Experimental study of hypergolic ignition by H₂O₂ droplets impacting a deep pool of NaBH₄-based fuel

In spite of hypergolic systems common use in rocket engine ignitors, the dynamics of hypergolic reaction at fluid–fluid interfaces remain underexplored. This study investigates the ignition dynamics of hydrogen peroxide (H₂O₂) droplets impacting deep pools of a sodium borohydride (NaBH₄)-based hypergolic liquid fluid. Unlike prior studies employing confined geometries (e.g., petri dishes or test tubes), the present setup minimizes wall effects and reveals several previously unreported phenomena. Key parameters–including NaBH₄ concentration (3, 6, 9 wt%), droplet height (H = 30, 100, 300 mm), pool depth (D = 10, 20, 30 mm), and isopropyl alcohol (IPA) additives–were systematically varied. High-speed shadowgraphy (6,300 fps) captured transient events such as crater formation, mist ejection, miscible plume with “tail-chasing” decomposition, two-stage ignitions, and two distinct modes of droplet combustion (evaporative and decomposition). H = 100 mm and higher NaBH₄ concentrations improved ignition reliability, while the influence of D was weaker. IPA had minimal influence on ignition but prolonged the ensuing combustion. Ignition delay times (IDT) for surface mist ranged from 16 to 55 ms, whereas plume ignitions ranged from 39 to 130 ms. Compared to confined geometries, the deep pool setup exhibited richer fluid dynamic behaviors, and comparable IDT to petri dish tests under certain conditions.