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Flame front characteristics of turbulent premixed flames diluted with CO2 and H2O at high pressure and high temperature
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Proceedings of the
Combustion Institute
Proceedings of the Combustion Institute 34 (2013) 1429–1436
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Flame front characteristics of turbulent premixed flames diluted with CO2 and H2O at high pressure and high temperature Jinhua Wang a,b,⇑, Futoshi Matsuno b, Masaki Okuyama b, Yasuhiro Ogami b, Hideaki Kobayashi b, Zuohua Huang a a
State Key Laboratory of Multiphase Flows in Power Engineering, Xi’an Jiaotong University, Xi’an 710049, PR China b Institute of Fluid Science, Tohoku University, Sendai, Miyagi 980-8577, Japan Available online 12 July 2012
Abstract Flame front structure characteristics of turbulent premixed flames including the local radius of curvature, fractal inner cutoff scale and local flame angle were calculated from the experimental OH-PLIF images of CH4/air flames and those diluted with CO2 and superheated H2O at 0.5 MPa and 573 K. The convex and concave structures of the flame front were detected and statistical analysis including PDF and ADF of the local radius of curvature and the local flame angle was conducted. Results showed that the flame front of turbulent premixed flames at high pressure and high temperature is a wrinkled flame front with small scale convex and concave cusps superimposed on large scale branches, while the whole flame front tends to be convex to the unburned mixture. The effect of EGR gas dilution on the flame front structure of turbulent premixed flames is dominated by CO2 rather than H2O dilution. The quantitative flame front characteristics reveal the complicated effect of CO2 dilution on turbulent premixed flames characteristics observed in our previous research. In the case of CO2 dilution, some local wrinkled structures become sharp and propagate deep into the burned mixture, resulting in a decrease of the fractal inner cutoff scale, the formation of a thick flame brush and the enlargement of the mean flame volume, while the overall wrinkled scale increases significantly due to the suppression of the concave structure with CO2 dilution, leading to a decrease of the turbulent flame front area, and subsequently to a decrease of ST/SL. Ó 2012 The Combustion Institute. Published by Elsevier Inc. All rights reserved. Keywords: Turbulent premixed flames; Flame front; Dilution; High pressure; High temperature
1. Introduction Exhaust gas recirculation (EGR) and flue gas recirculation (FGR) are widely used in internal combustion engines and gas turbines for reducing
NOx emissions by decreasing the flame temperature with EGR gas dilution [1,2]. Although studies of EGR gas dilution on internal combustion engines [1], gas turbines [2] and laminar flames [3–5] have been extensively performed, the
⇑ Corresponding author at: State Key Laboratory of Multiphase Flows in Power Engineering, Xi’an Jiaotong University, Xi’an 710049, PR China. Fax: +86 29 82668789. E-mail addresses: [email protected], jhwang@flame.ifs.tohoku.ac.jp (J. Wang),
1540-7489/$ - see front matter Ó 2012 The Combustion Institute. Published by Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.proci.2012.06.154
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fundamental study of the effects of EGR gases on turbulent premixed flames has not been well conducted, especially under relevant engine conditions, i.e., high pressure and high temperature. The effects of CO2 and superheated H2O (which are the main components of EGR gases) dilution on turbulent premixed flame characteristics have been studied by the present authors [6,7]. Findings showed that the effect of EGR gases on the structure of turbulent premixed flames is dominated by CO2 rather than H2O dilution. The turbulent burning velocity, ST, normalized by the laminar burning velocity, SL, became smaller, and the mean volume of the turbulent flame region increased in the case of CO2 dilution, while it scarcely changed with H2O dilution. Findings also showed that the flame wrinkled scale which was characterized as the fractal inner cutoff scale decreased with CO2 dilution. The simultaneous decrease in inner cutoff scale and ST/SL combined with the enlargement of the mean flame volume indicated the complicated effect of CO2 dilution on the flame front structure. Generally, the ST/ SL increases with the decrease of fractal inner cutoff scale, which generates increased surface wrinkling and enlarges the flame front area [8–10]. It seems that some local flame front is more wrinkled and some of the wrinkled cusps propagate deep and make a thick flame brush with CO2 dilution, but that the overall scale of wrinkling becomes larger, leading to a decrease of ST/SL. However, the quantitative flame front information was not well presented in the previous studies and the mechanism of the dilution effect on the overall flame dynamics is still not well understood. In order to clarify the effect of EGR gas dilution on the interaction between turbulence and flame, and to understand the difference between CO2 and H2O dilution, further quantitative investigation on flame front structure is needed. Flame front structure reveals the interaction between turbulence and flame, and it can also be used for turbulent combustion modeling. To date a complete geometric description of turbulent premixed flame front has been provided, such as fractal dimension and cutoff scales [8–10], flame curvature and angle statistics [11,12]. The effects of Lewis number [12,13], pressure [14–16] and turbulence intensity on the flame front structure have been studied. However, few studies have been performed on the effects of dilution, especially at high pressure and high temperature. The flame curvature and its statistical properties such as PDF (probability density function) have usually been used to indicate the wrinkled flame front structure. However, it cannot properly indicate the small scale structure on the flame front because the small scale cusps with large curvature are located far from the center and may even fall out of the figure of the PDF of curvature [11,12,15]. This problem will be even more severe at high pressure because
