Oxygen-enriched combustion has the potential to save energy, hence, reduce carbon dioxide emissions. In this study, an inherently safe technique of rapidly mixed type tubular flame combustion has been experimentally examined to extend it to higher oxygen concentrations. Results show that when the oxygen concentration of the oxidizer is less than about 40%, stable tubular flame combustion can be obtained. Above 40%, however, oscillatory combustion occurs. With an increase of the oxygen concentration, the oscillatory combustion becomes stronger and its lower limit in the overall equivalence ratio becomes smaller. Further decrease in the equivalence ratio, however, results in stable tubular flame combustion and subsequent extinction. For pure oxygen, stable tubular flame combustion can be obtained in the range of the equivalence ratio from 0.11 to 0.18. With an increase of oxygen concentration the equivalence ratio at extinction decreases gradually, while the fuel concentration at extinction takes an almost constant value of 4.7%. The process leading to the oscillatory combustion is different from that in the conventional premixed type tubular flame combustion; under higher oxygen concentrations, diffusion flames are anchored at the slit exits, which inhibit mixing of fuel and oxidizer, resulting in an intense combustion downstream in a strong swirling flow.
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