Fully resolved simulations of micro-unit composite fuel in solid propellants: Al@AP

Micro-unit composite fuel (Al@AP) is a promising strategy for achieving controllable 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 propellant 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, two extreme pressure conditions (e.g., condensed phase vs. vacuum) are considered to examine the pressure effect on the combustion process of micro-unit composite fuel. The simulation results demonstrate that pressure has a negligible effect on the rapid combustion stage of burning Al particles, as it is primarily governed by the diffusion mechanism. Under vacuum conditions, the reaction products of propellants drive the burning Al in the gas flow, leading to the detachment of the metal fuel from the oxidizer. The reaction mechanism transitions to surface reactions on Al particle with gaseous products. In terms of 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, as observed in conventional composites. 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.