High-sensitivity interference-free diagnostic for measurement of methane in shock tubes

A sensitive CW laser absorption diagnostic for in-situ measurement of methane mole fraction at high temperatures is developed. The selected transitions for the diagnostic are a cluster of lines near 3148.8 cm⁻¹ from the R-branch of the ν₃ band of the CH₄ absorption spectrum. The selected transitions have 2–3 times more sensitivity to CH₄ concentration than the P-branch in the 3.3 μm region, lower interference from major interfering intermediate species in most hydrocarbon reactions, and applicability over a wide range of pressures and temperatures. Absorption cross-sections for a broad collection of hydrocarbons were simulated to evaluate interference absorption, and were generally found to be negligible near 3148.8 cm⁻¹. However, minor interference from hot bands of C₂H₂ and C₂H₄ was observed and was characterized experimentally, revealing a weak dependence on wavelength. To eliminate such interferences, a two-color on-line and off-line measurement scheme is proposed to determine CH₄ concentration. The colors selected, i.e., for on-line (3148.81 cm⁻¹) and off-line (3148.66 cm⁻¹), are characterized between 0.2–4 atm and 500 K–2100 K by absorption coefficient measurements in a shock tube. Minimum detectable levels of CH₄ in shock tube experiments are reported for this range of temperatures and pressures. An example measurement is shown for sensitive detection of CH₄ in a shock tube chemical kinetics experiment. - Highlights: • Methane sensor designed for combustion studies in shock tubes. • Selected line was optimal over 900–1400 K. • The effect of interference from 35 common species was minimized. • Precise control of beam quality done using fiber coupled delivery and custom optics. • An example measurement of CH₄ was shown in propane pyrolysis experiment