Modeling and quantification of beam steering uncertainties in high-pressure laser absorption spectroscopy of non-uniform physical fields

Abstract

Beam steering induced by refractive index gradients poses a significant challenge for laser absorption spectroscopy in high-pressure, non-uniform combustion environments. This work establishes a physics-based framework to model and quantify beam steering effects on path-integrated absorbance and two-line H 2 O thermometry. A three-point perturbation approach reconstructs optimum ray trajectories between fixed source and detector locations, providing a direct quantitative mapping between refractive-index gradients and thermometric bias. Across three representative flames, including a single-peak axisymmetric flame, a flat flame, and a co-flow diffusion flame, the framework reveals that beam-steering-induced temperature and H 2 O mole fraction deviations are governed by the magnitude and spatial distribution of thermochemical gradients. Inferred temperature deviations range from a few percent in weak-gradient regions to more than 30% near steep boundary layers, and remain strictly positive, indicating consistently higher deflected-ray temperatures. A pressure-dependence study demonstrates that beam steering effects intensify rapidly from 1 to 400 atm. Deviations are negligible below 10 atm, increase nonlinearly in the 10–100 atm regime, and become substantial at 400 atm, where n −1 approaches the 10 −1 level and the most sensitive line pair exhibits mean temperature deviations of about 6.2% with ray-wise extremes near 54.6%. The analysis also shows that line-pair selection strongly influences diagnostic robustness, with certain combinations yielding substantially smaller deviations and reduced spatial variability. These findings establish beam steering can be an important source of uncertainty in high-pressure laser absorption spectroscopy and provide guidance for optimized line selection, optical layout, and compensation strategies.