Microscopic study on diesel spray under cavitating conditions by injecting fuel into water

Cavitation is a phenomenon which can significantly enhance spray breakup and affect the mixture preparation by introducing air or vapour bubbles. Attempts were made to capture the bubbles due to cavitation by injecting fuel into water based on the difference of density and refraction between water and vapour bubbles apart from the attempts to directly visualize vapour bubbles in an enlarged transparent nozzle. Tests were carried out with the employment of highly resolved microscope and ultra-high speed camera. The spray morphology and primary breakup characteristics in the near field were also investigated. It was found that irregularly shaped bubbles due to cavitation under low pressure can be observed in the nozzle. The increase of injection pressure (thereby increase of velocity) could significantly prolong the bubbles. At the nozzle outlet, the significant variation of pressure prolonged air bubbles into “thread” before collapsing. Vapour bubbles after exiting the injector nozzle were very small and the resolution of 3 µm/pixel was insufficient to capture the bubbles through direct visualization. A tree-shaped spray tip with significant radial propagation was observed. The sucking-in of water when the needle started to lift, the flow regime and the strong water resistance were closely related to the formation of tree-shaped spray tip. In addition, although the increase of fuel temperature generally enhanced the atomization of spray, the primary breakup of spray during the initial stage and end stage when the spray velocity was low surprisingly deteriorated for hot fuel.