236 research outputs found
Rankine-Hugoniot Relations in Relativistic Combustion Waves
As a foundational element describing relativistic reacting waves of relevance
to astrophysical phenomena, the Rankine-Hugoniot relations classifying the
various propagation modes of detonation and deflagration are analyzed in the
relativistic regime, with the results properly degenerating to the
non-relativistic and highlyrelativistic limits. The existence of
negative-pressure downstream flows is noted for relativistic shocks, which
could be of interest in the understanding of the nature of dark energy. Entropy
analysis for relativistic shock waves are also performed for relativistic
fluids with different equations of state (EoS), denoting the existence of
rarefaction shocks in fluids with adiabatic index \Gamma < 1 in their EoS. The
analysis further shows that weak detonations and strong deflagrations, which
are rare phenomena in terrestrial environments, are expected to exist more
commonly in astrophysical systems because of the various endothermic reactions
present therein. Additional topics of relevance to astrophysical phenomena are
also discussed.Comment: 34 pages, 9 figures, accepted for publication in Ap
Pointwise Green function bounds and stability of combustion waves
Generalizing similar results for viscous shock and relaxation waves, we
establish sharp pointwise Green function bounds and linearized and nonlinear
stability for traveling wave solutions of an abstract viscous combustion model
including both Majda's model and the full reacting compressible Navier--Stokes
equations with artificial viscosity with general multi-species reaction and
reaction-dependent equation of state, % under the necessary conditions of
strong spectral stability, i.e., stable point spectrum of the linearized
operator about the wave, transversality of the profile as a connection in the
traveling-wave ODE, and hyperbolic stability of the associated Chapman--Jouguet
(square-wave) approximation. Notably, our results apply to combustion waves of
any type: weak or strong, detonations or deflagrations, reducing the study of
stability to verification of a readily numerically checkable Evans function
condition. Together with spectral results of Lyng and Zumbrun, this gives
immediately stability of small-amplitude strong detonations in the small
heat-release (i.e., fluid-dynamical) limit, simplifying and greatly extending
previous results obtained by energy methods by Liu--Ying and Tesei--Tan for
Majda's model and the reactive Navier--Stokes equations, respectively
Canonical solutions for unsteady flow fields
The initial value problem of one-dimensional gas-dynamics involving discontinuous, nonuniform initial data is discussed. Canonical solutions which are valid in a small x, t region aroung a discontinuity, and which include the first order effects of nonuniformities in the data, are derived explicitly. The theory is derived by considering a group of elementary piston problems. Solutions with a shock or with a centered expansion wave are worked out individually in order to relate initial flow properties and their gradients to the speed and acceleration of the discontinuity waves. They are then combined to represent the solution of a general initial value problem by regarding the piston path as a contact line. In addition, problems with chemical reaction are discussed in terms of elementary piston problems which involve strong detonation waves, Chapman-Jouguet detonation waves, and deflagration waves
Stability of cosmological detonation fronts
The steady state propagation of a phase transition front is classified,
according to hydrodynamics, as a deflagration or a detonation, depending on its
velocity with respect to the fluid. These propagation modes are further divided
into three types, namely, weak, Jouguet, and strong solutions, according to
their disturbance of the fluid. However, some of these hydrodynamic modes will
not be realized in a phase transition. One particular cause is the presence of
instabilities. In this work we study the linear stability of weak detonations,
which are generally believed to be stable. After discussing in detail the weak
detonation solution, we consider small perturbations of the interface and the
fluid configuration. When the balance between the driving and friction forces
is taken into account, it turns out that there are actually two different kinds
of weak detonations, which behave very differently as functions of the
parameters. We show that the branch of stronger weak detonations are unstable,
except very close to the Jouguet point, where our approach breaks down.Comment: 34 pages, 11 figures. v2: typos corrected and minor change
LES study of deflagration to detonation mechanisms in a downsized spark ignition engine
