Breakup of fuel sprays under cavitating and flash boiling conditions

Abstract

Cavitation and flash boiling are important phenomena for fuel sprays of IC engines and can significantly boost spray breakup by introducing vapour bubbles, enhancing combustion, energy conversion efficiency and reducing emissions. In this study, attempts were made to capture the bubbles either due to cavitation or flash boiling under cavitating and flash boiling conditions by injecting fuels into liquid. The differences in density and refraction between water and vapour bubbles enable the bubbles to be visualized. The evolution of vapour bubbles in transparent nozzle was investigated. The direct visualization of vapour bubbles was carried out with the employment of highly resolved microscope and ultrahigh-speed camera. The spray morphology and primary breakup characteristics in the near field and macroscopic characteristics in the far field were also investigated. It was found that vapour bubbles due to cavitation or flash boiling were very small with the diameter of less than 9 µm after the primary breakup at the nozzle outlet. A tree-shaped spray tip with significant radial propagation was observed for diesel and isooctane fuels but not for dieseline (75% gasoline and 25% diesel) under liquid ambient condition. The increase of fuel temperature generally enhanced the spray atomization for the steady stage but surprisingly suppressed the primary breakup during the initial stage and end stage when the spray velocity is low. In addition, the strong disturbance of flow led to considerable radial propagation for cold diesel spray (20 °C) while the strong cavitation or flipping flow resulted in the significant radial expansion for hot gasoline spray (140 °C).