Influence of Fuel Surface Tension and Viscosity on Internal Flow Bubble Formation and Atomization in Flash Boiling Sprays

Flash boiling spray can achieve atomization effects similar to high-pressure subcooled sprays at lower injection pressures, which reduces engine hardware costs and aligns with the trend toward increased efficiency and miniaturization for hybrid vehicle engines. Thus, it is considered a promising next-generation fuel atomization technology. The atomization characteristics and spray structure of flash boiling sprays differ significantly from those of subcooled sprays, with distinct mechanisms in the primary breakup stage being a major cause of these differences. Previous studies have established the bubble-rupture primary breakup model, which is closely linked to bubble generation during internal flow. However, existing research has primarily focused on controlling the superheat index and injection pressure, without sufficiently considering the impact of fuel properties on bubble generation. As carbon neutrality goals advance, alternative fuels such as e-fuels (e.g., e-gasoline), low-carbon fuels (e.g., methanol), and zero-carbon fuels (e.g., ammonia, hydrogen) are likely to replace conventional fuels. The differing physical properties of these fuels could significantly affect the performance of flash boiling sprays. Therefore, understanding the impact of fuel properties on bubble formation and spray atomization is crucial for practical applications. This study investigates the effects of fuel properties using n-pentane, methanol, and water—liquids with varying viscosities and surface tensions—through a two-dimensional transparent nozzle experimental setup. High-speed microscopic backlight experiments are performed to analyze bubble generation in internal flow, while PDI experiments explore the influence of fuel surface tension and viscosity on external spray atomization. This research aims to refine the bubble-rupture breakup mechanism and provide a theoretical foundation for the practical application of flash boiling spray technology.