normal upper N normal upper O Subscript x N O x NOₓ emission modelling for composite cycle engines with recirculation and water injection: A reduced-order physics-based approach

The composite cycle engine (CCE) offers a promising pathway for significantly reducing fuel burn in future aircraft. By integrating a piston system into the core of a gas turbine, the CCE can utilise higher peak cycle temperatures and pressures and thus, achieve a step-change in thermal efficiency. These conditions also promote increased levels of normal upper N normal upper O Subscript x N O x NOₓ formation. This paper presents a methodology for the preliminary estimation of normal upper N normal upper O Subscript x N O x NOₓ emissions within both the piston engine and the subsequent Joule combustor. Fast-responding parametric models capable of capturing key normal upper N normal upper O Subscript x N O x NOₓ sensitivities, particularly the impact of combustion vitiation through water injection and exhaust gas recirculation (EGR), are established. Piston normal upper N normal upper O Subscript x N O x NOₓ is modelled using a two-zone approach, which demonstrates good correlation with reference engine data from literature. For the Joule combustor, a stochastic flow description is employed to predict normal upper N normal upper O Subscript x N O x NOₓ formation, with validation against an in-service combustor confirming the model’s predictive capability. Initial studies indicate high baseline piston normal upper N normal upper O Subscript x N O x NOₓ emissions, though significant mitigation is achievable through water injection or EGR. This work establishes a framework for future research into the emissions and necessary mitigation strategies for the CCE and other advanced combustion technologies.