Effect of buoyancy by fuel on flickering mode transition in jet diffusion flames

Abstract

This study investigates the transition of flickering instability from varicose to sinuous mode in buoyant jet diffusion flames by varying the Reynolds number (ReD). Fuels tested include neat methane, ethane, propane, and their diluted mixtures with He, N2, and CO2. These flames generate positive buoyancy due to burnt gas, while the fuel stream can exert either positive or negative buoyancy, depending on its density relative to air. The buoyancy effects are characterized using two Richardson numbers: Rib (burnt gas) and RiF (fuel stream). As ReD increases, the flickering mode has a transition from varicose to sinuous. Two distinct regimes are identified: Regime I, where both the flame and fuel exhibit positive buoyancies (RiF > 0); Regime II, where negative and positive buoyancies coexist (RiF < 0). In Regime I, mode transition is promoted by increasing ReD and decreasing Rib and RiF; in Regime II, it is enhanced by increasing ReD and decreasing Rib and |RiF|, with RiF playing a dominant role. Flame asymmetry, quantified by an asymmetry factor (As), reveals strong correlations with ReD, Rib, and |RiF|. From a dimensional analysis considering fuel-specific heat release, As/ΔH∗R,i is expressed as a function of ReD, Rib, and |RiF|, showing Rib’s greater influence in Regime I and RiF’s in Regime II. A critical Reynolds number(ReD,cr) for the transition is defined as the point where dAs/dReD peaks. Phenomenological correlations for ReD,cracross different fuel mixtures emphasize the need to incorporate the normalized heat of combustion (ΔH∗R,i). The findings highlight the significant role of RiF, especially when the fuel is heavier than air and its effect opposes the buoyancy of burnt gas, in governing the instability mode transition.