turbulent premixed flames at high pressure possess a much finer wrinkled structure with a much smaller scale of convex and concave cusps compared with that at ordinary pressure [16,17]. This fine wrinkled structure of turbulent premixed flames at high pressure is mainly caused by small scale turbulence interaction combined with flame instability at high pressure, which has been previously discussed by the authors [17]. However, the quantitative flame front geometric information was not well presented. The local radius of curvature of the flame front can be viewed as a length scale and was used in the present study with a focus on the small convex and concave structure on the flame front. The objective of the present study was to clarify the effects of CO2 and superheated H2O dilution on flame dynamics of CH4/air turbulent premixed flames based on quantitative flame front structure characteristics, especially at high pressure and high temperature. The local radius of curvature, fractal inner cutoff scale and local flame angle of CH4/air flames and those diluted with CO2 and H2O were calculated and statistical analysis was conducted. The effects of CO2 and H2O dilution and turbulence intensity on the interaction between flame and turbulence were examined based on statistical analysis and length scales. 2. Experimental setup and procedure Experiments were performed using the HighPressure Combustion Test Facility at the Institute of Fluid Science, Tohoku University. An electric air heater with a total power of 10 kW and a water evaporator with a 1.2 kW heater were installed in the high pressure chamber to supply air with superheated H2O at high temperature. A stainless steel nozzle-type burner with an outlet diameter of 20 mm was used. The turbulence was generated by a perforated plate installed 40 mm upstream of the outlet. Turbulence measurement was conducted using a constant-temperature hot-wire anemometer (Dantec, Streamline 90N). More details have been described elsewhere [17,18]. Bunsen-type turbulent premixed flames of CH4/ air mixtures and those diluted with CO2 and H2O were stabilized at the nozzle burner outlet in the high-pressure chamber. A large amount of fresh air was supplied to the chamber to keep the pressure constant at 0.5 MPa and to prevent the chamber from filling with burned gases. Automatic feedback of electric power to both the air heater and water evaporator was activated to keep the mixture temperature at the burner outlet at 573 ± 5 K. Air was diluted with CO2 or superheated H2O upstream of the electric air heater and used as oxidizer. The CH4 and oxidizer were premixed and supplied to the burner. The strength of dilution was defined by using the dilution ratio, Z, as Z CO2 ¼ ½CO2 =ð½air þ ½CO2 Þ for CO2
J. Wang et al. / Proceedings of the Combustion Institute 34 (2013) 1429–1436
dilution, Z H2 O ¼ ½H2 O=ð½air þ ½H2 OÞ for H2O dilution, where [air], [CO2] and [H2O] denote the mole fraction of air, CO2 and H2O in the mixtures, respectively. The equivalence ratio of the mixture was 0.9 and dilution ratios of 0.05 and 0.1 for CO2 and H2O dilution were performed. The properties of the mixtures are summarized in Table 1. The laminar burning velocity, SL, for mixtures used in this study was estimated by using the PREMIX code [19] and CHEMKIN-II database [20] with GRI-Mech 2.11 [21]. OH-PLIF measurements were performed to visualize the instantaneous flame front. An NdYAG laser (Spectra Physics, GCR-250-10) and a dye laser with a frequency doubler (Spectron, SL4000) were used. A high resolution ICCD camera (ANDOR, DH574-18F) with 1024 1024 pixels was used and the finest pixel resolution was 0.054 mm/pixel at the measurement plane. A description of the OH-PLIF apparatus and the selected wavelength are stated elsewhere [6,7]. The flame front was tracked from the OH-PLIF image by binarizing with a threshold value based on the histogram of the intensity of the images. About 1500 points of each OH-PLIF image were successfully tracked due to the high resolution and high sensitivity OH-PLIF technique used in this study. It is sufficient to fit the continuous flame front curve for subsequent calculation. A 3-order smoothing scheme was used to remove the digitization noise from the binarization process. The smooth flame front was then discretized with an interval of 0.50 mm, which was the smallest inner cutoff scale of all the flames in the present study. The use of this interval and discretization is to remove the digitization noise from the pixels since the inner cutoff scale can be viewed as the smallest scale of the flame front [12]. The discretized points were fitted using a 3-order spline interpolation scheme, which was then employed to calculate the local radius of curvature, R, and local flame angle, a, as follows: 2 2 3=2 xðsÞ0 þ yðsÞ0 R¼ ð1Þ xðsÞ0 yðsÞ00 yðsÞ0 xðsÞ00 a ¼ arctanðyðsÞ0 =xðsÞ0 Þ
ð2Þ
where xðsÞ and yðsÞ are coordinates of the flame front spline curve and s is the length of the accumulated flame from the starting point. For the local radius of curvature, the positive direction was
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defined as being convex to the unburned mixture. The local flame angle was defined from the flame front normal vector to the upward vertical axis in a counterclockwise direction. The fractal inner cutoff scale, ei, was calculated by using 50 OH-PLIF images with the circle method [18]. Sixty images were calculated for each condition and statistical analysis of the local radius of curvature and local flame angle was conducted including the PDF and ADF (accumulated distribution function) distributions as follows: Z R ADFðRÞ ¼ PDFdR ð3Þ 0
The range for the calculation of the radius of curvature is from 100 to 100 mm. The interval for PDF calculation is 0.02 mm for the radius of curvature, and 6° for the local flame angle, respectively. 3. Results and discussion 3.1. Effects of CO2 and H2O dilution on the local radius of curvature Figure 1 shows typical OH-PLIF images of CH4/air turbulent premixed flames and those with CO2 and H2O dilution at 0.5 MPa and 573 K. It is seen that the strongly wrinkled flame front can be well recognized by using the OH-PLIF technique with high resolution and high sensitivity in this study; thus, it should be appropriate to be used for the calculation of flame front characteristics. It is seen that all the flames in this study possess a fine, convoluted flame front structure with convex and concave cusps superimposed on large scale flame branches, which is a general characteristic of turbulent premixed flames at high pressure [17]. Although the flame height varies with the flow conditions, as shown in Fig. 1a and b, deep wrinkled flame cusps tend to be observed and these deep cusps become sharp; meanwhile, the smooth flame fronts also tend to increase with CO2 dilution. This means that the wrinkled flame front becomes small scale deep wrinkled cusps superimposed on a much larger scale smooth flame front with CO2 dilution. This tendency is more significant with the increase of dilution ratio from Z CO2 ¼ 0:05 to Z CO2 ¼ 0:1 (Fig. 1b and e). However, the flame front wrinkled structure shows a very small difference with H2O dilution
Table 1 The properties of the mixtures in this study, at 0.5 MPa and 573 K. Z CO2 ;H2 O
S L;CO2 (cm/s)
S L;H2 O (cm/s)
LeCH4 , CO2
LeCH4 , H2O
0.0 0.05 0.1
59.57 42.58 30.04
59.57 47.86 37.53
1.006 0.973 0.951
1.006 0.996 0.987
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a
b
c
a
0.012
u'/SL 1.2 ZCO2,H2O=0.05
0.010
CH4/air CH4/air/CO2 CH4/air/H2O
PDF
0.008 0.006 0.004 0.002
e
d
f -3
-2
-1
0
1
2
3
R (mm)
b CH4/air/CO2
0.4
CH4/air/H2O
Fig. 1. OH-PLIF images at 0.5 MPa and 573 K: (a) Z = 0, u0 /SL ffi 1.2; (b and c) Z CO2 ;H2 O ¼ 0:05, u0 /SL ffi 1.2; (d) Z = 0, u0 /SL ffi 2.6; (e and f) Z CO2 ;H2 O ¼ 0:1, u0 / SL ffi 2.6.
