1. Introduction

The planar laser-induced fluorescence [1] is a high-sensitivity and high-resolution laser spectral diagnostic technique developed in the 1980s. The emergence of PLIF technology has made great success in the visualization of the combustion field in flames, the dynamic evolution of the combustion process [2, 3], the temperature imaging [4, 5], and the quantitative measurements of the free radical concentrations under low pressure [6, 7]. PLIF technology is a noncontact measurement technology. Compared with the traditional contact measurement technology, PLIF technology not only has the unique advantages of noncontact and noninterference

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and eproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

to the combustion and flow fields but also can dynamically display and image two- or threedimensional space structure of free radicals interested in combustion fields. These merits are unattainable and difficult to achieve by the traditional contact and other laser spectral diagnostic techniques, such as coherent anti-Stokes Raman scattering (CARS), degenerate four-wave mixing technology (DFWM), cavity ring down spectroscopy technology (CRDS), tunable diode laser absorption spectroscopy (TDLAS), and direct absorption spectroscopy.

is of great significance for deep understanding, modification, and validation of reaction mechanism and chemical kinetic models. This chapter takes OH as an example to explain. Similar

The energy-level structure consists of the electronic, vibrational, and rotational energy levels. When numerous OH radicals in the flame absorb a certain wavelength laser from the ground state to the excited state, one part of molecules will shift to other vibrational or rotational energy levels due to the effect of vibrational energy transfer (VET) and rotational energy transfer (RET), one part of which will be transferred to the pre-dissociated state, and some of which will be quenched due to the collision of atoms and molecules in the surrounding environment. After above three physical processes, in fact, the OH radicals no longer involve the process of fluorescence radiation. Therefore, only a fairly small number of OH radicals can emit fluorescence from the excited state to the ground state. The physical process is shown in Figure 1.

The main purpose of the LIF technology is to determine the total number density of OH radicals by using the observable intensity of the OH fluorescence signal and then to obtain the local physical properties of the measured molecules. However, there are many physical processes difficult to observe directly in the LIF process, such as VET, RET, collisional quenching, and predissociation, which will greatly affect the intensity of the OH fluorescence signal. Therefore, if the concentration of fluorescence signal is directly obtained by observation, one will cause great deviation. Generally, the time scales of VET, RET, and collisional quenching effects are much less than the lifetime of OH fluorescence. Therefore, it is rather

Under the linear excitation, the LIF signal intensity of the excited molecule in position x at

<sup>m</sup><sup>f</sup> <sup>B</sup>ð Þ <sup>x</sup>; <sup>T</sup> Anm

Anm þ Q xð Þ ; T; Nc

ð

I xð Þ ; ω

Quantitative Planar Laser-Induced Fluorescence Technology

http://dx.doi.org/10.5772/intechopen.79702

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<sup>c</sup> g xð Þ ; <sup>ω</sup>; <sup>T</sup>; Nc <sup>d</sup><sup>ω</sup> (1)

ω

difficult to directly measure these physical processes on the OH fluorescence signal.

atmospheric pressure can be expressed as follows [9]:

<sup>I</sup>LIFð Þ¼ <sup>x</sup>; <sup>ω</sup>; <sup>T</sup>; Nc <sup>ℏ</sup>ωCexp BmnN<sup>0</sup>

Figure 1. The LIF process of hydroxyl radical [9].

conclusions exist for other combustion intermediates or radicals.

Admittedly, PLIF technology also has its own shortcomings. The biggest obstacle is that the PLIF technology is rather difficult to achieve quantitative measurement. The greatest difficulty in quantification of the PLIF lies in the fact that the electronic collisional quenching rate of the molecules to be measured is not exactly the same at the different positions of flame under the normal and high pressures. Worse still, there is a big difference for the distribution of electronic quenching rates in different types of flames. In addition, the calibration factor is also a parameter, which is easy to change with the observation conditions. In a general way, the collisional quenching rates of the molecules to be measured in flame have little effect on the fluorescence signal under the low pressure, so it can be often neglected. However, the collisional quenching rates of molecules turn out to be very sensitive to the combustion environment in flames under normal and high pressures. Therefore, the profiles of the collisional quenching rates become difficult to be measured in real time due to the diversification of the flame structure. In this case, an additional consideration is needed to accurately deduce the concentration distribution information of the molecules to be measured using the fluorescence signals.

In order to eliminate the detrimental impact of quenching effect on the quantitative measurement of species concentration, Versluis et al. [8] proposed to combine the traditional laserinduced fluorescence (LIF) with absorption spectroscopy to excite the molecules using the two opposite direction laser beams, counteracting many influencing factors, such as collisional quenching effect, pressure, and the calibration constant of the Detector and optical system, so as to deduce species concentration profiles. However, the bidirectional LIF technology is highly depending on the spatial coincidence of the beam (especially for the sheet laser beam) and the signal-to-noise ratio (SNR) of the fluorescence signal, which leads to the lack of systematic research on this technology.
