Interactions of shock tube exhaust flows with laminar and turbulent flames

The interactions of flow features emitting from open-ended shock tubes with free-standing propane flames have been investigated using high-speed schlieren imaging and high-frequency pressure measurements, with additional data from validated numerical modeling. Both compressed air-driven interactions with non-pre-mixed laminar diffusion flames (small-scale) and explosively-driven interactions with turbulent non-pre-mixed turbulent flames (large-scale) were tested for various flame locations and shock tube stagnation pressures (and therefore Mach numbers). In the small-scale tests it was observed that the flames were not significantly influenced by the passage of either the initial shock if placed close to the tube exit, or the weaker pressure waves downstream if the flame was placed further away. Four types of interaction were classified, three of which led to permanent extinguishment of the flames. The most effective mechanism of extinguishment for a flame in-line with the exhaust was the axial exhaust jet of expanding air, which served to push the flame off the fuel source either at close range (Type I) or more slowly at a distance (Type II), after which rapid cessation of combustion occurred. With the flame positioned to one side of the path of the jet, strong loop vortices achieved a similar overall outcome of extinguishment, albeit with very different flame behavior in reaction to the strong turbulence and vorticity induced by the passing flow (Type III). In all cases bar one, the disruption to the fire triangle caused by these flow effects was sufficient to extinguish – rapidly and permanently – the flame. However, at a sufficient lateral offset of the flame from the shock tube exit, the strength of rotating flow being entrained into the diffusing vortex ring was not sufficient to remove and disperse the heat from the extinguished flame (Type IV), such that re-ignition could occur. By contrast, in the large-scale tests with a significantly different shock pressure profile and a flame approximately 1 order of magnitude greater, extinguishment in all cases for all shock strengths and locations was achieved by the shock itself (accelerating combustion) and the following “blast wind” impulsively moving the flame off the fuel source, with the vortices having negligible effect at the given testing locations (Type V).