ADF
CH4/air
u'/SL 1.2 ZCO2,H2O=0.05
0.5
CH4/air CH4/air/CO2 CH4/air/H2O
0.3 0.2 0.1 0.0 -20
-15
-10
-5
0
5
10
15
20
R (mm)
Fig. 2. PDF and ADF distributions of the local radius of curvature: (a) PDF distribution; (b) ADF distribution.
0.6 0.5
CH4/air CH4/air/CO2 CH4/air/H2O
u'/SL 1.0 ZCO2,H2O=0.05
0.4
ADF
(Fig. 1c and f). This is the general characteristic of CO2 and H2O dilution from the direct visualization of the instantaneous flame front images. The effects of dilution and the difference between CO2 and H2O dilution on flame front structure is confirmed from analysis of quantitative flame front characteristics in the following section. Figure 2 shows PDF and ADF distributions of the local radius of curvature. It is seen from Fig. 2a that the PDF of the radius of curvature shows an obvious bias to positive radius. The peak of PDF distribution of local radius of curvature decreases and its profile becomes flat with CO2 dilution, indicating a less wrinkled structure with CO2 dilution. The position of the peak of PDF distribution tends to become larger, indicating that the most frequent wrinkled scale increases with CO2 dilution. The effect of H2O dilution is much weaker and the PDF of the local radius of curvature is almost the same as that of CH4/air flame. Figure 2b shows the ADF distribution of the radius of curvature in the range from 20 to 20 mm. As calculated by Eq. (3), ADF distribution indicated the size distribution of the local radius of curvature on the flame front. This shows that most of the radius of curvature on the flame front is within the region of small radius with a small scale structure, indicating that the flame front of turbulent premixed flames is a wrinkled flame front with a wide range of scales and that the small scale structure is predominant, as shown in Fig. 1. In the following section, the local radius of curvature at a small scale range of 5 to 5 mm will be presented to discuss the small scale structure in detail.
0.3 0.2 0.1 0.0 -5
-4
-3
-2
-1
0
1
2
3
4
5
R (mm) Fig. 3. Variations of ADF distribution of the local radius of curvature with CO2 and H2O dilution.
Figure 3 shows variations of ADF distribution of the local radius of curvature with CO2 and H2O dilution. It is seen that the radius of curvature with positive value is always more than that of the negative ones, which is also shown later in Fig. 5. This means that the convex structure which is convex to
J. Wang et al. / Proceedings of the Combustion Institute 34 (2013) 1429–1436
0.6
u'/SL 1.0 u'/SL 1.2 u'/SL 2.6
CH4/air 0.5
ADF
0.4 0.3 0.2 0.1 0.0
b
0.6
u'/SL 1.0 u'/SL 1.2 u'/SL 2.6
CH4/air/CO2 ZCO2=0.05
0.5
ADF
0.4 0.3 0.2 0.1 0.0 -5
-4
-3
-2
-1
0
1
2
3
4
5
R (mm) Fig. 4. Variations of ADF distribution of the local radius of curvature with u0 /SL: (a) CH4/air; (b) CH4/air/ CO2.
the unburned mixture is always more frequent than the concave ones for turbulent premixed flames; this would be a general characteristic of the turbulent premixed flames at high pressure. This phenomenon can also be observed directly from flame visualization as discussed by Kobayashi et al. [17] and was validated by quantitative flame front calculation in the present study. Previous research using the PDF of curvature to indicate the flame front structure generally showed a nearly symmetrical distribution with a mean of approximately zero [12,14,15,22–24] although some studies have found a slightly positive bias which was convex to the unburned mixture [11,16,25]. It has been argued that the overall effect of flame curvature on the burning rate integrates to zero if the curvature distribution is symmetric since the effect of positive and negative curvature will cancel each other to some degree [13,22]. For now, there is no consensus as to the distribution of local flame curvature and the local flame curvature on the overall burning rate is still not well understood. This may be due to the fact that the flat pattern on the flame front with a smaller curvature which is near zero is locate at the center of the PDF of curvature, while the small scale structure
has a large local curvature which is far from the center and may even be out of the range of previous research [11,12,16]. Thus, the small scale structure with large curvature was not properly presented by using the PDF of curvature in previous research. This problem is even more severe at high pressure and high temperature as the flame front is more wrinkled with a much smaller scale structure which corresponds to a much larger curvature. By using the local radius of curvature in the present study, the small scale structure and its distribution can be well presented and a higher probability of positive structure which is convex to unburned mixtures is clearly indicated. It is also seen from Fig. 3 that the ADF distribution of the local radius of curvature at both positive and negative radius obviously decreased with CO2 dilution. The flame curvature has a weak effect on local burning velocity because the Lewis number of all the mixtures in this study is close to unity, as shown in Table 1 [13,26]. This means that the variation of ADF distribution of the local radius of curvature mainly influences the turbulent flame front area by varying the wrinkled scale. The decrease of ADF distribution of the local radius of curvature indicated that the wrinkled scale of the flame front tends to become larger and less convoluted with CO2 dilution. This seems to be contradictory to the direct observation of OHPLIF images as shown in Fig. 1, that the flame front is more wrinkled with CO2 dilution. This is discussed in a later section based on the comparison of characteristic length scales of the flame front. In the case of H2O dilution, the ADF distribution of the local radius of curvature is almost the same as that of CH4/air mixtures without dilution. Figure 4 shows variations of ADF distribution of the local radius of curvature with u0 /SL, where u0 is turbulence intensity. It shows that the ADF distribution of the local radius of curvature increased for both positive and negative radius with
Convex and concave precentages (%)
a
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ZCO2,H2O=0.05
CH4/air CH4/air/CO2 CH4/air/H2O
56
solid: convex open: concave
52 48 44 40 36 0.5
1.0
1.5
2.0
2.5
3.0
u'/SL
Fig. 5. Variations of convex and concave percentages on the flame front with u0 /SL and CO2/H2O dilution.
