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Modelling of turbulent stratified flamesDarbyshire, Oliver Richard January 2012 (has links)
Due to concerns about pollutant emission combustion systems are increasingly being designed to operate in a lean premixed mode. However, the reduction in emissions offered by lean premixed combustion can be offset by its susceptibility to instabilities and ignition and extinction problems. These instabilities, caused by the coupling of unsteady heat release and pressure fluctuations can cause significant damage to combustion devices. One method of avoiding these problems whilst still operating a globally lean system is to employ a stratified premixed mode where areas of richer mixture are used to enhance the stability of the flame. In this thesis a computational modelling methodology for the simulation of stratified premixed flames is developed. Firstly, several sub-models for the dissipation rate of a reacting scalar are evaluated by the simulation of two laboratory scale flames, a turbulent stratified V-flame and a dump combustor fed by two streams of different mixture strength. This work highlights the importance of this quantity and its influence on the simulation results. Any model for stratified combustion requires at least two variables to describe the thermochemical state of the gas: one to represent the mixing field and another to capture the progress of reaction. In turbulent stratified flames the joint probability density function (pdf) of these variables can be used to recover the mean reaction rates. A new formulation for this pdf based on copula methods is presented and evaluated alongside two alternative forms. The new method gives improved results in the simulation of the two test cases above. As it is likely that practical stratified combustion devices will have some unsteadiness to the flow the final part of this work applies the modelling methodology to an unsteady test case. The influence of the unsteady velocity forcing on the pollutant emissions is investigated. Finally the methodology is used to simulate a developmental, liquid fuelled, lean burn aero-engine combustor. Here the model gives reasonable predictions of the measured pollutant emissions for a relatively small computational cost. As such it is hoped that the modelling methodology presented can be useful in the iterative industrial design process of stratified combustion systems.
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Computational modeling of Lorentz force induced mixing in alkali seeded diffusion flamesThompson, Jon Ira 21 November 1994 (has links)
Lorentz forces provide a unique method for the control and mixing of gas
flows without the physical intrusion of objects into the flow. Lorentz forces arise
when an electric current is passed through a volume in the presence of a magnetic
field. The interaction between the electric current and the electric and magnetic
fields produces a body force which affects the flow. These forces have been
investigated experimentally by other researchers and show promise as a way to
accelerate combustion in diffusion flames by increasing the mixing rate of fuel and
oxidant streams. Theoretical and numerical models were developed to gain insight
into this process.
Alkali metal seeding raises the electrical conductivity of a flame by two to
three orders of magnitude. This has two significant effects: the Lorentz force
becomes stronger for the same applied electric current and magnetic field, and the
alkali seed concentration becomes a dominant factor in determining electrical
conductivity of seeded gases. This makes electrical conductivity much easier to
predict, and so the Lorentz body force produced is easier to determine.
A theoretical basis for numerical modeling of reactive flows with variable
body forces has been developed. Many issues are important in simulating gas
flows. Conservation of chemical species must be carefully maintained. Mass
transport by gaseous diffusion, which limits combustion rates in a diffusion flame,
must be appropriately modeled. Viscous action is also important, since it promotes
mixing of the fuel and oxidant streams. Convective, conductive, and diffusive
transport of energy must be carefully treated since energy transport directly affects
the fluid flow.
A numerical model of an incompressible gas flow affected by Lorentz forces
was written and tested. Although assumptions made in the model, such as
isothermal conditions and uniform density, are not found in diffusion flames, the
numerical model predicts velocity vector patterns similar to those observed in actual
Lorentz force tests on diffusion flames.
A simulation code for compressible, reactive gas flows which include
Lorentz forces has also been written. Several parts of the model have been
validated, and the approach used appears likely to produce successful simulations.
Further validation studies will be required, however, before complete modeling of
the diffusion flame can proceed. / Graduation date: 1995
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A study of turbulent flame propagationMcNutt, Dinah Georgianna. January 1982 (has links)
Thesis: M.S., Massachusetts Institute of Technology, Department of Mechanical Engineering, 1982 / Includes bibliographical references. / by Dinah Georgianna McNutt. / M.S. / M.S. Massachusetts Institute of Technology, Department of Mechanical Engineering
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