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The combination of a burner and a combustion chamber is an important factor controlling flame characteristics. However, to our knowledge, this factor has yet to be investigated systematically. In the present study, coaxial jet diffusion flames in cylindrical combustion chambers have been studied in terms of inner diameters of the combustion chambers, global equivalence ratios, and turbulence in airflow. A fuel nozzle is composed of a stainless steel tube having an inner diameter (i.d.) of 2 mm with a coaxial pilot burner of 3.19 mm i.d., surrounded by two air coaxial tubes of 12 mm i.d. and 30 mm i.d., respectively. The inner and outer air tubes are for higher and lower airflows, respectively, and the turbulence in the airflow is changed by the velocity difference. The main fuel is propane. Hydrogen is used for the pilot flame, with a volumetric fuel ratio of 0.3. Each wall of the combustion chamber is made of a heat-resistant glass Pyrex tube so that each flame can be visualized. The inner diameter of the furnace is varied in order to investigate the effect of furnace size on the flame characteristics. The increase in the diameter of the combustion chamber has been found to enhance the exhaust gas self-recirculation, because the NOx emission decreases. The increase in turbulence in the airflow strengthens the entrainment of the exhaust gas transported upstream by the recirculation vortex. The increase in the global equivalence ratio from 0.2 to 0.8 in the present study decreases the oxygen concentration of the exhaust gas and leads to diluted combustion through the exhaust gas self-recirculation. A proper combination of these factors has been found to yield a low NOx combustion.

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 D 1 , 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 ∆Ua 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 D 1 U F ∆Ua, 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.

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