Multiscale Analysis of an Ammonia Dual-Role Brayton–Rankine Combined Cycle: System-Level Feasibility and Evaporator Design Limits

Ammonia (NH3) is a carbon-free fuel with growing potential for decarbonizing power generation, but single-cycle combustion faces inherent efficiency limits. This study evaluates a dual-role combined cycle in which ammonia serves as both the Brayton-cycle fuel and the working fluid of a bottoming Rankine cycle, coupling one-dimensional (1D) GT-SUITE system analysis with three-dimensional (3D) CONVERGE CFD analysis of the waste-heat-recovery (WHR) evaporator. Across 15 combinations of Rankine pressure (PR = 15–55 bar) and evaporator tube count (Nt = 40, 70, 100), valid Rankine output required working-fluid evaporation, and system efficiency reached a maximum of 31.98% (Nt = 100, PR = 55 bar), a 4.32-percentage-point gain over gas-turbine-only operation. Component-wise exergy destruction analysis shows combustion irreversibility dominates system losses, with the WHR evaporator as the second-largest contributor, explaining the modest Rankine work output. CFD results reveal a design-critical boundary between PR = 35 and 45 bar, where shell-side flow non-uniformity roughly doubles the tube-to-tube outlet temperature spread (38 K → 74–78 K) and risks a pinch-point violation; a single baffle removes the pinch violation but not the temperature spread, pointing to shell geometric asymmetry as the cause. Integrating the 1D and 3D results, we present a design operating-envelope map and recommend an operating point (Nt ≥ 70, PR ≈ 35 bar) balancing efficiency, stability, and heat-transfer-area economy, identifying the WHR evaporator as the key constraint for future design work.