Quantitative CH measurements in atmospheric-pressure, premixed flames of C1–C4 alkanes

Abstract

The rapid formation of nitric oxide (NO) within the flame front of hydrocarbon flames, occurring via the prompt-NO formation route, is strongly coupled to the concentration of the methylidyne radical, [CH]. This work presents absolute measurements of [CH] taken in atmospheric-pressure, premixed, stagnation flames of methane, ethane, propane, and n-butane. One-dimensional (1D) CH fluorescence profiles are extracted from 2D Planar Laser-Induced Fluorescence (PLIF) measurements made quantitative through normalization by the Rayleigh scattering signal of nitrogen. Axial velocity profiles are measured by Particle Tracking Velocimetry (PTV) and, along with mixture composition and temperature measurements, provide the required boundary conditions for flame simulations based on the 1D hydrodynamic model of Kee et al. (1989). A time-resolved, four-level, LIF model considering rotational energy transfer in both the ground and excited electronic states is used to convert the modeled CH concentration profiles into units compatible with the quantitative CH-LIF measurements. Large variations in the CH concentrations predicted by four thermochemical mechanisms are observed for all fuels and equivalence ratios considered. A detailed study of the mechanisms, through reaction path and sensitivity analysis, shows that the principal reactions impacting CH formation are: (a) involved in the CH formation route (CH3→CH2*→CH2→CH), (b) bypass and remove carbon atoms from the CH formation route, or (c) affect the pool of reaction partners in the aforementioned reactions. The order of magnitude variability in the model predictions is caused by significant disagreements among the mechanisms in terms of rate coefficients and reactions included in these pathways. This data set is made available and provides validation and optimization targets for future combustion model revisions.