes three distinctive stages: (1) initial exponential acceleration in the incompressible regime, (2) moderation of the acceleration process due to gas compression, so that the exponential acceleration state goes over to a much slower one, (3) eventual saturation to a steady (or statistically steady) high-speed deflagration velocity, which may be correlated with the Chapman-Jouguet deflagration speed. The possibility of deflagration-to-detonation transition is demonstrated.

  • 10.
    Bychkov, Vitaly
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Valiev, Damir
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Eriksson, Lars-Erik
    Chalmers, Dept Appl Mech, S-41296 Gothenburg, Sweden.
    Physical mechanism of ultrafast flame acceleration2008In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 101, no 16, p. 164501-Article in journal (Refereed)
    Abstract [en]

    We explain the physical mechanism of ultra-fast flame accelerationin obstructed channels used in modern experiments on detonationtriggering.  It is demonstrated that delayed burning between theobstacles creates a powerful jet-flow, driving the acceleration.This mechanism is much stronger than the classical Shelkinscenario of flame acceleration due to non-slip at the channelwalls.  The mechanism under study isindependent of the Reynolds number, with turbulence playing only asupplementary role. The flame front accelerates exponentially; theanalytical formula for the growth rate is obtained. The theory isvalidated by extensive direct numerical simulations and comparisonto previous experiments.

  • 11.
    Demirgok, Berk
    et al.
    Department of Mechanical and Aerospace Engineering, West Virginia University, Morgantown, WV 26506, USA.
    Ugarte, Orlando
    Department of Mechanical and Aerospace Engineering, West Virginia University, Morgantown, WV 26506, USA.
    Valiev, Damir
    Department of Mechanical and Aerospace Engineering, Princeton University, Princeton, NJ 08544, USA.
    Akkerman, V’yacheslav
    Department of Mechanical and Aerospace Engineering, West Virginia University, Morgantown, WV 26506, USA.
    Effect of thermal expansion on flame propagation in channels with nonslip walls2015In: Proceedings of the Combustion Institute, ISSN 0082-0784, E-ISSN 1878-027X, Vol. 35, no 1, p. 929-936Article in journal (Refereed)
    Abstract [en]

    Propagation of premixed flames in narrow channels is investigated by means of extensive numerical simulations of a complete system of combustion and hydrodynamic equations, incorporating transport properties (thermal conduction, diffusion and viscosity) and Arrhenius chemical kinetics. The system includes mass conservation and Navier–Stokes equations as well as those for the energy and species balance. A flame propagates from the closed end of a channel to the open one. An initially planar flame front gets corrugated due to wall friction and thereby accelerates. It is shown that a flame exhibits an exponential state of acceleration only when the thermal expansion coefficient Θ exceeds a certain critical value Θ>Θc. The quantity Θc is tabulated as a function of the Reynolds number related to the flame propagation, Re, being Θc≈6 for Re=5∼20. The major flame characteristics such as the flame propagation speed and acceleration rate are scrutinized. It is demonstrated that the acceleration promotes with Θ   but weakens with Re. In this respect, the present computational results support the theoretical prediction of Bychkov et al  . Physical Review E 72 (2005) 046307 in a wide range of Θ   and Re. While very good quantitative and qualitative agreement between numerical and theoretical results is found for realistically large thermal expansion, Θ>=8, agreement deteriorates with decreasing Θ. Specifically, while the theory and modeling do not quantitatively agree for Θc<Θ<8, they nevertheless demonstrate a qualitative resemblance (the exponential state of acceleration). Finally, no exponential acceleration at Θ<Θc denotes that the theory completely breaks in that case, but this fits other works in the field and thereby allows reconciling various formulations on the flame acceleration.

  • 12.
    Dion, Claude
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Demirgok, Berk
    Dept. Mechanical and Aerospace Engineering, West Virginia University, 26506 Morgantown, USA.
    Akkerman, Vyacheslav
    Dept. Mechanical and Aerospace Engineering, West Virginia University, 26506 Morgantown, USA.
    Valiev, Damir
    Umeå University, Faculty of Science and Technology, Department of Applied Physics and Electronics.
    Bychkov, Vitaly
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Acceleration and Extinction of Flames In Channels With Cold Walls2015In: Proceedings of the 25th International Colloquium on the Dynamics of Explosions and Reactive Systems / [ed] M.I. Radulescu, 2015Conference paper (Refereed)
  • 13.
    Dion, Claude
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Demirgök, Berk
    Dept. Mechanical and aerospace Engineering, West Virginia University.
    Akkerman, Vyacheslav
    Dept. Mechanical and aerospace Engineering, West Virginia University.
    Valiev, Damir
    Umeå University, Faculty of Science and Technology, Department of Applied Physics and Electronics.
    Bychkov, Vitaly
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Flames in channels with cold walls: acceleration versus extinction2015In: MCS 9, 2015Conference paper (Refereed)
    Abstract [en]

