8/10/2019 bstracts Volume 41 Issue 2 2000 [Doi 10.1016%2Fs0140-6701%2800%2990965-2] -- 0000988 Laser-Induced Spark
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09 Combustion burners, combustion systems)
1 983 Effects of NO on the ignition of hydrogen and
hydrocarbons by heated counterflowing air
Tan, Y.
et al Combustion Flame, 1999, 119, (3). 346-355.
Experim ents were carried out to study the influence of NO in air on the
ignition temperature of hydrogen and hydrocarbons in a nonpremixed
counterflowing system. These experiments were performed from 0.5 to b
atm, with the NO concentration varying from 100 ppm to 15,OOO ppm, It is
shown that addition of a small amount of NO in air significantly reduces the
ignition temperatu re of all fuels. For hydrogen, under certain pressures,
NO eventually becom es an inhibitor at higher levels of addition. Thus there
appears to exist an optimal NO concentration at which the catalytic effect is
the most pronounced , and this optimal concentration was found to also
depend on the system pressure. Numerical simulation was performed in the
hydrogen case to better understand the kinetics of the observ ed
phenomenon. It was found that at low NO concentrations, the ignition
temperature was determined by the interaction of the Hz-Oz.NO
subsystem, whereas at high NO concentrations the ignition temperature
was mostly affected by the NO, chemistry. For hydrocarbons. the minimum
temperature was much le:,s pronounced and in most cases nonexistent.
Furtherm ore. the extent of temperatur e decrease depended on the nature
of the fuel.
1 984
An extended coal combustion model
Backreedy . R. I. ef ul.
Fuel,
1999, 78, (14), 1745-1754.
Current coal combus tion models are a useful tool in research but they use
simplif ied coal particle de volatilization and combus tion steps in order to
meet computationa l limitations. The availability of more advance d
compu ters enables the use of more detailed steps for devolatilization and
the use of more realistic char combustion processes. In the present work the
devolatilization rates were calculated using the coal devolatilization model
FG-D VC. In this way devolatilization rates and the yields of char and
volatile were obtained. A drop tube reactor was operated at I623 K to
collect char samples. from Thoresby coal. at different sampling positions or
residence ttmec. and proximate and ultimate analysis were conducted on
these char samples to confirm the results. The detailed char combustion
suh-models being developed for CFD codes require char properties such as
densities. surface areas and pore size distributions but a simplified model
was used here. In this case the use of a simple global char oxidation model
together with an empirical derived
volatile and FG-DVC predicted
devolatilization rate data seem to give good agreement with the experi-
mental re sults available for the char burnout. Howev er, there still remains
consider able unccrtaintv in the use of char burnout models including the
one used here which -is not sufficiently accurate in predictin g carbon
burnout in all conditions.
1 985
Gravity effects on steady two-dimensional partially
premixed methane-air flames
Shu. Z. er ul.
Combustion Flnmc. lY9Y.
118, (I-2). 91-107.
Under normal-gravity conditions the flame heat release produces hoth flow
dilatation and buoyancy effects. While it may be possible to minimiz e
gravitational effects in a fully premixed flame by isolating buoyancy effects
to the lower-density postflame region or plume, this cannot be accom-
plished in nonpremix ed flames. It is known that partially premix ed flames
can contain two reaction zones, one with a premix ed-like structure and the
other consisting of a transport-limited nonpremixed zone (in which mixing
and entrainment effects are significant). For these reasons it is important to
understand the fundamental interaction between flow dilatation and
buoyancy effects in partially premix ed flames. A detailed n umerical study
is conducted to characterize the effect of buoyancy on the structure of two-
dimension al partially premix ed methane-air flames. The computation al
model is validated by comparison with the experimentally obtained
chemiluminescent emission from excited-C, free radical species as well
as with velocity vecto rs obtained using particle image velocimetry. Both the
experiments and simulations indicate the presence of two reaction z ones
that are synergistically coupled, with each region providing heat and/or
chemica l species for the other. While the inner prem ixed flame is only
weakly affec ted by gravity, the outer flame shows significant spatial
differences for the two cases due to buoyancy-ind uced entrainment, since
advection of air into the outer reaction zone increases in the presence of
gravity. The presence of gravity induces more compact flames, influences
the velocity profiles in the post-inner flame region and increases the normal
strain rate. Although the spatial differe nces between the O- and l-g flames
are more significa nt on the lean side, the state relationships in that region
are relatively unaffected by gravity. On the other hand , the inner (rich- side)
reaction zone shifts toward less-rich locations in the presenc e of gravity.
possibly due to the enhanced buoyant mixing. The l-g flames exhihit a
larger energy loss in the form of CO and HI emissions.
1 986 Investigation of catalytic combustion within a fin
boundary layer
Griffin, G. 1. and Wood, D. G.
Combustion
Flame, 1999. 118, (l-2), 3-
12.
A mathema tical model of a catalytic fin, a flat plate coated w ith a catalyst,
operating under steady-state conditions where air carrying a fuel flows
parallel to the surface, is developed. The model equations are derived from
the basic equations of change and model predictions of fin and boundary
layer temperature are compared with experimental data for the combustion
of propane and carbon monoxide (CO) over the flat plate coated with
platinum (Pt)/alumina catalyst. Good qualitative agreeme nt is found
between the results of the experimen ts and the model predic tions, although
the model generally pred icts higher fin temperatur es and ignition of
reaction to occur at lower tem peratures.
