Nonlinear deflection tomography of inhomogeneous flame temperature and concentration based on topology evolution with prior smoothing

Tomographic absorption spectroscopy (TAS) is a promising temperature and concentration monitoring technology because it has fewer optical access requirements and species selectivity. However, the inhomogeneous distribution of temperature and species concentration in a practical reaction flow can cause laser deflection, which may affect the imaging quality. Thus, this study aimed to develop a novel tomographic absorption deflection spectroscopy (TADS) technology for in-situ, quantitative, and simultaneous measurements of temperature and H2O concentration. The proposed system is based on the gas mixing equation and ray tracing, considering the inhomogeneous gradient refractive index and laser bending. Moreover, multi-prior information, including smooth regularization, non-negative constraints, and topological shape constraints, is coupled to mitigate the ill-posedness of inverse problems, thereby improving the accuracy and robustness of multiparameter tomography. Numerical verification and a comparison of algorithms for multimodal flames indicate that the proposed TADS technology exhibits excellent anti-noise performance and can significantly improve reconstruction quality. Additionally, a proof-of-concept based on experimental data of a steady methane diffusion flame is performed to demonstrate the feasibility of TADS as a practical combustion diagnostic. The reconstruction results of instantaneous temperature and H2O concentration on six different cross-sections show that the topological structure, dynamics, and parameter distribution can be quantitatively demonstrated. This technology is expected to enhance the practicability of laser absorption spectroscopy in the in-situ measurement of complex combustion reaction flows with high temperature, high pressure, and strong distortion.