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 nonmonotonic (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 stationary 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 observed. 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 characterized 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 & ndash;2.7 Hz by the repetitive action of positive (by burnt gas) and negative (when the fuel heavier than air) buoyancies.