Hypergolic ignition induced by head-on collision of bi-propellant droplets: Monoethanolamine-based fuel and hydrogen peroxide

Hypergolic ignition by the binary collision of H₂O₂ and Monoethanolamine (MEA)-NaBH₄ droplets has been experimentally studied with an emphasis on droplet inter-mixing, heat transfer, droplet swelling, and flame propagation. The entire ignition is phenomenally categorized into five stages. The H₂O₂ droplet penetrates the MEA- NaBH₄ droplet after the collision and coalesces with it. The H₂O₂ fluid rapidly restores its sphericity inside the coalesced droplet, which is theoretically proved to be driven by interfacial tension. The average measured restoration time is 14.0 ms. Large-scaled bubbles are generated at the interfacial structure, leading to swelling of the coalesced droplet. The existence of apparent interface tension tends to limit the rate of droplet inflation at the beginning. As more bubbles are produced, the interface tension wanes, and the droplet swells rapidly. The droplet surface expansion rate is identified via the gray-level information in recorded images. A theoretical model quantifying the swelling rate was established based on the underlying mechanism for the swelling of the reacting droplet, which can be attributed to the flash boiling of the superheated H₂O₂ fluid. As bubbles accumulate inside the coalesced droplet, a dual-bell-shaped droplet configuration is formed. A flame ring structure is observed when the ignition occurs. Luminant flame sustains more than 10 ms until the fuel vapor is depleted. © 2023