    The present work considers the problem of premixed flame front acceleration in microchannelswith smooth cold non-slip walls in the context of the deflagration-to-detonationtransition; the flame accelerates from the closed channel end to the open one. Recently, anumber of theoretical and computational papers have demonstrated the possibility of powerfulflame acceleration for micro-channels with adiabatic walls. In contrast to the previous studies,here we investigate the case of flame propagation in channels with isothermal cold walls. Theproblem is solved by using direct numerical simulations of the complete set of the Navier-Stokes combustion equations. We obtain flame extinction for narrow channels due to heat lossto the walls. However, for sufficiently wide channels, flame acceleration is found even for theconditions of cold walls in spite of the heat loss. Specifically, the flame accelerates in thelinear regime in that case. While this acceleration regime is quite different from theexponential acceleration predicted theoretically and obtained computationally for theadiabatic channels, it is consistent with the previous experimental observations, whichinevitably involve thermal losses to the walls. In this particular work, we focus on the effectof the Reynolds number of the flow on the manner of the flame acceleration.

  • 14.
    Feng, Ruixue
    et al.
    Tsinghua University.
    Zhong, Hongtao
    Tsinghua University.
    Valiev, Damir
    Umeå University, Faculty of Science and Technology, Department of Applied Physics and Electronics.
    Influence of gas expansion on the interaction between spatially periodic shear flow and premixed flame2017Conference paper (Other academic)
  • 15.
    Gruber, Andrea
    et al.
    SINTEF Energy Research, 7465 Trondheim, Norway.
    Chen, Jacqueline H.
    Combustion Research Facility, Sandia National Laboratories, Livermore, CA 94550, USA.
    Valiev, Damir
    Combustion Research Facility, Sandia National Laboratories, Livermore, CA 94550, USA.
    Law, Chung K.
    Department of Mechanical and Aerospace Engineering, Princeton University, Princeton, NJ 08544, USA.
    Direct numerical simulation of premixed flame boundary layer flashback in turbulent channel flow2012In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 709, p. 516-542Article in journal (Refereed)
    Abstract [en]

    Direct numerical simulations are performed to investigate the transient upstream propagation (flashback) of premixed hydrogen–air flames in the boundary layer of a fully developed turbulent channel flow. Results show that the well-known near-wall velocity fluctuations pattern found in turbulent boundary layers triggers wrinkling of the initially flat flame sheet as it starts propagating against the main flow direction, and that the structure of the characteristic streaks of the turbulent boundary layer ultimately has an important impact on the resulting flame shape and on its propagation mechanism. It is observed that the leading edges of the upstream-propagating premixed flame are always located in the near-wall region of the channel and assume the shape of several smooth, curved bulges propagating upstream side by side in the spanwise direction and convex towards the reactant side of the flame. These leading-edge flame bulges are separated by thin regions of spiky flame cusps pointing towards the product side at the trailing edges of the flame. Analysis of the instantaneous velocity fields clearly reveals the existence, on the reactant side of the flame sheet, of backflow pockets that extend well above the wall-quenching distance. There is a strong correspondence between each of the backflow pockets and a leading edge convex flame bulge. Likewise, high-speed streaks of fast flowing fluid are found to be always colocated with the spiky flame cusps pointing towards the product side of the flame. It is suggested that the origin of the formation of the backflow pockets, along with the subsequent mutual feedback mechanism, is due to the interaction of the approaching streaky turbulent flow pattern with the Darrieus–Landau hydrodynamic instability and pressure fluctuations triggered by the flame sheet. Moreover, the presence of the backflow pockets, coupled with the associated hydrodynamic instability and pressure–flow field interaction, greatly facilitate flame propagation in turbulent boundary layers and ultimately results in high flashback velocities that increase proportionately with pressure.

