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CRITICAL BEHAVIOR OF AN IGNITION MODEL IN CHEMICAL COMBUSTION.TONELLATO, PETER JOHN. January 1985 (has links)
A model for the hot slab ignition problem is analyzed to determine critical conditions based on the parameters of the system. Activation energy asymptotics, a singular perturbation approach, is applied to the governing equation resulting in a Volterra integral equation of the second kind whose solution represents the temperature perturbation at the surface of the hot slab. The system is said to be supercritical for given parameter values when the temperature perturbation blows up in small finite time, an indication of ignition, or subcritical when the blow up time is large, indicating that heat loss effects overcome the hot slab ignition mechanisms. Comparison principles for integral equations are used to construct upper and lower solutions of the equation. The exact solution as well as the upper and lower solutions depend on two parameters ε, the Zeldovich number a measure of the heat release and λ, the scaled hot slab size. Upper and lower bounds on the transition region, delineating the super-critical from the sub-critical region, are derived based upon the lower and upper solution behavior. The product integration method is used to compute solutions of the Volterra equation for values of ε and λ in the transition region. The computations indicate that a critical curve, λ(c) lying between the analytic bounds, exists.
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Numerical analysis of combustion inside a char particle porePianki, Francis Owen January 1981 (has links)
No description available.
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Fluidized bed utilization of South Australian coalsWildegger-Gaissmaier, Anna Elisabeth. January 1988 (has links) (PDF)
Bibliography: 208-218.
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Mathematical modeling of multistep chemical combustion: The hydrogen-oxygen system.Elele, Nwabuisi N. O. January 1988 (has links)
A model of premixed lean Hydrogen-Oxygen flame is studied by singular perturbation techniques based on high activation energy. The model is built from four reaction steps consisting of two chain branching steps, a chain propagating step, and a recombination step. The analysis, in this case, gives rise to a layer phenomenon different from what is currently seen in combustion literature. First, there is a basic layer similar to those obtained for the one step reaction model. Then embedded in the first layer is a thinner layer giving rise to an interesting system of nonlinear boundary value problems. This system of nonlinear problems does not meet standard existence criterium and also involves an unknown parameter. Hence existence results are called for. Existence is proved for both the boundary value problem and the unknown parameter, and numerical solutions are obtained in support of the existence results. A numerical estimate of the unknown parameter is obtained. A generalization of the model for different reaction parameter ranges is made. Two new thin layers emerge. The structure of one of the new thin layers turns out to be exactly the same as that just described, hence the existence results do carry over. The boundary value problem resulting from the second of the new thin layers turned out to be quite simple and a solution could be written down explicitly.
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A linear eddy model for steady-state turbulent combustionGoldin, Graham Mark 12 1900 (has links)
No description available.
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Mathematical modelling of the flow and combustion of pulverized coal injected in ironmaking blast furnaceShen, Yansong, Materials Science & Engineering, Faculty of Science, UNSW January 2008 (has links)
Pulverized coal injection (PCI) technology is widely practised in blast furnace ironmaking due to economic, operational and environmental benefits. High burnout of pulverized coal in the tuyere and raceway is required for high PCI rate operation. A comprehensive review reveals that although there have been a variety of PCI models, there is still an evident need for a more realistic model for PCI operation in blast furnace. Aiming to build a comprehensive PCI model of a full-scale blast furnace, this thesis presents a series of three-dimensional mathematical models, in terms of model development, validation and application, in a sequence from a pilot-scale to a full-scale, from a simple to complicated geometry, from a coal only system to a coupled coal/coke system. Firstly a three-dimensional model of pulverized coal combustion is developed and applied to a pilot-scale PCI test rig. This model is validated against the measurements from two pilot-scale test rigs in terms of gas species composition and coal burnout. The gas-solid flow and coal combustion are simulated and analysed. The results indicate that the model is able to describe the evolutions of coal particles and provide detailed gas species distributions. It is also sensitive to various parameters and hence robust in examining various blast furnace operations. This model is then extended to examine the combustion of coal blends. The coal blend model is also validated against the experimental results for a range of coal blends conditions. The overall performance of a coal blend and the individual behaviours of its component coals are analysed. More importantly, the synergistic effect of coal blending on overall burnout is examined and the underlying mechanisms are explored. It is indicated that such synergistic effect can be optimized by adjusting the blending fraction, so as to compensate for the decreased burnout under high coal rate operation. The model provides an effective tool for the optimum design of coal blends. As a scale-up phase, the coal combustion model is applied to the blowpipe-tuyereraceway region of a full-scale blast furnace, where the raceway