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Measurements of Turbulent Premixed Flames

CHAPTER V RESULTS AND DISCUSSION

5.5 Measurements of Turbulent Premixed Flames

In order to verify the applicability of developed sensor system for simultaneous measurements of equivalence ratio and temperature in turbulent flame, the sensor system is calibrated with the laminar premixed flames as described in Sec.5.3.2. The obtained correlation equations that relate intensity ratio of C2*/CH*, C2*/OH*, and CH*/OH* to the equivalence ratio as shown in Fig. 23 are used for equivalence ratio measurements in turbulent flame. For temperature measurement, the instrument constant C3 appeared in Eq.

(19) must be determined. Thermocouple temperature measurements are performed in laminar premixed flames at h = 9 mm and at the radial location where the maximum emission intensity occurs. Comparison of the measured and calculated adiabatic equilibrium temperature as a function of equivalence ratio is shown in Fig. 29. Fig. 29 shows that the measured flame temperatures are much lower than the adiabatic flame temperatures. The lower measured flame temperatures could be due to the effects of radiation heat loss and mixing of the ambient air with the burned gas. Nonetheless, the measured flame temperatures and the intensity ratios of C2*(1, 0)/ C2*(0, 0) are used to obtain the instrument

constant C3 from Eq. (19). An average value of 1.7759 is obtained for the φ = 1.0-1.5 flames.

The φ = 0.85-0.95 flames are not considered for the determination of the instrument constant because the low emission intensity of C2*(1, 0) band in these flames may result in large uncertainty in temperature measurements.

Simultaneous measurements of OH*, CH*, C2*(1, 0), and C2*(0, 0) chemiluminescence emissions are made in turbulent premixed methane-air jet flames (φ = 1.0 and 1.3) to examine the applicability of the developed sensor system for simultaneous measurements of equivalence ratio and temperature. The exit velocity and Reynolds number are 2.5 m/s and 3200 for both φ = 1.0 and 1.3 turbulent flames. The radial distributions of average OH*, CH*, and C2*(0, 0) intensities at h = 10 mm for φ = 1.0 and 1.3 turbulent flames are shown in Figs. 30 and 31, respectively. Note that at each radial location a total of 50 spectra are measured. It can be seen that for φ = 1.0 flame the OH* intensity is much higher than the CH* and C2*(0, 0) intensities. While for φ = 1.3 flame, the CH* and C2*(0, 0) intensities are increased remarkably. By applying the equations obtained from the laminar premixed flames, the histograms of measured equivalence ratio in turbulent premixed stoichiometric CH4-air flame at h = 10 mm and r = -9 mm are shown in Fig. 32. The measured average equivalence ratios from the emission intensity ratio of C2*/CH*, C2*/OH*, and CH*/OH* are φ = 0.984, 0.987, and 0.992, respectively. The rms of the local equivalence ratio is less than 0.015. Similar measurements of the local equivalence ratio in turbulent premixed rich CH4-air flame at h = 10 mm and r = -9.18 mm are shown in Fig. 33. The measured average equivalence ratios from the emission intensity ratio of C2*/CH*, C2*/OH*, and CH*/OH* are φ = 1.29, 1.275, and 1.231, respectively. The rms of the local equivalence ratio is also less than 0.015. Comparison of Figs. 32 and 33 suggests that the emission intensity ratio of CH*/OH* provides better result for equivalence ratio measurement in fuel-lean to stoichiometric flames. While the intensity ratio of C2*/CH* is more suitable for equivalence ratio measurement in fuel-rich flames.

Radial distributions of mean and rms equivalence ratio at h = 10 mm measured by the emission intensity ratio of C2*/CH*, C2*/OH*, and CH*/OH* are compared in Figs. 34 for φ

= 1.0 turbulent premixed flames. The measured mean equivalence ratio indicates that the stoichiometry of the flame is relatively uniform in the inner flame region. The decrease of the equivalence ratio near the jet outer boundary is due to mixing with the ambient air. Note that the equivalence ratio measured by the emission intensity ratio of C2*/CH* shows no decrease near the jet boundary. This is because weak intensity of C2* is observed in this region. Fig. 35 shows the radial distributions of mean and rms equivalence ratio at h = 10 mm for φ = 1.3 turbulent premixed flames. The intensity ratio of C2*/CH* estimates an equivalence ratio closer to a preset value of 1.3 than the intensity ratio of C2*/OH* and CH*/OH*. This fact suggests that the intensity ratio of C2*/CH* is more suitable for

equivalence ratio measurement in turbulent rich flames.