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the increase of u0 /SL in the case of CO2 dilution (Fig. 4b), while the effect is much weaker for CH4/air flame without dilution (Fig. 4a) in the experimental range. This means that the effect of turbulence flow on flame front wrinkling is much more obvious and that the flame response against turbulence flow is passive with CO2 dilution. The Lewis number of the mixtures decreased with CO2 dilution, as shown in Table 1. The characteristic flame instability scale, li, indicated the combined effect of the hydrodynamic instability and the diffusive-thermal instability, which corresponds to the wavelength at the maximum growth rate of flame front disturbance calculated using Sivashinsky’s formulation [27]. The li decreased from li = 0.600 mm at Z = 0 to li = 0.467 mm at Z CO2 ¼ 0:05. This means that the flame wrinkling caused by turbulence is amplified due to the promotion of the intrinsic flame instability with CO2 dilution. Figure 5 shows variations of convex and concave percentages on the flame front with u0 /SL. The convex and concave percentages correspond to the ADF value at the boundary of the calculation range of ±100 mm, which indicates the proportion of convex and concave structure on the flame front. As discussed in Fig. 3, the convex structure on the flame front is much more frequent than concave ones, which is well indicated in Fig. 5 for all the conditions in this study. The difference between convex and concave percentages is about 6% in the case of CH4/air flames. It is noted that the sum of the convex and concave percentages is less than unity for all the conditions which would be due to the existence of the flat patterns on the flame front. It is seen that both the convex and concave percentages show a very slight increase with the increase of u0 /SL in the experimental range for CH4/air flames. In the case of CO2 dilution, both the convex and concave percentages show a much more remarkable increase with the increase of u0 / SL compared to that of CH4/air flames without dilution, which indicates the passive flame response against flow turbulence as discussed in Fig. 4. The convex percentage decreases slightly while the concave percentage is much less (about 4% in this study) for CO2 dilution compared with that of CH4/air flames. This indicates that the concave structure propagating to the burned mixture was suppressed significantly with CO2 dilution. This would be due to the relatively larger heat capacity of CO2, which suppresses the propagation of the concave structure to the burned mixture. It is observed from Fig. 1b that deep, sharp concave cusps were formed due to the suppression of concave branch propagation to the burned mixture, leading to the breakage of the concave cusps; this effect also increases the smooth flame front structure because the propagation of the concave branch cannot be well developed. This indicates that the separation of convex and concave structures is very
important for understanding the flame response against turbulence flow. It is also seen that the convex and concave percentages with H2O dilution are almost the same as those of CH4/air flames without dilution, indicating that the flame response against turbulent flow is not passive in the case of H2O dilution. As mentioned Section 1, our previous research [6,7] showed a complicated effect of CO2 dilution on the flame front structure, and this will now be discussed in detail based on various characteristic flame front wrinkled scales combined with the scale of turbulence flow. The characteristic radius of curvature, R0.5, which is a characteristic length scale of the wrinkled flame front, was defined as the absolute local radius of curvature at ADF = 0.5 as shown in Fig. 6a, where only the scale was considered regardless of the direction. This means that 50% of the wrinkled structure was located at a scale smaller than R0.5, which can be considered as the average radius of curvature on the flame front. Figure 6b shows variations of the characteristic radius of curvature, R0.5, and the inner cutoff scale, ei, with u0 /SL. The scale of turbulence flow, lv, which is the average scale of the vortex tubes in isotropic turbulence and is about 10gk (gk is Kolmogorov scale) as reported by Tanahashi et al. [28], is also given. It is seen that R0.5 of CO2 dilution is much larger than that of CH4/air flames without dilution, indicating a larger average wrinkled scale with CO2 dilution. As discussed in Fig. 5, CO2 dilution suppresses the flame front propagation to the burned mixture leading to the breakage of the concave cusps and the formation of sharp, deep flame cusps. Meanwhile, the suppression of CO2 dilution also smoothes the flame front because the concave branch cannot be well developed and this effect is predominant and leads to an increase of average scale on the flame front with CO2 dilution. Both ei and R0.5 are the characteristic scales of the wrinkled flame front of turbulent premixed flames. However, the physical meanings are quite different. ei represents the smallest scale corresponding to the smallest wrinkled cusps on the flame front, while R0.5 means the average scale of the flame front wrinkled structure. It can be seen that both ei and R0.5 decreased with the increase of u0 /SL mainly caused by the small scale turbulence interaction because the turbulence scale, lv, decreased with the increase of u0 /SL. It was noted that the decrease of ei is more obvious compared to the R0.5 with the increase of u0 /SL, which means that the small scale cusps tend to be finer and sharper. However, the overall wrinkled scale decreased slightly with the increase of u0 / SL. It is seen that ei decreased but R0.5 obviously increased with CO2 dilution, which revealed the complicated effects of CO2 dilution on the flame front structure. It is seen that the small scale cusps are sharp and propagate deep into the burned mixture, as shown in Fig. 1b and e; meanwhile, the overall flame front is smoother due to the suppres-
J. Wang et al. / Proceedings of the Combustion Institute 34 (2013) 1429–1436
a
0.040
1.0
u'/SL 1.2 ZCO2,H2O=0.05
CH4/air CH4/air/CO2 CH4/air/H2O
0.035
0.8
ADF
0.6
0.030
PDF
CH4/air CH4/air/CO2 CH4/air/H2O
u'/SL≅1.2 ZCO2,H2O=0.05
0.025 0.020 0.015
0.4
burned
0.010 0.2
0.000
0.0 0
2
4
6
8
10
-150 -100 -50
50
100 150
Fig. 7. Variations of PDF distribution of the local flame angle with CO2 and H2O dilution.
