3.2. Saturated LIF

The laser source is usually composed of pump laser and dye laser to obtain a laser beam with different excitation wavelengths. The sheet-forming optics mainly consists of one cylindrical concave lens and other two vertical cylindrical convex lenses, achieving the transformation from line beam to sheet beam. The imaging acquisition and data storage system is mainly composed of ICCD camera and data storage module to acquire the weak fluorescence signals in a real time. The digital delay control system is used to control the synchronization between the laser and the ICCD camera. The burner is used to produce an objective flame to be researched. According to the characteristics of the actual research, the PLIF system will be

From the view of using purposes, LIF technology can be briefly divided into two categories: qualitative and quantitative. In this section, the current research status of quantitative LIF will

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

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 propor-

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 intro-

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

slightly different, but the main assembly is still composed of the above five parts.

3. Review on the developments of the quantitative LIF

be reviewed.

3.1. Calibration LIF

90 Laser Technology and its Applications

duced as follows.

calibration LIF, has been first proposed.

tional relationship with the LIF signal intensity.

When the excited energy density is higher than the threshold energy density of saturation excitation, the intensity of fluorescence signal is only related to the molecular number density, stimulated absorption, stimulated radiation, and spontaneous radiation but independent with excitation energy and the electronic quenching rate. This case is known as the saturated LIF.

In the saturated LIF, the measured fluorescence signal can directly reflect the number density of the stimulated molecules. The main drawback of the saturated LIF is that the output laser pulse is difficult to reach the required saturated excitation energy density. Therefore, it is difficult to achieve the planar concentration measurement for the species. In addition, because the laser pulse has a certain energy profile in time and space, it is easy to arise the so-called wing effect at the edge of energy profile. In other words, the laser energy density at the edge is less than the threshold energy density. Therefore, the excitation in this location still belongs to the linear excitation, leading to the fact that fluorescence signal is still affected by the collisional quenching effect. The researches on quantitative measurement of species concentration using saturated LIF mainly include:

Carter et al. [12] used saturated LIF to measure the OH concentration distributions in C2H6/O2/N2 flames under the high pressure. The experimental results indicate that the maximum OH concentration measured by saturated LIF is 1.10 1016, 1.05 1016, 1.18 1016 and 0.98 10<sup>16</sup> molecules/cm<sup>3</sup> , respectively, under the pressure of 0.98, 6.1, 9.2, and 12.3 atm.

Kohse-Höinghaus et al. [13] measured the concentrations of CH and OH radicals in a premixed C2H2/O2 flames under the low pressure using saturation LIF. The experimental results show that the concentration of CH and OH radicals in acetylene/oxygen flame is 1.1 1013 cm<sup>3</sup> (<sup>T</sup> = 1750 80 K, height at 2.6 mm) and 8.9 1014 cm<sup>3</sup> (<sup>T</sup> = 2000 100 K, height at 7.5 mm), respectively, under the pressure of 13 mbar and the equivalence ratio of 1.2.

#### 3.3. Laser-induced pre-dissociative fluorescence (LIPF)

LIPF has also been recognized a kind of quantitative LIF, which is proposed to solve the problem that the fluorescence signal is susceptible to collisional quenching effect in linear LIF. In the LIPF, the fluorescence quantum efficiency can be written as

$$\varphi = \frac{A\_{\text{mu}}}{A\_{\text{mu}} + Q(T, N\_c) + P} \tag{5}$$

gate width of nanosecond, the detected fluorescence signals are bound to be seriously affected by the influence of electronic quenching effect. Nevertheless, if the picosecond or a shorter laser pulse is considered to use, the molecules will be distributed in the upper energy state before the collisional quenching occurs. In this moment, if the picosecond detector can be employed to collect the fluorescence emitting from the molecule, the fluorescence signal will no longer be influenced by the collisional quenching effect. This is known as the short-duration pulsed LIF.

Quantitative Planar Laser-Induced Fluorescence Technology

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Thus, the short-duration pulsed LIF is also a kind of quantitative LIF, which is independent of the collisional quenching effect. The shortcoming of short-duration pulsed LIF lies in the fact that it is difficult to output a laser beam with a pulse width of picosecond, and the gate width of ICCD camera is not easy to reach at the order of picosecond. Furthermore, as the laser pulse width and the gate width of the detector are all at the order of picosecond, the collected fluorescence signals will be relatively weak, and its SNR is extraordinary low, which are not convenient for practical engineering applications. The researches on quantitative measure-

Bormann et al. [16] used the single-pulse picosecond LIF to obtain the relative OH concentration profiles in a premixed stoichiometric CH4/O2 flame under the normal pressure. The experimental results indicate that the number of OH is exceedingly few in the preheating zone above the burner outlet. Most of the OH radicals are distributed near the flame front and the

Brockhinke et al. [17] measured the OH concentration distributions in the hydrogen/air opposed diffusion flame using picosecond LIF and determined the concentration distributions of H atoms by three-photon LIF in the same flame. The experimental results suggest that the peak OH concentration is about 2.5 1016 molecules/cm<sup>3</sup> and the peak concentration of the H atom is around at 2.1 1016 molecules/cm<sup>3</sup> in the stagnation surface of diffusion flame.

