Abstract
Laser-Induced Grating Spectroscopy (LIGS) is applied to premixed CH4/air laminar flat flames under operating pressures of 1 to 6 bar. For the first time, temperature and water concentration have been acquired simultaneously in a reacting flow environment using LIGS. A 1064 nm pulsed laser is used as pump to generate a temporary stationary intensity grating in the probe volume. Water molecules in the flame products absorb the laser energy and generate a thermal grating if sufficiently high energies are delivered by the laser pulses, here more than 100 mJ per pulse. Such energies allow the electric field to polarize the dielectric medium, resulting in a detectable electrostrictive grating as well. This creates LIGS signals containing both the electrostrictive and the thermal contributions. The local speed of sound is derived from the oscillation frequency of LIGS signals, which can be accurately measured from the single shot power spectrum. Data show that the ratio between the electrostrictive and the thermal peak intensities is an indicator of the local water concentration. The measured values of speed of sound, temperature, and water concentration in the flames examined compare favorably with flame simulations with Chemkin, showing an estimated accuracy of 0.5 to 2.5% and a precision of 1.4–2%. These results confirm the potential for 1064 nm LIGS-based thermometry for high-precision temperature measurements of combustion processes.
Highlights
Temperature is a key parameter in combustion and reacting flows as it affects engine efficiency, pollutant emissions, and noise
The Laser-Induced Grating Spectroscopy (LIGS) signal oscillation frequency, and the corresponding speed of sound, are extracted from the power spectrum of the signal, while water concentration is obtained from the relative amplitude of the electrostrictive and thermal peaks
A closer look at the first oscillations of the signal in the case of the flame shows that the electrostrictive contributions in air and in the flame yield a first peak within a few nanoseconds after the laser pulse while the much larger thermal contribution in the flame signal appears after a longer time delay of roughly 15 ns
Summary
Temperature is a key parameter in combustion and reacting flows as it affects engine efficiency, pollutant emissions, and noise. We demonstrate Laser-Induced Grating Spectroscopy (LIGS) as a promising technique to measure local temperature and water concentration in high pressure combustion environments. Demands a significantly simpler set-up and data analysis than CARS It relies on the measurement of the modulation frequency of a laser-induced transient grating signal, which is related to the local speed of sound. The first harmonic of the Nd:YAG laser at 1064 nm is ideal to achieve the high energy outputs required to yield LIEGS and shows promise to produce LITGS using water as an absorber, which is available for free in combustion products. Results show that 1064 nm LIGS is a non-intrusive, accurate, low cost technique to measure temperature and water concentration in combustors. Comparisons are based on speed of sound, temperature and water concentration (Sec. 4)
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