Cavitation detection and characterization for small scale nozzles and fuel injectors

Abstract

Cavitation occurs when the liquid pressure drops below a critical threshold causing rapid bubble growth and violent collapse. The presence of cavitation inside fuel injector nozzles has been linked not only to damage associated with cavity collapse near the walls but has been found to enhanced fuel spray atomization. Proper fuel atomization increases engine performance while reducing fuel emissions. The majority of laboratory experimental studies found in the literature are highly reliant on optical access to the working fuel, requiring transparent material, specific geometry, and relatively slow flows to enable even minimally time-resolved optical images with ex-pensive high-speed cameras. While such studies are appropriate for gaining intuition into the types and spatial locations of cavitation phenomena possible in nozzle flows at high Re, there is a need for techniques which can not only be reliably employed at idealized laboratory conditions, but also can be deployed to study real steel fuel injectors while also yielding quantitative information. The objective of this work is to develop alternative non-intrusive acoustic and vibration methods to experimentally study cavitation phenomena in fuel injectors. First, a study was conducted utilizing a combination of optical and acoustic techniques to determine onset and activity of cavitation in small scaled nozzles. Experiments are conducted with acrylic nozzles of various geometry. Unfocused single element transducers are used for acoustic sensing, while digital imaging is used for optical study. Cavitation onset thresholds and development are studied as functions of flow rate and nozzle geometry. Substantial agreement between optical and acoustic methods was observed for both onset and development regimes of cavitation in nozzles. A second study was conducted utilizing laser Doppler vibrometry to measure the vibration response of a commercial fuel injector. An attempt was made to use injector flexural oscillations to determine the void fraction for different fuel injector conditions. Experiments were performed at the Oak Ridge National Laboratory on a commercial grade field injector using cyclopentane fuel with varying injection pressure and fuel temperature as control parameters. Frequency shift and mode shape are measured and correlated with cavitation-inducing control parameters. Analysis suggests that observed frequency shifts may allow inference of dynamic void fraction during cavitation in nozzles.