  • 16.
    Gruber, Andrea
    et al.
    SINTEF Energy Research, Trondheim, Norway.
    Kerstein, Alan R.
    SINTEF Energy Research, Trondheim, Norway.
    Valiev, Damir
    Combustion Research Facility, Livermore, CA, United States.
    Law, Chung K.
    Princeton University, Princeton, NJ, United States.
    Kolla, Hemanth
    Combustion Research Facility, Livermore, CA, United States.
    Chen, Jacqueline H.
    Combustion Research Facility, Livermore, CA, United States.
    Modeling of mean flame shape during premixed flame flashback in turbulent boundary layers2014In: Proceedings of the Combustion Institute, ISSN 0082-0784, E-ISSN 1878-027XArticle in journal (Refereed)
    Abstract [en]

    Direct numerical simulations of freely-propagating premixed flames in the turbulent boundary layer of fully-developed turbulent channel flows are used for a priori validation of a new model that aims to describe the mean shape of the turbulent flame brush during flashback. Comparison with the DNS datasets, for both fuel-lean and fuel-rich mixture conditions and for Damköhler numbers lower and larger than unity, shows that the model is able to capture the main features of the flame shape. Although further a priori and a posteriori validation is required, particularly at higher Reynolds numbers, this new simple model seems promising and can potentially have impact on the design process of industrial combustion equipment.

  • 17.
    Kodakoglu, Furkan
    et al.
    Department of Mechanical and Aerospace Engineering, West Virginia University, Morgantown, WV 26506, USA.
    Demir, Sinan
    School of Aerospace Engineering, Georgia Institute of Technology, Atlanta, GA 30332, USA .
    Valiev, Damir
    Umeå University, Faculty of Science and Technology, Department of Applied Physics and Electronics. Center for Combustion Energy, Department of Energy and Power Engineering, Tsinghua University, Beijing, 100084, China .
    Akkerman, V’yacheslav
    Department of Mechanical and Aerospace Engineering, West Virginia University, Morgantown, West Virginia 26506, USA.
    Towards Descriptive Scenario of a Burning Accident in anObstructed Mining Passage: An Analytical Approach2019Conference paper (Refereed)
  • 18.
    Modestov, Mikhail
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Bychkov, Vitaly
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Brodin, Gert
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Valiev, Damir
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Marklund, Mattias
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Matyba, Piotr
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Edman, Ludvig
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Model of the electrochemical conversion of an undoped organic semiconductor film to a doped conductor film.2010In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 81, no 8, p. 081203(R)-Article in journal (Refereed)
    Abstract [en]

    We develop a model describing the electrochemical conversion of an organic semiconductor (specifically, the active material in a light-emitting electrochemical cell) from the undoped nonconducting state to the doped conducting state. The model, an extended Nernst-Planck-Poisson model, takes into account both strongly concentration-dependent mobility and diffusion for the electronic charge carriers and the Nernst equation in the doped conducting regions. The standard Nernst-Planck-Poisson model is shown to fail in its description of the properties of the doping front. Solving our extended model numerically, we demonstrate that doping front progression in light-emitting electrochemical cells can be accurately described.

  • 19.
    Modestov, Mikhail
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Bychkov, Vitaly
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Valiev, Damir
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Marklund, Mattias
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Growth rate and the cutoff wavelength of the Darrieus-Landau instability in laser ablation2009In: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics, ISSN 1539-3755, E-ISSN 1550-2376, Vol. 80, no 4, p. 046403-046412Article in journal (Refereed)
    Abstract [en]

    The main characteristics of the linear Darrieus-Landau instability in the laser ablation flow are investigated. The dispersion relation of the instability is found numerically as a solution to an eigenvalue stability problem, taking into account the continuous structure of the flow. The results are compared to the classical Darrieus- Landau instability of a usual slow flame. The difference between the two cases is due to the specific features of laser ablation: sonic velocities of hot plasma and strong temperature dependence of thermal conduction. It is demonstrated that the Darrieus-Landau instability in laser ablation is much stronger than in the classical case. In particular, the maximum growth rate in the case of laser ablation is about three times larger than that for slow flames. The characteristic length scale of the Darrieus-Landau instability in the ablation flow is comparable to the total distance from the ablation zone to the critical zone of laser light absorption. The possibility of experimental observations of the Darrieus-Landau instability in laser ablation is discussed.

