Pin Fin Effects in an Ammonia Reactor for Hydrogen Production Applied to a Carbon-Free Turbofan

Abstract In pursuit of net-zero aviation and sustainable fuel, this study focuses on developing a cracking unit (CK) for a carbon-free turbofan engine concept utilizing ammonia as a hydrogen carrier. A simplified, single-step surface reaction mechanism was implemented using ANSYS FLUENT and CHEMKIN PRO R1, demonstrating strong agreement with data from the literature. Compared to a volumetric reaction model, the surface reaction significantly reduced computational cost by eliminating the need to resolve the small length scale of the catalyst layer. This chemical model, paired with the SST k-ω and transition SST turbulent models for their accurate boundary layer and free stream prediction, is employed in CFD. An investigation was conducted on three channels with varying heights, with and without pin fins. Pin fins were assessed based on their impact on flow behavior, conversion efficiency, energy consumption, and heat transfer performance. With pins, the outer reactor wall temperature significantly increased, with the smaller height channels showing the largest effect due to reduced heat transfer distance. Turbulent kinetic energy (TKE) analysis revealed that pin fins enhance mixing through flow separation, redistributing heat and aiding ammonia adsorption to the catalyst. The effects of heat transfer enhancement and increased catalyst surface area improved conversion by ∼19% compared to the smooth cases. With increased conversion efficiency, the reactor saw increased power consumption due to the endothermic nature of the reaction. The cracking unit is an essential part of the hardware for the proposed turbofan; these findings are an important step for decarbonization in the aviation industry.