PLIF AND IMAGE CHEMILUMINESCENCE
4.10 Combined OH-CH 2 O PLIF Results
4.10.1 Resulting Combined OH-CH 2 O PLIF Data
It was only possible to generate combined results for the seven base acoustic drive frequencies. This was limited by the available OH PLIF datasets which were only collected for the seven base drive frequencies. The reduced OH-CH2O data is presented in its entirety in appendix H. Figure 4-29 shows a sample of a combined OH-CH2O PLIF image of the flame under steady- state conditions (no acoustic forcing). All the image particulars are the same as the previous steady state images. The horizontal axis represents the radius from the centerline of the burner, and the vertical axis represents the height above the nozzle exit plane. The stagnation body height is at
Figure 4-29: Combined OH-CH2O PLIF image of experimental burner flame under steady conditions (no acoustic forcing). Collected data appears on the left, and is mirrored across the centerline for a visual representation of the entire flame. φ = 0.85, VR = 4.0.
z = 21 mm, which is shown as the top line of the graph frame. The data has been mirrored across the centerline of the burner to complete the visual appearance. The intensity scale presented has been normalized around the maximum intensity in the image.
Figure 4-30 shows typical results for the phase resolved OH-CH2O (combined) PLIF images. The presented images are for the 55 Hz acoustic drive frequency data. Only 12 of the 24 generated images are displayed. The phase angles shown follow the same conventions as described previously.
Examining the phase resolved images reveals a suspicious feature. Focusing on the images from 180° to 330°, it can be seen that the maximum intensity portion of the flame sheet splits into two peaks. In the 270° and 300° phase images, the flame sheet actually appears to separate. This is highly suspect and may be a product of small positional or scaling errors in the combination of the PLIF images from the two species. Extraordinary care was taken to insure that all images were scaled and registered properly. During the experiments, the imaging system was adjusted so that one pixel equated to a 100um x 100um region in the image plane. Furthermore, during the data processing for this combined case, great care was taken to assure the mated images were registered
Figure 4-30: Phase-resolved OH-CH2O (combined) PLIF images for acoustic forcing at 55 Hz. The horizontal axis is the radius (mm) from the centerline of the burner. The vertical axis is the distance (mm) above the nozzle exit plane. Phase angle θ = 0º corresponds to the rising-edge zero crossing of the local unsteady pressure. Burner operating parameters are: φ
= 0.85, VR = 4.0.
Drive Combustion Forced Rayleigh Frequency (Hz) Response, H(s) Index, Rf
Mag Phase
55 1306 16.4 1256
75 1142 -125.3 -660
100 626 99.2 -100
150 65.9 152.6 -58.5
220 44.5 -7.9 44.0
265 76.7 -156.5 -70.3
375 66.8 -125.6 -38.9
Table 4-5: Global unsteady combustion responses as discerned using the combined OH- CH2O PLIF data for the seven base drive frequencies. Positive phases correspond to the heat release lagging the pressure.
to each other appropriately. Nonetheless, small errors in the procedure may have produced these unusual results. In any case, the data are presented (and further processed) with the awareness of this blemish.
Following the usual methodology, the intensities of the 24 images for each drive frequency case were fully integrated (including the r dependence) and then processed by FFT in order to generate the global combustion response and forced Rayleigh index for the seven acoustic drive frequencies.
The results are shown in Table 4-5.
These data points can then be plotted on the graphs for the global combustion response function and the global forced Rayleigh index that were generated by bulk chemiluminescence measurements for the relevant test condition in chapter 3. These are shown in Figures 4-31 and 4- 32.
Examining the data in the magnitude curve, a matching trend can be seen in the low frequency portion of the response. Similar to the OH data plotted in Figure 4-11, the magnitudes of the four lower frequency-combined OH-CH2O data points fall under the chemiluminescence baseline curve. The size of this shortfall, however, is only about half of that seen in the OH data.
Phase (deg)
Frequency (Hz)
100.0 1000.0
-2160 -1800 -1440 -1080 -720 -360
20.0
Magnitude
Frequency (Hz)
100.0 1000.0
1.0E-2 1.0E-1 1.0E+0 1.0E+1 1.0E+2 1.0E+3
20.0
Figure 4-31: Combustion response function - points computed from the combined OH- CH2O PLIF data (shown as crosshairs) superimposed on the curves computed from the bulk chemiluminescence experiments. Phase data points were placed on the appropriate branch for closest proximity to the curve. Here, negative phases correspond to I’ lagging p’.
Figure 4-32: Forced Rayleigh index - points computed from the combined OH-CH2O PLIF data (shown as crosshairs) superimposed on the curves computed from the bulk
chemiluminescence experiments.
100.0 1000.0
-2000.0 -1000.0 0.0 1000.0 2000.0
20.0
Frequency (Hz) Forced Rayleigh Index (Rf)
As was typical in the previously plotted data, the three higher frequency data points in the OH- CH2O data show values significantly higher than the baseline curve.
Reviewing the phase plot, the four data points for the drive frequencies from 55 Hz to 150 Hz as well as the one for 265 Hz all show very good agreement with the chemiluminescence baseline. The remaining two higher frequency data points show some disagreement with the chemiluminescence curve; however, these errors are similar in magnitude to those seen in the CH PLIF data.