  • 20.
    Modestov, Mikhail
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Bychkov, Vitaly
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Valiev, Damir
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Marklund, Mattias
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Internal structure of planar electrochemical doping fronts in organic semiconductors2011In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 115, no 44, p. 21915-21926Article in journal (Refereed)
    Abstract [en]

    The internal structure of electrochemical doping fronts in organic semiconductors is investigated using an extended drift-diffusion model for ions, electrons, and holes. The model also involves the injection barriers for electrons and holes in the partially doped regions in the form of the Nernst equation, together with a strong dependence of the electron and hole mobility on concentrations. It is shown that the internal structure of the doping fronts is controlled by a balance between the diffusion and mobility processes. The asymptotic behavior of the concentrations and the electric field is studied analytically inside the doping fronts. The numerical solution for the front structure confirms the most important findings of the analytical theory: a sharp head of the front in the undoped region, a smooth relaxation tail in the doped region, and a plateau at the critical point of transition from doped to undoped regions. The theoretically predicted complex structure of the doping fronts is in agreement with the previous experimental data. The acceleration of the p- and n-fronts toward each other in light-emitting electrochemical cells is described. The theoretical predictions for the planar front acceleration are in a good quantitative agreement with the experimental measurements for the backside of the curved doping fronts.

  • 21.
    Qu, Zhechao
    et al.
    Umeå University, Faculty of Science and Technology, Department of Applied Physics and Electronics.
    Ghorbani, Ramin
    Umeå University, Faculty of Science and Technology, Department of Applied Physics and Electronics.
    Valiev, Damir
    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.
    Calibration-free scanned wavelength modulation spectroscopy – application to H2O and temperature sensing in flames2015In: Optics Express, ISSN 1094-4087, E-ISSN 1094-4087, Vol. 23, no 12, p. 16492-16499Article in journal (Refereed)
    Abstract [en]

    A calibration-free scanned wavelength modulation spectroscopy scheme requiring minimal laser characterization is presented. Species concentration and temperature are retrieved simultaneously from a single fit to a group of 2f/1f-WMS lineshapes acquired in one laser scan. The fitting algorithm includes a novel method to obtain the phase shift between laser intensity and wavelength modulation, and allows for a wavelengthdependent modulation amplitude. The scheme is demonstrated by detection of H2O concentration and temperature in atmospheric, premixed CH4/air flat flames using a sensor operating near 1.4 μm. The detection sensitivity for H2O at 2000 K was 4 × 10−5 cm−1 Hz-1/2, and temperature was determined with a precision of 10 K and absolute accuracy of ~50 K. A parametric study of the dependence of H2O and temperature on distance to the burner and total fuel mass flow rate shows good agreement with 1D simulations.

  • 22.
    Rutkowski, Lucile
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Johansson, Alexandra C.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Khodabakhsh, Amir
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Valiev, Damir
    Umeå University, Faculty of Science and Technology, Department of Applied Physics and Electronics.
    Lodi, Lorenzo
    Yurchenko, Sergey
    Polyansky, Oleg L.
    Tennyson, Jonathan
    Schmidt, Florian M.
    Umeå University, Faculty of Science and Technology, Department of Applied Physics and Electronics.
    Foltynowicz, Aleksandra
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Detection of OH and H2O in an Atmospheric Flame by Near-Infrared Optical Frequency Comb Spectroscopy2017In: 2017 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC), IEEE, 2017Conference paper (Refereed)
    Abstract [en]

    Absorption spectroscopy is attractive for combustion diagnostics because it allows in-situ and calibration-free quantification of reactants/products and thermometry. However, spectra measured at atmospheric pressure in the near-infrared telecom range, where laser sources and optical components are readily available, suffer from strong water interference. Cavity-enhanced optical frequency comb spectroscopy (CE-OFCS) is well suited for detection of other species, as it provides broad bandwidth with high signal-to-noise ratio and resolution, and allows de-convolving the spectra hidden among water transitions. Here we report detection of OH in the presence of H2O in an atmospheric premixed methane/air flat flame by CE-OFCS at 1.57 μm. We demonstrate a new water line list that is more accurate than HITEMP [1] and we isolate the OH lines by dividing spectra taken at different heights above the burner (HABs) to retrieve OH concentration and flame temperature.

