| Summary: | This study focused on the biogas-air premixed flames formed on a flat burner under the conditions of oxygen enrichment and hydrogen addition to improve the combustion characteristics of biogas-air premixtures. The objectives of this study were to study the effects of oxygen concentration in the oxidizer and hydrogen concentration in the fuel on the intrinsic instability of premixed flames, and to study the effects of flame instability on the flame shape and behavior. Photographs of flames were taken by a digital single-lens reflex camera, and the emission of light from flames was acquired by a photodiode. Cellular structures of flame fronts were identified by photographs taken. The light emission was analyzed to clarify the fluctuating characteristics by time series analysis. Cellular flames were observed at equivalent ratios near the lower flammable limit under the conditions of oxygen enrichment and hydrogen addition. The cellular-flame range decreased with increasing oxygen concentration in the oxidizer and increased with increasing hydrogen concentration in the fuel. Chaotic time series analysis was performed on the light emission to reconstruct the attractor. As a result, regardless of the oxygen concentration, the attractor at the lower flammable limit exhibited quasi-periodic behavior slightly. On the other hand, the attractor near the lower flammable limit became more complex with increasing hydrogen concentration in the fuel. These results indicated that the oxygen enrichment and hydrogen addition affected the intrinsic instability and flame shape and behavior.
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