A Swirling-Flame Combustor for Lean Mixtures

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Abstract
A combustor for lean mixtures, employing heat recirculation and a new gas burner generating a swirling central flame, was developed and tested. Stability curves for burning lean mixtures of methane/air were determined. The leanest mixture which was burned before extinction, at a preheat temperature of 540° c and a flow rate of 1000 × 10−6 m3/s, contained 3.2% of methane. This figure is 60% of the normal flammability limit. The stability curves were determined at mean preheat temperatures of 35, 460 and 555° c, and for burners with some different geometrical parameters. Special attention has been paid to explore the effect of entrainment of uncontrolled amounts of air on the stability curves. It was found that the major characteristic property of the present apparatus is that for almost a constant preheat temperature, the concentration of fuel in the air, F/(F +A), is continuously decreasing by increasing the flow rate through the system; the latter being an important practical feature of the apparatus. On the other hand, the spouted and crater bed combustors, exhibited an optimal flow rate at which the above ratio attained a minimum value. By increasing the flow rate, the ratio F/(F +A) is increased approaching the lower limit of flammability. A qualitative model, based on diffusion-convection mass transfer of the fuel in the mixture to the reaction zone, is developed; it predicts some characteristic behaviours determined in the experiments.