The flow field characteristics of confined flames have been investigated for propane nonpremixed flames in cylindrical furnaces. The effects of furnace inner diameter D 1 , air inlet velocity difference ∆U a , and global equivalence ratio φ on the flow field are related to NOx emission. The emission index of NOx, EINOx, decreases roughly with the increases of the above parameters. This decrease is observed as a consequence of flame stretch and dilution by the burned gas. The flame stretch is related to the velocity difference introduced by multiple inlets, and the dilution is attributable to the recirculation structure formed at the bottom of the furnace. The present investigation shows the mechanism of burner/chamber configuration inciting the flow field and indirectly controlling NOx emission.

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Confined flames are widely used in the industrial field. The flame characteristics can be strongly dominated by the combination of a burner and furnace geometries, which were not paid much attention before. In the present study, flow fields in confined flames are discussed in terms of the flame characteristics. The flow characteristics of confined flames have been investigated for propane nonpremixed flames in cylindrical furnaces. The effects of the inner diameter of the cylindrical furnace D1, the turbulence at the flame boundary, and the global equivalence ratio φ are examined in terms of the relation between the emission of NOx and the flow fields. The emission index of NOx, EINOx, decreases roughly with these parameters. The decrease in EINOx is thought to be related to the dilution of mixtures by the burned gas and the flame stretch. The dilution is attributable to vortices formed at the bottom of the furnace, and the flame stretch is attributable to the air velocity difference ΔUa created by two air nozzles. In the present study, it was found that the increases in D1, ΔUa, and φ enlarge and strengthen recirculation vortices to dilute the flame.

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