Mesoscale thermal-chemical coupling in heterogeneous energetic materials: Impact of elliptical microstructure on heat flux dynamics and unsteady combustion

The combustion of heterogeneous energetic materials involves complex coupling between condensed-phase heat conduction, interfacial pyrolysis, and gas-phase flame propagation. While spherical particle assumptions are prevalent in thermal modeling, practical propellant grains exhibit significant morphological anisotropy. This study investigates the impact of microstructural heterogeneity on the thermo-chemical stability of Nitrate Ester Plasticized Polyether (NEPE) propellants. Specifically, the modulation of burning rate uniformity and heat flux distribution by the morphology and orientation of elliptical particles is analyzed. A high-density mesoscale packing model utilizing the Axis-Aligned Bounding Box (AABB) and Gilbert-Johnson-Keerthi (GJK) algorithms is developed to resolve the thermal interaction between elliptical particles and the binder matrix. By coupling condensed-phase energy equations with a semi-global gas-phase reaction mechanism, the regulation of heat feedback by particle orientation is compared. Results indicate that within the present two-dimensional framework, the 45° elliptical configuration reduces the normalized heat-flux non-uniformity and the burning-rate coefficient of variation (CV) relative to the circular-particle reference, suggesting a more spatially uniform heat-feedback pathway under identical modeling assumptions. In the reduced AP/HMX/binder domain, HMX morphology has a stronger influence on the mean regression rate than AP morphology because elongated HMX-rich interfaces intensify AP/HMX/binder diffusion-flame coupling and modify solid-phase heat-conduction pathways. Under the imposed rapid depressurization condition, global flame weakening is primarily pressure-driven, whereas particle morphology controls early-stage heat-release localization and regression heterogeneity. The flame structure evolves from an early diffusion-flame-dominated regime to a weaker premixed-flame-like reaction stage. These findings indicate that oblique elliptical morphology can redistribute local heat-feedback pathways in a two-dimensional mesoscale framework, providing qualitative thermal-combustion insight for future studies that combine combustion modeling with mechanical, manufacturing, and three-dimensional microstructural constraints.