Experimental and Theoretical Investigation of Flame Propagation Through Compositionally Stratified Methane -Air Mixtures
Kang, Taekyu
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https://hdl.handle.net/2142/83907
Description
Title
Experimental and Theoretical Investigation of Flame Propagation Through Compositionally Stratified Methane -Air Mixtures
Author(s)
Kang, Taekyu
Issue Date
2008
Doctoral Committee Chair(s)
Kyritsis, Dimitrios C.
Department of Study
Mechanical Engineering
Discipline
Mechanical Engineering
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Engineering, Mechanical
Language
eng
Abstract
"Flame propagation in compositionally stratified methane-air mixtures was studied experimentally and theoretically. Stratification was established in a controlled manner using a convective-diffusive balance with very slow fuel-air mixture inflows into an optically accessible test chamber and its equivalence ratio distribution in the fresh mixture was measured with both acetone PLIF and Raman spectroscopy. For a flame propagating from stoichiometric to leaner methane-air mixtures, the flame speed was shown to be higher than the one corresponding to a homogeneous mixture of the local equivalence ratio. This flame speed increase was more prominent when the flame encountered leaner mixtures and higher equivalence ratio gradients. A significant extension of the lean flammability limit (ϕ≈0.4) was observed. Theoretical analysis was performed in order to rationalize the experimental results of the flame speed increase and was able to correctly predict the results. For a flame propagating from stoichiometric to richer methane-air mixtures, flame speed increase with respect to ""quasi-homogeneous"" propagation and a significant extension of the rich flammability limit (ϕ≈1.7) were also observed. Flame front instabilities at extremely lean mixtures (ϕ≈0.55) were quantitatively explained by considering diffusional-thermal effects. In order to highlight some of the theoretical aspects of stratified combustion, a commercial CFD software was employed to study temperature and radical distributions (OH, H) in flames propagating through step equivalence ratio changes from the unity equivalence ratio. The results indicated that larger equivalence ratio departures from unity resulted in the flame traveling longer distances in order to reach steady propagation. Interestingly, the transition of premixed flames to diffusion flames was observed when the premixed flames propagated through from stoichiometric to extremely high equivalence ratios (ϕ≈1.7)."
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