This study examines self-pulsation spray oscillations in a gas-centered swirl coaxial (GCSC) injector using high-speed photography and numerical simulation. For the first time, high-speed photography captured the unsteady gas–liquid flow inside the GCSC injectors' recess chamber, revealing three spray modes: stable liquid-film breakup, intermittent oscillatory breakup, and purely aerodynamic breakup. Mid- to high-frequency oscillations are governed by the gas–liquid momentum flux ratio and the recess ratio (RR). The core mechanism is a “pressure-relaxation” cycle triggered by periodic gas–liquid blockage in the recess. In this cycle, high-speed airflow expands and compresses the liquid film, causing pressure to build up and then release through the nozzle as a closed-loop oscillation. In stable film-breakup mode at M < 0.1, high-frequency oscillations can arise from Kelvin–Helmholtz waves and viscous shear, even without gas–liquid blockage. Frequency analysis shows that oscillation frequency is closely related to momentum ratio, liquid Reynolds number, and gas Weber number. High-frequency oscillations occur when 0.1 ≤ M < 0.8 and Rel >15 000 (J < 10, Weg < 6000) but disappear when M ≥ 0.8 and Rel < 20 000 (J ≥ 10, Weg ≥ 6000), leaving low- and medium-frequency instabilities. A Strouhal number correlation shows a peak at a recess ratio of 1.4 (∼2–4 kHz), a drop to ∼0.5–2 kHz at RR = 1.7, and no high-frequency oscillations at RR ≈ 1. These findings offer insight into self-pulsation mechanisms and provide guidance for optimizing injector design to mitigate high-frequency combustion instabilities in rocket thrust chambers.

Laser-induced explosions inside droplets can generate cavitation. This study investigates the transient cavitation process induced by laser-induced shock waves in spherical droplets. Modeling a mixed fluid of water, vapor, and air using a barotropic equation of state. And all phases are compressible. Using an explicit density based solver, 3D (three-dimensional) simulations were conducted for two eccentricity conditions of ε=0.4 and ε=0.6. The results successfully captured the coupling effect between shock waves and two-phase interfaces, as well as the development law of cavitation bubble clouds inside spherical droplets. A prediction of wavefront evolution process was established using ray parameter equations, providing a path diagram for the development of shock/rarefaction waves. Experiments, 3D simulations, and ray parameter equations confirm each other, revealing the core physical mechanism behind the generation of cavitation bubble clouds: the spatial convergence of rarefaction waves within spherical droplets leads to cavitation.

Extrapolating pintle-injector atomization from subscale tests to full-scale engines requires accounting for the spray-development space available within the thrust chamber. The objective of this study is to determine whether atomization similarity persists during injection-element enlargement and how the available chamber space constrains extrapolation. Compressible volume-of-fluid and Lagrangian-particle simulations are conducted for single gas–liquid orifice-type pintle injection elements at three prescribed geometric scales under matched inlet velocities and momentum conditions, and the resulting sprays are assessed against reference chamber geometries. The primary spray topology, interfacial-wave structure, and breakup position relative to injector size remain broadly similar across the scales, with the normalized liquid-column breakup height remaining approximately 6–7. For the sampled Lagrangian droplets, the median diameter increases from 86 to 308 and 565 μm with injector enlargement, while its value normalized by the element dimension decreases. Nonproportional injector–chamber scaling sharply reduces the radial development margin, with the normalized radial clearance decreasing by 55.7% between the 500 and 850 kN configurations, increasing potential wall-interaction risk. These findings distinguish element-level atomization similarity from chamber-level geometric compatibility and show that preserving local breakup behavior alone does not ensure full-scale applicability. By linking controlled injection-element enlargement with the spray-development space available in representative chambers, this study extends element-level atomization analysis to the assessment of geometric constraints on subscale extrapolation and provides a basis for matching injector dimensions to available chamber space in high-thrust pintle engines.