| 1. |
- Ivanov, Mikhail F., et al.
(författare)
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Ignition of deflagration and detonation ahead of the flame due to radiative preheating of suspended micro particles
- 2015
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Ingår i: Combustion and Flame. - : Elsevier BV. - 0010-2180 .- 1556-2921. ; 162:10, s. 3612-3621
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Tidskriftsartikel (refereegranskat)abstract
- We study a flame propagating in the gaseous combustible mixture with suspended inert solid micro particles. The gaseous mixture is assumed to be transparent for thermal radiation emitted by the hot combustion products, while particles absorb and reemit the radiation. Thermal radiation heats the particles, which in turn transfer the heat to the surrounding unburned gaseous mixture by means of thermal heat transfer, so that the gas phase temperature lags that of the particles. We consider different scenarios depending on the spatial distribution of the particles, their size and the number density. In the case of uniform spatial distribution of the particles the radiation causes a modest increase of the temperature ahead of the flame and corresponding modest increase of the combustion velocity. In the case of non-uniform distribution of the particles (layered dust cloud), such that the particles number density is relatively small in the region just ahead of the flame front and increases in the distant regions ahead of the flame, the preheating caused by the thermal radiation may trigger additional independent source of ignition. Far ahead of the flame, where number density of particles increases forming a dense cloud of particles, the radiative preheating results in the formation of a temperature gradient with the maximum temperature sufficient for ignition. Depending on the steepness of the temperature gradient formed in the unburned mixture, either deflagration or detonation can be initiated via the Zel'dovich's gradient mechanism. The ignition and the resulting combustion regimes depend on the number density profile and, correspondingly, on the temperature profile (temperature gradient), which is formed in effect of radiation absorption and gas-dynamic expansion. The effect of radiation preheating as stronger as smaller is the normal flame velocity. The effect of radiation heat transfer in the case of coal dust flames propagating in layered particle-gas deposits cloud can result in the spread of combustion wave with velocity up to 1000 m/s and it is a plausible explanation of the origin of dust explosion in coal mines.
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| 2. |
- Elperin, T., et al.
(författare)
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Tangling clustering instability for small particles in temperature stratified turbulence
- 2013
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Ingår i: Physics of fluids. - : AIP Publishing. - 1070-6631 .- 1089-7666. ; 25:8, s. 085104-
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Tidskriftsartikel (refereegranskat)abstract
- We study tangling clustering instability of inertial particles in a temperature stratified turbulence with small finite correlation time. It is shown that the tangling mechanism in the temperature stratified turbulence strongly increases the degree of compressibility of particle velocity field. This results in the strong decrease of the threshold for the excitation of the tangling clustering instability even for small particles. The tangling clustering instability in the temperature stratified turbulence is essentially different from the inertial clustering instability that occurs in non-stratified isotropic and homogeneous turbulence. While the inertial clustering instability is caused by the centrifugal effect of the turbulent eddies, the mechanism of the tangling clustering instability is related to the temperature fluctuations generated by the tangling of the mean temperature gradient by the velocity fluctuations. Temperature fluctuations produce pressure fluctuations and cause particle accumulations in regions with increased instantaneous pressure. It is shown that the growth rate of the tangling clustering instability is root Re (l(0)/L-T)(2)/(3Ma)(4) times larger than that of the inertial clustering instability, where Re is the Reynolds number, Ma is the Mach number, l(0) is the integral turbulence scale, and L-T is the characteristic scale of the mean temperature variations. It is found that depending on the parameters of the turbulence and the mean temperature gradient there is a preferential particle size at which the particle clustering due to the tangling clustering instability is more effective. The particle number density inside the cluster after the saturation of this instability can be by several orders of magnitude larger than the mean particle number density. It is also demonstrated that the evaporation of droplets drastically changes the tangling clustering instability, e. g., it increases the instability threshold in the droplet radius. The tangling clustering instability is of a great importance, e. g., in atmospheric turbulence with temperature inversions.
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| 3. |
- Elperin, T., et al.
(författare)
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Turbulent diffusion of chemically reacting gaseous admixtures
- 2014
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Ingår i: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics. - 1539-3755 .- 1550-2376. ; 90:5, s. 053001-
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Tidskriftsartikel (refereegranskat)abstract
- We study turbulent diffusion of chemically reacting gaseous admixtures in a developed turbulence. In our previous study [Phys. Rev. Lett. 80, 69 (1998)] using a path-integral approach for a delta-correlated in a time random velocity field, we demonstrated a strong modification of turbulent transport in fluid flows with chemical reactions or phase transitions. In the present study we use the spectral tau approximation that is valid for large Reynolds and Peclet numbers and show that turbulent diffusion of the reacting species can be strongly depleted by a large factor that is the ratio of turbulent and chemical times (turbulent Damk "ohler number). We have demonstrated that the derived theoretical dependence of a turbulent diffusion coefficient versus the turbulent Damkohler number is in good agreement with that obtained previously in the numerical modeling of a reactive front propagating in a turbulent flow and described by the Kolmogorov-Petrovskii-Piskunov-Fisher equation. We have found that turbulent cross-effects, e.g., turbulent mutual diffusion of gaseous admixtures and turbulent Dufour effect of the chemically reacting gaseous admixtures, are less sensitive to the values of stoichiometric coefficients. The mechanisms of the turbulent cross-effects differ from the molecular cross-effects known in irreversible thermodynamics. In a fully developed turbulence and at large Peclet numbers the turbulent cross-effects are much larger than the molecular ones. The obtained results are applicable also to heterogeneous phase transitions.