OOIOO987
Laser-induced ignition using a conical cavity in
CH4-air mixtures
Morsy, M. H. et ni
Comhustio~i Flame, 1999, 119. (4), 473-482.
A method that confines all the available energy in the vicinity of the ignition
point during a laser-indu ced ignition process is propose d. It utilizes
multiple reflection by a conical cavity surface when a small-dia meter laser
beam is directed into the cavity. S hadowg raphs of the early stages of the
combustion process for quiescent methane/air mixtures show a hot gas jet to
emerge from the cavity. During subsequent flame propagation, both
similarities with and differences from conventional spark ignition proc esses
are observe d, de pending on the cavity size and the concentration of
mixtures. With laser cavity ignition,
the chamber pressure increases
relatively rapidly and higher maximum pressure can he achieved. As a
result, the combus tion duration for laser cavity ignition is decreas ed relative
to laser-induced spark ignition. A model, which simulates flame kernel
development and the subsequent combustion process. is tested numerically
using the KIVA-II code. The associated flow, pressure and temperatur e
profiles are evaluated and satisfactory agreeme nt achieved betw een the
experiment and calculated results.
00100988 Laser-induced spark ignition of CH /air mixtures
Phuoc, T. X. and White. F. P. Combustion Name, 1999. 119. (3). 203-
216.
Laser-induced spark ignition of CHd-air mixtures w as experimentally
investigated using a nanosecond pulse at IO64 nm from a Q-switched Nd-
Yag laser. Laser irradiance in the order of 1012 to 1013 Wic mwa s found to
he sufficient to ignite a mixture having fr om 6.5 to 17% methane bv volume
(equivalence ratio, E R. from 0.66 to 1.05). The dependence of the
breakd own threshold laser energy, E,,,,.
on the gas pressure was in
agreemen t with the electron cascade theory. Depen ding on the Iascr
energy, ,,.
the spark abs orption coefficient in the range from 0. I to ahout
100 cm was calculated using the electron-ion inverse brrmsstrahlung
process. The minimum ignition energy was ahout one order of magnitude
higher than the minim um ignition energy obtained by the electric spark
ignition. It had its lowest value r emaining at about 3 to 4 mJ for a mixture
having 10 to 15% methane by volume (ER = 1.05X to 1.68) and it increased
sharply tow ard the far-lean and the far-rich sides of the stoichiometry. The
average length and radius of the spark for a stoichiomet ric or near-
stoichiometrie methane-air mixture were about 0.8 mm and 0.3 mm,
respectively. For lean or rich methane-air mixtures. the average long axis of
the spark size varied from about 0.8 to 2 mm, whereas for the short axis it
varied from about 0.4 to 1.2 mm depending on the methane volume
fraction.
1 989
Liquids for fossil fuel combustion improvement
and harmful gas reduction
Yamashita, Y. Jpn. Kokai Tokk yo Koho JP II 21,574 [99 21,574] (Cl.
ClOLlO/ OO), 26 Jan 1999, Appl. 97/213,797, 4 Jul 1997, 2 pp. (In Japanese)
Liquids containing harmless fermented material or their mixtures are added
to fossil fuels for combustion improvement and reduction of harmful gas
emissions.
00/00990 The mobilisation of sodium and potassium during
coal combustion and gasification
Thompson, D. and Argent, B. B.
Fuel,
1999, 78, (14). 1679-1689.
The mobilization of sodium and potassium during coal combustion and
gasification has been modelled using the FACT thermodynamic databases
and computation package. Account has been taken of the formation of
silicate and fused salt melts and consideration givaen to deposition taking
place during the subsequent cooling of the gases formed. Under
combustion conditions,
sodium and potassium are predicted to be
mobilized to a small extent. The mobilized fraction is predicted to begin
to be deposite d as salt melt from about 1100 K. The mobility agrees with the
limited amount of published data. Under gasifier conditions the predicted
mobilization is greater than observed for potassium-possibly due to the
slownes s of its release from clays.
00/00991 Modeling of lean premixed combustion in
stationary gas turbines
Brews ter, B. S. et al.
Progress in Energv Comhustiorf Sciertce,
1999. 25,
(4) 353-385.
Lean premixed combustion (LPC) of natural gas is of considerable interest
in land-based gas turbines for power generation. However, modeling such
combustots and adequately addressing the concerns of LPC, which include
emissions of nitrogen oxides, carbon monoxide and unburned hydrocar-
bons, remains a significant challenge. In this paper, characteris tics of
published simulations of gas turbine combustion are summarized and
methods of modelling turbulent combustion are reviewed. The velocity-
composition PDF method is selected for implementation in a new
comprehensive model that uses an unstructured-grid flow solver. Reduced
mechan isms for methane combus tion are evaluated in a partially stirred
reactor model. Comprehensive model predictions of swirl-stabilized LPC of
natural gas ate compared with detailed measurements obtained in a
laboratory-scale combustot. The model is also applied to industrial
combustor geometries.
Fuel and Energy bstracts March 2000 109
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