Temperature measurement by induced fluorescence in gas phase

Numerous measurement techniques based on optical diagnostics have been developed over the last few decades. The arrival of lasers was the driving force behind them. Today, many of them have become indispensable tools for understanding complex phenomena (combustion, reaction chemistry, interaction between chemistry and turbulence) and for providing a source of sufficiently accurate data to meet the expectations of highly resolved results from numerical calculations (RANS, LES and DNS). Several quantities characterize combustion and can be measured using optical diagnostics, such as temperature, gas and disperse phase velocity, droplet or solid particle size, droplet evaporation and the concentration of certain chemical species.

These characteristics can also be measured using specific sensors or appropriate techniques, such as thermocouples for temperature measurement, hot wires for velocity or temperature measurement, and gas chromatographs for species specification, coupled with mass spectrometry, for example. However, optical techniques are preferred because they are normally non-intrusive. What's more, they can achieve in situ measurements in reactive or non-reactive, homogeneous or heterogeneous environments. Laser techniques can also provide high spatial and temporal resolution. Typically, the duration of a laser pulse is of the order of 10 ^-12 to 10 ^-9 s. On these time scales, the flow or chemical reactions studied are considered to be frozen.

Laser diagnostic techniques are based on the interaction between the light emitted by the laser and the material being probed. This results in two types of light emission from the probed medium: the first is elastic emission, i.e. the scattered light is at the same wavelength as the excitation source light; the second is inelastic emission. In this configuration, the internal relaxation of molecules from the excited state to the ground state causes light to be emitted at a different wavelength from the excitation light source. In this case, more complex energy transfers are involved. Incident light is absorbed by a molecule or atom, which, in order to return from the excited energy state to the ground state, will re-emit an energy flux in several ways, either by scattering, luminescence and/or heating.

In the case of scattering, light is scattered at a different wavelength from the incident light. The spectral shifts introduced depend on the vibration-rotation energies of the excited molecule. Raman scattering can be used to measure species concentration, while analysis of the collected spectra provides temperature measurements.

This type of measurement has the advantage of being...