B13-118 NEAR EXTINCTION FLAME STRUCTURE OF MICRO-DIFFUSION FLAMES

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Near extinction flame structures of the small-scale diffusion flames (we call "microflame") are investigated numerically. Microflames considered in the present study are sustained over the circular port burner, whose diameter is hundreds micron or less, in normal-gravity environment. Since the flame size is so small, buoyancy is almost negligible and diffusion controls transport processes. In order to exclude artificial thermal loss to the wall to lead extinction, a burner wall is set to be adiabatic throughout the present study. Scale effects on the flame stabilization and extinction are discussed with various combinations of jet diameter, d, and ejected velocity, U. Numerical model consists the convective-diffusive heat and mass transport with an exothermic chemical reaction. One-step irreversible reaction is applied in the present study. Our numerical results show that there is extinction limit in flame size of less than 1(mm)^3 in volume, and minimum heat release from the combustion zone is about 1W. From the analysis of the flame structure near the extinction limit, it is found that extinction limit by size is caused by insufficient reaction time in the small flame; it is similar to the extinction caused in very fast flow field (i.e. blow off) and the limit could be summarized by Damkohler number (Da) defined as the ratio of chemical reaction time to the diffusive transport time (∿[size]^<-1>). Once Da plays a role on the flame behavior in the small size conditions, Re cannot solely describe the flame size characteristics; scale modeling for flame size cannot predict by Re only. Special treatment to increase Da needs to be performed in order to reduce the minimum size of combustor.
一般社団法人日本機械学会の論文
2003-11-30

B13-118 NEAR EXTINCTION FLAME STRUCTURE OF MICRO-DIFFUSION FLAMES

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