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• 1.
Umeå University, Faculty of Science and Technology, Physics.
Umeå University, Faculty of Science and Technology, Physics.
Flames with Realistic Thermal Expansion in a Time-Dependent Turbulent Flow2005In: Combustion, Explosion and Shock Waves, Vol. 41, p. 363-Article in journal (Refereed)
• 2.
Umeå University, Faculty of Science and Technology, Department of Physics.
Umeå University, Faculty of Science and Technology, Department of Physics.
Velocity of weakly turbulent flames of finite thickness2005In: Combustion theory and modelling, ISSN 1364-7830, E-ISSN 1741-3559, Vol. 9, no 2, p. 323-351Article in journal (Refereed)

The velocity increase of a weakly turbulent flame of finite thickness is investigated using analytical theory developed in previous papers. The obtained velocity increase depends on the flow parameters: on the turbulent intensity, on the turbulent spectrum and on the characteristic length scale. It also depends on the thermal and chemical properties of the burning matter: thermal expansion, the Markstein number and the temperature dependence of transport coefficients. It is shown that the influence of the finite flame thickness is especially strong close to the resonance point, when the wavelength of the turbulent harmonic is equal to the cut off wavelength of the Darrieus-Landau instability. The velocity increase is almost independent of the Prandtl number. On the contrary, the Markstein number is one of the most important parameters controlling the velocity increase. The relative role of the external turbulence and the Darrieus-Landau instability for the velocity increase is studied for different parameters of the flow and the burning matter. The velocity increase for turbulent flames in methane and propane fuel mixtures is calculated for different values of the equivalence ratio. The present theoretical results are compared with previous experiments on turbulent flames. In order to perform the comparison, the theoretical results of the present paper are extrapolated to the case of a strongly corrugated flame front using the ideas of self-similar flame dynamics. The obtained theoretical results are in a reasonable agreement with the experimental data, taking into account the uncertainties of both the theory and the experiments. It is shown that in many experiments on turbulent flames the Darrieus-Landau instability is more important for the flame velocity than the external turbulence.

• 3.
Umeå University, Faculty of Science and Technology, Department of Physics.
Umeå University, Faculty of Science and Technology, Department of Physics.
Flow-flame interaction in a closed chamber2008In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 20, no 5, p. 055107-055121Article in journal (Refereed)

Numerous studies of flame interaction with a single vortex andrecent simulations of burning in vortex arrays in open tubesdemonstrated the same tendency for the turbulent burning rate$\propto U_{rms}\lambda^{2/3}$, where  $U_{rms}$ is theroot-mean-square velocity and $\lambda$ is the vortex size. Here itis demonstrated that this tendency is not universal for turbulentburning. Flame interaction with vortex arrays is investigated forthe geometry of a closed burning chamber using direct numericalsimulations of the complete set of gas-dynamic combustion equations.Various initial conditions in the chamber are considered, includinggas at rest and several systems of vortices of different intensitiesand sizes. It is found that the burning rate in a closed chamber(inverse burning time) depends strongly on the vortex intensity; atsufficiently high intensities it increases with $U_{rms}$approximately linearly in agreement with the above tendency. On thecontrary, dependence of the burning rate on the vortex size isnon-monotonic and qualitatively different from the law$\lambda^{2/3}$. It is shown that there is an optimal vortex size ina closed chamber, which provides the fastest total burning rate. Inthe present work the optimal size is 6 times smaller than thechamber height.

• 4.
Umeå University, Faculty of Science and Technology, Physics.
Umeå University, Faculty of Science and Technology, Physics.
Numerical Study of Turbulent Flame Velocity2007In: Combustion and Flame, ISSN 0010-2180, Vol. 151, no 3, p. 452-471Article in journal (Other (popular science, discussion, etc.))
• 5.
Umeå University, Faculty of Science and Technology, Physics.
Umeå University, Faculty of Science and Technology, Physics. Umeå University, Faculty of Science and Technology, Physics.
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)

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.

• 6.
Umeå University, Faculty of Science and Technology, Physics.
Umeå University, Faculty of Science and Technology, Physics. Umeå University, Faculty of Science and Technology, Physics.
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)

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.