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| 4. |
- Ivanov, M. F., et al.
(författare)
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Influence of radiation absorption by microparticles on the flame velocity and combustion regimes
- 2015
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Ingår i: Journal of Experimental and Theoretical Physics. - 1063-7761 .- 1090-6509. ; 121:1, s. 166-178
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Tidskriftsartikel (refereegranskat)abstract
- Thermal radiation from hot combustion products has virtually no effect on the flame propagation in a gas medium. We consider a different situation when even a small concentration of microparticles suspended in a gas absorbs the thermal radiation and heats the gas mixture ahead of the combustion wave front by transferring it to the gas. The mixture heating ahead of the flame front can lead either to a moderate increase in the combustion wave velocity for a fast flame or to its significant increase for a slow flame, depending on the gas mixture reactivity and the normal laminar flame velocity. For a slow flame, the heat transfer by radiation from the combustion products can become the dominant mechanism compared to the ordinary molecular thermal conduction that determines the combustion wave structure and velocity. The radiative heating for a spatially nonuniform distribution of particles ahead of the flame front is shown to give rise to a temperature gradient that, in turn, can lead to the ignition of different combustion regimes, depending on the radiation absorption length. In accordance with the Zeldovich gradient mechanism, both deflagration and detonation regimes can be formed in this case. A hydrogen-oxygen flame is used as an example to illustrate the ignition of different combustion wave propagation regimes, depending on the radiation absorption length.
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| 5. |
- Kiverin, A. D., et al.
(författare)
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Mechanisms of ignition by transient energy deposition : Regimes of combustion wave propagation
- 2013
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Ingår i: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics. - 1539-3755 .- 1550-2376. ; 87:3, s. 033015-
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Tidskriftsartikel (refereegranskat)abstract
- Regimes of chemical reaction wave propagating in reactive gaseous mixtures, whose chemistry is governed by chain-branching kinetics, are studied depending on the characteristics of a transient thermal energy deposition localized in a finite volume of reactive gas. Different regimes of the reaction wave propagation are initiated depending on the amount of deposited thermal energy, power of the source, and the size of the hot spot. The main parameters which define regimes of the combustion waves facilitated by the transient deposition of thermal energy are acoustic time scale, duration of the energy deposition, ignition time scale, and size of the hot spot. The interplay between these parameters specifies the role of gasdynamical processes, the formation and steepness of the temperature gradient, and speed of the spontaneous wave. The obtained results show how ignition of one or another combustion regime depends on the value of energy, rate of the energy deposition, and size of the hot spot, which is important for the practical use and for risk assessment.
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| 6. |
- Liberman, Mikhail A.
(författare)
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Unsteady combustion processes controlled by detailed chemical kinetics
- 2014
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Ingår i: Active Flow and Combustion Control 2014. - Cham : Springer. - 9783319119663 - 9783319119670 ; , s. 317-341
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Bokkapitel (refereegranskat)abstract
- A correct description of unsteady, transient combustion processes controlled by chemical kinetics requires knowledge of the detailed chemical reaction mechanisms for reproducing combustion parameters in a wide range of pressures and temperatures. While models with fairly simplified gas-dynamics and a one-step Arrhenius kinetics in many cases makes possible to solve the problem in question in explicit analytical form, many important features of combustion can not be explained without account of the reactions chain nature, describing qualitatively a few major properties of the phenomena in question with some poor accuracy if any, often rendering misinterpretation of a verity of combustion phenomena. However, for modeling real three-dimensional and turbulent flows we have to use reduced chemical kinetic schemes, since the use of detailed reaction mechanisms consisting up to several hundreds species and thousands reactions is difficult or practically impossible to implement. In this lecture we consider the option of a reliable reduced chemical kinetic model for the proper understanding and interpretation of the unsteady combustion processes using hydrogen-oxygen combustion as a quintessential example of chain mechanisms in chemical kinetics. Specific topics covered several of the most fundamental unsteady combustion phenomena including: the regimes of combustion wave initiated by initial temperature non-uniformity; ignition of combustion regimes by the localized transient energy deposition; the spontaneous flame acceleration in tubes with no-slip walls; and the transition from slow combustion to detonation.
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| 7. |
- Moskalenko, S. A., et al.
