329 research outputs found
Study of combustion experiments in space
The physical bases and scientific merits were examined of combustion experimentation in a space environment. For a very broad range of fundamental combustion problems, extensive and systematic experimentation at reduced gravitational levels (0 g 1) are viewed as essential to the development of needed observations and related theoretical understanding
Ultrasonic in-situ determination of the regression rate of the melting interface in burning metal rods
Results of tests in which metallic rods are burned in oxygen enriched atmospheres often include the determination of the regression rate of the melting interface for the burning test specimen. This regression rate is used as an indication of a metallic material's relative flammability and its general ability to sustain burning under the test conditions. This paper reports on the development and first application of an ultrasonic measurement system that enables in situ measurement of the regression rate of the melting interface in burning metal rods. All other methods currently used for determining this parameter are based on posttest, visual interrogation, which is costly and often inaccurate. The transducer and associated equipment used to drive and record the transducer's output signal are described and typical results for iron rods burning in pure oxygen at different gauge pressures are given along with a comparison of these results with regression gates obtained from visual interrogation. The excellent sensitivity, accuracy and reliability of the new ultrasonic transducer are demonstrated, thus indicating the transducer's great potential. (C) 1999 Acoustical Society of America. [S0001-4966(99)00702-X]
Nonlinear equation for curved stationary flames
A nonlinear equation describing curved stationary flames with arbitrary gas
expansion , subject to the
Landau-Darrieus instability, is obtained in a closed form without an assumption
of weak nonlinearity. It is proved that in the scope of the asymptotic
expansion for the new equation gives the true solution to the
problem of stationary flame propagation with the accuracy of the sixth order in
In particular, it reproduces the stationary version of the
well-known Sivashinsky equation at the second order corresponding to the
approximation of zero vorticity production. At higher orders, the new equation
describes influence of the vorticity drift behind the flame front on the front
structure. Its asymptotic expansion is carried out explicitly, and the
resulting equation is solved analytically at the third order. For arbitrary
values of the highly nonlinear regime of fast flow burning is
investigated, for which case a large flame velocity expansion of the nonlinear
equation is proposed.Comment: 29 pages 4 figures LaTe
Experiments on shock induced combustion of isolated regions of hydrogen-oxygen mixtures
Hydrodynamic Stability Analysis of Burning Bubbles in Electroweak Theory and in QCD
Assuming that the electroweak and QCD phase transitions are first order, upon
supercooling, bubbles of the new phase appear. These bubbles grow to
macroscopic sizes compared to the natural scales associated with the Compton
wavelengths of particle excitations. They propagate by burning the old phase
into the new phase at the surface of the bubble. We study the hydrodynamic
stability of the burning and find that for the velocities of interest for
cosmology in the electroweak phase transition, the shape of the bubble wall is
stable under hydrodynamic perturbations. Bubbles formed in the cosmological QCD
phase transition are found to be a borderline case between stability and
instability.Comment: preprint # SLAC-PUB-5943, SCIPP 92/56 38 pages, 10 figures (submitted
via `uufiles'), phyzzx format minor snafus repaire
Anomalous roughness with system size dependent local roughness exponent
We note that in a system far from equilibrium the interface roughening may
depend on the system size which plays the role of control parameter. To detect
the size effect on the interface roughness, we study the scaling properties of
rough interfaces formed in paper combustion experiments. Using paper sheets of
different width \lambda L, we found that the turbulent flame fronts display
anomalous multi-scaling characterized by non universal global roughness
exponent \alpha and the system size dependent spectrum of local roughness
exponents,\xi_q, whereas the burning fronts possess conventional multi-affine
scaling. The structure factor of turbulent flame fronts also exhibit
unconventional scaling dependence on \lambda These results are expected to
apply to a broad range of far from equilibrium systems, when the kinetic energy
fluctuations exceed a certain critical value.Comment: 33 pages, 16 figure
Asymptotics for turbulent flame speeds of the viscous G-equation enhanced by cellular and shear flows
G-equations are well-known front propagation models in turbulent combustion
and describe the front motion law in the form of local normal velocity equal to
a constant (laminar speed) plus the normal projection of fluid velocity. In
level set formulation, G-equations are Hamilton-Jacobi equations with convex
( type) but non-coercive Hamiltonians. Viscous G-equations arise from
either numerical approximations or regularizations by small diffusion. The
nonlinear eigenvalue from the cell problem of the viscous G-equation
can be viewed as an approximation of the inviscid turbulent flame speed .
