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Ignition and Flame Propagation Studies Over a Flat Fuel Surface.

pdf icon Ignition and Flame Propagation Studies Over a Flat Fuel Surface. (3298 K)
Amos, B. T.

NIST GCR 92-604; 148 p. March 1992.


National Institute of Standards and Technology, Gaithersburg, MD

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fuels; ignition; flame propagation; combustion; diffusion flames; predictive models; premixed flames; radiation; surface temperature; radiation ignition


Numerical studies are performed which show the evolution of the combustion process over a flat fuel surface subjected to an external source of radiation. Ignition is caused either by the high temperature of the fuel surface or by radiation absorption by the fuel vapor. The surface is assumed to be either in a zero gravity, initially stagnant air environment or in a stagnation point flow field. Regardless of the source of ignition considered or the type of the flow field, the same sequence of events is predicted. This sequence of events begins with a pre-ignition, radiation dominated phase in which fuel and air mix above the fuel surface. After ignition occurs, there is a period of weak chemical reaction, which is followed by a period of stronger reaction in which a premixed flame front develops. Before dying out the premixed flame front separates the fuel from the oxygen and leaves behind a diffusion flame. The combustion and radiation processes are shown to have a large effect on the flow field in the stagnation point flow cases. For the case in which ignition is caused by gas phase absorption, the radiation required to cause ignition is so high that an opposed jet flow is created. In the case in which ignition is caused by the hot fuel surface, the radiation is lower and the boundary layer remains almost intact. For both types of ignition the premixed flame fronts produced heat fast enough that the expanding gas is able to drive the incoming flow back from the fuel surface. After the premixed flame front dies out leaving the diffusion flame the incoming flow again dominates and a boundary layer reappears.