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The fractal character and induced structures of a propane/air diffusion flame


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Li, HL (1994) The fractal character and induced structures of a propane/air diffusion flame. Other Degree thesis, University of Tasmania.

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Signal recovery techniques have been used to measure the periodic structures formed in
a co-annular Propane/ Air diffusion flame due to acoustic excitation of the flow from
upstream. The flow response was sensed using the quantitative schlieren technique, and
consistent results were obtained on the basis of both axial and transverse optical beam
deflections and appropriate analysis of signal records. The two dominant modes of
response were identified as a series of alternating ring disturbances along the flow axis
at the higher frequency and a series of alternating ring disturbances containing on axis
disturbances of opposite sign at the lower frequency. The former mode was essentially
associated with the fuel jet shear layer, whilst the latter was associated with the outer
annular shear layer surrounding the air flow from the outer nozzle. The strength of
disturbances was consistent with mixing fluctuations between cold fuel gas and the
products of combustion. Structures induced in the outer shear layer weakened rapidly
with the distance from the nozzle, indicating a relatively sudden breakup of coherent
structures caused by the excitation near the nozzle. These results provide conclusive
evidence that signal recovery from noise by phase reference to an acoustic excitation is
an effective means of determining structures and strength of induced disturbance of
diffusion flame.
The fractal texture of Propane/ Air diffusion flame were obtained by analysing the
fractal distribution of flame schlieren images based on the box counting method which
is an efficient method used to analyse the fractal texture of diffusion flame schlieren
images with a variance of approximately 0.04 in the fractal dimension. The results
suggest that diffusion flames do exhibit fractal behaviour and that the value of fractal
dimension are around of 2.3~2.47, which varies with the excitation and character of
turbulent flow. The Power Spectrum Density (PSD) of propane/air diffusion flame has also been investigated. These observations of PSD appeared to be generally consistent
with the fractal dimensions. A numerical simulation investigation into the fractal
relationship between the original field and its schlieren images showed that there is no
simple universal relationship between the fractal dimension of the schlieren image and
that of the actual spatial field which gave rise to the image. However, it seems that the
fractal dimension of the diffusion flame and its schlieren image are similar, whilst the
fractal dimension of the schlieren image of a fairly homogeneous turbulent mixing field
may exceed that of the field itself appreciably. Contours of differentiated images have
potential for revealing any regular character in diffusion flames, the present work
showing clear evidence of structures at approximately ±25 o to the flame axis.

Item Type: Thesis (Other Degree)
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Date Deposited: 29 Jul 2014 01:46
Last Modified: 11 Mar 2016 05:55
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