3.5. Bidirectional LIF

<sup>φ</sup> <sup>¼</sup> Anm

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

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

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

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

the same conditions is much smaller than that of methane/air flames, with a value of about

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

In the linear LIF, the duration of the excitation laser pulse (pulse width) is at the order of

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

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

, the collision quenching rate is about 109 s

concentration in flames can be obtained by using this quantitative relationship.

quantitative concentration measurements using LIPF mainly include:

1.6 � 104 <sup>s</sup>

�1

92 Laser Technology and its Applications

�1

value of about 1.1 � <sup>10</sup><sup>16</sup> molecules/cm<sup>3</sup>

.

nanosecond, and the collision quenching rate is commonly 109

1.5 � 1015 molecules/cm<sup>3</sup>

approximately in this flame.

3.4. Short-duration pulsed LIF

around at 1 � 1010 <sup>s</sup>

Anm <sup>þ</sup> Q Tð Þþ ; Nc <sup>P</sup> (5)

. For propane/air flame, the peak OH concentration on

–1010 s �1

, slightly less than the

, and the pre-dissociative rate is

�1

. Therefore, the effects of A and Q on the fluorescence quantum efficiency

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

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 the quantitative measurements of OH concentration distributions in an opposed diffusion flame. Because the opposed diffusion flame belongs to a kind of symmetrical flame, they used only one beam to excite the OH radicals and combined the mirror symmetry method to achieve the quantitative measurements of one-dimensional OH concentration distributions. Their experimental results indicate that the OH concentration is about 7.8 � 1015 molecules/cm3 at the height of 1.8 mm from the burner nozzle. In addition, Tian et al. [20] also used bidirectional LIF to quantitatively measure the concentration of iron atoms in a premixed laminar propylene/oxygen/argon flat flames. However, the two opposite directional beams have not been employed in their experiments. Instead, the mirror symmetry method has been used to obtain the variations of the iron atom concentration with the axial heights.

Judging from the existing literature statistics, the current species concentration measurements based on bidirectional LIF/PLIF technology are still fairly scarce. Although the bidirectional LIF/PLIF has great advantages beyond other quantitative LIF/PLIF technologies, such as no collisional quenching effect, no special excitation conditions (e.g., large energy, short pulses, etc.) and no additional calibration, it has a high requirement for the spatial coincidence of the beams and the SNR of the fluorescence signal. In addition, the experimental expression of effective peak absorption cross section provided by Versluis et al. has a limitation, which is not applicable to the case of weak absorption. In view of this problem, we have supplemented and corrected the experimental measurement equation in this chapter. These difficulties have resulted in the fact that the research of species concentration measurement based on the bidirectional PLIF is almost at a standstill. Therefore, it is necessary to conduct the in-depth research in order to promote the further development of the bidirectional LIF/PLIF.
