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

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, showing strong agreement with experimental data from 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-? and transition SST turbulence models for 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 for their impact on flow behavior, conversion efficiency, power consumption, and heat transfer. With pins, the outer reactor wall temperature increased significantly, with 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 combined effects of heat transfer enhancement and increased catalyst surface area improved conversion by ~19% compared to smooth cases. With increased conversion efficiency, the reactor experienced greater 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 represent an important step toward decarbonization in the aviation industry.