The histograms of measured mean and rms temperatures in turbulent premixed stoichiometric and rich CH4-air flames at h = 10 mm and r = -9 mm are shown in Fig. 36.

The measured mean temperatures using the emission intensity ratio of C2*(1, 0)/C2*(0, 0) for φ = 1.0 and 1.3 are 1825 and 1682 K, respectively. And the rms of the local temperature is 79 and 13 K for φ = 1.0 and 1.3, respectively. Because the Reynolds number for both turbulent flames is about the same, the higher fluctuation of the measured temperature for φ = 1.0 flame could be due to the lower C2* emissions in stoichiometric flame. Radial distributions of the measured mean and rms temperatures for φ = 1.0 and 1.3 turbulent flames at h = 10 mm are shown in Fig. 37. It can be seen that the mean temperatures of φ = 1.0 flame are, as expected, higher than those of φ = 1.3 flame. The higher fluctuation of the rms temperatures for φ = 1.0 flame could be, again, due to the lower C2* emissions in stoichiometric flame.

CHAPTER VI

SUMMARY AND CONCLUSIONS

In the present study, a low cost, robust, non-laser based optical sensor system has been developed and applied to turbulent premixed CH4-air flames for simultaneous measurements of equivalence ratio and temperature. The sensor system uses Cassegrain optics to eliminate the chromatic aberrations and to improve the spatial resolution for light collection. Two types of light detection units, one is the spectroscopic unit with the PMT array and the other is the spectrometer with the LN-CCD camera, are tested in the experiments to verify their applicability. The PMT array provides fast data acquisition rate for chemiluminescence emission measurements but gives no information on the broadband CO2* emissions. On the other hand, the LN-CCD camera measures the entire spectral range of OH*, CH*, C2*(1, 0), C2*(0, 0), C2*(0, 1) as well as the broadband CO2* emissions but gives slow data acquisition rate (about 1.5 s per image frame). Although the LN-CCD camera gives slow data acquisition rate, the simultaneous measurements of C2*(1, 0) and C2*(0, 0) emissions provide a method for flame temperature measurement using the intensity ratio of C2*(1, 0)/C2*(0, 0).

Both types of light detection units are applied to the laminar premixed flames to demonstrate the capability of the light detection units for chemiluminescence emission measurements.

The measured radial distributions of OH*, CH*, and C2*(0, 0) suggest that although the Cassegrain optics improves the spatial resolution, the light emissions outside the focal volume are also collected by the Cassegrain optics. The maximum intensity occurs at the radial position which indicates the location of flame front. The flame front position varies with the equivalence ratio. Comparisons of the measured maximum intensity for φ = 0.85-1.5 at h = 3 and 9 mm indicate that at both heights the maximum OH*, CH*, and C2* intensities occur approximately at φ = 0.95-1.05, 1.15-1.2, and 1.3 respectively. These findings are in good agreement with experimental results of Kojima et al. [36], but depart from those measured by Jeong et al. [60]. The chemiluminescence intensity ratio of C2*/CH*, C2*/OH*, and CH*/OH* ratios increase linearly with increasing equivalence ratio over the range from φ = 0.85 to 1.3. The high degree of correlation suggests that the local flame stoichiometry at the flame front of premixed methane-air flames can be determined by the spatially resolved chemiluminescence measurements. However, it seems difficult to measure the equivalence ratio in methane-air flames for φ > 1.35 by using this system because nonlinear relationship is observed within this range. The log curve fit equations of the dependence of C2*/CH*, C2*/OH*, and CH*/OH* on the equivalence ratio are used for equivalence ratio measurements in turbulent flames. For all calibration conditions and for a given value of equivalence ratio, the C2*/CH*, C2*/OH*, and CH*/OH* ratios are within ±10% of the value given by each curve fit equation, which leads to an uncertainty of approximately ±0.1 for determination of equivalence ratio for 0.85 ≤ φ ≤ 1.3. In addition, the maximum intensity ratios of C2*(1, 0)/ C2*(0, 0) measured in the laminar flames are used to obtain the instrument

constant C3 for turbulent flame temperature measurements. An average value of 1.7759 is obtained for the φ = 1.0-1.5 flames. The φ = 0.85-0.95 flames are not considered for the determination of the instrument constant because the low emission intensity of C2*(1, 0) band in these flames may result in large uncertainty in temperature measurements.

