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 N2, He, and Ar. A non-monotonic (decreasing and then increasing) liftoff height (HL) behavior with jet velocity (U0) was observed previously for methane fuel diluted with N2. To further elucidate the fuel Schmidt number (ScF) effect in exhibiting such a non-monotonic (U-shaped) behavior, various diluents (N2, 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 ScF and fuel density; nozzle-attached flame, stationary lifted flame, and oscillating flame. Among stationary lifted flames, two distinct HL behaviors with U0 were observed; monotonic and non-monotonic HL behaviors. A critical Schmidt number (ScF,cr1) existed over which monotonically increasing behavior was observed. A second critical Schmidt number (ScF,cr2) also existed such that U-shaped behavior was observed for ScF,cr2<ScF<ScF,cr1. An oscillating lifted flame was observed for ScF<ScF,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 HL cases (including the increasing regime in U-shaped behavior), HL behavior can be characterized in terms of ScF, the density difference between fuel and air, ScF,cr1, and U0. While the decreasing HL regime in the U-shaped behavior can be characterized with ScF 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.