Abstract
The flame characteristics of confined flames are investigated for propane and hydrogen jet nonpremixed flames in cylindrical furnaces. The flame characteristics can be strongly dominated by the combination of the burner and furnace geometries. In the present study, 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 emission of NOx. 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. The EINOx characteristics are evaluated by the furnace Reynolds number Re, c based on the inner diameter of the furnace reflecting the dilution and the strain rate S∼Da-1 based on the air velocity difference, where Da indicates the Damköhler number. The EINOx is well characterized by the factor D1UFΔUa, which is proportional to Re, cDa-1. This result indicates that the confined nonpremixed flame is characterized by the furnace volume, not the flame volume, which is related to the reaction rate of the jet nonpremixed flame with coflowing air based on the flamelet model. This may imply that the furnace combustion should be modified from the flamelet concept owing to the dilution.