• 7.
Nuclear Safety Institute of Russian Academy of Sciences B. Tulskaya 52, 115191 Moscow, Russia.
Department of Physics and Power Engineering, Moscow Institute of Physics and Technology, 141700 Dolgoprudny, Moscow Region, Russia. Umeå University, Faculty of Science and Technology, Department of Physics.
Turbulent flow produced by Piston Motion in a Spark-ignition engine2009In: Flow Turbulence and Combustion, ISSN 1386-6184, E-ISSN 1573-1987, Vol. 82, no 3, p. 317-337Article in journal (Refereed)

Turbulence produced by the piston motion in spark-ignition engines is studied by 2D axisymmetric numerical simulations in the cylindrical geometry as in the theoretical and experimental work by Breuer et al (Flow Turb. Combust. 74 (2005) 145, Ref. [1]). The simulations are based on the Navier-Stokes gas-dynamic equations including viscosity, thermal conduction and non-slip at the walls. Piston motion is taken into account as a boundary condition. The turbulent flow is investigated for a wide range of the engine speed, 1000-4000 rpm, assuming both zero and non-zero initial turbulence. The turbulent rms-velocity and the integral length scale are investigated in axial and radial directions. The rms-turbulent velocity is typically an order-of-magnitude smaller than the piston speed. In the case of zero initial turbulence, the flow at the top-dead-center may be described as a combination of two large-scale vortex rings of a size determined by the engine geometry. When initial turbulence is strong, then the integral turbulent length demonstrates self-similar properties in a large range of crank angles. The results obtained agree with the experimental observations of [1].

• 8.
Princeton University.
Princeton University. Umeå University, Faculty of Science and Technology, Department of Physics.
Self-similar accelerative propagation of expanding wrinkled flames and explosion triggering2011In: Physical Review E, Statistical, nonlinear and soft matter physics, ISSN 1539-3755 (print), 1550-2376 (online), Vol. 83, p. 026305-Article in journal (Refereed)

﻿The formulation of Taylor on the self-similar propagation of an expanding spherical piston with constant velocity was extended to an instability-wrinkled deflagration front undergoing acceleration with RF∝tα, where RF is the instantaneous flame radius, t the time, and α a constant exponent. The formulation describes radial compression waves pushed by the front, trajectories of gas particles, and the explosion condition in the gas upstream of the front. The instant and position of explosion are determined for a given reaction mechanism. For a step-function induction time, analytic formulas for the explosion time and position are derived, showing their dependence on the reaction and flow parameters including thermal expansion, specific heat ratio, and acceleration of the front.﻿

• 9.
Princeton University.
Princeton University. Umeå University, Faculty of Science and Technology, Department of Physics. CTH.
Analysis of flame acceleration induced by wall friction in open tubes2010In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 22, p. 053606-Article in journal (Refereed)

Spontaneous flame acceleration leading to explosion triggering in open tubes/channels due to wall friction was analytically and computationally studied. It was first demonstrated that the acceleration is affected when the thermal expansion across the flame exceeds a critical value depending on the combustion configuration. For the axisymmetric flame propagation in cylindrical tubes with both ends open, a theory of the initial (exponential) stage of flame acceleration in the quasi-isobaric limit was developed and substantiated by extensive numerical simulation of the hydrodynamics and combustion with an Arrhenius reaction. The dynamics of the flame shape, velocity, and acceleration rate, as well as the velocity profile ahead and behind the flame, have been determined.

• 10. Baikov, I. V.
Umeå University, Faculty of Science and Technology, Physics. Umeå University, Faculty of Science and Technology, Physics.
Radiation of a neutrino mechanism for type II supernovae2007In: Astronomy Reports, Vol. 51, p. 274-Article in journal (Refereed)
• 11.
Umeå University, Faculty of Science and Technology, Department of Physics.
Umeå University, Faculty of Science and Technology, Department of Physics. Umeå University, Faculty of Science and Technology, Department of Physics. Umeå University, Faculty of Science and Technology, Department of Physics. Umeå University, Faculty of Science and Technology, Department of Physics.
Magnetic Richtmyer-Meshkov instability in a two-component Bose-Einstein condensate2010In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 82, no 4, p. 043608-Article, review/survey (Refereed)
• 12.
Umeå University, Faculty of Science and Technology, Physics.
Umeå University, Faculty of Science and Technology, Physics.
Explosion triggering by an accelerating flame2006In: 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. 73, no 6, p. 066305-Article in journal (Refereed)

The analytical theory of explosion triggering by an accelerating flame is developed. The theory describes the structure of a one-dimensional isentropic compression wave pushed by the flame front. The condition of explosion in the gas mixture ahead of the flame front is derived; the instant of the explosion is determined provided that a mechanism of chemical kinetics is known. As an example, it is demonstrated how the problem is solved in the case of a single reaction of Arrhenius type, controlling combustion both inside the flame front and ahead of the flame. The model of an Arrhenius reaction with a cutoff temperature is also considered. The limitations of the theory due to the shock formation in the compression wave are found. Comparison of the theoretical results to the previous numerical simulations shows good agreement.

• 13.
Umeå University, Faculty of Science and Technology, Physics.
Umeå University, Faculty of Science and Technology, Physics. Umeå University, Faculty of Science and Technology, Physics. 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)

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.”

