Visualization Study of the Effects of Fuel Viscosity and Surface Tension on Cavitation Flow in Full-Scale Transparent Nozzles

Abstract The pursuit of cleaner combustion in engines has positioned alternative fuels as a critical enabler, yet their diverse range of viscosities and surface tensions poses a fundamental challenge to the regulation of nozzle internal flow behavior. In this study, full-scale transparent nozzles and high-speed microscopic imaging are employed to directly visualize the fuel internal flow. Additionally, glycerol–AES (sodium fatty alcohol ether sulfate)–aqueous solutions are formulated to decouple the effects of viscosity and surface tension, enabling a quantitative and independent investigation of their impact on cavitation flow. The results show that cavitation inside the nozzle undergoes clear temporal evolution during injection. Three distinct cavitation regimes were observed and defined, which are geometry-induced cavitation, needle-originated string cavitation, and hole-sac string cavitation. Their evolution is governed by the coupled effect of flow separation, vortex dynamics, and nozzle geometry. Surface tension is shown to have a negligible influence on string cavitation, in contrast, fuel viscosity plays a critical and non-monotonic role by modulating vortex evolution. Higher viscosity delays the inception of string cavitation while enhancing its stability and duration. Another key finding is that the relative position between the nozzle hole inlet and the conical surface of the needle valve tip significantly affect the initial position and reflux zone position of the fuel flow and separation in the sac, thereby determining whether increased viscosity enhances or suppresses the cavitation intensity. This study provides the experimental basis and the theoretical guidance for application of alternative fuels and design and optimization of nozzle structures.