Effects of Thrust Scaling on Pintle-Injector Atomization Under Chamber Geometric Constraints

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.