A Study of the Mechanisms Leading to Re-ignition in a ‘‘worst Case” Fire Scenario

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 vaned 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 caracterization 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.