A Study of Cavitation - Ignition Bubble Combustion

We present the results of an experimental and computational study of the physics and chemistry of cavitation-ignition bubble combustion (CIBC), a process that occurs when combustible gaseous mixtures are ignited by the high temperatures found inside a rapidly collapsing bubble. The CIBC process is similar to that in a diesel engine. However, the length scales are very small (μm to nm) and the time scales are very brief (μs to fs). Furthermore, the process takes place inside a micro-spherical chamber with variable dimensions rather than in a macro-cylindrical metal chamber with fixed dimensions. We computationally model the CIBC process using a 1and 0-dimensional time-dependent compressible fluid-dynamics code that includes finite-rate chemistry. The computational model of the CIBC process indicates that gas-phase reactions within the bubble occur and produce CO and other by-products of combustion, heat and mechanical energy release through a bubble volume-expansion phase. The model shows that the CIBC process is sensitive to the fuel-air mixture ratio, the initial bubble diameter, and the acoustic pressure forcing amplitude. We experimentally demonstrate the CIBC process using an ultrasonically excited cavitation flow reactor. In the flow reactor, we subject gaseous mixtures of C3H8-air and CH4-air bubbles in liquid water, and methanol vapor-air bubbles in liquid methanol, to circa 100 W of acoustic power at a frequency of 20 kHz. We measure small amounts (up to 160 ppm) of carbon monoxide (CO) emitted as a byproduct of the gas-phase chemical reactions or combustion within the collapsing bubbles. We find that the CO production is proportional to the acoustic power level delivered to the CIBC flow reactor. The results of the model were found to be consistent with the measured experimental results. Based on the experimental data, and supported by the results of the computational model and previous reports of the “micro-diesel effect” in industrial hydraulic systems, we determined that gas-phase chemical reactions that are initiated by the high temperatures within a collapsing bubble are indeed possible and exist in ultrasonicallyand hydrodynamically-induced cavitation. Using the results from the computer model of CIBC of a CH4-air bubble in water, we find that, in theory, it may be possible to develop a hydraulic-CIBC engine that produces net power. The results of the model indicate that such an engine would require at minimum the following conditions to operate at a break-even point: a liquid flow rate of about 114 liter ⋅ min (31 gal ⋅ min) with a 10 percent bubble void fraction of approximately 10 μm diameter CH4-air bubbles in water, with an equivalence ratio of about 1.25, and a pressure drop of about 21 kPa (3 psid) provided by a venturi with a pressure recovery factor of at least 85 percent. A significant result of this finding is that a hydraulic-CIBC engine requires a venturi or other pressure recovering device to operate in a self-sustaining mode.