Numerical simulations of the laminar premixed CH4-air Bunsen flames at φ = 0.85, 1.0, and 1.2 are performed to investigate the flame structures. Comparisons of the computed OH*, CH*, and C2* mass fraction isopleths reveal that the CH* emissions exist in a very narrow region around the potential core of the jet flame. While the OH* and C2* emissions distribute in a broader region. The double peaks of C2* emission appeared at the flame tip for the φ = 1.2 flame are observed which suggest that the calculation of this flame may not reach the steady state condition and this flame requires further investigations. Comparisons of the measured and predicted radial profiles of OH*, CH*, and C2* emissions for the φ = 0.85, 1.0, and 1.2 flames at h = 3 mm indicate that the calculated OH* and C2* intensity profiles are much broader that the measured data. However, the measured and calculated peak intensity locations are within ± 1 mm.

Simultaneous measurements of OH*, CH*, C2*(1, 0), and C2*(0, 0) chemiluminescence emissions are made in turbulent premixed methane-air jet flames (φ = 1.0 and 1.3) to examine the applicability of the developed sensor system for simultaneous measurements of equivalence ratio and temperature. The obtained correlation equations that relate intensity ratio of C2*/CH*, C2*/OH*, and CH*/OH* to the equivalence ratio and the instrument constant are used to determine the equivalence ratio and temperature in turbulent flame. The measured histograms of equivalence ratio show that the average equivalence ratios from the intensity ratio of C2*/CH*, C2*/OH*, and CH*/OH* are φ = 0.984, 0.987, and 0.992, respectively, in turbulent premixed stoichiometric CH4-air flame at h = 10 mm and r = -9 mm.

The rms of the local equivalence ratio is less than 0.015. The measured average equivalence ratios from the emission intensity ratio of C2*/CH*, C2*/OH*, and CH*/OH* are φ = 1.29, 1.275, and 1.231, respectively, in turbulent premixed rich CH4-air flame at h = 10 mm and r = -9.18 mm. The rms of the local equivalence ratio is also less than 0.015. Comparison of the measured mean equivalence ratios in both turbulent flames suggests that the emission intensity ratio of CH*/OH* provides better result for equivalence ratio measurement in fuel-lean to stoichiometric flames. While the intensity ratio of C2*/CH* is more suitable for equivalence ratio measurement in fuel-rich flames. The histograms of measured mean and rms temperatures in turbulent premixed stoichiometric and rich CH4-air flames at h = 10 mm and r = -9 mm show that the mean temperatures using the emission intensity ratio of C2*(1, 0)/C2*(0, 0) for φ = 1.0 and 1.3 are 1825 and 1682 K, respectively. And the rms of the local temperature is 79 and 13 K for φ = 1.0 and 1.3, respectively. The higher fluctuation of the

measured temperature for φ = 1.0 flame could be due to the lower C2* emissions in stoichiometric flame.

SELF EVALUATION

In the present study, a low cost, robust, non-laser based optical sensor has been developed for simultaneous measurements of local equivalence ratio and temperature in premixed hydrocarbon flames. The developed sensor is applied to the turbulent premixed CH4-air flames and demonstrated its capability for simultaneous measurements of local equivalence ratio and temperature. Although the use of chemiluminescence intensity ratios of C2*/CH*, C2*/OH*, and CH*/OH* for local equivalence ratio measurements has been reported in the literature, simultaneous measurements of local equivalence ratio and temperature were not reported. Therefore, we will submit the results of the present study to SCI Journals for publication as soon as possible. In addition, we will continuously improve the non-laser based optical sensor system for future real-time active control in industrial burners and hazardous waste incinerators.

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Table 1. Operating conditions for laminar premixed CH4-air Bunsen flames.

