Lift-off characteristics of non-premixed jet flames in laminar/turbulent transition

Abstract

The stabilization mechanism of a laminar lifted flame has been explained theoretically. However, variation of the stabilization mechanism of the lifted flame in the transitional regime between laminar and turbulent flames has not been clarified. In this study, lift-off heights were estimated in relation to variation of the fuel tube diameter, fuel jet velocity, and proportions of methane and propane. With increase of the fuel jet velocity, stable lifted flames could be obtained under various conditions and the soot emission reduced. The stabilization mechanisms were explained based on the concepts of laminar and turbulent mixing cores. These were classified into five regimes: laminar similarity, laminar near-core, external mixing, transitional, and fully turbulent. Improved relationships between the flow rate and the lift-off height were proposed in two regimes (external mixing regime and the fully turbulent one). In addition, the blowout mechanism was explained using the turbulent intensity. Finally, an overall stabilization diagram showing the various stabilization mechanisms of the lifted flames was presented. (c) 2021 The Combustion Institute. Published by Elsevier Inc. All rights reserved.