High-temperature iso-butene absorption diagnostic for shock tube kinetics using a pulsed quantum cascade laser near 11.3 μm

A high-bandwidth absorption sensing technique for iso-butene (iC(4)H(8)) was developed to measure transient species concentration behind reflected shock waves for combustion kinetics studies. Direct measurements of iC(4)H(8) were enabled by monitoring absorption in the infrared near 11.3 mu m using a novel pulsed external-cavity quantum cascade laser (ECQCL) with a repetition rate of 600 kHz. Optimal wavelength selection for high-temperature combustion gases was first determined by a spectral survey at 1000 K near the peak of the absorption band (878 cm(-1) to 892 cm(-1)) using the ECQCL. Absorption cross section measurements of iC(4)H(8) at 881.4 cm(-1), the selected high-temperature wavelength, were then conducted from 800 K to 1800 K behind reflected shocks to characterize temperature dependence at modest pressures (4-8 atm). The species-specific technique was subsequently demonstrated by time-resolved (100 kHz) measurements of iso-butene decay during thermal decomposition (1280-1480 K). First-ever shock tube measurements of iC(4)H(8) yields from iso-octane pyrolysis (1070-1300 K) were also produced, with a detection limit of similar to 100 ppm. Experimental results were compared to recent kinetic models to illustrate the potential of this diagnostic for analyzing combustion chemistry. (C) 2014 The Combustion Institute. Published by Elsevier Inc. All rights reserved.