A mid-IR laser diagnostic for HCN detection

Hydrogen cyanide (HCN) is a major source of prompt-NO x formation especially in fuel-bound nitrogen systems. To date, there is still a significant disagreement between experimental data and theoretical predictions of the rate coefficients of combustion reactions involving HCN as a prompt-NO x precursor. Accurate modeling of NO x formation would greatly benefit from a diagnostic capable of performing high-fidelity measurements of HCN formation/consumption time-histories. In this study, a laser diagnostic is developed for sensitive and selective HCN sensing by probing its most intense absorption feature in the mid-infrared (MIR). The diagnostic is based on difference-frequency generation (DFG) between a CO 2 gas laser and an external-cavity quantum cascade laser in a nonlinear orientation-patterned gallium arsenide crystal which results in a DFG laser tunable over 11.56 - 15 & mu;m. HCN measurements were carried out at the peak of the Q -branch of its strong & nu;2 vibrational band near 14 & mu;m. Pressure dependence of the absorption cross-section was investigated at room temperature over the pressure range of 0.07 - 1.07 bar. Temperature-dependent absorption cross-section measurements were conducted behind reflected shock waves over the temperature range of 850 - 3000 K. The diagnostic was demonstrated in reactive experiments in a shock tube where HCN mole fraction time-histories were measured during the thermal decomposition of isoxazole (C 3 H 3 NO) and the first-order rate coefficients of C 3 H 3 NO & RARR; HCN + CH 2 CO reaction were determined. & COPY; 2022 The Combustion Institute. Published by Elsevier Inc. All rights reserved.