COUNTERFLOW DIFFUSION FLAMES OF OXYGEN AND N-ALKANE HYDROCARBONS (CH_4-C_(16)H_(34)) AT SUBCRITICAL AND SUPERCRITICAL CONDITIONS

摘要

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 (delta) and the global heat-release rate ((q) over dot) of oxygen/hydrocarbon systems are closely related to the pressure-weighted strain rate, delta similar to 1/root pa and (q) over dot similar to root pa. The latter correlation is further modified to account for the pressure effect on peak flame temperature, (q) over dot similar to p(0.536)root a for the oxygen/methane system, (q) over dot similar to p(0.534)root a for the oxygen-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.