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

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posted on 2023-05-27, 08:24 authored by Li, HL
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.


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