In Vitro Microbubble Nucleation and Sanal Flow Choking Induced Combustion Instabilities in Liquid Rocket Propulsion

Combustion instability presents a persistent challenge in liquid rocket propulsion, particularly in environments where multiphase flow interactions exacerbate instability mechanisms. This study investigates the role of microbubble nucleation, growth, and rupture in hydrocarbon fuels under reduced-pressure conditions, highlighting their influence on Sanal flow choking and combustion dynamics. Controlled in vitro experiments were conducted using petrol, diesel, and kerosene within a custom-built vacuum chamber equipped with AI-assisted imaging to characterize void fraction and bubble size distributions. Petrol exhibited the highest void fractions and largest bubbles between 250–450 mmHg, owing to its high volatility and low surface tension. Diesel, with greater viscosity and boiling point, showed minimal nucleation, while kerosene displayed stable vaporization behavior—indicating more favorable thermal balance for propulsion applications. The formation of vapor bubbles alters the fluid’s acoustic properties, significantly reducing the effective speed of sound to 10–100 m/s, as described by classical models (Wood, 1930; Silberman, 1957; Leighton, 1994; Prosperetti, 2015). This acoustic softening enables localized sonic conditions at low bulk velocities, even in the absence of geometric restrictions, leading to Sanal flow choking. When the internal-to-external bubble pressure ratio exceeds a critical threshold during rupture, steep pressure gradients and localized shock waves are generated (Kumar and Radhakrishnan, Springer Nature 2024), triggering microdetonation-like events and intensifying combustion instability. These effects are further amplified in high-altitude and cryogenic environments where low ambient pressures favor cavitation and phase transitions. The results underscore the need for precise control over thermodynamic conditions, fuel volatility, and void fraction management. By integrating classical acoustic models with emerging multiphase dynamics, this study provides new insight into the mechanisms driving instability and informs injector design, atomization strategies, and pressure control for enhanced propulsion system reliability in both terrestrial and spaceflight environments.