  • 23.
    Rutkowski, Lucile
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Johansson, Alexandra C.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Valiev, Damir
    Umeå University, Faculty of Science and Technology, Department of Applied Physics and Electronics.
    Khodabakhsh, Amir
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Tkacz, Arkadiusz
    Schmidt, Florian M.
    Umeå University, Faculty of Science and Technology, Department of Applied Physics and Electronics.
    Foltynowicz, Aleksandra
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Detection of OH in an atmospheric flame at 1.5 μm using optical frequency comb spectroscopy2016In: Photonics Letters of Poland, ISSN 2080-2242, E-ISSN 2080-2242, Vol. 8, no 4, p. 110-112Article in journal (Refereed)
    Abstract [en]

    We report broadband detection of OH in a premixed CH4/air flat flame at atmospheric pressure using cavity-enhanced absorption spectroscopy based on an Er:fiber femtosecond laser and a Fourier transform spectrometer. By taking ratios of spectra measured at different heights above the burner we separate twenty OH transitions from the largely overlapping water background. We retrieve from fits to the OH lines the relative variation of OH concentration and flame temperature with a height above the burner and compare them with the 1D simulations of flame structure.

  • 24.
    Rutkowski, Lucile
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Khodabakhsh, Amir
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Johansson, Alexandra C.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Valiev, Damir M.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Lodi, Lorenzo
    Qu, Zhechao
    Umeå University, Faculty of Science and Technology, Department of Applied Physics and Electronics.
    Ghorbani, Ramin
    Umeå University, Faculty of Science and Technology, Department of Applied Physics and Electronics.
    Polyansky, Oleg L.
    Jin, Yuwei
    Tennyson, Jonathan
    Schmidt, Florian M.
    Umeå University, Faculty of Science and Technology, Department of Applied Physics and Electronics.
    Foltynowicz, Aleksandra
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Measurement of H2O and OH in a Flame by Optical Frequency Comb Spectroscopy2016In: Proceedings Conference on Lasers and Electro-Optics, 2016Conference paper (Refereed)
    Abstract [en]

    We measure broadband H2O and OH spectra in a flame using near-infrared cavity-enhanced Fourier transform optical frequency comb spectroscopy, we retrieve temperature and OH concentration, and compare water spectra to an improved line list.

  • 25.
    Ugarte, Orlando
    et al.
    Department of Mechanical and Aerospace Engineering, West Virginia University, Morgantown, West Virginia 26506-6106, USA.
    Bychkov, Vitaly
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Sadek, Jad
    Department of Mechanical and Aerospace Engineering, West Virginia University, Morgantown, West Virginia 26506-6106, USA.
    Valiev, Damir
    Umeå University, Faculty of Science and Technology, Department of Applied Physics and Electronics. Center for Combustion Energy, Tsinghua University, Beijing, China.
    Akkerman, V’yacheslav
    Department of Mechanical and Aerospace Engineering, West Virginia University, Morgantown, West Virginia 26506-6106, USA.
    Critical role of blockage ratio for flame acceleration in channels with tightly spaced obstacles2016In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 28, no 9, article id 093602Article in journal (Refereed)
    Abstract [en]

    A conceptually laminar mechanism of extremely fast flame acceleration in obstructed channels, identified by Bychkov et al. ["Physical mechanism of ultrafast flame acceleration," Phys. Rev. Lett. 101, 164501 (2008)], is further studied by means of analytical endeavors and computational simulations of compressible hydrodynamic and combustion equations. Specifically, it is shown how the obstacles length, distance between the obstacles, channel width, and thermal boundary conditions at the walls modify flamepropagation through a comb-shaped array of parallel thin obstacles. Adiabatic and isothermal (cold and preheated) side walls are considered, obtaining minor difference between these cases, which opposes the unobstructed channel case, where adiabatic and isothermal walls provide qualitatively different regimes offlame propagation. Variations of the obstructed channel width also provide a minor influence on flamepropagation, justifying a scale-invariant nature of this acceleration mechanism. In contrast, the spacing between obstacles has a significant role, although it is weaker than that of the blockage ratio (defined as the fraction of the channel blocked by obstacles), which is the key parameter of the problem. Evolution of the burning velocity and the dependence of the flame acceleration rate on the blockage ratio are quantified. The critical blockage ratio, providing the limitations for the acceleration mechanism in channels with comb-shaped obstacles array, is found analytically and numerically, with good agreement between both approaches. Additionally, this comb-shaped obstacles-driven acceleration is compared to finger flameacceleration and to that produced by wall friction.

