Quantitative volumetric laser absorption imaging of methane and temperature in flames utilizing line-mixing effects

Abstract

A mid-infrared volumetric laser absorption imaging technique was developed for quantitative three-dimensional measurements of methane concentration and temperature in combustion flows. The spectroscopic method utilizes temperature-dependent collisional line mixing amongst a cluster of rovibrational transitions of the R(15) maniold of the v3 asymmetric stretch band of methane. Notably, line mixing effects are shown to increase sensitivity of the ratiometric thermometry, and a first-order model enables quantitative inference of species. 3D volumetric imaging is demonstrated experimentally on a methane-oxygen laminar flame doublet (<cm) backlit with tunable radiation from an interband cascade laser near 3.16 μm. 2D images were collected at six different projection angles on a high-speed mid-infrared camera, yielding an aggregate of 27,648 unique lines of sight capturing the scene with a pixel resolution of ∼70 μm. A linear tomographic reconstruction of spectrally-resolved absorbance with a 3D masked Tikhonov regularization method was performed to obtain methane spectra at every grid point, which enables subsequent interpretation of local gas properties. Importantly, this method enables quantitative temperature and species imaging in the fuel decomposition regions of flames.