Influences of Nitrogen Dilution in the Near Flow Field of Transition Regime Lifted Natural Gas Jet Flames

Flame liftoff height data were obtained on lifted methane jet flames diluted with nitrogen at transition regime Reynolds numbers. The data were analyzed to better understand and model natural gas combustion phenomena, in particular the effects of dilution in the near transition regime of methane jet flames. Images of the stable lifted flames were obtained at low Reynolds numbers from 2000 to 3800. This regime of Reynolds numbers was chosen due to the instability of laminar, lifted methane flames at these lower Reynolds numbers. Amongst other applications, lifted flames are often utilized in boilers and industrial burners to reduce thermal stresses, and are thus of importance in natural gas and low calorific fuel gas combustion. Radial and axial locations of the lowest flammable regions were observed. Since definitive models depicting lifted flames in the transition regime between laminar and turbulent flows have not yet been developed, the data was compared to the approximations of a turbulent flame model. The mass fraction model proposed by Tieszen et al (1996), which approximates the flammable regions based on mass fraction of fuel, was utilized in the analysis. For the conditions examined, turbulent lifted methane flames exist between mass fractions of 0.05 and 0.15. Applying this model showed that experimental results occurred outside of the leanest combustion zone predicted. At low Reynolds numbers, the model’s limit for radial location was the most pronounced factor which distinguished the predicted and observed values. These findings could be a result of the instabilities in the transition regime that result from lower stream velocities, such as unsteady vortical structures or small radial variance. It may also be a result of the data occurring at the lower limit of the model’s applicable regime. Some examination of the data also shows a correlation between mass flow rate of the diluent and the deviation of the experimental data from the theoretical data. Further investigations will continue to observe the effects of dilution into higher Reynolds number jet flames under dilution and co-flow, since they are important in understanding fundamental behavior seen in syngas and low calorific gas combustion.