Bidirectional LIF has been recognized as a non-calibration linear LIF, which is independent of the collisional quenching effect. In the bidirectional LIF, the number density of the stimulated molecules is only related to the effective peak absorption cross section of the measured molecules and the forward and backward fluorescence signals. It has no relevance with the collisional quenching effect, the calibration constant of the detection system, and the energy density of the excited laser. Using bidirectional LIF/PLIF to map the concentration distributions, the two laser beams (or sheet beams) propagating through flame in the opposite direction are required to excite the molecules in the flame, so as to obtain the forward and backward LIF/PLIF signals. With combining the effective peak absorption cross sections of the molecules by other measure-

The available literature shows that the embryonic form of the bidirectional LIF is first proposed by the Stepowski [18]. After that, Versluis et al. [8] have further developed it and given a more concise and explicit expression for the concentration measurements in the high absorptive flames. The first application of bidirectional LIF/PLIF to the quantification of the twodimensional OH concentration distributions in a methane/oxygen torch flame is investigated by Versluis et al. Besides that, Brackmann et al. [19] also employed bidirectional LIF to achieve

ment methods, the number density of the excited molecules can then be obtained.

ments of species concentration using short-duration pulsed LIF mainly include:

edge of the flame.

3.5. Bidirectional LIF

where P represents the pre-dissociative rates of the molecules in the excited state.

Generally speaking, if the ground state molecules can be excited to a suitable upper level, then there is a relationship of Q ≪ P. Taking the vibrational band (3,0) excitation of OH radical as an example, the typical spontaneous emission rate A in the upper level is approximately 1.6 � 104 <sup>s</sup> �1 , the collision quenching rate is about 109 s �1 , and the pre-dissociative rate is around at 1 � 1010 <sup>s</sup> �1 . Therefore, the effects of A and Q on the fluorescence quantum efficiency can be neglected. In the LIPF, it can be considered that the fluorescence quantum efficiency is only affected by the pre-dissociative effect, but has no obvious relevance with the spontaneous emission and the collisional quenching effects. If the calibration factor of LIF signal would be obtained by other methods (calibration or direct measurement), the measured molecular concentration in flames can be obtained by using this quantitative relationship.

Using LIPF to measure the concentration fields of the stimulated molecules can immunize the LIF signal from the interference of collision quenching effect and thus reduce the difficulty for the quantitative measurements. However, it will bring in another trouble that the higher dissociative rate will lead to the decrease of fluorescence quantum efficiency, which makes the fluorescence signal further weakened and difficult to capture. In addition, compared with the traditional linear LIF excitation wavelength, LIPF usually needs to excite the measured species to a higher excitation level. At the same time, the energy density of excitation laser should also be increased as high as possible, so as to meet a higher SNR requirement. These experimental conditions are rather incompetent for the common lasers. The researches on the quantitative concentration measurements using LIPF mainly include:

Yuan et al. [14] quantitatively measured the variations of the OH concentrations with the axial heights in a premixed methane/air and propane/air flat flames at the range of 1–5 atm and the equivalence ratio of 0.7–1.3. The experimental results indicate that the OH concentration in the methane/air flame reaches the peak at around 2 mm from the burner surface, with a numerical value of about 1.1 � <sup>10</sup><sup>16</sup> molecules/cm<sup>3</sup> . For propane/air flame, the peak OH concentration on the same conditions is much smaller than that of methane/air flames, with a value of about 1.5 � 1015 molecules/cm<sup>3</sup> .

Brown et al. [15] measured the OH concentration profiles in a hydrogen/air diffusion flame using the LIPF and compared the experimental results with the numerical simulations. The experimental results show that the peak concentration of OH radical is 9.3 � 1016 molecules/cm3 approximately in this flame.

#### 3.4. Short-duration pulsed LIF

In the linear LIF, the duration of the excitation laser pulse (pulse width) is at the order of nanosecond, and the collision quenching rate is commonly 109 –1010 s �1 , slightly less than the excited laser pulse width, while the fluorescence lifetime of the excited state molecules is around at a few nanoseconds. Therefore, if the nanosecond laser pulse is used to excite the molecules, and then the fluorescence emitted from excited molecules is collected by an ICCD camera with a gate width of nanosecond, the detected fluorescence signals are bound to be seriously affected by the influence of electronic quenching effect. Nevertheless, if the picosecond or a shorter laser pulse is considered to use, the molecules will be distributed in the upper energy state before the collisional quenching occurs. In this moment, if the picosecond detector can be employed to collect the fluorescence emitting from the molecule, the fluorescence signal will no longer be influenced by the collisional quenching effect. This is known as the short-duration pulsed LIF.

Thus, the short-duration pulsed LIF is also a kind of quantitative LIF, which is independent of the collisional quenching effect. The shortcoming of short-duration pulsed LIF lies in the fact that it is difficult to output a laser beam with a pulse width of picosecond, and the gate width of ICCD camera is not easy to reach at the order of picosecond. Furthermore, as the laser pulse width and the gate width of the detector are all at the order of picosecond, the collected fluorescence signals will be relatively weak, and its SNR is extraordinary low, which are not convenient for practical engineering applications. The researches on quantitative measurements of species concentration using short-duration pulsed LIF mainly include:

Bormann et al. [16] used the single-pulse picosecond LIF to obtain the relative OH concentration profiles in a premixed stoichiometric CH4/O2 flame under the normal pressure. The experimental results indicate that the number of OH is exceedingly few in the preheating zone above the burner outlet. Most of the OH radicals are distributed near the flame front and the edge of the flame.

Brockhinke et al. [17] measured the OH concentration distributions in the hydrogen/air opposed diffusion flame using picosecond LIF and determined the concentration distributions of H atoms by three-photon LIF in the same flame. The experimental results suggest that the peak OH concentration is about 2.5 1016 molecules/cm<sup>3</sup> and the peak concentration of the H atom is around at 2.1 1016 molecules/cm<sup>3</sup> in the stagnation surface of diffusion flame.