• 14.
Umeå University, Faculty of Science and Technology, Department of Physics.
Princeton University. Umeå University, Faculty of Science and Technology, Department of Physics. 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)
• 15.
Umeå University, Faculty of Science and Technology, Physics.
Princeton University. Umeå University, Faculty of Science and Technology, Physics. 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)

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.

• 16.
Umeå University, Faculty of Science and Technology, Department of Physics.
Umeå University, Faculty of Science and Technology, Department of Physics. Umeå University, Faculty of Science and Technology, Department of Physics. Umeå University, Faculty of Science and Technology, Department of Physics.
Nonliner dynamics of corrugated doping fronts in organic optoelectronic devices2012In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 85, no 24, p. 245212-Article in journal (Refereed)

Recently, it was demonstrated that electrochemical doping fronts in organic semiconductors exhibit a new fundamental instability growing from multidimensional perturbations [ Bychkov et al.  Phys. Rev. Lett. 107 016103 (2011)]. In the instability development, linear growth of tiny perturbations goes over into a nonlinear stage of strongly distorted doping fronts. Here we develop the nonlinear theory of the doping front instability and predict the key parameters of a corrugated doping front, such as its velocity, in close agreement with the experimental data. We show that the instability makes the electrochemical doping process considerably faster. We obtain the self-similar properties of the front shape corresponding to the maximal propagation velocity, which allows for a wide range of controlling the doping process in the experiments. The developed theory provides the guide for optimizing the performance of organic optoelectronic devices such as light-emitting electrochemical cells.

• 17.
Umeå University, Faculty of Science and Technology, Department of Physics.
Umeå University, Faculty of Science and Technology, Department of Physics. Umeå University, Faculty of Science and Technology, Department of Physics.
The Rayleigh-Taylor instability and internal waves in quantum plasmas2008In: Physics Letters A, ISSN 0375-9601, E-ISSN 1873-2429, Vol. 372, no 17, p. 3042-3045Article in journal (Refereed)

Influence of quantum effects on the internal waves and the Rayleigh-Taylor instability in plasma is investigated. It is shown that quantum pressure always stabilizes the RT instability. The problem is solved both in the limit of short-wavelength perturbations and exactly for density profiles with layers of exponential stratification. In the case of stable stratification, quantum pressure modifies the dispersion relation of the inertial waves. Because of the quantum effects, the internal waves may propagate in the transverse direction, which was impossible in the classical case. A specific form of pure quantum internal waves is obtained, which do not require any external gravitational field.

• 18.
Umeå University, Faculty of Science and Technology, Department of Physics.
Umeå University, Faculty of Science and Technology, Department of Physics. Department of Mechanical and Aerospace Engineering, Princeton University, Princeton, New Jersey 08544-5263, USA. Umeå University, Faculty of Science and Technology, Department of Physics. Umeå University, Faculty of Science and Technology, Department of Physics. Umeå University, Faculty of Science and Technology, Department of Physics. Department of Mechanical and Aerospace Engineering, Princeton University, Princeton, New Jersey 08544-5263, USA. Umeå University, Faculty of Science and Technology, Department of Physics. 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)

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.

• 19.
Umeå University, Faculty of Science and Technology, Department of Physics.
Umeå University, Faculty of Science and Technology, Department of Physics. Umeå University, Faculty of Science and Technology, Department of Physics. Department of Applied Mechanics, Chalmers University of Technology 41246 Göteborg, Sweden.
The Rayleigh-Taylor instability in inertial  fusion, astrophysical plasma and flames2007In: Plasma Physics and Controlled Fusion, ISSN 0741-3335, E-ISSN 1361-6587, Vol. 49, no 12B, p. B513-B520Article in journal (Refereed)

Previous results are reviewed and new results are presented on the Rayleigh-Taylor instability in inertial confined fusion, flames and Supernovae including gravitational and thermonuclear explosion mechanisms. The instability couples micro-scale plasma interaction with laser radiation, with neutrino, or thermonuclear reactions to large-scale hydrodynamic phenomena. In inertial fusion the instability stops target compression. In Supernovae the instability produces large-scale convection, which determines fate of the star. The instability is often accompanied by mass flux through the unstable interface, which may have both stabilizing or destabilizing influence. Destabilization happens due to the Darrieus-Landau instability of a deflagartion front. Still, it is unclear if the instabilities lead to well-organized large-scale structures (bubbles) or to relatively isotropic turbulence (mixing layer)

• 20.
Umeå University, Faculty of Science and Technology, Department of Physics.
Combustion Phenomena in Modern Physics: I. Inertial Confinement Fusion2015In: Progress in Energy and Combustion Science, ISSN 0360-1285, E-ISSN 1873-216X, Vol. 47, p. 32-59Article, review/survey (Refereed)