R0.5
ZCO2,H2O=0.05
0
Local flame angle,α (°)
abs(R) (mm) 1.4
α
0.005
R0.5
b
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R0.5, , lv (mm)
1.2 1.0
CH4/air/H2O
0.8
CH4/air/CO2
CH4/air
0.6
lv
0.4 0.5
1.0
1.5
2.0
2.5
3.0
u'/SL Fig. 6. Definition of R0.5, variations of characteristic length scales of turbulent flame and flow turbulence with u0 /SL and CO2/H2O dilution: (a) definition of R0.5; (b) R0.5, ei and lv.
sion of the propagation of the concave structure with CO2 dilution, as discussed in Fig. 5. The sharp and deep small scale cusps lead to the decrease of inner cutoff scale, formation of a thick flame brush, and enlargement of the mean flame volume. The overall wrinkled scale becomes larger, leading to a decrease of the turbulent flame front area, and subsequently to the decrease of ST/SL with CO2 dilution. It is also found that the R0.5 of H2O dilution slightly increases and almost remains the same as that of the CH4/air flames without dilution. This would be the reason that only H2O dilution had a weak effect on ST/SL in our previous research [7]. 3.2. Effect of CO2 and H2O dilution on local flame angle Local flame angle indicates the distribution of flame front normal vectors and is an important flame front geometric property as well as having potential application in turbulent flame modeling [29]. As defined above, the local flame angle is measured from the flame front normal vector to the
upward vertical axis in the counterclockwise direction as shown in Fig. 7. The positive value indicates that the flame front is concave to the unburned mixture and that the larger angle corresponds to the flamelet propagating upward. The PDF distribution of the local flame angle is given in Fig. 7. It is seen that the PDF distribution of the local flame angle of CH4/air flame shows a relatively symmetrical shape and has almost the same distribution as that of H2O dilution. This means that the local flame propagation is relatively isotropic for CH4/ air flames and that there is almost no influence from H2O dilution. The local flame angle shows higher probability at a positive degree and lower probability at a negative degree; the peak of PDF distribution is located at about 120° with CO2 dilution. This indicates that the local flame tends to propagate upward and that the most frequent structure is the concave cusps propagating upward due to the breakage of the concave branches with CO2 dilution. 4. Conclusions 1. The flame front of turbulent premixed flames at high pressure and high temperature is a wrinkled flame front with small scale convex and concave cusps superimposed on large scale branches, while the whole flame front tends to be convex to unburned mixture. 2. The statistical local radius of curvature and local flame angle with H2O dilution were almost the same as that of CH4/air flames, while ADF and PDF distribution of the local radius of curvature and characteristic radius of curvature showed an obvious increase of average radius of curvature on the flame front with CO2 dilution. This indicated that the
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effect of EGR gas dilution on the structure of turbulent premixed flames is dominated by CO2 rather than H2O dilution. 3. The quantitative flame front characteristics reveal the complicated effects of CO2 dilution on turbulent premixed flames characteristics observed in our previous research [6,7]. In the case of CO2 dilution, some local wrinkled structures become sharp and propagate deep into the burned mixture, resulting in a decrease of fractal inner cutoff scale, formation of a thick flame brush and enlargement of the mean flame volume, while the overall wrinkled scale increases significantly due to the suppression of concave structure with CO2 dilution, leading to a decrease of the turbulent flame front area, and subsequently to a decrease of ST/SL.
Acknowledgement Jinhua Wang acknowledges the Japan Society for the Promotion of Science for a JSPS Postdoctoral Fellowship grant.
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¨ .L. Gu¨lder, G.J. Smallwood, R. Wong, et al., [9] O Combust. Flame 120 (2000) 407–416. [10] H. Kobayashi, H. Kawazoe, Proc. Combust. Inst. 28 (2000) 375–382. [11] I.G. Shepherd, W.M.T. Ashurst, Proc. Combust. Inst. 24 (1992) 485–491. [12] T.-W. Lee, G.L. North, D.A. Santavicca, Combust. Flame 93 (1993) 445–456. [13] C.J. Rutland, A. Trouve´, Combust. Flame 94 (1993) 41–57. [14] A. Soika, F. Dinkelacker, A. Leipertz, Combust. Flame 132 (2003) 451–462. [15] T. Lachaux, F. Halter, C. Chauveau, I. Go¨kalp, I.G. Shepherd, Proc. Combust. Inst. 30 (2005) 819– 826. ¨ .L. [16] C. Cohe´, F. Halter, C. Chauveau, I. Go¨kalp, O Gu¨lder, Proc. Combust. Inst. 31 (2007) 1345–1352. [17] H. Kobayashi, T. Nakashima, T. Tamura, K. Maruta, T. Niioka, Combust. Flame 108 (1997) 104–117. [18] H. Kobayashi, T. Kawahata, K. Seyama, T. Fujimari, J.-S. Kim, Proc. Combust. Inst. 29 (2002) 1793–1800. [19] R.J. Kee, J.F. Grcar, M.D. Smooke, J.A. Miller, Sandia report SAND89-8009, Sandia National Laboratories, 1993. [20] R.J. Kee, F.M. Rupley, J.A. Miller, Sandia report SAND85-8240, Sandia National Laboratories, 1991. [21] C.T. Bowman, R.K. Hanson, D.F. Davidson, et al., GRI-Mech homepage, 1994, available at http:// www.me.berkeley.edu/gri_mech. [22] D. Bradley, P.H. Gaskell, A. Sedaghat, X.J. Gu, Combust. Flame 135 (2003) 503–523. ¨ .L. Gu¨lder, Proc. Combust. Inst. 32 [23] F.T.C. Yuen, O (2009) 1747–1754. [24] M.Z. Haq, C.G.W. Sheppard, R. Woolley, D.A. Greenhalgh, R.D. Lockett, Combust. Flame 131 (2002) 1–15. [25] K.N.C. Bray, R.S. Cant, Proc. R. Soc. Lond. A 434 (1991) 217–240. [26] S. Kadowaki, Combust. Sci. Technol. 162 (2001) 223–234. [27] G.I. Sivashinsky, Annu. Rev. Fluid Mech. 15 (1983) 179–199. [28] M. Tanahashi, S.-J. Kang, T. Miyamoto, S. Shiokawa, T. Miyauchi, Int. J. Heat Fluid Flow 25 (2004) 331–340. [29] T.C. Chew, K.N.C. Bray, R.E. Britter, Combust. Flame 80 (1990) 65–82.
