Comprehensive CO detection in flames using femtosecond two-photon laser-induced fluorescence

100-kHz carbon monoxide TP-PLIF imaging using ultra-narrow-linewidth burst-mode OPO

A 100-kHz rate two-photon planar laser-induced fluorescence (TP-PLIF) imaging of carbon monoxide (CO) is successfully demonstrated utilizing a narrow-linewidth optical parametric oscillator (OPO) generating light at ∼230.1 nm. A specially designed injection-seeded burst-mode OPO was constructed and characterized for this purpose. This OPO efficiently converts the 355-nm output of a high-energy nanosecond burst-mode laser to ∼230.1 nm following parametric splitting and mixing processes. Generation of an ultra-narrow-linewidth 230.1 nm laser pulse is crucial for effectively exciting CO via a two-photon process from the ground X1Σ+ to the B1Σ+ electronic state-enabling PLIF imaging over a large area. The experimental setup is capable of tracking high-speed flow structures of a CO/N2 mixture, showcasing detection speeds 100 times greater than those achieved with previous femtosecond laser sources. This substantial increase in repetition rate will allow time-resolved CO-TP-PLIF measurements in highly dynamic hypersonic boundary layers and detonation-driven combustion processes for revealing chemical kinetics and turbulent aerodynamics.

Modeling of Carbon Monoxide Two-Photon Laser-Induced Fluorescence (LIF) Spectra at High Temperature and Pressure

In this study, quantitative model of two-photon excitation and fluorescence spectra of carbon monoxide based on up-to-date spectroscopic constants collected during an extensive literature survey was developed. This semi-classical model takes into account Hönl-London factors, quenching effects (collisional broadening and shift), ionization and stark effect (broadening and shift), whereas predissociation is neglected. It was specifically developed to first reproduce with a high confidence level the behavior of our experimental spectra obtained from laser-induced fluorescence (LIF) measurements, and then to allow us to extrapolate the fluorescence signal amplitude in other conditions than those used in these experiments. Synthetic two-photon excitation and fluorescence spectra of CO were calculated to predict the fluorescence signal at high pressures and temperatures, which are representative of gas turbine operating conditions. Comparison between experimental and calculated spectra is presented. Influence of temperature on both excitation and fluorescence spectra shapes and amplitudes is well reproduced by the simulated ones. It is then possible to estimate flame temperature from the comparison between experimental and calculated shapes of numerical excitation spectra. Influence of pressure on both excitation and fluorescence spectra was also investigated. Results show that for temperature below 600 K and pressure above 0.1 MPa, the usual Voigt profile is not suitable to reproduce the shape of the excitation spectrum. We found that the Lindholm profile is well suited to reproduce the pressure-dependence of the spectrum in the range 0.1 to 0.5 MPa at 300 K, and 0.1 to 0.7 MPa at 860 K. Beyond 0.7 MPa, in this temperature range, it is shown that the Lindholm profile does no longer match the spectral profiles, in particularly the red wing. Further analyses taking into account the line mixing phenomenon at higher pressure are thus discussed.

Pressure-scaling characteristics of femtosecond two-photon laser-induced fluorescence of carbon monoxide

Broadband femtosecond (fs) two-photon laser-induced fluorescence (TP-LIF) of the B¹Σ⁺←X¹Σ⁺, Hopfield-Birge system of carbon monoxide (CO) is believed to have two major advantages compared to narrowband nanosecond excitation. It should (i) minimize the effects of pressure-dependent absorption line broadening and shifting, and (ii) produce pressure-independent TP-LIF signals as the effect of increased quenching due to molecular collisions is offset by the increase in number density. However, there is an observed nonlinear drop in the CO TP-LIF signal with increasing pressure. In this work, we systematically investigate the relative impact of potential deexcitation mechanisms, including collisional quenching, forward lasing, attenuation of the source laser by the test cell windows or by the gas media, and a 2+1 photoionization process. As expected, line broadening and collisional quenching play minor roles in the pressure-scaling behavior, but the CO fs TP-LIF signals deviate from theory primarily because of two major reasons. First, attenuation of the excitation laser at high pressures significantly reduces the laser irradiance available at the probe volume. Second, a 2+1 photoionization process becomes significant as the number density increases with pressure and acts as a major deexcitation pathway. This work summarizes the phenomena and strategies that need to be considered for performing CO fs TP-LIF at high pressures.