The overarching objective of the present endeavor is to demonstrate the universal character of combustion phenomena for various areas of modern physics, focusing on inertial confinement fusion (ICF) in this review. We present the key features of laser deflagration, and consider the similarities and differences between the laser plasma flow and the slow combustion front. We discuss the linear stage of the Rayleigh-Taylor instability in laser ablation, short-wavelength stabilization of the instability due to the mass flow, and demonstrate the importance of the concepts and methods of combustion science for an understanding of the corresponding ICF processes. We show the possibility of the Darrieus-Landau instability in the laser ablation flow and discuss the specific features of the instability at the linear and nonlinear stages as compared to the combustion counterpart of this phenomenon. We consider the nonlinear stage of the Rayleigh-Taylor instability in the ICF and generation of ultra-high magnetic field by the instability, and show that proper understanding of vorticity production in the laser plasma and, hence, of the magnetic field generation requires concepts from combustion science.

• 21.
Umeå University, Faculty of Science and Technology, Department of Physics.
Umeå University, Faculty of Science and Technology, Department of Physics. Umeå University, Faculty of Science and Technology, Department of Physics.
Magnetohydrodynamic instability in plasmas with intrinsic magnetization2010In: Physics of Plasmas, ISSN 1070-664X, E-ISSN 1089-7674, Vol. 17, no 11, p. 112107-112112Article in journal (Refereed)

From a magnetofluid description with intrinsic magnetization, a new plasma instability is obtained. The plasma magnetization is produced by the collective electron spin. The instability develops in a nonuniform plasma when the electron concentration and temperature vary along an externally applied magnetic field. Alfvén waves play an important role in the instability. The instability properties are numerically investigated for a particular example of an ultrarelativistic degenerate plasma in exploding white dwarfs.

• 22.
Umeå University, Faculty of Science and Technology, Department of Physics.
Umeå University, Faculty of Science and Technology, Department of Physics. Umeå University, Faculty of Science and Technology, Department of Physics.
The Darrieus-Landau instability in fast deflagration and laser ablation2008In: Physics of Plasmas, ISSN 1070-664X, E-ISSN 1089-7674, Vol. 15, no 3, p. 032702-Article in journal (Refereed)

The problem of the Darrieus-Landau instability at a discontinuous deflagration front in a compressible flow is solved. Numerous previous attempts to solve this problem suffered from the deficit of boundary conditions. Here, the required additional boundary condition is derived rigorously taking into account the internal structure of the front. The derived condition implies a constant mass flux at the front; it reduces to the classical Darrieus-Landau condition in the limit of an incompressible flow. It is demonstrated that in general the solution to the problem depends on the type of energy source in the flow. In the common case of a strongly localized source, compression effects make the Darrieus-Landau instability considerably weaker. Particularly, the instability growth rate is reduced for laser ablation in comparison to the classical incompressible case. The instability disappears completely in the Chapman-Jouguet regime of ultimately fast deflagration.

• 23.
Umeå University, Faculty of Science and Technology, Department of Physics.
Umeå University, Faculty of Science and Technology, Department of Physics. Umeå University, Faculty of Science and Technology, Department of Physics.
The structure of weak shocks in quantum plasmas2008In: Physics of Plasmas, ISSN 1070-664X, E-ISSN 1089-7674, Vol. 15, no 3, p. 032309-032322Article in journal (Refereed)

The structure of a weak shock in a quantum plasma is studied, taking into account both dissipation terms due to thermal conduction and dispersive quantum terms due to the Bohm potential. Unlike quantum systems without dissipations, even a small thermal conduction may lead to a stationary shock structure. In the limit of zero quantum effects, the monotonic Burgers solution for the weak shock is recovered. Still, even small quantum terms make the structure nonmonotonic with the shock driving a train of oscillations into the initial plasma. The oscillations propagate together with the shock. The oscillations become stronger as the role of Bohm potential increases in comparison with thermal conduction. The results could be of importance for laser-plasma interactions, such as inertial confinement fusion plasmas, and in astrophysical environments, as well as in condensed matter systems.

• 24.
Umeå University, Faculty of Science and Technology, Physics.
Umeå University, Faculty of Science and Technology, Physics. Umeå University, Faculty of Science and Technology, Physics.
Increase of Flame Velocity in a Rotating Gas and the Renormalization Approach to Turbulent Burning2007In: Combustion Science and Technology, ISSN 0010-2202, Vol. 179, no 7, p. 1231-1259Article in journal (Refereed)
• 25.
Umeå University, Faculty of Science and Technology, Physics.
Umeå University, Faculty of Science and Technology, Physics. Umeå University, Faculty of Science and Technology, Physics.
The Role of Bubble Motion for Turbulent Burning in Taylor-Couette Flow2006In: Focus on Combustion Research, Nova Science Publishers, Hauppauge, NY, USA , 2006, p. 187-207Chapter in book (Other academic)
• 26.
Umeå University, Faculty of Science and Technology, Physics.
Umeå University, Faculty of Science and Technology, Physics. Umeå University, Faculty of Science and Technology, Physics.
Theory and modeling of accelerating flames in tubes2005In: 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. 72, no 4, p. 046307-Article in journal (Refereed)

