Counterflow Diffusion Flames of Oxygen and N-Alkane Hydrocarbons (CH₄-C₁₆H₃₄) at Subcritical and Supercritical Conditions

Abstract

This article presents an investigation of counterflow diffusion flames of oxygen and n-alkanes (CH4-C16H34) over the entire thermodynamic fluid regime and a wide range of flow strain rates. The formulation incorporates fundamental thermodynamics and transport theories, along with detailed chemistry. An improved two-point flame-controlling continuation method is employed to capture the complete flame response along the S-curve. Two selected members of the n-alkane family, methane and n-heptane, are studied in detail. The results confirm that the flame thickness () and the global heat-release rate () of oxygen/hydrocarbon systems are closely related to the pressure-weighted strain rate, and . The latter correlation is further modified to account for the pressure effect on peak flame temperature, for the oxygen/methane system, for the oxygen/n-heptane system. The inlet temperature appears to have a negligible effect on flame characteristics. General similarities are developed in the mixture-fraction space in terms of flame temperature, species concentrations, flame thickness, and heat-release rate for all pressures under consideration. This suggests that the flame behaviors at high pressure can be evaluated by their counterpart at low pressure. The common features for the n-alkane family are identified. The flame properties of a given hydrocarbon fuel can be predicted from those of another hydrocarbon fuel at the same flow conditions. The results contribute to the establishment of a tabulated chemistry library for the modeling of supercritical combustion of oxygen and hydrocarbon fuels.