Mechanism of periodic spray-morphology evolution of heavy aviation fuel driven by gas-liquid coupling in an air-assisted fuel injection premixing chamber

Excellent atomization performance can significantly enhance combustion efficiency and reduce pollutant emissions, making it a key technology for meeting increasingly stringent environmental requirements and high-performance aerospace propulsion demands. This study reveals a previously unobserved phenomenon in air-assisted fuel injection (AAFI) systems: a periodic, cyclic variation of the spray morphology. Using diffused background illumination (DBI) imaging over 120 consecutive tests, we discovered for the first time that the spray systematically alternates between a “spindle-shaped” (SS) and an “anchor-shaped” (AS) form. These morphologies exhibit starkly contrasting characteristics. The spindle-shaped spray is dominated by axial penetration (mean: 42.3 mm), whereas the anchor-shaped spray features significant radial expansion (mean cone angle: 78.5°). Quantitative image analysis confirms the anchor-shaped spray possesses a considerably higher optical density, with its mean grayscale value being 6.3 % higher than the SS spray. Further investigation traced the origin of this cyclic behavior to the premixing chamber. Here, complex gas-liquid coupling effects lead to the dynamic deposition of fuel on internal surfaces. This periodic fuel accumulation and stripping governs the actual ejected fuel mass (mf), which in turn dictates the spray's evolution. A high mf leads to lower injection velocity and dominant radial expansion (the AS), while a low mf results in higher velocity and dominant axial momentum (the SS). These findings reveal the intrinsic physical link between the fluid dynamics within the AAFI premixing chamber and the external spray morphology, demonstrating the determinative role of the internal flow characteristics in shaping the macroscopic spray structure.