An Experimental Study of Flame Characteristics of Jet Diffusion Flames in Cylindrical Furnaces (1st Report, Effect of Inner Diameter of Furnace on NOx Emission Properties)

Abstract

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.

Effect of Flow Field on NOx Emission Properties of Jet Nonpremixed Flames in Cylindrical Furnaces

“…The present paper reports the flow field characteristics of confined jet nonpremixed flames as a complement to previous studies (7) (8) . These studies revealed that NOx emission would significantly decrease by increasing the inner diameter of the furnace, increasing the turbulence at the flame boundary, and increasing the global equivalence ratio.…”

“…Figures 1 and 2 show the experimental apparatus used for the measurement of velocity fields. Pyrex tubes of previous investigations (7) (8) , which were used as furnace walls, are replaced by steel cylinders with inner diameters D 1 of 92 mm and 180 mm, respectively. The replacement of the furnace resulted in a slight increase in NOx on the basis of the decrease in the radiation heat loss.…”

Numerical Evaluation of the Effect of Global Equivalence Ratio on Confined Turbulent Non-Premixed Flames

“…Recirculation transports high-temperature burnt gases upstream and introduces a mixing process that is related to flame stabilization and dilution (3) . The comprehensive experimental studies performed by Noda et al (4)(5)(6) revealed that in confined cylindrical furnaces, NOx emission can be significantly decreased by increasing the inner diameter of the furnace, turbulence at the flame boundary, and global equivalence ratio. The influence of recirculation vortices generated on the basis of confined configuration was reported to be a major contributor to dilution by burnt gases (4,5) .…”

“…The comprehensive experimental studies performed by Noda et al (4)(5)(6) revealed that in confined cylindrical furnaces, NOx emission can be significantly decreased by increasing the inner diameter of the furnace, turbulence at the flame boundary, and global equivalence ratio. The influence of recirculation vortices generated on the basis of confined configuration was reported to be a major contributor to dilution by burnt gases (4,5) . The decrease in NOx emissions is possibly related to the dilution of mixtures by the burnt gases and flame stretch (4) .…”

“…The influence of recirculation vortices generated on the basis of confined configuration was reported to be a major contributor to dilution by burnt gases (4,5) . The decrease in NOx emissions is possibly related to the dilution of mixtures by the burnt gases and flame stretch (4) . Calculation for these interactions can be numerically simulated on the basis of mathematical models.…”