Hypergolic droplet‒film interaction dynamics at high temperatures

Liquid film cooling serves as a critical thermal protection mechanism in rocket thrusters. The interaction between oxidizer droplet, which is deposited from mainstream region of thrust chamber, and fuel film on the wall inevitably influences cooling efficiency, which is poorly understood in existing research. This study experimentally investigated hypergolic reaction between white fuming nitric acid droplet and ionic liquid fuel film at elevated wall temperature T_w using synchronized high-speed and infrared thermography. Results show that reaction progresses through inertia-dominant spreading, mixing, and culminates in intense liquid-phase explosion (micro-explosion). An elevated T_w intensifies micro-explosion, increasing the risk of wall exposure and leading to the decline of cooling efficiency. Paradoxically, the increase in local film temperature inversely correlates with T_w, which is related to reduced explosion delay time. These findings first provide thermal and hydrodynamic data essential for the design of future thermal protection measures for small hypergolic liquid rocket thrusters and offer theoretical basis for optimizing liquid film cooling systems in bipropellant propulsion architectures.