φ CH4 (l/min) Air (l/min)

0.80 0.37 4.35

0.85 0.39 4.33

0.90 0.41 4.31

0.95 0.43 4.29

1.00 0.45 4.27

1.05 0.47 4.24

1.10 0.49 4.22

1.15 0.51 4.21

1.20 0.53 4.19

1.25 0.55 4.17

1.30 0.57 4.15

1.35 0.59 4.13

1.40 0.60 4.11

1.45 0.62 4.09

1.50 0.64 4.07

Table 2. Physical properties of the C2* vibrational bands [51].

λ (nm) υ′, υ″ Eυ’ (erg) Arel,υ’, υ”

1 468.48 4, 3 7.67 × 10-13 0.358

2 469.76 3, 2 6.02 × 10-13 0.477

3 471.52 2, 1 4.34 × 10-13 0.318

4 473.71 1, 0 2.63 × 10-13 0.637

5 509.77 2, 2 4.34 × 10-13 1.000

6 512.93 1, 1 2.63 × 10-13 1.000

7 516.52 0, 0 0.88 × 10-13 1.000

Table 3. Reaction mechanism for the excited state species OH(A), CH(A), and C2(d) [50].

) (

E / RT exp

AT

k

= na

Reaction A (cm3/mol/s) n Ea (cal/mol)

C2H + O = CH* + CO 1.08E+13 0 0

C2H + O2 = CH* + CO2 2.17E+10 0 0

CH* = CH 1.85E+06 0 0

CH* + N2 = CH + N2 3.03E+02 3.4 -381

CH* + O2 = CH + O2 2.48E+06 2.14 -1720

CH* + H2O = CH + H2O 5.30E+13 0 0

CH* + H2 = CH + H2 1.47E+14 0 1361

CH* + CO2 = CH + CO2 2.40E-01 4.3 -1694

CH* + CO = CH + CO 2.44E+12 0.5 0

CH* + CH4 = CH + CH4 1.73E+13 0 167

CH + O2 = OH* + CO 3.25E+13 0 0

OH* = OH 1.45E+06 0 0

OH* + N2 = OH+ N2 1.08E+11 0.5 -1238

OH* + O2 = OH + O2 2.10E+12 0.5 -482

OH* + H2O = OH + H2O 5.92E+12 0.5 -861

OH* + H2 = OH + H2 2.95E+12 0.5 -444

OH* + CO2 = OH + CO2 2.75E+12 0.5 -968

OH* + CO = OH + CO 3.23E+12 0.5 -787

OH* + CH4 = OH + CH4 3.36E+12 0.5 -635

CH2 + C = C2* + H2 7.50E+13 0 0

C2* = C2 8.33E+03 0 0

Fig. 1. Schematic diagram of the Raman/Rayleigh/CO LIF line imaging and crossed PLIF imaging experiment [44].

Fig. 2. Schematic diagram of Cassegrain optics.

Fig. 3. Schematic diagram and photograph of the measurement system using the PMTs.

Cassegrain Optics

LN CCD Camera Spectrometer

Optical Fiber

Fig. 4. Photograph of measurement system using a spectrometer coupled with a LN-CCD camera.

(a) Laminar (b) Turbulent

Fig. 5. Photograph of laminar (a) and turbulent (b) premixed CH4/air flames at φ = 1.0.

Fig. 6. Typical emission spectra of the two C2* bands between 460 and 520 nm for CH4 flame under conditions of TN2 + O2 = 20°C [51]. The numbers in the figure correspond to those given in Table 2.

200 0

-50 x

r

56.5 30

CH4/air u = 1 m/s T = 300 K

p = const.

p = const.

p = const.

Stainless

Ambient air

g

v

u

Units: mm

Fig. 7. Computational domain with boundary conditions.

(a)

Position X μm

Position Y μm

-40 -30 -20 -10 0 10 20 30 40

-40 -30 -20 -10 0 10 20 30 40

(b)

0 0.1

-0.1 0

-0.1

-0.2

-0.3 0.1 0.2 0.3

Intensity

1

0

Radial Position (mm)

Axial Position (mm)

-0.01 0 0.01

1.0 0.8 0.6 0.4 0.2 0.0

0.0 0.1

0.2 -0.1

-0.2

-0.02

0.3 -0.3

0.02

Intensity

(c)

Fig. 8. The light-collection-rate distribution around the probe volume. (a) at the focal point, (b) 2-D distribution, and (c) 3-D profile.

