Reacting shock bubble interaction

In this work, experimental results are presented for the first time on the shock-initiated combustion of a spherical density inhomogeneity. A converging lens geometry, in which a heavy bubble free-falls in a lighter fluid, is used to focus a planar shock wave resulting in the ignition of a mixture of H2, O2 and Xe (55%, 30% and 15%, respectively) in N2. The experiments are performed at the Wisconsin Shock Tube Laboratory in a 9 m vertical shock tube with a 25.4 cm square cross-section. A pneumatic injector is used to generate a 4 cm diameter soap film bubble filled with the combustible mixture. The injector retracts flush into the side of the tube releasing the bubble into a state of free-fall. The initial conditions are captured with two high speed CMOS cameras orientated perpendicular to one another. Simultaneous Mie scattering and chemiluminescence diagnostics are used to image the post-shock bubble morphology and combustion, respectively. The ignition delay times are measured for several incident shock strengths (1.3 < M < 2.9, where M is the Mach number) and range between 5 μs and 400 μs. The ignition delay times are used in combination with 1D gas dynamics to estimate the effect of shock-focusing. It is observed that two limiting cases emerge, with the combustion characteristics depending heavily on the incident shock strength. A comparison to non-combusting mixtures is made by matching the density of the bubble mixture without the fuel (49% Xe, 51% O2).