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Experiments were performed to analyze the interaction of an explosively driven shock wave and a propane flame. A 30 gram explosive charge was detonated at one end of a shock tube with a 3 m length and 0.6 m diameter to produce a shock wave which propagated down the tube and out into the atmosphere. A propane flame source was positioned at various locations outside of the shock tube to investigate the flame response to different strength shock waves. Synchronized high-speed digital cameras were used to image the events. One grayscale camera performed retroreflective shadowgraph imaging to visualize the shock wave motion and flame response. The other camera was a color camera which imaged the flame and shock tube exit directly. Piezoelectric pressure gages were used to record free-field pressures at various locations throughout the test setup. The explosively driven shock tube was shown to produce a repeatable shock wave and a large vortex ring. Digital streak images show the shock wave and vortex ring expansion and propagation throughout the field of view. The high-speed shadowgraph images show that the shock wave extinguishes the propane flame by pushing it off of the fuel source. Even a weak shock wave was found to be capable of extinguishing the propane flame.
Explosively-driven shock waves were observed to extinguish flames by blowing the flames off of the fuel source with the velocity that the shock wave imparted to the ambient air. Experiments were performed with varied positions of the flame source relative to the explosively-driven shock tube, and in all cases the flame was extinguished by the same mechanism. This is contrary to results obtained in the laboratory with a smaller shock tube, where the vortex ring was instrumental in extinguishing the candle flame sources. One test showed no flame extinguishment, with the flame source located almost 8m from the shock tube exit. If the imparted air velocity is the only extinguishment mode for all of the scenarios tested, then the minimum air velocity needed to extinguish the flame is between 15-30 m/s.
These tests documented the propagation velocity of a shock wave produced with a length of detcord that was contained in a 0.6 m diameter shock tube. High-speed shadowgraph imaging allowed tracking of the shock wave and imaging of its temporal evolution. The shock wave velocity is initially constant after exiting the shock tube, but then begins to decay the same as a free-air blast. The constant velocity region was found to be approximately one shock-tube radius in length. Free-field pressure gages showed the shock wave overpressure was similar to a free-air blast.
Future testing will focus on determining the scalability of this method for extinguishing flames and evaluating performance with different flame sources. The flame source will be varied in an attempt to control and/or modify the flame attachment methods. One major flame condition of interest is a pool fire. Variations in the geometry of the shock tube will also be explored, including modifying the exit plane geometry to change the shock velocity and vortex ring characteristics. A series of tests to vary the explosive mass, shock speed at the fire, and scale of the fire is of interest. Evaluating the changing role of the vortex ring as the shock wave strength decreases with decreasing explosive driver is also of interest. Experiments to identify the minimum shock Mach number that will blow out the flame should be performed via more tests at distances between 3-8m from the shock tube exit. Computational fluid dynamics (CFD) modeling is also currently underway to gain further insight.
Research and Experimentation was performed in conjunction with the University of New South Wales.