Two-photon-excited fluorescence of CO: experiments and modeling

A model based on rate-equation analysis has been developed for simulation of two-photon-excited laser-induced fluorescence of carbon monoxide (CO) in the Hopfield-Birge band at 230 nm. The model has been compared with experimental fluorescence profiles measured along focused beams provided by lasers emitting nano-, pico-, and femtosecond pulses. Good quantitative agreement was obtained between simulations and experimental data obtained in premixed CH₄/C₂H₄-air flames. For excitation with femtosecond pulses, experimental and simulated fluorescence signals showed quadratic dependence on laser power under conditions of low laser irradiance, whereas different sublinear dependencies were obtained at higher irradiances due to photoionization. Simulations of CO signal versus femtosecond laser linewidth suggest the strongest signal for a transform-limited pulse, which is sufficiently broad spectrally to cover the CO Q-branch absorption spectrum. Altogether, the developed rate-equation model allows for analysis of two-photon excitation fluorescence to arrange suitable diagnostic configurations and retrieve quantitative data for CO as well as other species in combustion, such as atomic oxygen and hydrogen.

A Review of Femtosecond Laser-Induced Emission Techniques for Combustion and Flow Field Diagnostics

The applications of femtosecond lasers to the diagnostics of combustion and flow field have recently attracted increasing interest. Many novel spectroscopic methods have been developed in obtaining non-intrusive measurements of temperature, velocity, and species concentrations with unprecedented possibilities. In this paper, several applications of femtosecond-laser-based incoherent techniques in the field of combustion diagnostics were reviewed, including two-photon femtosecond laser-induced fluorescence (fs-TPLIF), femtosecond laser-induced breakdown spectroscopy (fs-LIBS), filament-induced nonlinear spectroscopy (FINS), femtosecond laser-induced plasma spectroscopy (FLIPS), femtosecond laser electronic excitation tagging velocimetry (FLEET), femtosecond laser-induced cyano chemiluminescence (FLICC), and filamentary anemometry using femtosecond laser-extended electric discharge (FALED). Furthermore, prospects of the femtosecond-laser-based combustion diagnostic techniques in the future were analyzed and discussed to provide a reference for the relevant researchers.

Spectroscopic investigation of high-pressure femtosecond two-photon laser-induced fluorescence of carbon monoxide up to 20 bar

Extension of laser diagnostic methods to high pressure is critically important because most practical combustion systems operate at elevated pressures. However, increased collisional quenching, fluorescence trapping, and other spectroscopic complications make high-pressure laser diagnostics extremely challenging. As the pressure increases, collisional effects broaden and shift excitation spectral lines, reducing the excitation quantum efficiency of the laser-induced fluorescence (LIF) technique, in particular when using conventional narrowband, nanosecond (ns)-duration laser pulses. In this work, spectroscopic investigation of broadband, femtosecond two-photon LIF (fs-TPLIF) of carbon monoxide (CO) was performed in a high-pressure static gas cell, up to total pressures of 20 bar. The fluorescence emission spectrum broadened marginally at the highest pressure investigated and hence can be neglected in most cases. The subquadratic dependence of the CO fs-TPLIF signal on the laser fluence increased as the pressure is increased. Moreover, the CO fs-TPLIF signal decays more slowly with increasing pressure compared to the previously reported ns-TPLIF data. The signal drop when the total pressure is increased from 1 to 13 bar is approximately 30% in the fs excitation, as compared to approximately a 60% drop in the ns excitation in CO/N₂/O₂ mixtures. The pressure effects on the fluorescence signal were observed to be similar in the Ångström and third positive bands, suggesting that the third positive band could also be used for CO measurements with broadband fs laser pulses. Furthermore, experimentally investigated are the effect of pressure on the CO fluorescence signal in different quenching gases using fixed CO mole fractions, as well as number densities. Overall, the fs-TPLIF scheme is shown to be a promising diagnostics tool for CO detection in practical combustion systems at elevated pressures.

Femtosecond, two-photon, laser-induced fluorescence (TP-LIF) measurement of CO in high-pressure flames

Quantitative, kiloherz-rate measurement of carbon monoxide mole fractions by femtosecond two-photon, laser-induced fluorescence (TP-LIF) was demonstrated in high-pressure, luminous flames over a range of fuel-air ratios. Femtosecond excitation at 230.1 nm was used to pump CO two-photon rovibrational X¹Σ⁺→B¹Σ⁺ transitions in the Hopfield-Birge system and avoid photolytic interferences with excitation irradiance ∼1.7×10¹⁰  W/cm². The effects of excitation wavelength, detection scheme, and potential sources of de-excitation were also assessed to optimize the signal-to-background and signal-to-noise ratios and achieve excellent agreement with theoretically predicted CO mole fractions at low and high pressure.