In the present work, an underwater shock wave was applied to an explosive bubble, which then generated a spherical shock wave after shrinking process. A special care was taken for arrangement of the bubble and a pressure transducer so as to measure pressure behind the spherical shock wave originating from single bubble expansion. The bubble was made of a stoichiometric ethylene-oxygen mixture and its initial equivalent radius ranged from 1.0 mm to 2.2 mm. An incident underwater shock wave (ISW) was driven by gaseous detonation which propagated towards water surface, giving a peak pressure behind ISW (Pi) from 6 MPa to 23 MPa. Shadowgraph images of the bubble show that it starts to shrink after passage of ISW and then emits a light indicating combustion during the shrinking phase. When the bubble turns to expansion, it generates a shock wave (BSW) propagating spherically into the surrounding water. The experimental results reveal that the non-dimensional maximum pressure behind BSW (Ppeak/Pi) is almost inversely proportional to the non-dimensional measurement distance from the bubble based on the initial radius of the bubble. The energy conversion efficiency from the bubble energy to the shock energy is dependent on the momentum acquired by water around the bubble, which is estimated based on the concept of the Kelvin impulse. The measured shrinking time of the bubble is found to be in good agreement with the Rayleigh collapse time.
Active radical species are generated on thermally-excited titanium dioxide (TiO2) under air conditions. Oxidation and decomposition of unburned compounds, such as CO and acetaldehyde, were investigated using this activation of TiO2. The materials used were TiO2 bead and TiO2/SiO2 composite bead. The supporting ratio of TiO2 on the composite bead used was 14% or 20%. Oxidation of CO to CO2 was 70% at 500 ℃ using TiO2 bead, though that was about 20% using TiO2/SiO2 bead. Decomposition of acetaldehyde was over 90% at 300 ℃ using TiO2 bead or TiO2/SiO2 bead. The existence of TiO2 particles on the surface and in the nano-pore of silica would allow the effective decomposition of acetaldehyde. It was assumed that the thermally-excitation of TiO2 was caused by the formation of lattice defect, that is oxygen defect, in TiO2 under high temperature followed by the active radical generation.