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Measurements of Fractal Properties of Premixed Turbulent Flames and Their Relation to Turbulent Burning Velocities

Published online by Cambridge University Press:  05 May 2011

S. I. Yang  and
S. S. Shy
S. I. Yang*
Affiliation:
Department of Mechanical Engineering, National Central University, Chung-Li, Taiwan 32054, R.O.C.
S. S. Shy*
Affiliation:
Department of Mechanical Engineering, National Central University, Chung-Li, Taiwan 32054, R.O.C.
*
*Graduate student
**Professor
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Abstract

The fractal properties of premixed transient flames propagating downwards through a near-isotropic turbulent flow field in a fan-stirred cruciform burner were investigated. The long vertical section of the cruciform burner was used to provide a downward propagating premixed flame at 1 atm. The large horizontal vessel equipped with a pair of counter-rotating fans and perforated plates at each end was used to generate near-isotropic turbulence. Turbulent flame front images were obtained using high-speed laser sheet imaging for both methane-air and propane-air mixtures. The nondimensional turbulent intensity (u′/SL), Reynolds number based on the integral length scale, and turbulent Karlovitz number were varied from 1 to 10, from 698 to 6032, and from 0.05 to 1.43, respectively. Hundreds of runs for each experimental condition were carried out to obtain sufficient images of these turbulent transient flame fronts just in the central uniform region. These images were then processed to extract fractal dimension, inner and outer cutoffs using both the circle and the caliper methods. It was found that the mean fractal dimension is only 2.18, nearly independent of u′/SL, in support of recent Bunsen-flame results found by Gülder and his co-workers. This contradicts the findings of many previous studies in which the fractal dimension may approach asymptotically to a value of 2.33 when u′/SL > 3. The inner (εi) and outer (ε0) cutoffs are found to be nearly constant for all flames studied, where ε0 is an order of magnitude greater than εi and it is smaller than the integral length scale of unreacted turbulence. Finally, the present fractal characteristics cannot predict turbulent burning velocities correctly when the available fractal closure model was used, indicating a limit of the fractal analysis on prediction of turbulent burning velocities.

Keywords

Fractal dimensionInner and outer cutoffsPremixed turbulent combustionTurbulent burning velocities
Type
Articles
Information
Journal of Mechanics , Volume 17 , Issue 2 , June 2001 , pp. 93 - 101
Copyright
Copyright © The Society of Theoretical and Applied Mechanics, R.O.C. 2001

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