CO concentration and temperature sensor for combustion gases using quantum-cascade laser absorption near 4.7 μm

Abstract

A sensor for sensitive in situ measurements of carbon monoxide and temperature in combustion gases has been developed using absorption transitions in the (v′=1←v″=0) and (v′=2←v″=1) fundamental bands of CO. Recent availability of mid-infrared quantum-cascade (QC) lasers provides convenient access to the CO fundamental band near 4.7 μm, having approximately 10⁴ and 10² times stronger absorption line-strengths compared to the overtone bands near 1.55 μm and 2.3 μm used previously to sense CO in combustion gases. Spectroscopic parameters of the selected transitions were determined via laboratory measurements in a shock tube over the 1100–2000 K range and also at room temperature. A single-laser absorption sensor was developed for accurate CO measurements in shock-heated gases by scanning the line pair v″=0, R(12) and v″=1, R(21) at 2.5 kHz. To capture the rapidly varying CO time-histories in chemical reactions, two different QC lasers were then used to probe the line-center absorbance of transitions v″=0, P(20) and v″=1, R(21) with a bandwidth of 1 MHz using fixed-wavelength direct absorption. The sensor was applied in successful shock tube measurements of temperature and CO time-histories during the pyrolysis and oxidation of methyl formate, illustrating the capability of this sensor for chemical kinetic studies.

Appendix

High-resolution absorption spectroscopy of the CO transition v″=1, R(22) at 2194.46 cm⁻¹ has been recorded in high temperature shock tube experiments (0.496 % CO/1 % H₂/Ar, hydrogen is added to accelerate CO vibrational relaxation). A representative line-shape for the R(22) transition at 2162 K, 1.3 atm is illustrated in Fig. 18. It is found that R(22) is blended with a weak, nearby transition v″=2, R(32) centered at 2194.44 cm⁻¹. Our observation is also proved by fitting the absorption data using the Voigt profile as shown in Fig. 19. The one-line Voigt fit gives a peak-normalized residual of 10 %, compared to 1.8 % using the two-line Voigt fit. This unknown transition is probably coming from an isotope of CO in the mixture.

Fig. 18

Absorption data of v″=1, R(22) taken at 2.5 kHz with 0.496 % CO/1 %⋅H₂/Ar. Vibrationally equilibrated reflected shock conditions: 2162 K, 1.3 atm. A blended transition (v″=2, R(32), 0.024 cm⁻¹ away) is observed

Fig. 19

One-line and two-line best-fit Voigt profiles for the absorption data in Fig.18. The residual of the fits are shown in the lower panels