The analytical theory of premixed laminar flames accelerating in tubes is developed, which is an important part of the fundamental problem of flame transition to detonation. According to the theory, flames with realistically large density drop at the front accelerate exponentially from a closed end of a tube with nonslip at the walls. The acceleration is unlimited in time; it may go on until flame triggers detonation. The analytical formulas for the acceleration rate, for the flame shape and the velocity profile in the flow pushed by the flame are obtained. The theory is validated by extensive numerical simulations. The numerical simulations are performed for the complete set of hydrodynamic combustion equations including thermal conduction, viscosity, diffusion, and chemical kinetics. The theoretical predictions are in a good agreement with the numerical results. It is also shown how the developed theory can be used to understand acceleration of turbulent flames.

• 27.
Umeå University, Faculty of Science and Technology, Physics.
Umeå University, Faculty of Science and Technology, Physics.
Dynamics of bubbles in supernovae and turbulent vortices2006In: Astronomy Reports, Vol. 50, p. 298-Article in journal (Refereed)
• 28.
Umeå University, Faculty of Science and Technology, Department of Physics.
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)

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.

• 29.
Umeå University, Faculty of Science and Technology, Department of Physics.
Umeå University, Faculty of Science and Technology, Department of Physics. 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)

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.

• 30. Denet, B.
Umeå University, Faculty of Science and Technology, Physics.
Low Vorticity and Small Gas Expansion in Premixed Flames2005In: Combustion Science and Technology, Vol. 177, p. 1543-Article in journal (Refereed)
• 31.
Umeå University, Faculty of Science and Technology, Department of Physics.
Dept. Mechanical and Aerospace Engineering, West Virginia University, 26506 Morgantown, USA. Dept. Mechanical and Aerospace Engineering, West Virginia University, 26506 Morgantown, USA. Umeå University, Faculty of Science and Technology, Department of Applied Physics and Electronics. 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)
• 32.
Umeå University, Faculty of Science and Technology, Department of Physics.
Dept. Mechanical and aerospace Engineering, West Virginia University. Dept. Mechanical and aerospace Engineering, West Virginia University. Umeå University, Faculty of Science and Technology, Department of Applied Physics and Electronics. Umeå University, Faculty of Science and Technology, Department of Physics.
Flames in channels with cold walls: acceleration versus extinction2015In: MCS 9, 2015Conference paper (Refereed)

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.

• 33.
Umeå University, Faculty of Science and Technology, Department of Physics.
Umeå University, Faculty of Science and Technology, Department of Physics. Nordita. Umeå University, Faculty of Science and Technology, Department of Physics. Umeå University, Faculty of Science and Technology, Department of Physics.
Anisotropic properties of spin avalanches in crystals of nanomagnets2013In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 87, no 1, article id 014409Article in journal (Refereed)

Anisotropy effects for spin avalanches in crystals of nanomagnets are studied theoretically with the external magnetic field applied at an arbitrary angle to the easy axis. Starting with the Hamiltonian for a single nanomagnet in the crystal, two essential quantities characterizing spin avalanches are calculated: the activation and Zeeman energies. The calculation is performed numerically for a wide range of angles and analytical formulas are derived within the limit of small angles. The anisotropic properties of a single nanomagnet lead to anisotropic behavior of the magnetic deflagration speed. Modifications of the magnetic deflagration speed are investigated for different angles between the external magnetic field and the easy axis of the crystals. Anisotropic properties of magnetic detonation are also studied, which concern, first of all, the temperature behind the leading shock and the characteristic time of spin switching in the detonation.

• 34.
Umeå University, Faculty of Science and Technology, Department of Physics.
Umeå University, Faculty of Science and Technology, Department of Physics. Umeå University, Faculty of Science and Technology, Department of Physics. Department of Applied Physics, Chalmers University of Technology, Göteborg, Sweden. Umeå University, Faculty of Science and Technology, Department of Physics.
Multidimensional instability and dynamics of spin-avalanches in crystals of nanomagnets2014In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 113, no 21, article id 217206Article in journal (Refereed)

We obtain a fundamental instability of the magnetization-switching fronts in superparamagnetic and ferromagnetic materials such as crystals of nanomagnets, ferromagnetic nanowires, and systems of quantum dots with large spin. We develop the instability theory for both linear and nonlinear stages. By using numerical simulations we investigate the instability properties focusing on spin avalanches in crystals of nanomagnets. The instability distorts spontaneously the fronts and leads to a complex multidimensional front dynamics. We show that the instability has a universal physical nature, with a deep relationship to a wide variety of physical systems, such as the Darrieus-Landau instability of deflagration fronts in combustion, inertial confinement fusion, and thermonuclear supernovae, and the instability of doping fronts in organic semiconductors.

