INTRACAVITY LASER ABSORPTION SPECTROSCOPY FOR COMBUSTION DIAGNOSTICS

Laser-based diagnostic measurements provide a rigorous test of our understanding of combustion chemistry. Intracavity Laser Absorption Spectroscopy (ICLAS) technique used for absolute concentration measurements of HCO and ¹CH2 radicals, playing an important role in combustion mechanism.

The experimental spectra of the HCO and CH₂ radicals were recorded for different equivalence ratios. The interpretation of the spectral data is presented and its potential for the combustion model development is discussed. (see Fig.1)

Fig.1 Experimental HCO(left) and CH2(right) profiles (filled triangles) and those calculated using the GRI mechanism (solid magenta line), Aramco mechanism (solid green line) and Konnov mechanism(solid blue line). Equivalence ratio is φ =1.0. Red diamond: temperatures measured by LIF.

New methodology for concentration measurements of CO, CO_2, CH₄, and H₂O and thermometry based on broadband detection of these species, using Fiber Laser Intracavity Absorption Spectroscopy (FLICAS) was developed and verified. The experiments were performed in a temperature-controlled flow-cell, which allowed evaluating the feasibility of the FLICAS technique for temperature and concentration diagnostics.

We have found that CO and CO₂ spectra are well described by HITRAN database allowing for accurate and simultaneous determination of temperature and concentration. Our study demonstrates that for CH₄ and H₂O the deficiencies of HITRAN database (especially for high temperature conditions) somewhat limit the use of these molecules as candidates for thermometry in combustion related devices. (see Fig.2)

Fig.2 Left panel: FLICAS spectrum of CO2 at room temperature along with simulation; Right panel: FLICAS spectrum of water vapors (A) at 1280 K along with simulation using original HITEMP database (C) and the modified one (B) (shifted lines shown with arrows), indicating the deficiencies of the spectral database.