Laser-based thermometry in premixed flat flames using wavelength modulation spectroscopy

TThis thesis presents the development and validation of a non-intrusive temperature measurement strategy for methane-air burner-stabilized premixed flames operating in a sub-adiabatic regime. The diagnostic approach is based on Tunable Diode Laser Absorption Spectroscopy (TDLAS) combined with Wavelength Modulation Spectroscopy (WMS). To ensure high signal stability and immunity against laser power fluctuations and background flame emission, a 2f/√4f harmonic normalization strategy was implemented. The thermometric method relies on the two-line intensity ratio of Carbon Monoxide (CO) absorption transitions in the near-infrared region around 2.3 µm. The linearity and robustness of the area-based ratio strategy were first validated through synthetic spectra generated from the HITEMP database, confirming a unique temperature dependence within the 1800–2100 K range. For the processing of experimental data, a specialized Python-based pipeline was developed, focusing on signal denoising via spline interpolation and numerical area integration to overcome the limitations of traditional fitting procedures on noisy datasets. Experimental axial scans were performed at different heights above the burner (HAB) for stoichiometric (ϕ = 1.0) and fuel-rich (ϕ = 1.2) conditions. The retrieved temperature profiles were compared with one-dimensional numerical simulations performed using the Cantera software. By employing the FFCM-1 kinetic mechanism and incorporating radiative heat loss models (RADCAL), the simulations showed excellent agreement with the experimental measurements. The results demonstrate that the synergy between advanced laser diagnostics and refined chemical-kinetic modeling provides a reliable metrological framework for characterizing the thermal structure of non-adiabatic flames, supporting the design of more efficient and sustainable combustion systems.