  • 26. Ugarte, Orlando
    et al.
    Demir, Sinan
    Demirgok, Berk
    Akkerman, V'yacheslav
    Bychkov, Vitaly
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Valiev, Damir
    Umeå University, Faculty of Science and Technology, Department of Applied Physics and Electronics.
    Effect of Wall Boundary Conditions on Flame Propagation in Micro-Chambers2016In: PROCEEDINGS OF THE ASME POWER CONFERENCE, 2015, The american society of mechanical engineers , 2016, article id V001T03A009Conference paper (Refereed)
    Abstract [en]

    Flame dynamics in micro-pipes have been observed to be strongly affected by the wall boundary conditions. In this respect, two mechanisms of flame acceleration are related to the momentum transferred in these regions: 1) that associated with flame stretching produced by wall friction forces; and 2) when obstacles are placed at the walls, as a result of the delayed burning occurring between them, a jet-flow is formed, intensively promoting the flame spreading. Wall thermal conditions have usually been neglected, thus restricting the cases to adiabatic wall conditions. In contrast, in the present work, the effect of the boundary conditions on the flame propagation dynamics is investigated, computationally, with the effect of wall heat losses included in the consideration. In addition, the powerful flame acceleration attained in obstructed pipes is studied in relation to the obstacle size, which determines how different this mechanism is from the wall friction. A parametric study of two-dimensional (2D) channels and cylindrical tubes, of various radiuses, with one end open is performed. The walls are subjected to slip and non-slip, adiabatic and constant temperature conditions, with different fuel mixtures described by varying the thermal expansion coefficients. Results demonstrate that higher wall temperatures promote slower propagation as they reduce the thermal expansion rate, as a result of the post-cooling of the burn matter. In turn, smaller obstacle sizes generate weaker flame acceleration, although the mechanism is noticed to be stronger than the wall friction-driven, even for the smaller sizes considered.

  • 27.
    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.

  • 28.
    Valiev, Damir
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Bychkov, Vitaly
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Akkerman, Vyacheslav
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Eriksson, Lars-Erik
    Chalmers, Dept Appl Mech, S-41296 Gothenburg, Sweden.
    Different stages of flame acceleration from slow burning to Chapman-Jouguet deflagration2009In: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics: Statistical Physics, Plasmas, Fluids, and Related Interdisciplinary Topics, ISSN 1063-651X, E-ISSN 1095-3787, Vol. 80, no 3, p. 036317-Article in journal (Refereed)
    Abstract [en]

    Numerical simulations of spontaneous flame acceleration are performed within the problem of flame transition to detonation in two-dimensional channels. The acceleration is studied in the extremely wide range of flame front velocity changing by 3 orders of magnitude during the process. Flame accelerates from realistically small initial velocity (with Mach number about 10−3) to supersonic speed in the reference frame of the tube walls. It is shown that flame acceleration undergoes three distinctive stages: (1) initial exponential acceleration in the quasi-isobaric regime, (2) almost linear increase in the flame speed to supersonic values, and (3) saturation to a stationary high-speed deflagration velocity. The saturation velocity of deflagration may be correlated with the Chapman-Jouguet deflagration speed. The acceleration develops according to the Shelkin mechanism. Results on the exponential flame acceleration agree well with previous theoretical and numerical studies. The saturation velocity is in line with previous experimental results. Transition of flame acceleration regime from the exponential to the linear one, and then to the constant velocity, happens because of gas compression both ahead and behind the flame front.

  • 29.
    Valiev, Damir
    et al.
    Princeton University.
    Bychkov, Vitaly
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Akkerman, Vyacheslav
    Princeton University.
    Eriksson, Lars-Erik
    CTH.
    Law, Chung K.
    Princeton University.
    Quasi-steady stages in the process of premixed flame acceleration in narrow channels2013In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 25, no 9, p. 096101-Article in journal (Refereed)
  • 30.
    Valiev, Damir
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Bychkov, Vitaly
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Akkerman, V'yacheslav
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Eriksson, Lars-Erik
    Chalmers, Dept Thermo & Fluid Dynam, SE-41296 Gothenburg, Sweden.
    Marklund, Mattias
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Heating of the fuel mixture due to viscous stress ahead of accelerating flames in deflagration-to-detonation transition2008In: Physics Letters A, ISSN 0375-9601, E-ISSN 1873-2429, Vol. 372, no 27-28, p. 4850-4857Article in journal (Refereed)
    Abstract [en]

