778 research outputs found
Nonlinear cellular instabilities of planar premixed flames: numerical simulations of the Reactive Navier-Stokes equations
Two-dimensional compressible Reactive Navier-Stokes numerical simulations of intrinsic planar, premixed flame instabilities are performed. The initial growth of a sinusoidally perturbed planar flame is first compared with the predictions of a recent exact linear stability analysis, and it is shown the analysis provides a necessary but not sufficient test problem for validating numerical schemes intended for flame simulations. The long-time nonlinear evolution up to the final nonlinear stationary cellular flame is then examined for numerical domains of increasing width. It is shown that for routinely computationally affordable domain widths, the evolution and final state is, in general, entirely dependent on the width of the domain and choice of numerical boundary conditions. It is also shown that the linear analysis has no relevance to the final nonlinear cell size. When both hydrodynamic and thermal-diffusive effects are important, the evolution consists of a number of symmetry breaking cell splitting and re-merging processes which results in a stationary state of a single very asymmetric cell in the domain, a flame shape which is not predicted by weakly nonlinear evolution equations. Resolution studies are performed and it is found that lower numerical resolutions, typical of those used in previous works, do not give even the qualitatively correct solution in wide domains. We also show that the long-time evolution, including whether or not a stationary state is ever achieved, depends on the choice of the numerical boundary conditions at the inflow and outflow boundaries, and on the numerical domain length and flame Mach number for the types of boundary conditions used in some previous works
Lift-Off Characteristics and Flame Base Structure of Coal Seeded Gas Jet Flames
An experimental study of the burner rim stability characteristics and the flame base structure of flames co-fired with pulverized coal and propane gas is presented. Lift-off and reattachment characteristics are examined as functions of propane concentration in the jet stream for lignite, bituminous and anthracite coals. The effects on flame base structure are studied in terms of temperature, product species concentration and radiation profiles. The addition of lignite and anthracite coals favours the lift-off transitions. Bituminous coal, on the other hand, makes the flame more stable. The peak values of temperature and concentrations of major combustion product species in the flame stabilization region strongly depend upon the rank of coal. Among the coals tested, bituminous coal produces the highest peak temperature and its flame emits maximum radiation from the stabilization region. Anthracite and lignite coals produce somewhat comparable stability characteristics and structure of the flame base. The effects of coal rank are explained by the differences in volatile matter, moisture and pyrolysis characteristics of coals.Yeshttps://us.sagepub.com/en-us/nam/manuscript-submission-guideline
Propagation of Avalanches in Mn-acetate: Magnetic Deflagration
Local time-resolved measurements of fast reversal of the magnetization of
single crystals of Mn12-acetate indicate that the magnetization avalanche
spreads as a narrow interface that propagates through the crystal at a constant
velocity that is roughly two orders of magnitude smaller than the speed of
sound. We argue that this phenomenon is closely analogous to the propagation of
a flame front (deflagration) through a flammable chemical substance.Comment: 5 pages, 5 figure
Classical generalized constant coupling model for geometrically frustrated antiferromagnets
A generalized constant coupling approximation for classical geometrically
frustrated antiferromagnets is presented. Starting from a frustrated unit we
introduce the interactions with the surrounding units in terms of an internal
effective field which is fixed by a self consistency condition. Results for the
magnetic susceptibility and specific heat are compared with Monte Carlo data
for the classical Heisenberg model for the pyrochlore and kagome lattices. The
predictions for the susceptibility are found to be essentially exact, and the
corresponding predictions for the specific heat are found to be in very good
agreement with the Monte Carlo results.Comment: 4 pages, 3 figures, 2 columns. Discussion about the zero T value of
the pyrochlore specific heat correcte
Kinetics of the Methyl-Vinyl Radical + O-2 Reactions Associated with Propene Oxidation
The bimolecular rate coefficients of reactions CH3CCH2 + O-2 (1) and cis/trans-CH3CHCH + O-2 (2a/3a) have been measured using a tubular laminar flow reactor coupled with a photoionization mass spectrometer (PIMS). These reactions are relevant in the combustion of propene. Pulsed excimer laser photolysis of a ketone or a bromide precursor molecule at 193 or 248 nm wavelength was used to produce radicals of interest homogeneously along the reactor. Time-resolved experiments were performed under pseudo-first-order conditions at low pressure (0.3-1.5 Torr) over the temperature range 220-660 K. The measured bimolecular rate coefficients were found to be independent of bath gas concentration. The bimolecular rate coefficients possess negative temperature dependence at low temperatures (T 420 K). Observed products of the reaction CH3CCH2 + O-2 were CH3 and H2CO, while for the reaction cis/trans-CH3CHCH + O-2, observed products were CH3CHO and HCO. Current results indicate that the reaction mechanism of both reactions is analogous to that of C2H3 + O-2. Methyl substitution of the vinyl radical changes its reactivity toward O-2 upward by ca. 50% if it involves the alpha-position and downward by ca. 30% if the methyl group takes either of the beta-positions, respectively.Peer reviewe
Gaseous emissions during concurrent combustion of biomass and non-recyclable municipal solid waste
Background: Biomass and municipal solid waste offer sustainable sources of
energy; for example to meet heat and electricity demand in the form of combined
cooling, heat and power. Combustion of biomass has a lesser impact than solid
fossil fuels (e. g. coal) upon gas pollutant emissions, whilst energy recovery
from municipal solid waste is a beneficial component of an integrated,
sustainable waste management programme. Concurrent combustion of these fuels
using a fluidised bed combustor may be a successful method of overcoming some of
the disadvantages of biomass (high fuel supply and distribution costs,
combustion characteristics) and characteristics of municipal solid waste
(heterogeneous content, conflict with materials recycling). It should be
considered that combustion of municipal solid waste may be a financially
attractive disposal route if a 'gate fee' value exists for accepting waste for
combustion, which will reduce the net cost of utilising relatively more
expensive biomass fuels. Results: Emissions of nitrogen monoxide and sulphur
dioxide for combustion of biomass are suppressed after substitution of biomass
for municipal solid waste materials as the input fuel mixture. Interactions
between these and other pollutants such as hydrogen chloride, nitrous oxide and
carbon monoxide indicate complex, competing reactions occur between
intermediates of these compounds to determine final resultant emissions.
Conclusions: Fluidised bed concurrent combustion is an appropriate technique to
exploit biomass and municipal solid waste resources, without the use of fossil
fuels. The addition of municipal solid waste to biomass combustion has the
effect of reducing emissions of some gaseous pollutants
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