Microbubble Nucleation and Acoustic Softening Trigger Multiphase Sanal Flow Choking and Shock Waves in Liquid Rockets

This study presents a comprehensive investigation into the role of microbubble nucleation, acoustic softening, and multiphase Sanal flow choking in triggering shock waves and flow instabilities within liquid rocket propulsion systems. Through integrated in silico simulations and experimental studies, we analyze how vapor pressure, heat capacity, and system pressure govern microbubble formation, flow choking, and subsequent shock generation. Elevated vapor pressures combined with low heat capacity in propellants promote bubble nucleation under transient decompression, particularly during starting transient and high-altitude operation. Diesel fuel simulations reveal that even minor bubble formation drastically reduces local sound speed—from ~1200 m/s in pure liquid to below 50 m/s in vapor-laden regions—leading to premature Sanal flow choking at low bulk flow velocities. This localized acoustic softening enables flow choking and shock wave initiation due to streamtube expansion well before classical choking conditions. Bubble collapse further amplifies pressure gradients, introducing thermodynamic discontinuities that mimic shock-like structures and potentially trigger microdetonation events, especially in reactive fuels. The findings have important implications for propulsion systems such as rotating detonation engines, pulse detonation engines, and cryogenic fuel systems where bubble-induced instabilities may compromise performance. Parametric studies demonstrate that increasing heat capacity delays vapor formation, while maintaining static pressures above vapor pressures across pipelines, valves, and injectors is critical to suppressing cavitation, choking, and shock formation. The study also addresses the compounded challenges in microgravity, where altered surface wettability enhances bubble growth. Hybrid wettability surfaces show promise in stabilizing flow by suppressing slug formation and controlling bubble dynamics. This research establishes a unified framework linking microfluidic behavior, acoustic softening, and multiphase flow choking, offering predictive guidelines for propellant formulation, pressure management, and flow control to enhance stability, reliability, and safety in liquid rocket propulsion.