Precision Thermometry of Flat Flames Using Spatially Resolved Multi-Color Laser Absorption Spectroscopy of Carbon Dioxide

TL;DR

This study develops an accurate, non-intrusive, spatially resolved flame thermometry method based on multi-color CO$_2$ laser absorption spectroscopy. By using a collinear dual-laser setup to scan tens of CO$_2$ transitions and a high-speed 2D beam-scanning system, the authors achieve ~1% single-shot temperature accuracy with 1 mm spatial resolution and planar/volumetric speeds of 200 Hz and 2 Hz. A physically constrained nonlinear inference framework combines a forward spectral model (Voigt CO$_2$ lines from HITEMP) with tomographic reconstruction to infer $T$ and $X_{CO_2}$ across measurement regions, validated on axisymmetric flat flames and a complex PKU flame. The results reveal core flame temperatures near $T_{core}\approx 2020$ K (below $T_{ad}$ due to heat loss and entrainment) and CO$_2$ enrichment consistent with equilibrium predictions at lean regimes, with strong contrast and resolution demonstrated in non-axisymmetric geometries. The method promises broad utility for combustion studies due to its high precision, robustness, and relative experimental simplicity.

Abstract

This work developed an accurate and robust absorption-based method for spatially resolved measurements of gas temperatures in flames and reacting flows, with typical single-measurement uncertainties on the order of 1\%. This method exploits narrow-linewidth laser absorption of hot CO$_2$ molecules, which can be generated from combustion or artificially seeded into the flow. A collinear dual-laser setup allowed for periodic scans over tens of CO$_2$ absorption transitions near the $ν_3$ bandhead every 100 $μs$, from which gas temperatures (as well as CO$_2$ concentrations) were determined with high sensitivity and robustness. Spatially resolved measurements were achieved using an electrically driven high-speed beam scanning system consisting of a 2-D galvo scanner and a pair of off-axis parabolic mirrors. An effective spatial resolution of 1 mm was achieved at a planar field measurement speed of 200 Hz and a volumetric field measurement speed of 2 Hz. A physically constrained nonlinear inference framework was also developed for the quantitative analysis of the measurement data. Proof-of-concept experiments were performed on axisymmetric flames stabilized on a Mckenna burner at various equivalence ratios and flow rates, and the results agreed asymptotically with the theoretical value of the adiabatic flame temperature. An additional experiment on a flame of complex geometry demonstrated an excellent level of resolution, precision, and contrast achieved by the current thermometry method. This method promises to provide good utility in future combustion studies due to its high performance metrics and relative ease of use.

Figures (11)

• Figure 1: A schematic of the current experimental setup. Panel (a): laser diagnostics system. Panel (b): axisymmetric flame with a stagnation plate. Panel (c): PKU flame.
• Figure 2: A schematic diagram of the current framework for analyzing the absorption data
• Figure 3: Example data obtained in the current study. Panel (a): raw signals of the photodetector (top) and the galvo scanner modulation (bottom). Panel (b): a zoomed-in view of the region highlighted in (a) illustrating the intensity (top) and wavenumber (bottom) modulations of the lasers. Panel (c): the absorbance signal extracted from the raw data within one modulation cycle of 100 $\rm{\mu s}$.
• Figure 4: Absorbance data obtained from a single-shot planar measurement of an axisymmetric flame at mass flow rates of $\dot{m}_{CH_4}$ = 2.50 SLPM, $\dot{m}_{air}$ = 24.15 SLPM, and $\dot{m}_{N_2}$ = 28.17 SLPM, with an equivalence ratio of $\phi$ = 1.00. The measurement was taken at a height of 5 mm above the burner surface.
• Figure 5: The radial distributions of gas temperature and $\rm CO_2$ mole fraction inferred from the absorbance data shown in Fig. \ref{['Fig4']}. The measurement was taken at a height of 5 mm above the burner surface.