Structure and extinction of non-premixed n-heptane flames

Abstract

An experimental and numerical study is performed to elucidate the structure and mechanisms of extinction of non-premixed n-heptane flames. Experiments are conducted on flames stabilized between two connterflowing streams. The fuel stream is a mixture of prevaporized n-heptane and nitrogen, and the oxidizer stream is a mixture of air and nitrogen. Concentration profiles of C₇H₁₆, O₂, N₂, CO₂ CO, H₂, CH₄, C₂H₂, C₂H₄, C₂H₆, C₃H₄, C₃H₆, C₃H₈, C₄-hydrocarbons, C₅-hydrocarbons and C₆-hydrocarbons are measured. The measurements are made by removing gas samples from the flame using a quartz microprobe and analyzing the samples using gas chromatographs. The identity of the species is established using a mass selective detector. Temperature profiles are measured using a thermocouple. In addition, critical conditions of extinction are measured, giving the mass fraction of reactants as a function of the strain rate. Numerical calculations are performed using detailed chemistry to determine the flame structure and critical conditions of extinction at conditions identical to those used in the experiments. Calculated and measured flame structures are found to agree reasonably well: however, a small shift is observed between the calculated and measured temperature and concentration profiles. In general, the measured profiles are broader than the calculated profiles. At given values of the mass fraction of oxygen in the oxidizer stream, the calculated strain rates at extinction are noticeably higher than those measured. Experiments are also performed on non-premixed flames stabilized in the counterflowing configuration over a liquid pool of n-heptane. Critical conditions of extinction are measured. Numerical calculations are performed at conditions used in these experiments, and critical conditions of extinction are obtained. At given values of the mass fraction of oxygen in the oxidizer stream, the calculated strain rates at extinction are noticeably higher than those measured. The differences between the calculated and measured strain rates at extinction for the liquid pool flame are higher than for n-heptane-vapor flames.