Experimental Study of Impingement and Reaction of Hypergolic Droplets

Abstract

Understanding hypergolic ignition is critical for the safe and successful operation of hypergolic engines. Presently, hypergolic ignition models do not capture the complex coupling of physical and chemical processes necessary to achieve ignition and rely on experimental results for validation. In some cases, the chemical kinetics of hypergolic propellants and the fluid dynamics of droplet collisions couple to produce unexpected phenomena. This research investigates contact between droplets and pools of liquid hypergolic propellants under various impact velocities and impact geometries in order to quantify the liquid-phase reactions and categorize the resulting interaction. A total 142 drop test experiments were performed. Three distinct types of impacts were identified: explosions, bounces, and splashes. The impact type was found to depend on the impact Weber number and impact angle. Splashes occurred above a critical Weber number of 250, regardless of impact angle. Explosions occurred for Weber numbers less than 250, and for impact angles less than seven degrees. If the impact angle was greater than seven degrees, the test resulted in a bounce. High-speed videos acquired at up to 100,000 frames per second provided a means to observe the liquid-phase chemical reactions in detail. From these videos, the delay between contact and first gas production was measured to be between 20 and 200 microseconds for monomethylhydrazine and red fuming nitric acid. This delay provides insight into the rates of the liquid-phase chemical reactions, and has helped calibrate liquid-based ignition models.