8/10/2019 bstracts Volume 41 Issue 2 2000 [Doi 10.1016%2Fs0140-6701%2800%2990965-2] -- 0000988 Laser-Induced Spark

1/1

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