Asymptotic and numerical study of the stabilization of diffusion flames byhot gas

Recirculating hot combustion products play an essential role in the stabili zation of diffusion flames in a combustor. An idealized one-dimensional con figuration is utilized to elucidate this process. It consists of three para llel streams: to the left, a layer of fuel at subignition temperature; an i ntermediate finite thickness layer of oxidizer at the same low temperature; and to the right, a layer of hot combustion products. Using numerical simu lations, three distinct ignition regimes are identified as the thickness of the intermediate layer is decreased, with corresponding ignition time vari ations spanning several orders of magnitude. The main speed-up mechanism is attributed to the abrupt transition from ignition via a classical triple f lame at the fuel-oxidizer boundary, to ignition at a competing site located at the boundary between the cold oxidizer and the hot products. High activ ation energy asymptotics leads to a practical procedure to relate the inter mediate layer thickness and the ignition time. (C) 1999 by The Combustion I nstitute.