Midinfrared polarization spectroscopy of OH and hot water in low pressure lean premixed flames

Mid-Infrared Polarization Spectroscopy Measurements of Species Concentrations and Temperature in a Low-Pressure Flame

We demonstrate quantitative measurements of methane (CH₄) mole fractions in a low-pressure fuel-rich premixed dimethyl ether/oxygen/argon flat flame (Φ = 1.87, 37 mbar) using mid-infrared (IR) polarization spectroscopy (IRPS). Non-intrusive in situ detection of CH₄, acetylene (C₂H₂), and ethane (C₂H₆) in the flame was realized by probing the fundamental asymmetric C-H stretching vibration bands in the respective molecules in the spectral range 2970-3340 cm^-1. The flame was stabilized on a McKenna-type porous plug burner hosted in a low-pressure chamber. The temperature at different heights above the burner (HAB) was measured from the line ratio of temperature-sensitive H₂O spectral lines recorded using IRPS. Quantitative measurements of CH₄ mole fractions at different HAB in the flame were realized by a calibration measurement in a low-pressure gas flow of N₂ with a small admixture of known amount of CH₄. A comprehensive study of the collision effects on the IRPS signal was performed in order to quantify the flame measurement. The concentration and temperature measurements were found to agree reasonably well with simulations using Chemkin. These measurements prove the potential of IRPS as a sensitive, non-intrusive, in situ technique in low pressure flames.

Comparison of an InSb Detector and Upconversion Detector for Infrared Polarization Spectroscopy

This paper compares the signal-to-noise ratio obtained using an InSb photodiode for infrared (IR) polarization spectroscopy to that obtained using an upconversion detector, and shows a factor 64 improvement by the change. Upconversion detection is based on using sum frequency generation to move the IR optical signal to near-visible wavelengths to improve the sensitivity.

Advanced Laser-Based Techniques for Gas-Phase Diagnostics in Combustion and Aerospace Engineering

Gaining information of species, temperature, and velocity distributions in turbulent combustion and high-speed reactive flows is challenging, particularly for conducting measurements without influencing the experimental object itself. The use of optical and spectroscopic techniques, and in particular laser-based diagnostics, has shown outstanding abilities for performing non-intrusive in situ diagnostics. The development of instrumentation, such as robust lasers with high pulse energy, ultra-short pulse duration, and high repetition rate along with digitized cameras exhibiting high sensitivity, large dynamic range, and frame rates on the order of MHz, has opened up for temporally and spatially resolved volumetric measurements of extreme dynamics and complexities. The aim of this article is to present selected important laser-based techniques for gas-phase diagnostics focusing on their applications in combustion and aerospace engineering. Applicable laser-based techniques for investigations of turbulent flows and combustion such as planar laser-induced fluorescence, Raman and Rayleigh scattering, coherent anti-Stokes Raman scattering, laser-induced grating scattering, particle image velocimetry, laser Doppler anemometry, and tomographic imaging are reviewed and described with some background physics. In addition, demands on instrumentation are further discussed to give insight in the possibilities that are offered by laser flow diagnostics.

Single-shot, planar infrared imaging in flames using polarization spectroscopy

Planar infrared visualization of species in flames is challenging due to the severe thermal radiation background and relatively weak fluorescence quantum yields from ro-vibration transitions. In this express, we report imaging of molecular species in a flame via an absorption-based coherent optical method, namely infrared polarization spectroscopy (IRPS). Single-shot, planar imaging of hydrogen fluoride (HF) has been achieved in a premixed CH(4)/O(2) Bunsen flame, being seeded with a small amount of SF(6). The HF molecule was excited through a rovibrational transition at around 2.5 µm, which belongs to the fundamental vibration band. High spatial resolution was guaranteed using an orthorgonal pump-probe geometry, and an effective suppression of thermal background emission was achieved owing to the coherent nature of the demonstrated two-dimensional IRPS. Other advantages, e.g. high temporal resolution and species-specificity, are also features of this laser-based technique, which make it suitable for imaging of non-fluorescent but infrared active gaseous molecules in harsh environments.

Spatially resolved trace detection of HCl in flames with mid-infrared polarization spectroscopy

Sensitive and nonintrusive detection of HCl in reactive gas flows with high spatial and temporal resolution manner has for the first time (to our knowledge) been demonstrated using mid-infrared polarization spectroscopy (IRPS). Trace levels of HCl were prepared in an atmospheric pressure premixed CH4/O2/Ar flat flame by seeding a small amount of chloroform into the Ar flow. Detection of HCl with IRPS in the burnt region of the stoichiometric flame was performed by probing the fundamental ro-vibration transitions with a 3.2 microm tunable pulsed laser. The quantitative nature, the detection sensitivity, and the potential spectral interferences from water were investigated.