• 35.
Umeå University, Faculty of Science and Technology, Department of Physics.
Umeå University, Faculty of Science and Technology, Department of Physics. Chalmers, Dept Appl Phys, SE-41296 Gothenburg, Sweden. Umeå University, Faculty of Science and Technology, Department of Physics.
Magnetic detonation structure in crystals of nanomagnets controlled by thermal conduction and volume viscosity2015In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 91, no 9, article id 094428Article in journal (Refereed)

Experimentally detected ultrafast spin avalanches spreading in crystals of molecular (nano) magnets [Decelle et al., Phys. Rev. Lett. 102, 027203 (2009)] have recently been explained in terms of magnetic detonation [Modestov et al., Phys. Rev. Lett. 107, 207208 (2011)]. Here magnetic detonation structure is investigated by taking into account transport processes of the crystals such as thermal conduction and volume viscosity. The transport processes result in smooth profiles of the most important thermodynamical crystal parameters, temperature, density, and pressure, all over the magnetic detonation front, including the leading shock, which is one of the key regions of magnetic detonation. In the case of zero volume viscosity, thermal conduction leads to an isothermal discontinuity instead of the shock, for which temperature is continuous while density and pressure experience jump. It is also demonstrated that the thickness of the magnetic detonation front may be controlled by applying the transverse-magnetic field, which is important for possible experimental observations of magnetic detonation.

• 36.
Umeå University, Faculty of Science and Technology, Department of Physics.
Institute for Theoretical Physics, Utrecht University, The Netherlands. Umeå University, Faculty of Science and Technology, Department of Physics. Department of Applied Physics, Division of Condensed Matter Theory, Chalmers University of Technology. Umeå University, Faculty of Science and Technology, Department of Physics.
Turbulence in binary Bose-Einstein condensates generated by highly nonlinear Rayleigh-Taylor and Kelvin-Helmholtz instabilities2014In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 89, p. 013631-Article in journal (Refereed)

Quantum turbulence (QT) generated by the Rayleigh-Taylor instability in binary immiscible ultracold 87Rb atoms at zero temperature is studied theoretically. We show that the quantum vortex tangle is qualitatively different from previously considered superfluids, which reveals deep relations between QT and classical turbulence. The present QT may be generated at arbitrarily small Mach numbers, which is a unique property not found in previously studied superfluids. By numerical solution of the coupled Gross-Pitaevskii equations we find that the Kolmogorov scaling law holds for the incompressible kinetic energy. We demonstrate that the phenomenon may be observed in the laboratory.

• 37.
Umeå University, Faculty of Science and Technology, Department of Physics.
Umeå University, Faculty of Science and Technology, Department of Physics. Umeå University, Faculty of Science and Technology, Department of Physics. Umeå University, Faculty of Science and Technology, Department of Physics.
Quantum swapping of immiscible Bose-Einstein condensates as an alternative to the Rayleigh-Taylor instability2012In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 85, no 1, p. 013630-Article, review/survey (Refereed)

We consider a two-component Bose-Einstein condensate in a quasi-one-dimensional harmonic trap, where the immiscible components are pressed against each other by an external magnetic force. The zero-temperature nonstationary Gross-Pitaevskii equations are solved numerically; analytical models are developed for the key steps in the process. We demonstrate that if the magnetic force is strong enough, then the condensates may swap their places in the trap due to dynamic quantum interpenetration of the nonlinear matter waves. The swapping is accompanied by development of a modulational instability leading to quasiturbulent excitations. Unlike the multidimensional Rayleigh-Taylor instability in a similar geometry of two-component quantum fluid systems, quantum interpenetration has no classical analog. In a two-dimensional geometry a crossover between the Rayleigh-Taylor instability and the dynamic quantum interpenetration is investigated.

• 38.
Umeå University, Faculty of Science and Technology, Department of Physics.
Umeå University, Faculty of Science and Technology, Department of Physics. Umeå University, Faculty of Science and Technology, Department of Physics. Umeå University, Faculty of Science and Technology, Department of Physics.
Parametric resonance of capillary waves at the interface between two immiscible Bose-Einstein condensates2012In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 86, no 2, p. 023614-Article in journal (Refereed)

We study the parametric resonance of capillary waves on the interface between two immiscible Bose-Einstein condensates pushed towards each other by an oscillating force. Guided by analytical models, we solve numerically the coupled Gross-Pitaevskii equations for a two-component Bose-Einstein condensate at zero temperature. We show that, at moderate amplitudes of the driving force, the instability is stabilized due to nonlinear modifications of the oscillation frequency. When the amplitude of the driving force is large enough, we observe a detachment of droplets from the Bose-Einstein condensates, resulting in the generation of quantum vortices (skyrmions). We analytically investigate the vortex dynamics, and conditions of quantized vortex generation.

