5.1. Measurements for the absolute OH concentration profiles in a partially premixed methane/air flat flame

The temperature distribution of CH4/O2/N2 partially premixed flame is first measured by UV absorption spectrometry. The average temperature of the premixed flame of methane/air is 1772 K, with the statistical uncertainty of 10.4%. Then, the axial distributions of OH effective peak absorption cross section for Q1(8) line (the corresponding wavelength of 309.240 nm) in the band (0,0) are determined by using the wavelength scanning method. The statistical average of the OH effective peak absorption cross section on the axial direction is 1.10 <sup>10</sup><sup>15</sup> cm<sup>2</sup> with the relative statistical uncertainty, which is 9.9%. The standard flat flame burner, designed by Hartung et al. [24], in the experiments is employed. More detailed experimental parameters can be found in the literature [25]. The variations of two-dimensional OH concentration fields with the equivalence ratios Φ (from 0.7 to 1.4) have been obtained in CH4/O2/N2 partially premixed flat flame by using bidirectional PLIF, as shown in Figures 4 and 5.

Figures 4 and 5 show the two-dimensional spatial distributions of OH concentration in the methane/air partially premixed flame and its variations with the equivalence ratios, respectively. The actual size of each image is 15 mm 46 mm, and the spatial resolution is 87.6 μm.

As can be seen from Figure 4, when the flame is burned in the lean-burn condition, the OH radicals are mainly distributed in a narrow band above the burner surface. With the increase of the axial distance, the OH concentration will decrease rapidly. On the other hand, the OH

Figure 4. Variations of two-dimensional OH concentrations with equivalence ratios (Φ = 0.7–1.0). (a) Φ = 0.7, (b) Φ = 0.8, (c) Φ = 0.9, (d) Φ = 1.0.

Figure 5. Variations of two-dimensional OH concentrations with equivalence ratios (Φ = 1.1–1.4). (a) Φ = 1.1, (b) Φ = 1.2, (c) Φ = 1.3, (d) Φ = 1.4.

radical group gradually moves toward both sides of the flame, and the amount of OH radicals in the middle region is decreasing gradually with the increase of the equivalence ratio from 0.7 to 1. When the flame is burning at the rich-burn condition, as shown in Figure 5, the OH concentration profiles have changed greatly. That is, as the equivalence ratio continues to increase from 1.1 to 1.4, the two strong OH radical bands are formed on both sides of the flame. Meanwhile, the OH radical density in the middle region of flame decreases sharply.

2. In the range of equivalence ratio 0.7–1.2, the calculated OH concentration is generally higher than the experimental values. It is not difficult to find that the OH concentration profiles based on the GRI-Mech 3.0 mechanism is in good agreement with the experimental variation trend only when the equivalent ratio equals to 0.7. The reason for the above difference may lie in the fact that the simulation gives the OH concentration distribution in the adiabatic state, but the actual flame is in a nonadiabatic state, and it is unavoidable to have some radiation loss. Therefore, the experimental measurement results of the OH concentrations are found to be always smaller than the corresponding calculation results. A rough calculation shows that the calculated temperature considering the radiation loss is about 150–250 K lower than that without taking into account the thermal radiation loss. 3. In the range of equivalence ratio 0.7–1.2, it is found that the OH concentration increases rapidly from zero to the maximum value. The experimental results indicate that the change speed of OH concentration is not as large as the calculated result and the measured OH concentrations are higher than the calculation results at the same axial position. It is presumed that the main reason for this phenomenon lies in the following: the width of the chemical reaction zone is rather narrow (about 1–2 mm) in the methane/air flat flame. When the flame reaches the stable combustion state, a large amount of heat will flow toward the burner surface and thus increase the flame temperature. Therefore, the temperature of the premixed gas inside the burner will be increased accordingly. As a consequence, the temperature of the

Quantitative Planar Laser-Induced Fluorescence Technology

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

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Figure 6. Comparison between experimental and numerical results of OH concentration in the central axis.