Proceedings of the
Combustion Institute
Proceedings of the Combustion Institute 34 (2013) 1429–1436
www.elsevier.com/locate/proci
Flame front characteristics of turbulent premixed flames diluted with CO2 and H2O at high pressure and high temperature Jinhua Wang a,b,⇑, Futoshi Matsuno b, Masaki Okuyama b, Yasuhiro Ogami b, Hideaki Kobayashi b, Zuohua Huang a a
State Key Laboratory of Multiphase Flows in Power Engineering, Xi’an Jiaotong University, Xi’an 710049, PR China b Institute of Fluid Science, Tohoku University, Sendai, Miyagi 980-8577, Japan Available online 12 July 2012
Abstract Flame front structure characteristics of turbulent premixed flames including the local radius of curvature, fractal inner cutoff scale and local flame angle were calculated from the experimental OH-PLIF images of CH4/air flames and those diluted with CO2 and superheated H2O at 0.5 MPa and 573 K. The convex and concave structures of the flame front were detected and statistical analysis including PDF and ADF of the local radius of curvature and the local flame angle was conducted. Results showed that the flame front of turbulent premixed flames at high pressure and high temperature is a wrinkled flame front with small scale convex and concave cusps superimposed on large scale branches, while the whole flame front tends to be convex to the unburned mixture. The effect of EGR gas dilution on the flame front structure of turbulent premixed flames is dominated by CO2 rather than H2O dilution. The quantitative flame front characteristics reveal the complicated effect of CO2 dilution on turbulent premixed flames characteristics observed in our previous research. In the case of CO2 dilution, some local wrinkled structures become sharp and propagate deep into the burned mixture, resulting in a decrease of the fractal inner cutoff scale, the formation of a thick flame brush and the enlargement of the mean flame volume, while the overall wrinkled scale increases significantly due to the suppression of the concave structure with CO2 dilution, leading to a decrease of the turbulent flame front area, and subsequently to a decrease of ST/SL. Ó 2012 The Combustion Institute. Published by Elsevier Inc. All rights reserved. Keywords: Turbulent premixed flames; Flame front; Dilution; High pressure; High temperature
1. Introduction Exhaust gas recirculation (EGR) and flue gas recirculation (FGR) are widely used in internal combustion engines and gas turbines for reducing
NOx emissions by decreasing the flame temperature with EGR gas dilution [1,2]. Although studies of EGR gas dilution on internal combustion engines [1], gas turbines [2] and laminar flames [3–5] have been extensively performed, the
⇑ Corresponding author at: State Key Laboratory of Multiphase Flows in Power Engineering, Xi’an Jiaotong University, Xi’an 710049, PR China. Fax: +86 29 82668789. E-mail addresses: [email protected], jhwang@flame.ifs.tohoku.ac.jp (J. Wang),
1540-7489/$ - see front matter Ó 2012 The Combustion Institute. Published by Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.proci.2012.06.154
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fundamental study of the effects of EGR gases on turbulent premixed flames has not been well conducted, especially under relevant engine conditions, i.e., high pressure and high temperature. The effects of CO2 and superheated H2O (which are the main components of EGR gases) dilution on turbulent premixed flame characteristics have been studied by the present authors [6,7]. Findings showed that the effect of EGR gases on the structure of turbulent premixed flames is dominated by CO2 rather than H2O dilution. The turbulent burning velocity, ST, normalized by the laminar burning velocity, SL, became smaller, and the mean volume of the turbulent flame region increased in the case of CO2 dilution, while it scarcely changed with H2O dilution. Findings also showed that the flame wrinkled scale which was characterized as the fractal inner cutoff scale decreased with CO2 dilution. The simultaneous decrease in inner cutoff scale and ST/SL combined with the enlargement of the mean flame volume indicated the complicated effect of CO2 dilution on the flame front structure. Generally, the ST/ SL increases with the decrease of fractal inner cutoff scale, which generates increased surface wrinkling and enlarges the flame front area [8–10]. It seems that some local flame front is more wrinkled and some of the wrinkled cusps propagate deep and make a thick flame brush with CO2 dilution, but that the overall scale of wrinkling becomes larger, leading to a decrease of ST/SL. However, the quantitative flame front information was not well presented in the previous studies and the mechanism of the dilution effect on the overall flame dynamics is still not well understood. In order to clarify the effect of EGR gas dilution on the interaction between turbulence and flame, and to understand the difference between CO2 and H2O dilution, further quantitative investigation on flame front structure is needed. Flame front structure reveals the interaction between turbulence and flame, and it can also be used for turbulent combustion modeling. To date a complete geometric description of turbulent premixed flame front has been provided, such as fractal dimension and cutoff scales [8–10], flame curvature and angle statistics [11,12]. The effects of Lewis number [12,13], pressure [14–16] and turbulence intensity on the flame front structure have been studied. However, few studies have been performed on the effects of dilution, especially at high pressure and high temperature. The flame curvature and its statistical properties such as PDF (probability density function) have usually been used to indicate the wrinkled flame front structure. However, it cannot properly indicate the small scale structure on the flame front because the small scale cusps with large curvature are located far from the center and may even fall out of the figure of the PDF of curvature [11,12,15]. This problem will be even more severe at high pressure because
