Numerical simulation of transcritical multiphase combustion using real-fluid thermochemical and transport properties

We scrutinize high-fidelity physical and numerical models for mixing and combustion in high-pressure thermal propulsion systems, where the operating pressure is typically above the critical pressures of the pure fuel and oxidizer, but lower than the cricondenbar values of the mixture. At such transcritical operating conditions, the fluid’s state and transport properties strongly deviate from ideal-gas or ideal-liquid models. The possible coexistence of vapor and liquid phases, governed by the local composition of the fluid mixture, further complicates the physical and numerical modeling. To this end, we present a comprehensive framework for the simulation of combustion at transcritical pressures using multiphase thermodynamics. The Navier–Stokes equations are solved for a multi-component working fluid with thermodynamic properties computed by solving suitable volumetric and caloric state equations combined with phase-splitting equations. The transport properties of the working fluid are modeled using high-pressure correction methods with appropriate structural mixing rules in the co-existence regime. Real-fluid effects on the diffusion driving force are quantified via the thermodynamic correction factor with a proposed extension to the multiphase transcritical regime. The finite-rate chemistry model includes real-fluid high-pressure effects in reaction source terms via the fugacity of the species. Computational results demonstrate the need for accurate models of thermochemical and transport properties and their impact on the predicted ignition behavior of transcritical flames. Novelty and Significance statement: This paper tackles the complexities of simulating transcritical multiphase combustion, highlighting the non-ideal behavior of fluid mixtures under high pressure and the potential coexistence of vapor and liquid phases dictated by local composition. Its significance extends beyond the pure usage of multiphase thermodynamics (MT), which couples the equation of state with vapor–liquid equilibrium calculations to accurately predict the thermodynamic properties of the mixture within the two-phase region. This work generalizes and extends the MT approach to transport properties and the diffusion driving force for real-fluid mixtures at transcritical pressures. Additionally, it introduces a finite-rate chemistry model that comprehensively addresses the real-fluid effects in high-pressure combustion based on species fugacity. The integration of these innovations provides a predictive model of unprecedented accuracy.