Baseline-free broadband absorption spectroscopy tomography for high-pressure combustion application

Abstract

Tunable Diode Laser Absorption Tomography (TDLAT) is a widely used technique for detecting spatial distributions of temperature and concentration fields in combustion systems. When pressure field measurement is required, absorption coefficient tomography is typically employed to simultaneously retrieve pressure, temperature, and concentration field distributions. However, the baseline submergence effect under high-pressure conditions poses challenges in accurately acquiring projected absorbance, while baseline distortions in unsteady combustion regions further exacerbate this issue. This study addresses the problem by introducing baseline-free inversion algorithms into the broadband absorption spectroscopy-based absorption coefficient tomography framework. The proposed method employs linearity-compliant baseline-free processing to pre-correct projection spectra, followed by tomographic reconstruction and parameter inversion of the preprocessed spectral lines. In the comparative assessment of absorption coefficient tomography integrated with three baseline-free methods—Interpolated Envelope Correction Method (IECM), molecular Free Induction Decay (m-FID), and Derivative Spectral Method (DSM)—the IECM-based approach exhibits the lowest inversion errors. Its reconstructed field morphology shows the closest agreement with the original distribution, and the inversion results closely approximate those from direct inversion under baseline-free conditions. In high-pressure testing at 40 bar, the IECM-based absorption coefficient tomography exhibits reconstruction errors comparable to those at 17 bar, demonstrating broad adaptability to high-pressure environments and showing significant potential for practical combustion diagnostics in such conditions.