    The role of viscous stress in heating of the fuel mixture in deflagration-to-detonation transition in tubes is studied both analytically and numerically. The analytical theory is developed in the limit of low Mach number; it determines temperature distribution ahead of an accelerating flame with maximum achieved at the walls. The heating effects of viscous stress and the compression wave become comparable at sufficiently high values of the Mach number. In the case of relatively large Mach number, viscous heating is investigated by direct numerical simulations. The simulations were performed on the basis of compressible Navier-Stokes gas-dynamic equations taking into account chemical kinetics. In agreement with the theory, viscous stress makes heating and explosion of the fuel mixture preferential at the walls. The explosion develops in an essentially multi-dimensional way, with fast spontaneous reaction spreading along the walls and pushing inclined shocks. Eventually, the combination of explosive reaction and shocks evolves into detonation.

  • 31.
    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.

  • 32.
    Valiev, Damir
    et al.
    Umeå University, Faculty of Science and Technology, Department of Applied Physics and Electronics.
    Gruber, Andrea
    SINTEF Energy Research, Trondheim, Norway.
    Law, Chung K.
    Princeton University, Princeton, USA.
    Chen, Jacqueline H.
    Sandia National Laboratories, Livermore, CA, USA.
    Numerical study of interaction between Darrieus-Landau instability and spatially periodic shear flow2015In: 25th International Colloquium on the Dynamics of Explosions and Reactive Systems / [ed] M.I. Radulescu, 2015Conference paper (Refereed)
  • 33.
    Valiev, Damir
    et al.
    Department of Mechanical and Aerospace Engineering, Princeton University, Princeton, NJ 08544-5263, United States; Combustion Research Facility, Sandia National Laboratories, Livermore, CA 94550, United States.
    Law, Chung K.
    Department of Mechanical and Aerospace Engineering, Princeton University, Princeton, NJ 08544-5263, United States.
    Chen, Jacqueline H.
    Combustion Research Facility, Sandia National Laboratories, Livermore, CA 94550, United States.
    Turbulence-affected Darrieus-Landau instability2013In: 8th US National Combustion Meeting 2013, Western States Section/Combustion Institute , 2013, Vol. 1, p. 907-912Conference paper (Refereed)
    Abstract [en]

    The present work studies the combined effects of weak turbulence and intrinsic flamefront hydrodynamic (Darrieus-Landau, DL) instability on the dynamics of premixed flames. Recent modeling [A. Gruber, J.H. Chen, D. Valiev, C.K. Law, J. Fluid Mech. 709 (2012) pp. 516-542] showed that DL instability contributes to the flashback of premixed hydrogen-air flames in the boundary layer of a turbulent channel flow. A concept of a turbulence-induced DL cutoff as a function of the laminar DL cutoff and the turbulence intensity was introduced for high turbulence intensities [S. Chaudhuri, V. Akkerman, C.K. Law, Phys. Rev. E 84 (2011) art. no. 026322]. It was shown qualitatively that increasing turbulence intensity limits the instability development to large scales, thereby moderating the entire effect of the instability. In order to investigate the low turbulence intensity limit in greater detail, an initially planar flame is subjected to oncoming turbulence in direct numerical simulations (DNS) with detailed hydrogen/air chemistry. A parametric study is performed by varying the turbulence intensities and the length scale of the domain. By examining the flame front shape and velocity evolution it is found that for certain ranges of turbulence intensity, the development of the DL instability is significantly affected by turbulence. It is shown that the DL instability of the flame front that is stable in the laminar case may be triggered by weak turbulence.

  • 34.
    Valiev, Damir
    et al.
    Umeå University, Faculty of Science and Technology, Department of Applied Physics and Electronics.
    Qu, Zhechao
    Umeå University, Faculty of Science and Technology, Department of Applied Physics and Electronics.
    Steinvall, Erik
    Umeå University, Faculty of Science and Technology, Department of Applied Physics and Electronics.
    Schmidt, Florian
    Umeå University, Faculty of Science and Technology, Department of Applied Physics and Electronics.
    Measurement and simulation of atomic potassium in the plume above potassium hydroxide in a methane-air flat flame2016Conference paper (Other academic)
  • 35.
    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.

1 - 35 of 35
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