Using 15 LES cycles of a high load/low speed spark ignition engine operating point, two different fresh gases autoignition regimes called knock and super-knock are analyzed. A direct “a posteriori” analysis of pressure waves and autoignition heat release observed in LES is proposed. It reveals that low to moderate knock intensity, corresponding to late spark timings (ST) is characterized by one or several random autoignition (AI) spots which consume the surrounding fresh gases without coupling with the AI heat release. On the contrary, the highest knock intensities correspond to what is usually called super-knock, a very intense knock observed under pre-ignition conditions or for very early ST, as done in this study. LES shows that the pressure waves generated by one or a couple of AI spots are strong enough to induce locally a strong fresh gases temperature increase leading itself to a substantial decrease of the AI delay. This allows to generate a coupling between the pressure wave and the AI reaction rate which reinforce each other, leading to maximum pressures and propagation speeds close to those of a detonation. These results therefore strongly support the hypothesis proposed in the literature that super-knock is characterized by a deflagration to detonation transition (DDT). An “a priori” analysis is also performed thanks to the use of a local detonation indicator based on Bradley’s DDT diagram. It is shown that this tool not only predicts the change of combustion regime as a function of the ST, but it also roughly succeeds in predicting the location and time of appearance of the DDT in the chamber. Unfortunately, the first AI spot is not always responsible for the DDT, implying that using cold flow LES to calculate the detonation indicator instead of a reacting LES as proposed here, would lead to a failure of the indicator in many cases
General concept for autoignitive reaction wave covering from subsonic to supersonic regimes
We consider a one-dimensional (1D) autoignitive reaction wave in reactive
flow system comprising unburned premixed gas entering from the inlet boundary
and burned gas exiting from the outlet boundary. In such a 1D system at given
initial temperature, it is generally accepted that steady-state solutions can
only exist if the inlet velocity matches either the velocity of deflagration
wave, as determined by the burning rate eigenvalue in the subsonic regime or
the velocity of detonation wave as dictated by the Chapman-Jouguet (CJ)
condition in the supersonic regime. In this study, we developed the general
concept of "autoignitive reaction wave" and theoretically demonstrate that two
distinct regimes that can maintain steady-state solutions both in subsonic and
supersonic conditions. Based on this theory, we selected inlet velocities that
are predicted to yield either steady-state or flashback solutions, and
conducted numerical simulations. This novel approach revealed that steady-state
solutions are possible not only at the velocity of the deflagration wave in the
subsonic regime and the velocity of the detonation wave in the supersonic
regime, but also across a broad range of inlet velocities. Furthermore, we
identify a highly stable "autoignitive reaction wave" that emerges when the
inlet velocity surpasses the velocity of detonation wave, devoid of the typical
shock wave commonly seen in detonation waves. This "supersonic autoignitive
reaction wave" lacks the instability-inducing detonation cell structure,
suggesting the potential for the development of novel combustor concepts.Comment: Prior to publication please use: "The following article has been
submitted to Physics of Fluids. After it is published, it will be found at
Link.
Detonations and deflagrations in cosmological phase transitions
We study the steady state motion of bubble walls in cosmological phase
transitions. Taking into account the boundary and continuity conditions for the
fluid variables, we calculate numerically the wall velocity as a function of
the nucleation temperature, the latent heat, and a friction parameter. We
determine regions in the space of these parameters in which detonations and/or
deflagrations are allowed. In order to apply the results to a physical case, we
calculate these quantities in a specific model, which consists of an extension
of the Standard Model with singlet scalar fields. We also obtain analytic
approximations for the wall velocity, both in the case of deflagrations and of
detonations.Comment: 31 pages, 14 figures. v2: several clarifications added, a change of
notation. v3: reference added. Version to appear in Nucl. Phys.
Quasi-steady stages in the process of premixed flame acceleration in narrow channels
The present paper addresses the phenomenon of spontaneous acceleration of a pre-mixed flame front propagating in micro-channels, with subsequent deflagration-to-detonation transition. It has recently been shown experimentally [M. Wu, M. Burke, S. Son, and R. Yetter, Proc. Combust. Inst. 31, 2429 (2007)], computationally [D. Valiev, V. Bychkov, V. Akkerman, and L.-E. Eriksson, Phys. Rev. E 80, 036317 (2009)], and analytically [V. Bychkov, V. Akkerman, D. Valiev, and C. K. Law, Phys. Rev. E 81, 026309 (2010)] that the flame acceleration undergoes different stages, from an initial exponential regime to quasi-steady fast deflagration with saturated velocity. The present work focuses on the final saturation stages in the process of flame acceleration, when the flame propagates with supersonic velocity with respect to the channel walls. It is shown that an intermediate stage may occur during acceleration with quasi-steady velocity, noticeably below the Chapman-Jouguet deflagration speed. The intermediate stage is followed by additional flame acceleration and subsequent saturation to the Chapman-Jouguet deflagration regime. We elucidate the intermediate stage by the joint effect of gas pre-compression ahead of the flame front and the hydraulic resistance. The additional acceleration is related to viscous heating at the channel walls, being of key importance at the final stages. The possibility of explosion triggering is also demonstrated
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