Prediction of propagating flames under high-pressure conditions with real-fluid combustion modeling

Abstract

The present study discusses computational fluid dynamics (CFD) modeling for combustion flows under high-pressure conditions, including supercritical pressure conditions. The real-fluid effects are considered in terms of thermodynamic properties, transport properties, and chemical kinetics. In the present model, the real-fluid effect on chemical kinetics is introduced via a modified equilibrium constant derived using the Gibbs free energy variation of chemical reactions and the fugacity. The results obtained with the present model are compared with available experimental data of high-pressure premixed H2/O2 propagating flames diluted by Ar or He. We demonstrate in the H2/O2/Ar flame case that the real-fluid model provides a better prediction accuracy for the negative pressure dependence of the mass burning rate compared to the ideal-gas model. The improved prediction accuracy is primarily attributed to the proper estimation of thermodynamic properties such as unburnt-gas enthalpy via an appropriate equation of state and a departure function. The negative pressure dependence of unburnt-gas enthalpy of the H2/O2/Ar mixture with the real-fluid model significantly affects the flame speed prediction under high-pressure conditions. On the other hand, although the H2/O2/He mixture shows a positive pressure dependence of enthalpy, differences in the mass burning rate between the real-fluid and ideal-gas models are not significant for the H2/O2/He flame case. In the He-diluted case, the real-fluid effect is undermined owing to the low density of the H2/O2/He mixture. Thus, the real-fluid effect appears differently in the prediction of propagating flames depending on the species composition and thermodynamic conditions. The present study suggests that the positive or negative pressure dependence of enthalpy (i.e., the isothermal Joule–Thomson coefficient) is a metric to identify the real-fluid effects that appear.