0 0.5 1 1.5 2 2.5 3

Radial distance (mm)

0 0.2 0.4 0.6 0.8 1 1.2

N o rm a liz e d O H * in te n s it y ( a .u )

5 mm slit 10 mm slit 15 mm slit Without mask

Fig. 9. Effect of the different slit widths of the eye mask on the OH* chemiluminescence intensity profiles (premixed CH4-air jet flame, φ = 1.35).

250 300 350 400 450 500 550 600 650 wavelength (nm)

0 2000 4000

In te n s it y ( a .u .)

0 2000 4000 0 1000 2000 3000

OH*(0, 0)

CH*(0, 0)

C2*(1, 0)

C2*(0, 0)

OH*(0, 0)

OH*(0, 0)

CH*(0, 0)

CH*(0, 0)

CH*(1, 0)

C2*(1, 0)

C2*(1, 0) C2*(0, 0)

C2*(0, 0)

C2*(0, 1) C2*(0, 1)

Φ = 0.85

Φ = 1.0

Φ = 1.3

CH*(1, 0)

Fig. 10. Flame emission spectra measured from laminar premixed methane-air flames at φ = 0.85, 1.0, and 1.3.

-6 -4 -2 0 2 4 6

Radial distance, r (mm)

0 0.25 0.5 0.75 1

In te n si ty ( a .u .)

OH*

CH*

C2*

0 0.25 0.5 0.75 1 1.25

h = 3 mm

h = 9 mm Φ = 0.85

Fig. 11. Radial distribution of the OH*, CH*, and C2* chemiluminescence intensities at h = 3 and 9 mm for φ = 0.85 flame.

-6 -4 -2 0 2 4 6

Radial distance, r (mm)

0 0.25 0.5 0.75 1

In te n si ty ( a .u .)

OH*

CH*

C2*

0 0.25 0.5 0.75 1 1.25

h = 3 mm

h = 9 mm Φ = 1.0

Fig. 12. Radial distribution of the OH*, CH*, and C2* chemiluminescence intensities at h = 3 and 9 mm for φ = 1.0 flame.

-6 -4 -2 0 2 4 6

Radial distance, r (mm)

0 0.25 0.5 0.75 1

In te n si ty ( a .u .)

OH*

CH*

C2*

0 0.25 0.5 0.75 1 1.25

h = 3 mm

h = 9 mm Φ = 1.3

Fig. 13. Radial distribution of the OH*, CH*, and C2* chemiluminescence intensities at h = 3 and 9 mm for φ = 1.3 flame.

0.8 1 1.2 1.4 1.6 1.8 2

Equivalence ratio

0 0.2 0.4 0.6 0.8 1 1.2

In te n s it y ( a .u .) OH*

CH*

C 2 * 0

0.2 0.4 0.6 0.8 1 1.2

In te n s it y ( a .u ) OH*

CH*

C 2 * (a)

h = 3 mm (b) h = 9 mm

Fig. 14. Variations of the maximum chemiluminescence intensities with the equivalence ratio at h = 3 and 9 mm.

0.9 1.05 1.2 1.35 1.5

Equivalence ratio

0 0.5 1 1.5 2 2.5 3

In te n s it y r a ti o

C2*/CH*

C2*/OH*

CH*/OH

*

0 0.5 1 1.5 2 2.5 3

In te n s it y r a ti o

C2*/CH*

C2*/OH*

CH*/OH*

h = 9 mm

h = 3 mm

(a)

(b)

Fig. 15. Correlation of the intensity ratios of C2*/CH*, C2*/OH* and CH*/OH* to the equivalence ratios at h = 3 and 9 mm.

-6 -4 -2 0 2 4 6 Radial distance, r (mm)

0 0.25 0.5 0.75 1

N o rm al iz ed i n tens it y ( a .u .)

OH*

CH*

C2*

0 0.25 0.5 0.75 1 1.25

h = 3 mm

h = 9 mm Φ = 0.85

Fig. 16. Radial distribution of the OH*, CH*, and C2* chemiluminescence intensities at h = 3 and 9 mm for φ = 0.85 flame.

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