A Study of the Mechanisms Leading to Re-Ignition in a Worst Case Fire Scenario
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Date
06/2000Author
Jomaas, Grunde
Roberts, B T
DuBois, Jacqueline
Torero, Jose L
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Abstract
A systematic evaluation of the stability of a re-circulation zone behind a backward
facing step under conditions expected in an aircraft engine nacelle has been conducted
together with the evaluation of the effects of the flow structure on a propane diffusion
flame established downstream of the step. The objective being to characterize a “worst
case” fire scenario. Characterization of the non-reacting re-circulation zone was
performed by means of flow visualization. The parameters varied were the flow velocity,
step height and surface temperature. Numerical modeling using a Large Eddie
Simulation (LES) code has been contrasted with the experimental results. It was
observed that for all conditions studied a flow re-circulation zone appears down-stream of
the step and is stable but not stationary. The temperature of the floor of the test section
was increased up to 600°C to explore the effect of buoyancy without the complexity of
the reacting flow. Heating of the incoming flow lead to an increase in the dimensions of
the re-circulation zone. However, de-stabilization of the flow did not occur. Comparison
between the numerical and experimental results shows good qualitative agreement. The
characterization of the flame established behind the backward facing step was followed by
a study of the re-ignition potential. The flame was extinguished by separating fuel from
oxidizer by means of a plate which was impulsively removed and re-ignition observed. It
was established that re-ignition is controlled by cooling and mass transport towards the
hot plate. A “worst case scenario” for re-ignition is given by maximizing the fuel mass
transfer while keeping the characteristic time for cooling of the fuel surface shorter than
the characteristic time to attain a flammable mixture.
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