• 39.
Umeå University, Faculty of Science and Technology, Department of Physics.
Umeå University, Faculty of Science and Technology, Department of Physics. Umeå University, Faculty of Science and Technology, Department of Physics. Umeå University, Faculty of Science and Technology, Department of Physics. Princeton University. Umeå University, Faculty of Science and Technology, Department of Physics.
Interface dynamics of a two-component Bose-Einstein condensate driven by an external force2011In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 83, no 4, p. 043623-Article in journal (Refereed)

The dynamics of an interface in a two-component Bose-Einstein condensate driven by a spatially uniform time-dependent force is studied. Starting from the Gross-Pitaevskii Lagrangian, the dispersion relation for linear waves and instabilities at the interface is derived by means of a variational approach. A number of diverse dynamical effects for different types of driving force is demonstrated, which includes the Rayleigh-Taylor instability for a constant force, the Richtmyer-Meshkov instability for a pulse force, dynamic stabilization of the Rayleigh-Taylor instability and onset of the parametric instability for an oscillating force. Gaussian Markovian and non-Markovian stochastic forces are also considered. It is found that the Markovian stochastic force does not produce any average effect on the dynamics of the interface, while the non-Markovian force leads to exponential perturbation growth.

• 40. Modestov, M.
Umeå University, Faculty of Science and Technology, Department of Physics. Umeå University, Faculty of Science and Technology, Department of Physics. Umeå University, Faculty of Science and Technology, Department of Physics. Department of Applied Physics, Chalmers University of Technology, Gothenburg, Sweden.
Evolution of the magnetic field generated by the Kelvin-Helmholtz instability2014In: Physics of Plasmas, ISSN 1070-664X, E-ISSN 1089-7674, Vol. 21, no 7, p. 072126-Article in journal (Refereed)

The Kelvin-Helmholtz instability in an ionized plasma is studied with a focus on the magnetic field generation via the Biermann battery (baroclinic) mechanism. The problem is solved by using direct numerical simulations of two counter-directed flows in 2D geometry. The simulations demonstrate the formation of eddies and their further interaction and merging resulting in a large single vortex. In contrast to general belief, it is found that the instability generated magnetic field may exhibit significantly different structures from the vorticity field, despite the mathematically identical equations controlling the magnetic field and vorticity evolution. At later stages of the nonlinear instability development, the magnetic field may keep growing even after the hydrodynamic vortex strength has reached its maximum and started decaying due to dissipation.

• 41.
Umeå University, Faculty of Science and Technology, Department of Physics.
Umeå University, Faculty of Science and Technology, Department of Physics. Fusion Science Center and Laboratory for Laser Energetics, University of Rochester, 250 East River Road, Rochester, New York 14623, USA . Department of Applied Mechanics, Chalmers University of Technology, 412 96 Göteborg, Sweden.
Bubble velocity in the nonlinear Rayleigh-Taylor instability at a deflagration front2008In: Physics of Plasmas, ISSN 1070-664X, E-ISSN 1089-7674, Vol. 15, no 4, p. 042703-042715Article in journal (Refereed)

The Rayleigh-Taylor instability at a deflagration front is studiedsystematically using extensive direct numerical simulations.  Itis shown that, for a sufficiently large gravitational field, theeffects of bubble rising dominate the deflagration dynamics. Itis demonstrated both analytically and numerically that thedeflagration speed is described asymptotically by the Layzertheory in the limit of large acceleration. In the opposite limitof small and zero gravitational field, intrinsic properties of thedeflagration front become important. In that case, the deflagrationspeed is determined by the velocity of a planar front and by theDarrieus-Landau instability. Because of these effects, thedeflagration speed is larger than predicted by theLayzer theory. An analytical formula for the deflagration speedis suggested, which matches two asymptotic limits of large andsmall acceleration. The formula is in good agreement withthe numerical data in a wide range of Froude numbers. Thepresent results are also in agreement with previous numericalsimulations on this problem.

• 42.
Umeå University, Faculty of Science and Technology, Department of Physics.
Umeå University, Faculty of Science and Technology, Department of Physics. Umeå University, Faculty of Science and Technology, Department of Physics. Umeå University, Faculty of Science and Technology, Department of Physics. Umeå University, Faculty of Science and Technology, Department of Physics. Umeå University, Faculty of Science and Technology, Department of Physics. 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)

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.

