710 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
Magnetic reconnection detonation in supernova remnants
As a key process that refreshes the interstellar medium, the dynamics and
radiative properties of the supernova remnant (SNR) expansion front not only
reflect the physical environment of the old interstellar medium (ISM)
surrounding the supernova, but they also provide information about the
refreshed ISM. However the expansion dynamics of SNRs cannot be simply
explained by the conventional law of spherical shock wave propagation; on the
other hand, the high energy radiation requires an additional electron
acceleration mechanism in the shock front beyond thermal collision. We consider
herein the detonation wave description of the SNR expansion, in which magnetic
reconnection follows the shock front and transfers the SNR magnetic field
energy to both fluid thermal energy and particle kinetic energy. The structure
of the magnetic reconnection detonation (MRD) is identified based on scaling
analysis in this paper. By applying the MRD description of the SNR expansion
shock to the example of the Crab Nebula, this paper shows that the MRD
description can explain both the accelerative expansion of the nebula as well
as the origin of the luminous expanding shell.Comment: Accepted for publication in Ap
On the Structure and Stabilization Mechanisms of Planar and Cylindrical Premixed Flames
The configurational simplicity of the stationary one-dimensional flames renders them intrinsically attractive for fundamental flame structure studies. The possibility and fidelity of studies of such flames on earth, however, have been severely restricted by the unidirectional nature of the gravity vector. To demonstrate these complications, let us first consider the premixed flame. Here a stationary, one-dimensional flame can be established by using the flat-flame burner. We next consider nonpremixed flames. First it may be noted that in an unbounded gravity-free environment, the only stationary one-dimensional flame is the spherical flame. Indeed, this is a major motivation for the study of microgravity droplet combustion, in which the gas-phase processes can be approximated to be quasi-steady because of the significant disparity between the gas and liquid densities for subcritical combustion. In view of the above considerations, an experimental and theoretical program on cylindrical and spherical premixed and nonpremixed flames in microgravity has been initiated. For premixed flames, we are interested in: (1) assessing the heat loss versus flow divergence as the dominant stabilization mechanism; (2) determining the laminar flame speed by using this configuration; and (3) understanding the development of flamefront instability and the effects of the flame curvature on the burning intensity
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Regulation of Wages and Hours Prior to 1938
Direct numerical simulations are performed to investigate the transient upstream propagation (flashback) of premixed hydrogen–air flames in the boundary layer of a fully developed turbulent channel flow. Results show that the well-known near-wall velocity fluctuations pattern found in turbulent boundary layers triggers wrinkling of the initially flat flame sheet as it starts propagating against the main flow direction, and that the structure of the characteristic streaks of the turbulent boundary layer ultimately has an important impact on the resulting flame shape and on its propagation mechanism. It is observed that the leading edges of the upstream-propagating premixed flame are always located in the near-wall region of the channel and assume the shape of several smooth, curved bulges propagating upstream side by side in the spanwise direction and convex towards the reactant side of the flame. These leading-edge flame bulges are separated by thin regions of spiky flame cusps pointing towards the product side at the trailing edges of the flame. Analysis of the instantaneous velocity fields clearly reveals the existence, on the reactant side of the flame sheet, of backflow pockets that extend well above the wall-quenching distance. There is a strong correspondence between each of the backflow pockets and a leading edge convex flame bulge. Likewise, high-speed streaks of fast flowing fluid are found to be always colocated with the spiky flame cusps pointing towards the product side of the flame. It is suggested that the origin of the formation of the backflow pockets, along with the subsequent mutual feedback mechanism, is due to the interaction of the approaching streaky turbulent flow pattern with the Darrieus–Landau hydrodynamic instability and pressure fluctuations triggered by the flame sheet. Moreover, the presence of the backflow pockets, coupled with the associated hydrodynamic instability and pressure–flow field interaction, greatly facilitate flame propagation in turbulent boundary layers and ultimately results in high flashback velocities that increase proportionately with pressure
Thermal-Diffusional Instability in White Dwarf Flames: Regimes of Flame Pulsation
Thermal-diffusional pulsation behaviors in planar as well as outwardly and
inwardly propagating white dwarf carbon flames are systematically studied. In
the 1D numerical simulation, the asymptotic degenerate equation of state and
simplified one-step reaction rates for nuclear reactions are used to study the
flame propagation and pulsation in white dwarfs. The numerical critical
Zel'dovich numbers of planar flames at different densities (, 3 and
4~g/cm) and of spherical flames (with curvature -0.01, 0,
0.01 and 0.05) at a particular density (~g/cm) are
presented. Flame front pulsation in different environmental densities and
temperatures are obtained to form the regime diagram of pulsation, showing that
carbon flames pulsate in the typical density of and
temperature of . While being stable at higher
temperatures, at relatively lower temperatures the amplitude of the flame
pulsation becomes larger. In outwardly propagating spherical flames the
pulsation instability is enhanced and flames are also easier to quench due to
pulsation at small radius, while the inwardly propagating flames are more
stable.Comment: ApJ, 841, 21 (2017), 25 pages in arxiv versio
Influence of gas compression on flame acceleration in the early stage of burning in tubes
The mechanism of finger flame acceleration at the early stage of burning in
tubes was studied experimentally by Clanet and Searby [Combust. Flame 105: 225
(1996)] for slow propane-air flames, and elucidated analytically and
computationally by Bychkov et al. [Combust. Flame 150: 263 (2007)] in the limit
of incompressible flow. We have now analytically, experimentally and
computationally studied the finger flame acceleration for fast burning flames,
when the gas compressibility assumes an important role. Specifically, we have
first developed a theory through small Mach number expansion up to the
first-order terms, demonstrating that gas compression reduces the acceleration
rate and the maximum flame tip velocity, and thereby moderates the finger flame
acceleration noticeably. This is an important quantitative correction to
previous theoretical analysis. We have also conducted experiments for
hydrogen-oxygen mixtures with considerable initial values of the Mach number,
showing finger flame acceleration with the acceleration rate much smaller than
those obtained previously for hydrocarbon flames. Furthermore, we have
performed numerical simulations for a wide range of initial laminar flame
velocities, with the results substantiating the experiments. It is shown that
the theory is in good quantitative agreement with numerical simulations for
small gas compression (small initial flame velocities). Similar to previous
works, the numerical simulation shows that finger flame acceleration is
followed by the formation of the "tulip" flame, which indicates termination of
the early acceleration process.Comment: 19 pages, 20 figure
Turbulence decay and cloud core relaxation in molecular clouds
The turbulent motion within molecular clouds is a key factor controlling star
formation. Turbulence supports molecular cloud cores from evolving to
gravitational collapse and hence sets a lower bound on the size of molecular
cloud cores in which star formation can occur. On the other hand, without a
continuous external energy source maintaining the turbulence, such as in
molecular clouds, the turbulence decays with an energy dissipation time
comparable to the dynamic timescale of clouds, which could change the size
limits obtained from Jean's criterion by assuming constant turbulence
intensities. Here we adopt scaling relations of physical variables in decaying
turbulence to analyze its specific effects on the formation of stars. We find
that the decay of turbulence provides an additional approach for Jeans'
criterion to be achieved, after which gravitational infall governs the motion
of the cloud core. This epoch of turbulence decay is defined as cloud core
relaxation. The existence of cloud core relaxation provides a more complete
understanding in the competition between turbulence and gravity on the dynamics
of molecular cloud cores and star formation.Comment: 18 pages, 1 figure, accepted for publication in Ap
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