Two-line jointly optimized tomographic absorption spectroscopy for flame measurement with adaptive hybrid regularization

We present an improved tomographic absorption spectroscopy (TAS) for accurate reconstruction of temperature and H₂O mole fraction distributions of the combustion field. The absorption lines at 7185.597 cm^-1 and 7444.352 cm^-1 of the H₂O molecule are selected to recover temperature using two-line thermometry. By jointly solving the absorption coefficients of the selected two lines with an adaptive hybrid regularization approach, the proposed method effectively suppresses artifacts and local distortions caused by limited projections and measurement noise. The regularization framework dynamically balances the smoothness and gradient preservation in both individual absorption coefficient fields and their ratio. Numerical tests demonstrate that our proposed method outperforms the algebraic reconstruction technique (ART) and Tikhonov regularization algorithms, reducing reconstruction errors by approximately 50%. Experimental measurement was conducted on a laminar premixed McKenna CH₄/air flame. The tomographic results showed good agreement with thermocouple measurements, demonstrating a mean deviation of 13.2 K in the core region, and successfully recovering the expected top-hat profile. This work advances the application of TAS in non-uniform combustion fields.