is simplified as a tube with a slight expansion. The in-furnace phenomena are simulated and analysed, focusing on the main coal plume. The effect of cooling gas conditions on combustion behaviours is investigated. Among the three types of cooling gas (methane, air, and oxygen), oxygen gives the highest coal burnout. Finally, a three-dimensional integrated mathematical model of pulverized coaVcoke combustion is developed. The model is applied to the blowpipe-tuyere-raceway-coke bed region of a full-scale blast furnace, which features a complicated raceway geometry and coke bed properties. The model is validated against the measurements in terms of coal burnout from a test rig and gas composition from a blast furnace, respectively. The model gives a comprehensive full-scale picture of the flow and thermo-chemical characteristics of PCI process. The typical operational parameters are then examined in terms of coal burnout and gas composition. It is indicated that the final burnout along the tuyere axis is insensitive to some operational parameters. The average burnout over the raceway surface can better represent the amount of unburnt coal particles entering the surrounding coke bed and it is also found to be more sensitive to the changes of most parameters. In addition, the underlying mechanisms of coal combustion are obtained. The coal burnout strongly depends on both oxygen availability and residence time. The existence of recirculation region gives a more realistic coal particle residence time and burnout. Compared with the fore-mentioned two models, this model is considered as a more comprehensive model of PCI operation for understanding the infurnace behaviours and provides more reliable information for the design of operational parameters.
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Fluidized bed utilization of South Australian coals / Anna Elisabeth Wildegger-Gaissmaier.Wildegger-Gaissmaier, Anna Elisabeth January 1988 (has links)
Bibliography: 208-218. / 339 leaves : ill. ; 30 cm. / Title page, contents and abstract only. The complete thesis in print form is available from the University Library. / Thesis (Ph.D.)--University of Adelaide, Dept. of Chemical Engineering, 1989
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A numerical study of solid fuel combustion in a moving bedKo, Daekwun 12 November 1993 (has links)
Coal continues to be burned by direct combustion in packed or moving bed in
small size domestic furnaces, medium size industrial furnaces, as well as small power
stations. Recent stringent restrictions on exhaust emissions call for a better
understanding of the process of combustion of coal in beds.
The present study is a prelude to developing methods of analysis to obtain this
improved understanding. A one-dimensional steady-state computational model for
combustion of a bed of solid fuel particles with a counterflowing oxidant gas has
been developed. Air, with or without preheating, is supplied at the bottom of the bed.
Spherical solid fuel particles (composed of carbon and ash) are supplied at the top of
the bed. Upon sufficient heating in their downward descent, the carbon in particles
reacts with oxygen of the flowing gas.
The governing equations of conservation of mass, energy, and species are
integrated numerically to obtain the solid supply rate whose carbon content can be
completely consumed by a given gas supply rate. The distributions of solid and gas
temperatures, of concentrations of various gas species, of carbon content in solid, and
of velocity and density of gas mixture are also calculated along the bed length. The
dependence of these distributions on the solid and gas supply rates, the air supply
temperature, the size of solid fuel particle, and the initial carbon content in solid is
also investigated.
The calculated distributions are compared with the available measurements
from literature to find reasonable agreement. More gas supply is needed for complete
combustion at higher solid supply rate. At a given gas supply rate, more solid fuel
particles can be consumed at higher gas supply temperature, for larger particle size,
and for lower initial carbon content in solid. The temperature of the bed becomes
higher for higher solid supply rate, higher gas supply temperature, larger solid
particle diameter, or lower initial carbon content in solid. These reasonable results
lead one to encourage extension of the model presented here to more complex
problems involving combustion of coals in beds including the effects of drying and
pyrolysis. / Graduation date: 1994
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A diagnostic quasi-dimensional model of heat transfer and combustion in compression-ignition engines.Hansen, Alan Christopher. 23 September 2013 (has links)
Investigations into the combustion of alternative fuels in
compression-ignition engines in South Africa have underlined the
inadequacies of existing zero-dimensional combustion models. The
major aspect of concern in these models was the computation of
heat transfer which had been singled out by a number of
researchers as the leading cause of inaccuracies in heat release
computations.
The main objective of this research was to develop a combustion
model that was less empirically based than the existing zerodimensional
models for use in evaluating the combustion and
resulting thermal stresses generated by alternative fuels. in
diesel engines. Particular attention was paid to the development
of a spatial and temporal model of convective heat transfer that
was based on gas flow characteristics and to the introduction of
a radiation heat transfer model that made use of fuel properties
and fuel-air ratio. The combustion process was divided into two
zones representing burnt and unburnt constituents and the
resulting temperatures in each zone were used in the calculations
of convective and radiative heat transfer. The complete model
was formulated in such a way that it could be applied with the
aid of a micro-computer.
