Effects of Schmidt number on non-monotonic liftoff height behavior in laminar coflow-jet flames with diluted methane and ethylene

Abstract

Stabilization characteristics of laminar lifted jet flames in a coflow were investigated experimentally to elucidate the effect of Schmidt number in methane and ethylene fuels diluted with N 2 , He, and Ar. A non- monotonic (decreasing and then increasing) liftoff height ( H L ) behavior with jet velocity ( U 0 ) was observed previously for methane fuel diluted with N 2 . To further elucidate the fuel Schmidt number (Sc F ) effect in exhibiting such a non-monotonic (U-shaped) behavior, various diluents (N 2 , He, and Ar) were added to the fuel streams and methane and/or ethylene fuels were used. The result showed three flame types in terms of Sc F and fuel density; nozzle-attached flame, stationary lifted flame, and oscillating flame. Among sta- tionary lifted flames, two distinct H L behaviors with U 0 were observed; monotonic and non-monotonic H L behaviors. A critical Schmidt number (Sc F,cr1 ) existed over which monotonically increasing behavior was ob- served. A second critical Schmidt number (Sc F,cr2 ) also existed such that U-shaped behavior was observed for Sc F,cr2 < Sc F < Sc F,cr1 . An oscillating lifted flame was observed for Sc F < Sc F,cr2 . The oscillating and stationary lifted flames can be categorized in terms of the density differences among the fuel, air, and burnt gas. For the increasing H L cases (including the increasing regime in U-shaped behavior), H L behavior can be char- acterized in terms of Sc F , the density difference between fuel and air, Sc F,cr1 , and U 0 . While the decreasing H L regime in the U-shaped behavior can be characterized with Sc F and/or the Richardson number (defined based on the density difference between fuel and air). Oscillating flames were observed with the frequency range of 2.1–2.7 Hz by the repetitive action of positive (by burnt gas) and negative (when the fuel heavier than air) buoyancies.