Infrared planar laser-induced fluorescence with a CW quantum-cascade laser for spatially resolved CO2 and gas properties

Abstract

The design and demonstration of a new infrared laser-induced fluorescence (IR-LIF) technique that enables spatially resolved measurements of CO2, temperature, and pressure, with potential for velocity, are presented. A continuous-wave, wavelength-tunable, quantum-cascade laser (QCL) near $$4.3\,\upmu \hbox {m}$$ with up to 120 mW was used to directly excite the asymmetric-stretch fundamental-vibration band of CO2 for approximately 200 to $$10^5$$ times more absorbance compared with previous IR-LIF techniques. This enabled LIF detection limits (signal-to-noise ratio of 1) of 20 and 70 ppm of CO2 in Ar and $$\hbox {N}_2$$ , respectively, at 1 bar and 296 K in static-cell experiments. Simplified and detailed kinetic models for simulating the LIF signal as a function of gas properties are presented and enable quantitative, calibration-free, IR-LIF measurements of CO2 mole fraction within 1–8 % of known values at 0.5–1 bar. By scanning the laser across two absorption transitions and performing a multi-line Voigt fit to the LIF signal, measurements of temperature, pressure, and $$\chi _{\hbox {CO}_2}$$ within 2 % of known values were obtained. LIF measurements of gas pressure at a repetition rate up to 200 Hz (in argon) are also presented. Planar-LIF (PLIF) was used to image steady and unsteady CO2–Ar jets at 330 frames per second with a spatial signal-to-noise ratio (SNR) up to 25, corresponding to a detection limit (SNR = 1) of 200 ppm with a projected pixel size of $$40\,\upmu \hbox {m}$$ . The gas pressure was measured within $$3 \pm 2$$ % of the known value (1 bar) at 5 Hz by scanning the QCL across the P(42) absorption transition and least-squares fitting a Voigt profile to the PLIF signal. Spatially resolved measurements of absolute CO2 mole fraction in a laminar jet are also presented.