Fully resolved simulations of micro-unit composite fuel in hydroxyl-terminated polybutadiene (HTPB): Al@AP

Micro-unit composite fuel (Al@AP) is a promising strategy for achieving stable and efficient combustion, due to its reduced pressure dependency and ability to prevent agglomeration. This research focuses on the heat and mass transfer as well as the reaction mechanism within the Al@AP structure through molecular modeling. The direct contact between the metal fuel and oxidizer leads to an immediate redox reaction at the corresponding interface, facilitating the diffusion of oxygen atoms from surrounding AP molecules into the Al particles. Heat is transferred from the Al particles to the surrounding oxidizer, instead of inward heat transfer from the AP-HTPB interface as observed in the conventional Al/AP composite. This novel assembly method significantly accelerates the reaction rate, more than doubling that of the conventional Al/AP composite. Moreover, the research explores combustion behavior at diverse pressure conditions (condensed phase vs. vacuum), revealing pressure's minimal impact on the rapid burning stage primarily driven by diffusion mechanism. Under vacuum, reaction products propel burning Al within gas flow, transitioning to surface reactions on Al particles with gaseous products. Regarding agglomerations, the coalescence of two burning Al droplets is observed to form a larger product owing to the reduced distances within AP particle. The Al droplets are almost consumed before coalescence due to the rapid combustion, resulting in no negative impact on the Al combustion. The above findings highlight the unique features of micro-unit composite fuel in the application of solid propellants, and serve as an atomistic guide for propellant manufacturing and design.