Theoretical study of the real-fluid laminar flame propagation under supercritical conditions by using the virial equation of state and the Enskog transport model

Abstract

A real-fluid computation model using the Virial equation of state and the Enskog transport model was developed in this work to simulate real-fluid laminar flame propagations, and it was also compared with the Redlich-Kwong-Ely-Hanley/Takahashi real-fluid model in literature. The real-fluid effects of equation of state, thermodynamics, chemical potential, thermal conductivity, and mass diffusion have been investigated on the freely propagating flame simulations of different fuels. The contribution and the source of uncertainties by different real-fluid properties are also comprehensively discussed. The overall effects of real-fluid behaviors on the laminar flame speed simulations in H₂O-dilute mixtures can reach 35 % at 100 atm, which reveal that accurate descriptions of real-fluid effects are crucial for flame speed predictions, especially using dilute gas with a high polarization, such as H₂O. The Redlich-Kwong-Ely-Hanley/Takahashi model shows severe over-predictions of the hydrogen laminar flame speeds due to its significant weakness in predictions of thermal conductivities of mixtures by comparing with the NIST data, while the present Virial-Enskog model exhibits more reasonable predictability. The real-fluid simulations of the laminar flame speeds of DME and n-heptane flames by using the Virial-Enskog model and the Redlich-Kwong-Ely-Hanley/Takahashi model are compared with the available experimental data. It shows a crucial real-fluid effect on the simulations of DME and n-heptane flame speeds, up to 8 % discrepancy from the ideal-gas flame speed prediction, even at 20–25 atm. Overall, the Virial-Enskog model provides better estimations of the real-fluid behaviors than the empirical Redlich-Kwong-Ely-Hanley/Takahashi model, especially at high pressures.