(författare)
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Two-dimensional cavity polaritons under the influence of the landau quantization, Rashba spin-orbit coupling and Zeeman splitting
- 2016
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Ingår i: IFMBE Proceedings. - Singapore : Springer. - 9789812877352 ; , s. 35-39
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Konferensbidrag (refereegranskat)abstract
- The properties of the two-dimensional cavity polaritons subjected to the action of a strong perpendicular magnetic and electric fields, giving rise to the Landau quantization (LQ) of the 2D electrons and holes accompanied by the Rashba spinorbit coupling, by the Zeeman splitting and by the nonparabolicity of the heavy-hole dispersion law were investigated. Our results are based on the exact solutions for the eigenfunctions and for the eigenvalues of the Pauli- type Hamilonian with third order chirality terms for heavy-holes and with first order chirality terms for electrons. They were obtained using the method proposed by Rashba [1]. We predict the drastic changements of the optical properties of the cavity polaritons. The main of them are related with the existence of a multitude of the polariton energy levels nearly situated on the energy scale, their origin being related with the LQ of the electrons and holes. Most of these levels have the nonmonotonous dependences on the magnetic field strength B with overlapping and intersections.
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| 8. |
- Moskalenko, S. A., et al.
(författare)
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Two-dimensional cavity polaritons under the influence of the perpendicular strong magnetic and electric fields. The gyrotropy effects
- 2015
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Ingår i: Solid State Communications. - : Pergamon Press. - 0038-1098 .- 1879-2766. ; 222, s. 58-64
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Tidskriftsartikel (refereegranskat)abstract
- The properties of the two-dimensional cavity polaritons subjected to the action of a strong perpendicular magnetic and electric fields, giving rise to the Landau quantization (LQ) of the 2D electrons and holes accompanied by the Rashba spin-orbit coupling, by the Zeeman splitting and by the nonparabolicity of the heavy-hole dispersion law are investigated. We use the method proposed by Rashba (1960) [1] and the obtained results are based on the exact solutions for the eigenfunctions and for the eigenvalues of the Pauli-type Hamilonians with third order chirality terms and nonparabolic dispersion law for heavy-holes and with the first order chirality terms for electrons. The selection rules of the band-to-band optical quantum transitions as well as of the quantum transitions from the ground state of the crystal to the magnetoexciton states depend essentially on the numbers n(e) and n(h) of the LQ levels of the (e-h) pair forming the magnetoexciton. It is shown that the Rabi frequency Omega(R) of the polariton branches and the magnetoexciton oscillator strength f(osc), increase with the magnetic field strength B as Omega(R) similar to root B and f(osc) similar to B. The optical gyrotropy effects may be revealed changing the sign of the photon circular polarization at a given sign of the wave vector longitudinal projection k(z) or equivalently changing the sign of k(z) at the same selected circular polarization.
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| 9. |
- Qian, Chengeng, et al.
(författare)
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On the mechanism of “tulip flame” formation : The effect of ignition sources
- 2023
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Ingår i: Physics of fluids. - : AIP Publishing. - 1070-6631 .- 1089-7666. ; 35:11
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Tidskriftsartikel (refereegranskat)abstract
- The initial stages of hydrogen-air flame propagation in tubes and the mechanism of tulip flame formation are investigated using a high-order numerical code to solve the fully compressible reactive Navier-Stokes equations for a spark or planar igniting flame at the closed end of a tube and propagating to the opposite closed or open end. It is shown that the mechanism of tulip flame formation is universal for both sparked and planar ignited flames in tubes with both ends closed. Flame front inversion results from the tulip-shaped profile of the unburned gas axial velocity near the flame front, which is the result of the superposition of the unburned gas flow generated by the accelerating flame and the reverse flow generated by the rarefaction wave during flame deceleration. In a half-open tube, this mechanism is valid for spark ignited flames. In the case of planar ignition, there is no rarefaction wave, but the growth of bulges on the sidewalls leads to the formation of a tulip flame.
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| 10. |
- Wang, Cheng, et al.
(författare)
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Influence of chemical kinetics on detonation initiating by temperature gradients in methane/air
- 2018
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Ingår i: Combustion and Flame. - : Elsevier BV. - 0010-2180 .- 1556-2921. ; 197, s. 400-415
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Tidskriftsartikel (refereegranskat)abstract
- Different simplified and detailed chemical models and their impact on simulations of combustion regimes initiating by the initial temperature gradient in methane/air mixtures are studied. The limits of the regimes of reaction wave propagation depend upon the spontaneous wave speed and the characteristic velocities of the problem. The present study mainly focus to identify conditions required for the development a detonation and to compare the difference between simplified chemical models and detailed chemistry. It is shown that a widely used simplified chemical schemes, such as one-step, two-step and other simplified models, do not reproduce correctly the ignition process in methane/air mixtures. The ignition delay times calculated using simplified models are in orders of magnitude shorter than the ignition delay times calculated using detailed chemical models and measured experimentally. This results in considerably different times when the exothermic reaction affects significantly the ignition, evolution, and coupling of the spontaneous reaction wave and pressure waves. We show that the temperature gradient capable to trigger detonation calculated using detailed chemical models is much shallower (the size of the hot spot is much larger) than that, predicted by simulations with simplified chemical models. These findings suggest that the scenario leading to the deflagration to detonation transition (DDT) may depend greatly on the chemical model used in simulations and that the Zel'dovich gradient mechanism is not necessary a universal mechanism triggering DDT. The obtained results indicate that the conclusions derived from the simulations of DDT with simplified chemical models should be viewed with great caution.
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