A High-Intensity Focused Ultrasonic Beam Generates Cavitation-Induced Micro-Combustion in Fuel

Cavitation induced by exposing micro-bubbles to a High-Intensity Focused Ultrasonic (HIFU) beam triggers impulse micro-combustion in fuel (kerosene). Mono-dispersed trains of 50-80 μm diameter microbubbles containing fuel vapor and air (oxygen) are exposed to the HIFU beam, causing their rapid collapse. The resulting high temperature and pressure initiate controlled localized combustion. The experiments involve high-speed (20-50 kHz) and ultra-high-speed (1-5 MHz) shadowgraphy and holography in a specialized facility that enables us to observe the onset of cavitation in the micro-bubbles, their fragmentation into bubble clouds, and their transition to sustained cavitation. Bright flashes, presumably involving transient plasma formation, followed by propagating shockwaves, appear during the bubble collapse, both in water and in fuel. However, thermal plumes form around the bubble clouds only in fuel, suggesting elevated temperatures resulting from sustained combustion. At high ultrasonic power, the process becomes self-sustained, as continuously generated new bubble clouds undergo periodic expansion and collapse. This process is amplified near solid walls, presumably due to the increased number of nucleation sites and enhanced pressure caused by the reflection of the ultrasonic beam.