Experimental Investigation of Cavitation-Bubble-Induced Atomization

Licentiatavhandling, 2019

Improving the efficiency and lowering the emissions of internal combustion engines (ICE) has been drawing increased attention because climate change caused by greenhouse gases and the need to reduce emission harmful to human health, and stricter legislation on the emissions from liquid-fueled transportation is implemented. Design of the liquid fuel atomizer, which atomizes the fuel delivered into the combustion chamber by the fuel injection system, is important because a good atomization, which facilitates the evaporation of fuel and improves the fuel-air mixture, leads to higher efficiency and lower emissions of ICE. Cavitation which originates inside the atomizer nozzle, especially for the high injection pressure ones, has great influence on the liquid atomization. Aiming at providing validation data of cavitation bubble collapse induced spray break-up for Computational Fluid Dynamics (CFD) simulation, which is a powerful tool for understanding the mechanism of atomization and the design of fuel atomizers, a series of experiments were carried out to investigate the influence of individual cavitation bubbles generated by laser light.

As a first step, to obtain information on laser-induced cavitation bubbles, an experiment was set up to investigate bubble formation in a glass cell (cuvette) filled with water. A pulsed Nd:YAG laser was used for bubble generation and for imaging, a schlieren set-up with an intensified CCD camera was used. Bubble generation, collapse, rebound, etc. was imaged, as well as the shock waves associated with these events. The relationship between bubble size and laser pulse energy was investigated.

As the next step, bubbles were generated close to the free water surface in the cuvette. A dimensionless variable γ, which is the distance of the initial bubble centroid to the free surface divided by the maximum bubble radius, was implemented. The experimental cases covered a wide range of γ values (from 0.86 to 2.24). Time resolved shadowgrams of bubbles and free surface deformations were used to analyze the bubble and free surface dynamics in different cases. The results showed the typical surface deformations of a fast spike jet and a slower thick jet at low γ values (<1.0), merged spike and thick jets when the γ value was in certain higher range (~1.1-1.3), and just protrusions for even higher γ values.

Finally, the effect of laser-induced cavitation bubbles on jet break-up was studied in a continuous flow rig, where the water exited from a transparent nozzle at various flow velocities. Shadow images of the jet or spray were recorded by a high-speed video camera. The break-ups induced by the bubble collapse, were measured, compared and analyzed under different injection pressures and bubble generating positions. The break-ups were categorized into small and massive breakups. The distance of the laser focus to the center axis of the nozzle was found to be the main factor that determined the type of break-up.