The forced Rayleigh index plot in Figure 4-32 shows the combined OH-CH2O PLIF global data points overlaid on the baseline curve generated from the chemiluminescence data. Unlike the forced Rayleigh index plot for the previously reviewed species, this plot is in fair (if not good) agreement for the four lower drive frequencies. In fact, for the 75 Hz and 100 Hz data points, the agreement is very good. The three higher frequency data points show some scatter; this is similar to what was seen in the previously reviewed data.
The spatially resolved response functions generated from the combined OH-CH2O data are presented next. The combustion response function plots are shown in Figure 4-33. Not surprisingly, these response images show larger regions of involvement at lower drive frequencies (150 Hz and less) and smaller regions at higher drive frequencies. The banded regions seen earlier in the formaldehyde images are still present in certain regions of each plot. For drive frequencies of 150 Hz and less, the banding can be seen in the responses for radii less than approximately 12 mm. For frequencies higher than this, the banding appears throughout, similar to the previous CH2O combustion response data.
As previously seen, the low frequency combustion response plots for OH and CH2O both exhibit the “head and tail” structure. In the case of formaldehyde, each band (upper and lower) has its own independent “head.” The lobes have clearly visible minima between them, and the phase
Figure 4-33: Spatially resolved combustion response function produced from OH-CH2O PLIF combination data. Magnitude plots appear in the left column while phase plots are shown on the right. Note that positive phase values correspond to I’ lagging p’. The coordinate system origin is along the burner centerline at the nozzle exit plane. Data for the seven base drive frequencies are shown. (See next page.)
Figure 4-33 (cont): Spatially resolved combustion response function produced from OH- CH2O PLIF combination data. Magnitude plots appear in the left column while phase plots are shown on the right. Data for the seven base drive frequencies are shown. (See previous page.) Note the change in magnitude scale versus the plots for 150 Hz and lower.
Figure 4-34: Spatially resolved forced Rayleigh index produced from OH-CH2O PLIF combination. Data for the seven base drive frequencies are shown. Plots for the four lower frequencies use the top magnitude legend. Plots for the three higher frequencies use the bottom magnitude legend.
jumps across lobes at the points of closest distance are typically not small. However, for the corresponding test cases in the combined OH-CH2O results, the two bands merge into a single
“head” region. Although the phases in each band are very different (typically 180° apart), the phases blend together, continuously and smoothly within the “head” of the response zone.
For the 220 Hz to 375 Hz cases, the results are very similar in appearance to the associated images generated independently from the CH and CH2O PLIF data. The thickness of the two bands at these drive frequencies is similar to that seen in the formaldehyde data. The phase behavior of the bands is relatively constant away from the edge of the flame (i.e. for r < 10 mm). For the 220 Hz and 265 Hz acoustic drive cases, the upper bands in the flame’s inner region are 180° out of phase with the oscillating pressure field while the lower bands are seen to be in phase. At the higher 375 Hz drive frequency, the phase shifts (as before) to the upper band leading the pressure by 90°
and the lower band lagging the pressure by 90°.
Figure 4-34 shows the plots of the spatially resolved forced Rayleigh index for the combined OH-CH2O data. Double banding is seen, once again, in the region away from the flame’s edge. The bands exhibit the same signs (polarities) as were seen in the CH and CH2O forced Rayleigh index plots. In all the images except the 75 Hz case, the upper band has negative (damping) polarity while the lower band has positive (driving) polarity. For the 75 Hz case, this is reversed. Inspecting the 375 Hz drive case, the blue damping band is found to be dominant. Careful examination reveals a very faint yellow band below the blue band at small radii. Nonetheless, the total integrated magnitude of the yellow driving band is clearly much smaller than that of the damping blue band.
Focusing on the 55 Hz and the 75 Hz drive cases, a definite similarity exists with the corresponding plots from the formaldehyde PLIF data. Both 55 Hz plots have a damping upper band and a driving lower band, while the reverse is true for the 75 Hz plots. The main difference can be seen in the missing gap in the lower band for the combined data. In the CH2O plots, the
lower band has a separation between its “head” and “tail,” allowing a narrow path of color to sneak through at the far end of the upper band down to a lower lobe. This is seen in both the 55 Hz and 75 Hz cases. However, in the OH-CH2O images, the lower bands for these drive frequencies are continuous through to the “head” region.
In the 100 Hz image, the pattern has become far less complicated than the poly-lobed monstrosity seen in the formaldehyde images. In this case, the bands are seen with one pair of alternating lobes. As the drive frequency increases, the alternating lobe pattern develops, once again appearing as a “twisted rope.” This is seen in the forced Rayleigh index images for the 150 Hz and 220 Hz cases. At 265 Hz, the alternating lobes vanish and the two response bands appear nearly continuous over their whole lengths. Finally, at 375 Hz, the alternations return but are very faint. In this image, virtually all of the yellow-red driving color has disappeared. The image is dominated by a single, blue damping band. A single yellow stripe is seen twisting over the top of the blue band at a radius of approximately 13 mm. A few other faint hints of yellow appear in various locations, including below the blue band at the root of the flame. All of these are very weak.