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- Liberman, M. A., et al.
(författare)
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Formation of the preheated zone ahead of a propagating flame and the mechanism underlying the deflagration-to-detonation transition
- 2009
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Ingår i: Physics Letters A. - : Elsevier BV. - 0375-9601 .- 1873-2429. ; 373:5, s. 501-510
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Tidskriftsartikel (refereegranskat)abstract
- The Letter presents analytical, numerical and experimental studies of the mechanism underlying the deflagration-to-detonation transition (DDT). Insight into how, when, and where DDT occurs is obtained by analyzing analytically and by means of multidimensional numerical simulations dynamics of a flame accelerating in a tube with no-slip walls. It is shown that the deflagration-to-detonation transition exhibits three separate stages of evolution corroborating majority experimental observations. During the first stage flame accelerates and generates shocks far ahead of the flame front. During the second stage the flame slows down, shocks are formed in the immediate proximity of the flame front and the preheated zone ahead of the flame front is created. The third stage is self-restructuring of the steep temperature profile within the flame, formation of a reactivity gradient and the actual formation of the detonation wave itself. The mechanism for the detonation wave formation, given an appropriate formation of the preheated zone, seems to be universal and involves a reactivity gradient formed from the initially steep flame temperature profile in the presence of the preheated zone. The developed theory and numerical simulations are found to be well consistent with extensive experiments of the DDT in hydrogen-oxygen and ethylene-oxygen mixtures in tubes with smooth and rough walls.
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- Liberman, M.A., et al.
(författare)
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On the Mechanism of Detonation Initiation in Unstable Flames
- 2006
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Ingår i: Combust Theory and Modeling.
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Tidskriftsartikel (refereegranskat)abstract
- The flame folding, induced by the classical Darrieus-Landau instability, is identified as a possible mechanism for the spontaneous transition from deflagration to detonation. A numerical simulation of premixed gas combustion spreading from the closed end of a semi-infinite, smooth-walled channel is performed. It is found that in sufficiently wide channels the Darrieus-Landau instability may invoke nucleation of hot spots within the folds of the developing wrinkled flame, triggering an abrupt transition from deflagrative to detonative combustion. The mechanism of the transition is the temperature increase due to the influx of heat from the folded reaction zone, followed by autoignition. The transition occurs when the pressure elevation at the accelerating reaction front becomes high enough to produce a shock capable of supporting detonation. The transition is found to be sensitive to the flame’s incipient speed and the reaction rate molecularity, favoring fast flames and high order reactions. To gain a better insight into the mechanism of the transition, a one-dimensional model is explored whose basic characteristics are borrowed from the two-dimensional system. Zel’dovich’s theory of soft initiation, requiring the fresh mixture to be appropriately preconditioned, is substantiated.
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- Moskalenko, S. A., et al.
(författare)
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Intra-Landau-level excitations of the two-dimensional electron-hole liquid
- 2009
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Ingår i: Journal of Physics. - : IOP Publishing. - 0953-8984 .- 1361-648X. ; 21:23, s. 235801-
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Tidskriftsartikel (refereegranskat)abstract
- The intra-Landau-level excitations of the two-dimensional electron-hole liquid are characterized by two branches of the energy spectrum. The acoustical plasmon branch with in-phase oscillations of electrons and holes has a linear dispersion law in the range of small wavevectors, with a velocity which does not depend on the magnetic field strength, and monotonically increases with saturation at higher values of the wavevectors. The optical plasmon branch with oscillations of electrons and holes in opposite phases has a quadratic dependence in the range of long wavelength, a weak roton-type behaviour at the intermediary values of the wavevectors and monotonically increases with saturation similar to the case of the acoustical branch. The influence of the supplementary in-plane electric field leads to the drift of the charged particles in the crossed electric and magnetic fields and to the energy spectrum as in the reference frame, where the e-h system is moving with the drift velocity. A perturbation theory using the Green function method is developed on the basis of a small parameter v(2)( 1 - v(2)), where v(2) is the filling factor and ( 1 - v(2)) displays the phase space filling effect.
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