Study of Liftoff Mechanism of Nonpremixed Jet Flame near Unity Schmidt Number

Nonpremixed jet flames of dimethyl ether (DME) were studied both experimentally and theoretically to investigate flame liftoff near unity Schmidt number. It was found experimentally that although the DME nonpremixed flames have a Schmidt number larger than unity it cannot be lifted directly by increasing the flow rate. Lifted flames can only be established by igniting the mixture in a narrow region downstream of the jet at low flow rates. The results also show that the liftoff flow rate is less than that of the blowout limit of the attached flame. Theoretically, the self-similar Landau-Squire solution for a round jet is revisited and the combined effects of stretch and flame curvature on triple flame propagation speed were considered. It was found that the critical Schmidt number for liftoff shifts around unity. The critical Schmidt number is less than unity for fuel Le numbers larger than 0.5 and larger than unity for Le numbers less than 0.5.