3.1. Calibration LIF

From Eq. (1), it is known that the following parameters should be measured accurately when using the PLIF image to deduce the concentration field of the species: the excitation laser wavelength, the experimental calibration constant, the Einstein stimulated absorption coefficient, the Boltzmann fraction, the fluorescence quantum yield (especially for the collisional quenching rate Q), and the convolution of the laser line shape and the molecular absorption line shape. However, it is difficult to obtain the exact values of the temperature, the fluorescence quantum yield, and other parameters at the same time in flames. Therefore, the quantification of the molecular concentration field is thought to be fairly difficult. In order to simplify the difficulty of quantification, calibrating these parameters with a standard flame, named calibration LIF, has been first proposed.

Using the calibration method to determine the species concentration field, the following simplification is needed: under the condition of linear excitation, it is considered that the concentration of the molecules to be measured is only related to the LIF signal intensity, calibration constant, flame temperature, and environmental pressure but independent of other factors. To further reduce the dependence of the Boltzmann fraction on temperature, it is always necessary to select an excited line, which is not sensitive to the changes of temperature. After the above simplification, it can be considered that the species concentration has a direct proportional relationship with the LIF signal intensity.

At present, extensive research for the measurements of the OH concentration spatial distribution has been studied by using the calibration LIF/PLIF. The typical research work is introduced as follows.

Arnold et al. [10] measured the OH concentration distributions in the premixed methane/air flame at pressures of 1, 5, and 20 bar by using the calibration LIF. The calibration factor was obtained by ultraviolet (UV) absorption spectroscopy. Jalbert [11] researched the variations of OH concentration with the flame heights in the premixed methane/air and hydrogen/air flames. And the influences of the equivalent ratio and flow rate on the OH concentration have also been investigated by using the calibration LIF.

Although the calibration LIF has ability to measure the species concentration profiles to a certain extent, there is more serious problem that should not be neglected: the calibration LIF ignores the fact that the collisional quenching rates vary with the spatial position in the flame. Therefore, the calibration LIF cannot be considered as a real quantitative LIF strictly. It can only be regarded as a semiquantitative LIF technology.
