Local temperature and composition measurements in pressurised CH₄/air flames with LIGS (Laser Induced Grating Spectroscopy)

Laser Induced Grating Spectroscopy (LIGS) -a seedless non intrusive point measurement technique- is applied in non-sooty laminar flat flames to determine temperature and water concentration. A 1064 nm pulsed seeded laser is used as pump laser. When sufficiently high energy is delivered by the pulses (around 100 mJ), the weak rotational transition of the water molecules in the probe volume is excited (thermalisation) and the electric field of the interference structure polarizes the dielectric medium (electrostriction). The LIGS signal is then probed with a CW laser at 532 nm. We demonstrate that the ratio between the electrostriction and the thermalisation peaks in the signals is an indicator of the water concentration, which is used to determine the gas composition. The speed of sound is extracted from the oscillation frequency of the signals from their FFTs. Once the composition is known, the temperature is determined with an iterative process. Temperature and composition data compare favourably with thermocouple measurements and flame simulations. A pressurised environment is needed to enhance the signal and obtain a sufficient number of peaks to enable a Fourier transform. These results confirm the potential for LIGS-based thermometry for high-precision combustion processes.