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

Abstract

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 3148.8cm−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 3.3μm 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 3148.8cm−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 (3148.81cm−1) and off-line (3148.66cm−1), are characterized between 0.2–4atm and 500K–2100K 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.