turbulent premixed flames at high pressure possess a much finer wrinkled structure with a much smaller scale of convex and concave cusps compared with that at ordinary pressure [16,17]. This fine wrinkled structure of turbulent premixed flames at high pressure is mainly caused by small scale turbulence interaction combined with flame instability at high pressure, which has been previously discussed by the authors [17]. However, the quantitative flame front geometric information was not well presented. The local radius of curvature of the flame front can be viewed as a length scale and was used in the present study with a focus on the small convex and concave structure on the flame front. The objective of the present study was to clarify the effects of CO2 and superheated H2O dilution on flame dynamics of CH4/air turbulent premixed flames based on quantitative flame front structure characteristics, especially at high pressure and high temperature. The local radius of curvature, fractal inner cutoff scale and local flame angle of CH4/air flames and those diluted with CO2 and H2O were calculated and statistical analysis was conducted. The effects of CO2 and H2O dilution and turbulence intensity on the interaction between flame and turbulence were examined based on statistical analysis and length scales. 2. Experimental setup and procedure Experiments were performed using the HighPressure Combustion Test Facility at the Institute of Fluid Science, Tohoku University. An electric air heater with a total power of 10 kW and a water evaporator with a 1.2 kW heater were installed in the high pressure chamber to supply air with superheated H2O at high temperature. A stainless steel nozzle-type burner with an outlet diameter of 20 mm was used. The turbulence was generated by a perforated plate installed 40 mm upstream of the outlet. Turbulence measurement was conducted using a constant-temperature hot-wire anemometer (Dantec, Streamline 90N). More details have been described elsewhere [17,18]. Bunsen-type turbulent premixed flames of CH4/ air mixtures and those diluted with CO2 and H2O were stabilized at the nozzle burner outlet in the high-pressure chamber. A large amount of fresh air was supplied to the chamber to keep the pressure constant at 0.5 MPa and to prevent the chamber from filling with burned gases. Automatic feedback of electric power to both the air heater and water evaporator was activated to keep the mixture temperature at the burner outlet at 573 ± 5 K. Air was diluted with CO2 or superheated H2O upstream of the electric air heater and used as oxidizer. The CH4 and oxidizer were premixed and supplied to the burner. The strength of dilution was defined by using the dilution ratio, Z, as Z CO2 ¼ ½CO2 =ð½air þ ½CO2 Þ for CO2
J. Wang et al. / Proceedings of the Combustion Institute 34 (2013) 1429–1436
dilution, Z H2 O ¼ ½H2 O=ð½air þ ½H2 OÞ for H2O dilution, where [air], [CO2] and [H2O] denote the mole fraction of air, CO2 and H2O in the mixtures, respectively. The equivalence ratio of the mixture was 0.9 and dilution ratios of 0.05 and 0.1 for CO2 and H2O dilution were performed. The properties of the mixtures are summarized in Table 1. The laminar burning velocity, SL, for mixtures used in this study was estimated by using the PREMIX code [19] and CHEMKIN-II database [20] with GRI-Mech 2.11 [21]. OH-PLIF measurements were performed to visualize the instantaneous flame front. An NdYAG laser (Spectra Physics, GCR-250-10) and a dye laser with a frequency doubler (Spectron, SL4000) were used. A high resolution ICCD camera (ANDOR, DH574-18F) with 1024 1024 pixels was used and the finest pixel resolution was 0.054 mm/pixel at the measurement plane. A description of the OH-PLIF apparatus and the selected wavelength are stated elsewhere [6,7]. The flame front was tracked from the OH-PLIF image by binarizing with a threshold value based on the histogram of the intensity of the images. About 1500 points of each OH-PLIF image were successfully tracked due to the high resolution and high sensitivity OH-PLIF technique used in this study. It is sufficient to fit the continuous flame front curve for subsequent calculation. A 3-order smoothing scheme was used to remove the digitization noise from the binarization process. The smooth flame front was then discretized with an interval of 0.50 mm, which was the smallest inner cutoff scale of all the flames in the present study. The use of this interval and discretization is to remove the digitization noise from the pixels since the inner cutoff scale can be viewed as the smallest scale of the flame front [12]. The discretized points were fitted using a 3-order spline interpolation scheme, which was then employed to calculate the local radius of curvature, R, and local flame angle, a, as follows: 2 2 3=2 xðsÞ0 þ yðsÞ0 R¼ ð1Þ xðsÞ0 yðsÞ00 yðsÞ0 xðsÞ00 a ¼ arctanðyðsÞ0 =xðsÞ0 Þ
ð2Þ
where xðsÞ and yðsÞ are coordinates of the flame front spline curve and s is the length of the accumulated flame from the starting point. For the local radius of curvature, the positive direction was
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defined as being convex to the unburned mixture. The local flame angle was defined from the flame front normal vector to the upward vertical axis in a counterclockwise direction. The fractal inner cutoff scale, ei, was calculated by using 50 OH-PLIF images with the circle method [18]. Sixty images were calculated for each condition and statistical analysis of the local radius of curvature and local flame angle was conducted including the PDF and ADF (accumulated distribution function) distributions as follows: Z R ADFðRÞ ¼ PDFdR ð3Þ 0
