Measurements of CO_2 concentration and temperature at high pressures using 1f-normalized wavelength modulation spectroscopy with second harmonic detection near 27 μm

Abstract

Tunable diode lasers (TDL) near 2.7 mum are used to measure high-resolution direct absorption and wavelength modulation with second harmonic (WMS-2f) spectra at high pressures for two CO(2) transitions near 3633.08 and 3645.20 cm(-1), belonging to the nu(1)+ nu(3) vibrational band. Important factors influencing the design of a high-pressure TDL sensor and the variation of WMS-2f line shape with changes in pressure and laser parameters are discussed. Measurements of line strength and line broadening parameters are carried out for the 3645.20 cm(-1) transition in an atmospheric-pressure, high-temperature cell. A room-temperature high-pressure cell is then used to measure the pressure shift for both CO(2) transitions. Deviation of the direct absorption and wavelength modulation spectroscopy (WMS) spectra from the Lorentzian profile is studied in a high-density (9.2 amagats) CO(2)-Ar mixture. The WMS spectra are shown to be negligibly affected by non-Lorentzian effects up to 10 atm and room temperature, in contrast with direct absorption. Measurements of CO(2) concentration and temperature are carried out in nonreactive shock-tube experiments (P approximately 8-12 atm, T~800-1200 K) to validate the accuracy and precision of wavelength-modulation-spectroscopy-based sensing. CO(2) time histories are then measured in heptane ignition experiments and compared with reaction kinetics mechanisms to demonstrate the use of this sensor in high-pressure combustion systems.