Planar concentration measurements of carbon monoxide (CO) using 2-photon planar LIF in a generic model combustor

Abstract

The further development of lean-burn technology for aero-engine applications requires an improved understanding of the interaction between wall cooling and the combustion processes. Wall cooling leads to combustion extinction and plays a significant role in the production of pollutants. Experimental investigation of the coupling between aerodynamics and combustion chemistry in the confines of pressurized combustion chamber is achieved by means of a dedicated burner, which burns very close to the wall, and an effusion cooled wall. While particle image velocimetry (PIV) lends itself for the measurement of the flow field, both the flame front as well as the temperature distribution can be estimated by quantitative concentration measurement of the OH radical using planar laser induced fluorescence (OH-PLIF). In this context PLIF on carbon monoxide (CO-PLIF) holds the potential of locating and quantifying areas of flame extinction and is the subject of the present feasibility study. In a first step the two-photon excitation of CO molecules at 230 nm was investigated from the spectroscopic view, in particular with respect to excitation and emission. From the respective excitation/emission spectra obtained in the flame of a calibrated matrix burner, the wavelengths can be detected at which CO is excited. Spectral analysis both reveal interference with other molecules (e.g. C2) and provide reliable information for a simulation that also includes the influence of the quasi-resonant (2+1) ionization of the CO molecule. In a second step planar measurements were performed on this matrix burner using a fuel-rich methane/air flame (phi = 1.45) as a reference flame with respect to which all other flames (phi = 0.8 – 1.3) are calibrated. This allows the influence of the position-dependent differences in the laser light sheet to be adequately considered and resulted in an even CO distribution for the different flames. The respective mean values of the CO concentration show good agreement with the results of equilibrium calculations. The present setup allows planar measurements of mean CO concentrations at a range of about 200 ppm while single pulse measurements are limited to roughly 1000 ppm. As the next step, the CO-PLIF measurement technique is to be applied on a pressurized single-sector combustion chamber.