| Summary: | 碩士 === 國立成功大學 === 航空太空工程學系 === 81 === Turbulent swirling flow has drawn considerable attention in the
past decades since it possesses wide-range applications in many
practical combustion devices. It promotes mixing of fuel and
air, reducing flame lengths and pollutant emission. It also
provides good flame stabilization such that the flame holding
mechanisms installed in the combustor can be eliminated.
However, due to its complexity associated with the coupling of
turbulence and swirling, it is still a challenging subject for
numerical studies. Turbulent swirling flow calculations can be
inaccurate due to at least three major reasons: limitations of
the turbulence modeling used, incorrect or inadequate
specifications of inlet boundary conditions, and error
introduced through the numerical diffusion. Furthermore,
multiple flow structures for confined swirling flows were
found at certain flow conditions by our group recently. This
finding has significant impacts on the swirling flow study and
is further investigated in this thesis. The numerical results
obtained in the present study reveal that, when the sizes of
internal and external recirculation zones are exhibited to be
comparable, multiple flow structures result. The internal and
external recirculation zone sizes, however, depend upon not
only the flow conditions but also the employed numerical
scheme, grid system, and physical models. It is found that the
previous arguments regarding k-.epsilon. model's performance
should be reexamined since some previous computational results,
which deviated significantly from the experimental results,
just fell into the other branch than the one for the
experimental conditions.
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