| Summary: | 博士 === 國立成功大學 === 機械工程學系碩博士班 === 92 === This investigation is aimed at studying the characteristics of laminar conical premixed flames in an impinging jet flow experimentally, numerically, and theoretically with emphasis on the effects of negative flame stretch from flame curvature, positive stretch from the flow field and preferential diffusion.
The results show that flame shapes exhibit double-solution characteristics in a certain range of methane concentration. Experimentally, by following different paths of adjusting methane concentration (decreasing from rich to lean or increasing from lean to rich), two different flame configurations (planar or conical flame) may exist at the same flow conditions: burner-to-plate distance, inlet velocity and methane concentration. At the higher (or lower) critical methane concentration, the transition from a flat flame to a conical flame (or from a conical flame to a flat flame) occurs. When the operating condition is in this region, the flame is unstable since an externally mechanical disturbance may transform the flame shape back and forth between conical flame and flat flame, i.e., flames in the double solution region are unstable. The double solution region is strongly influenced by the burner-to-plate separation distance (H/d) and inlet velocity. Lower H/d or lower inlet velocity decreases the double solution region; In other words, the flame is relatively stable at lower H/d or lower inlet velocity. Instability in the double solution region may strongly affect the flame shapes. Therefore, the design of low-Reynolds-number heating devices, such as domestic gas burners, should take the double solution region into consideration, especially for those used in lean premixed flame applications.
Stretch calculation along a conical flame in an impinging flow shows the conical flame still endures negative stretch. However, the effect of positive flow stretch due to the impinging flow reduces the extent of negative stretch. When the methane flame is outward open-tip or flat, it then receives positive stretch.
Theoretical analysis reveals that for a negatively-stretched conical flame in a positively-stretched flow, positive stretch is a positive effect to the lean methane flame (Le<1), but negative to rich methane flame (Le>1). The downstream heat loss is a negative effect to both rich and lean methane flame (Le>1 and Le<1). Experimental results agree well with the theoretical and numerical analysis qualitatively.
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