• 43.
Umeå University, Faculty of Science and Technology, Department of Physics.
Umeå University, Faculty of Science and Technology, Department of Physics. Umeå University, Faculty of Science and Technology, Department of Physics.
Pulsating regime of magnetic deflagration in crystals of molecular magnets2011In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 83, no 21, p. 214417-214416Article in journal (Refereed)

The stability of a magnetic deflagration front in a media of molecular magnets, such as Mn12 acetate, is considered. It is demonstrated that stationary deflagration is unstable with respect to one-dimensional perturbations if the energy barrier of the magnets is sufficiently high in comparison with the release of Zeeman energy at the front; their ratio may be interpreted as an analog to the Zeldovich number, as found in problems of combustion. When the Zeldovich number exceeds a certain critical value, a stationary deflagration front becomes unstable and propagates in a pulsating regime. Analytical estimates for the critical Zeldovich number are obtained. The linear stage of the instability is investigated numerically by solving the eigenvalue problem. The nonlinear stage is studied using direct numerical simulations. The parameter domain required for experimental observations of the pulsating regime is discussed.

• 44.
Umeå University, Faculty of Science and Technology, Department of Physics.
Umeå University, Faculty of Science and Technology, Department of Physics. Umeå University, Faculty of Science and Technology, Department of Physics.
The Rayleigh-Taylor instability in quantum magnetized plasma with para- and ferromagnetic properties2009In: Physics of Plasmas, ISSN 1070-664X, E-ISSN 1089-7674, Vol. 16, no 3, p. 032106-032117Article in journal (Refereed)

We investigate influence of magnetic field on the Rayleigh–Taylor instability in quantum plasmas with para- and ferromagnetic properties. Magnetization of quantum plasma happens due to the collective electron spin behavior at low temperature and high plasma density. In the classical case, without magnetization, magnetic field tends to stabilize plasma perturbations with wave numbers parallel to the field and with sufficiently short wavelengths. Paramagnetic effects in quantum plasma make this stabilization weaker. The stabilization disappears completely for short wavelength perturbations in the ferromagnetic limit, when the magnetic field is produced by intrinsic plasma magnetization only. Still, for perturbations of long and moderate wavelength, certain stabilization always takes place due to the nonlinear character of quantum plasma magnetization.

• 45.
Umeå University, Faculty of Science and Technology, Department of Physics.
Umeå University, Faculty of Science and Technology, Department of Physics. Umeå University, Faculty of Science and Technology, Department of Physics.
Ultrafast Spin Avalanches in Crystals of Nanomagnets in Terms of Magnetic Detonation2011In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 107, no 20, p. 207208-Article in journal (Refereed)

Recent experiments [W. Decelle et al., Phys. Rev. Lett. 102 027203 (2009)] have discovered ultrafast propagation of spin avalanches in crystals of nanomagnets, which is 3 orders of magnitude faster than the traditionally studied magnetic deflagration. The new regime has been hypothetically identified as magnetic detonation. Here we demonstrate unequivocally the possibility of magnetic detonation in the crystals, as a front consisting of a leading shock and a zone of Zeeman energy release. We study the key features of the process and find that the magnetic detonation speed only slightly exceeds the sound speed in agreement with the experimental observations. For combustion science, our results provide a unique physical example of extremely weak detonation.

• 46.
Umeå University, Faculty of Science and Technology, Department of Physics.
Umeå University, Faculty of Science and Technology, Department of Physics. Umeå University, Faculty of Science and Technology, Department of Physics. 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)

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.

• 47.
Umeå University, Faculty of Science and Technology, Department of Physics.
Umeå University, Faculty of Science and Technology, Department of Physics. Umeå University, Faculty of Science and Technology, Department of Physics. 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)

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.

• 48.
Umeå University, Faculty of Science and Technology, Physics.
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)
• 49.
Umeå University, Faculty of Science and Technology, Physics.
Umeå University, Faculty of Science and Technology, Physics. Umeå University, Faculty of Science and Technology, Physics.
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)

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.

• 50.
Umeå University, Faculty of Science and Technology, Physics.
Umeå University, Faculty of Science and Technology, Physics. Umeå University, Faculty of Science and Technology, Physics.
Violent folding of a flame front in a flame-acoustic resonance2006In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 97, no 16, p. 164501-Article in journal (Refereed)

The first direct numerical simulations of violent flame folding because of the flame-acoustic resonance are performed. Flame propagates in a tube from an open end to a closed one. Acoustic amplitude becomes extremely large when the acoustic mode between the flame and the closed tube end comes in resonance with intrinsic flame oscillations. The acoustic oscillations produce an effective acceleration field at the flame front leading to a strong Rayleigh-Taylor instability during every second half period of the oscillations. The Rayleigh-Taylor instability makes the flame front strongly corrugated with elongated jets of heavy fuel mixture penetrating the burnt gas and even with pockets of unburned matter separated from the flame front

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