Scaling of Gas-Jet Flame Lengths in Elevated Gravity

Abstract

There are two well known scaling laws for lengths of laminar jet diffusion flames on circular burners. The more prevalent of these is a linear relationship between normalized flame length and Reynolds number. The other, invoked in most past studies at elevated gravity, relates these lengths to a function of Reynolds and Froude numbers. The Reynolds scaling indicates stoichiometric flame lengths are independent of gravity level, while the Reynolds-Froude scaling indicates lengths decrease at elevated gravity. The ability of both approaches to correlate flame lengths is examined. Published lengths of laminar hydrogen, methane, ethane, and propane flames in 1–15 times earth gravity are considered. The Reynolds scaling yields better correlations of flame lengths for all fuels except hydrogen. The Reynolds-Froude scaling has a weaker theoretical basis and does not account as accurately for variations in fuel flowrate. Further, it does not admit microgravity flames and past predictions for its limiting behavior at low and high Froude number are not supported. Observed reductions in luminous flame length at elevated gravity are attributed to soot interference. The Reynolds scaling is recommended for flame lengths at microgravity, normal gravity, and elevated gravity.