Calibration and verification of the gas flow sub-models which
involved the squish, swirl and turbulence components necessitated
the use of published data. Good agreement for the squish and
swirl components was obtained between the present model and the
experimental data from three engines, two with a bowl-in-piston
and the other with a flat piston. These gas flow components
dominated the gas velocities in the combustion chamber and
provided a reliable foundation for the calculation of convective
heat transfer. In spite of the well documented difficulties of
characterising turbulence, after calibration the model generated
turbulence levels with acceptable trends and magnitudes. Tests were carried out on a naturally aspirated ADE 236 engine
involving the measurement of cylinder pressure and heat flux at
a single point. Motored engine data were used to verify the
convective heat transfer rates and to ascertain the effects of
soot deposition on the heat flux probe. Close correlation
between predicted and measured heat flux was achieved after
accounting for the effects of chamber geometry at the probe site.
Soot deposition on the probe caused a significant attenuation of
the heat flux within a short period of the engine running under
fired conditions.
The results from fired engine tests showed that the two zone
combustion model was providing plausible trends in the burnt and
unburnt zone temperatures and that the model generated combined
heat transfer rates which were credible not only on a global
basis but also in terms of point predictions in the combustion
chamber. The results also highlighted the considerable variation
in heat transfer that could occur from one point in the chamber
to another. Such variations added considerable weight to the
objective of moving away from a zero-dimensional model to a
quasi-dimensional type where predictions could be made on a more
localised rather than global basis. It was concluded that the
model was a definite improvement over zero-dimensional models and
competed favourably with existing quasi-dimensional models with
advantages in both simplicity and accuracy. / Thesis (Ph.D.)-University of Natal, Pietermaritzburg, 1989.
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Combustion modelling of pulverised coal boiler furnaces fuelled with Eskom coalsEichhorn, Niels Wilhelm January 1998 (has links)
A dissertation submitted to the Faculty of Engineering, University of the Witwatersrand,
Johannesburg, in fulfilment of the requirements for the degree of Master in Science in
Engineering,
Johannesburg September 1998 / Combustion modelling of utility furnace chambers provides a cost efficient means to
extrapolate the combustion behaviour of pulverised fuel (pf) as determined from drop
tube furnace (DTF) experiments to full scale plant by making use of computational fluid
dynamics (CFD). The combustion model will be used to assimilate essential
information for the evaluation and prediction of the effect of
• changing coal feedstocks
• proposed operational changes
• boiler modifications.
TRI comrnlssloned a DTF in 1989 which has to date been primarily used for the
comparative characterisation of coals in terms of combustion behaviour. An analysis of
the DTF results allows the determination of certain combustion parameters used to
define a mathematical model describing the rate at which the combustion reaction
takes place. This model has been incorporated into a reactor model which can
simulate the processes occurring in the furnace region of a boiler, thereby allowing the
extrapolation of the DTF determined combustion assessment to the full scale. This
provides information about combustion conditions in the boiler which in turn are used
in the evaluation of the furnace performance.
Extensive furnace testwork of one of Eskom's wall fired plant (Hendrina Unit 9) during
1996, intended to validate the model for the ar plications outlined above, included the
measurement {If :
• gas temperatures
• O2, C02, CO, NOx and S02 concentrations
• residence time distributions
• combustible matter in combustion residues extracted from the furnace
• furnace heat fluxes.
The coal used during the tests was sampled and subjected to a series of chemical and
other lab-scale analyses to determine the following:
• physical properties
• composition
• devolatilisation properties
" combustion properties
The same furnace was modelled using the University of Stuttgart's AIOLOS combustion
code, the results of Which are compared with the measured data.
A DTF derived combustion assessment of a coal sampled from the same site but from
a different part of the beneficiation plant, which was found to burn differently, was
subsequently used in a further simulation to assess the sensitivity of the model to char
combustion rate data. The results of these predictions are compared to the predictions
of the validation simulation.
It was found that the model produces results that compare well with the measured
data. Furthermore. the model was found to be sufficiently sensitive to reactivity
parameters of the coal. The model has thereby demonstrated that it can be used in the
envisaged application of extrapolating DTF reactivity assessments to full scale plant. In
using the model, it has become apparent that the evaluations of furnace modifications
and assessments of boiler operation lie well within the capabilities of the model. / MT2017
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