An important problem in turbulent combustion theory is to study properties of
, in particular how depends on the flow amplitude . In this
paper, we will study the behavior of as at
any fixed diffusion constant . For the cellular flow, we show that
Compared with the inviscid G-equation (), the diffusion dramatically slows
down the front propagation. For the shear flow, the limit
\nit where
is strictly decreasing in , and has zero derivative at .
The linear growth law is also valid for of the curvature dependent
G-equation in shear flows.Comment: 27 pages. We improve the upper bound from no power growth to square
root of log growt
The Thermonuclear Explosion Of Chandrasekhar Mass White Dwarfs
The flame born in the deep interior of a white dwarf that becomes a Type Ia
supernova is subject to several instabilities. We briefly review these
instabilities and the corresponding flame acceleration. We discuss the
conditions necessary for each of the currently proposed explosion mechanisms
and the attendant uncertainties. A grid of critical masses for detonation in
the range - g cm is calculated and its
sensitivity to composition explored. Prompt detonations are physically
improbable and appear unlikely on observational grounds. Simple deflagrations
require some means of boosting the flame speed beyond what currently exists in
the literature. ``Active turbulent combustion'' and multi-point ignition are
presented as two plausible ways of doing this. A deflagration that moves at the
``Sharp-Wheeler'' speed, , is calculated in one dimension
and shows that a healthy explosion is possible in a simple deflagration if the
front moves with the speed of the fastest floating bubbles. The relevance of
the transition to the ``distributed burning regime'' is discussed for delayed
detonations. No model emerges without difficulties, but detonation in the
distributed regime is plausible, will produce intermediate mass elements, and
warrants further study.Comment: 28 pages, 4 figures included, uses aaspp4.sty. Submitted to Ap
Low temperature vibrational spectra, lattice dynamics, and phase transitions in some potassium hexahalometallates: K2[XY6] with X=Sn or Te and Y=Cl or Br
An efficient mixed-precision, hybrid CPU-GPU implementation of a fully implicit particle-in-cell algorithm
Recently, a fully implicit, energy- and charge-conserving particle-in-cell
method has been proposed for multi-scale, full-f kinetic simulations [G. Chen,
et al., J. Comput. Phys. 230,18 (2011)]. The method employs a Jacobian-free
Newton-Krylov (JFNK) solver, capable of using very large timesteps without loss
of numerical stability or accuracy. A fundamental feature of the method is the
segregation of particle-orbit computations from the field solver, while
remaining fully self-consistent. This paper describes a very efficient,
mixed-precision hybrid CPU-GPU implementation of the implicit PIC algorithm
exploiting this feature. The JFNK solver is kept on the CPU in double precision
(DP), while the implicit, charge-conserving, and adaptive particle mover is
implemented on a GPU (graphics processing unit) using CUDA in single-precision
(SP). Performance-oriented optimizations are introduced with the aid of the
roofline model. The implicit particle mover algorithm is shown to achieve up to
400 GOp/s on a Nvidia GeForce GTX580. This corresponds to 25% absolute GPU
efficiency against the peak theoretical performance, and is about 300 times
faster than an equivalent serial CPU (Intel Xeon X5460) execution. For the test
case chosen, the mixed-precision hybrid CPU-GPU solver is shown to over-perform
the DP CPU-only serial version by a factor of \sim 100, without apparent loss
of robustness or accuracy in a challenging long-timescale ion acoustic wave
simulation.Comment: 25 pages, 6 figures, submitted to J. Comput. Phy
- âŠ