Effects of coaxial airflow swirl number on combustion and flame characteristics of methane/air and n-butane/air flames in a miniature-scale swirl burner

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Abstract
In this paper, the effects of coaxial airflow swirl number on blow-out limit, flame characteristics, and emission performance of non-premixed methane/air and n-butane/air flames in a miniature-scale swirl burner are investigated experimentally. Swirl numbers ranging from 0 to 0.72 have been chosen to be examined. It was observed that increasing the vane angle does not increase the blow-out limit monotonically and a swirler with 30 ° vane angle ( 0.50 swirl number) results in the greatest blow-out limit. Besides, it was found out that methane outperforms n-butane in the blow-out limit at the same fuel flow rates. The intermittency distribution approach was used to determine flame properties such as lift-off height, length, and width. It was found that the lift-off variation with coaxial airflow Reynolds number at the specified fuel flow rates yields an inverted V-shape for S n ≥ 0.50. The tip of these plots indicates the airflow rate at which the swirl shows its influence, and after this point, the lift-off height begins to decrease. It was also observed that the airflow related to this point decreases with an increase in the swirl number at a certain fuel flow rate. Furthermore, the results revealed that at high airflow rates, the flame width rises as the swirl number increases. Also, raising the swirl number and airflow Reynolds number reduced the flame length. It was also observed that n-butane/air flames have longer and wider flames than methane/air flames. Additionally, adding swirl to the airflow rate typically reduces N O x and CO concentrations and n-butane/air flames exhibit lower levels of N O x and higher levels of CO than methane/air flames under the same operating conditions.

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