Experimental and Numerical Studies on Acoustic Softening and Flow Choking in Liquid Propulsion System

Recent advances in liquid propulsion research highlight the critical influence of bubble nucleation, acoustic softening, and multiphase flow choking in driving severe pressure transients, particularly in pressure-gain configurations. Through combined experimental measurements and numerical simulations, this study demonstrates that bubble nucleation in diesel fuel under reduced static pressure significantly decreases the local speed of sound, enabling Sanal Flow Choking (SFC), wherein localized Mach numbers rise sharply despite low bulk velocities. A key observation is that bubble collapse under SFC conditions produces intense pressure spikes and physically driven solver divergence, reflecting rapid localized energy release within a compressible multiphase medium rather than numerical error. Vacuum-chamber experiments confirm pressure-driven microbubble formation, growth, and phase transformation, while CFD simulations qualitatively capture vapor generation, acoustic softening, and void-fraction–induced compressibility effects. The results show that increasing void fraction produces pronounced acoustic softening, driving the effective sound speed toward collapse. Experimental data corroborate predictions from Wood’s equation, revealing sound-speed reductions to below 10 m/s and demonstrating that choking can occur even in creeping bubbly flows. These findings underscore the pivotal role of bubble dynamics in altering the acoustic and thermodynamic response of propulsion-grade fuels, with implications for multiphase SFC-induced choking and the onset of shock- or detonation-like pressure events in liquid and cryogenic propulsion feedlines and injectors.