Species quantification during n-heptane autoignition using filtered natural emission of species

Abstract

Quantitative species concentration measurements taken during hydrocarbon oxidation are critical for the development of kinetic mechanisms. Spectroscopic methodologies are often employed to take such measurements due to their unobtrusive nature. A new methodology for species concentration measurements, called the filtered natural emission of species (FNES), has been proposed as it provides greater spatial resolution and a broader range of viable test conditions than other spectroscopic methods, such as laser absorption spectroscopy (LAS). To validate these claims, the concentrations of CO, CO2 and H2O were measured during the oxidation of a lean, n-heptane mixture in a rapid compression machine using FNES. The mixture was tested at an average pressure of 22 bar in the temperature range between 710 and 784 K. The measured data was compared with results of 0-D and 3-D numerical models using multiple kinetic mechanisms and available data found in literature. The empirical data of this work agreed well with the peak concentration measurements reported in the literature; however, the FNES methodology was able to provide insight into combustion processes when inhomogeneities caused by flow or boundary layer effects existed due to its unique field-of-view. As such, elevated CO concentrations were observed after the peak concentration when using the FNES methodology. This observation was not reported in literature nor captured by the 0-D numerical models; however, the 3-D model was able to explain the phenomenon as the result of the temperature field near the wall. 3-D simulations are recommended for the benefit of future works. In addition, the measured species concentrations displayed a great sensitivity to temperature, which was a function of the detailed kinetic mechanisms. Different kinetic mechanisms were found to significantly alter the results of the measured concentrations. Kinetic mechanisms must be chosen with care for accurate modeling of temperature for use of species concentration measurements.