Unraveling the Transition from Standard to Reactive Atomization of Hypergolic Propellants

The rocket propulsion community is actively seeking nontoxic, stable propellants to replace hazardous hydrazine-based options. This study provides a comprehensive analysis of the hypergolic ignition and spray characteristics of a newly developed eco-friendly self-igniting fuel (named PAHyp 3) with rocket-grade hydrogen peroxide (RGHP), using an impinging jet apparatus. Using synchronized high-speed visible shadowgraph and infrared imaging, the transition from standard atomization to reactive bubbly atomization was revealed by modulating propellant reactivity. Infrared imaging aids in tracking spray breakup, mixing, and the liquid-to-gas transition of droplets, providing valuable insights. To better understand the mixing and reactive bubbly dynamics before ignition, a simplified oxidation mechanism is proposed based on infrared imaging of hypergolic and non-hypergolic samples. This work reveals key multiphase reaction behaviors, including bubble nucleation and growth, bubbly droplet burst (microexplosion), and gas-phase combustion dynamics, along with insights into the reaction kinetics. Unlike non-hypergolic propellants, hypergolic combinations exhibit unique droplet dynamics characterized by bubble nucleation, expansion, and rapid consumption via internal and external evaporation. These mechanisms result in accelerated droplet disintegration and a shorter lifespan. The findings provide insights applicable to other hydrogen-peroxide-based bipropellants as well as to conventional hydrazine-based systems.