The range for the calculation of the radius of curvature is from 100 to 100 mm. The interval for PDF calculation is 0.02 mm for the radius of curvature, and 6° for the local flame angle, respectively. 3. Results and discussion 3.1. Effects of CO2 and H2O dilution on the local radius of curvature Figure 1 shows typical OH-PLIF images of CH4/air turbulent premixed flames and those with CO2 and H2O dilution at 0.5 MPa and 573 K. It is seen that the strongly wrinkled flame front can be well recognized by using the OH-PLIF technique with high resolution and high sensitivity in this study; thus, it should be appropriate to be used for the calculation of flame front characteristics. It is seen that all the flames in this study possess a fine, convoluted flame front structure with convex and concave cusps superimposed on large scale flame branches, which is a general characteristic of turbulent premixed flames at high pressure [17]. Although the flame height varies with the flow conditions, as shown in Fig. 1a and b, deep wrinkled flame cusps tend to be observed and these deep cusps become sharp; meanwhile, the smooth flame fronts also tend to increase with CO2 dilution. This means that the wrinkled flame front becomes small scale deep wrinkled cusps superimposed on a much larger scale smooth flame front with CO2 dilution. This tendency is more significant with the increase of dilution ratio from Z CO2 ¼ 0:05 to Z CO2 ¼ 0:1 (Fig. 1b and e). However, the flame front wrinkled structure shows a very small difference with H2O dilution
Table 1 The properties of the mixtures in this study, at 0.5 MPa and 573 K. Z CO2 ;H2 O
S L;CO2 (cm/s)
S L;H2 O (cm/s)
LeCH4 , CO2
LeCH4 , H2O
0.0 0.05 0.1
59.57 42.58 30.04
59.57 47.86 37.53
1.006 0.973 0.951
1.006 0.996 0.987
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a
b
c
a
0.012
u'/SL 1.2 ZCO2,H2O=0.05
0.010
CH4/air CH4/air/CO2 CH4/air/H2O
0.008 0.006 0.004 0.002
e
d
f -3
-2
-1
0
1
2
3
R (mm)
b CH4/air/CO2
0.4
CH4/air/H2O
Fig. 1. OH-PLIF images at 0.5 MPa and 573 K: (a) Z = 0, u0 /SL ffi 1.2; (b and c) Z CO2 ;H2 O ¼ 0:05, u0 /SL ffi 1.2; (d) Z = 0, u0 /SL ffi 2.6; (e and f) Z CO2 ;H2 O ¼ 0:1, u0 / SL ffi 2.6.
ADF
CH4/air
u'/SL 1.2 ZCO2,H2O=0.05
0.5
CH4/air CH4/air/CO2 CH4/air/H2O
0.3 0.2 0.1 0.0 -20
-15
-10
-5
0
5
10
15
20
R (mm)
Fig. 2. PDF and ADF distributions of the local radius of curvature: (a) PDF distribution; (b) ADF distribution.
0.6 0.5
CH4/air CH4/air/CO2 CH4/air/H2O
u'/SL 1.0 ZCO2,H2O=0.05
0.4
ADF
(Fig. 1c and f). This is the general characteristic of CO2 and H2O dilution from the direct visualization of the instantaneous flame front images. The effects of dilution and the difference between CO2 and H2O dilution on flame front structure is confirmed from analysis of quantitative flame front characteristics in the following section. Figure 2 shows PDF and ADF distributions of the local radius of curvature. It is seen from Fig. 2a that the PDF of the radius of curvature shows an obvious bias to positive radius. The peak of PDF distribution of local radius of curvature decreases and its profile becomes flat with CO2 dilution, indicating a less wrinkled structure with CO2 dilution. The position of the peak of PDF distribution tends to become larger, indicating that the most frequent wrinkled scale increases with CO2 dilution. The effect of H2O dilution is much weaker and the PDF of the local radius of curvature is almost the same as that of CH4/air flame. Figure 2b shows the ADF distribution of the radius of curvature in the range from 20 to 20 mm. As calculated by Eq. (3), ADF distribution indicated the size distribution of the local radius of curvature on the flame front. This shows that most of the radius of curvature on the flame front is within the region of small radius with a small scale structure, indicating that the flame front of turbulent premixed flames is a wrinkled flame front with a wide range of scales and that the small scale structure is predominant, as shown in Fig. 1. In the following section, the local radius of curvature at a small scale range of 5 to 5 mm will be presented to discuss the small scale structure in detail.
0.3 0.2 0.1 0.0 -5
-4
-3
-2
-1
0
1
2
3
4
5
R (mm) Fig. 3. Variations of ADF distribution of the local radius of curvature with CO2 and H2O dilution.
Figure 3 shows variations of ADF distribution of the local radius of curvature with CO2 and H2O dilution. It is seen that the radius of curvature with positive value is always more than that of the negative ones, which is also shown later in Fig. 5. This means that the convex structure which is convex to
J. Wang et al. / Proceedings of the Combustion Institute 34 (2013) 1429–1436
0.6
u'/SL 1.0 u'/SL 1.2 u'/SL 2.6
CH4/air 0.5
ADF
0.4 0.3 0.2 0.1 0.0
b
0.6
u'/SL 1.0 u'/SL 1.2 u'/SL 2.6
CH4/air/CO2 ZCO2=0.05
0.5
ADF
0.4 0.3 0.2 0.1 0.0 -5
-4
-3
-2
-1
0
1
2
3
4
5
R (mm) Fig. 4. Variations of ADF distribution of the local radius of curvature with u0 /SL: (a) CH4/air; (b) CH4/air/ CO2.
the unburned mixture is always more frequent than the concave ones for turbulent premixed flames; this would be a general characteristic of the turbulent premixed flames at high pressure. This phenomenon can also be observed directly from flame visualization as discussed by Kobayashi et al. [17] and was validated by quantitative flame front calculation in the present study. Previous research using the PDF of curvature to indicate the flame front structure generally showed a nearly symmetrical distribution with a mean of approximately zero [12,14,15,22–24] although some studies have found a slightly positive bias which was convex to the unburned mixture [11,16,25]. It has been argued that the overall effect of flame curvature on the burning rate integrates to zero if the curvature distribution is symmetric since the effect of positive and negative curvature will cancel each other to some degree [13,22]. For now, there is no consensus as to the distribution of local flame curvature and the local flame curvature on the overall burning rate is still not well understood. This may be due to the fact that the flat pattern on the flame front with a smaller curvature which is near zero is locate at the center of the PDF of curvature, while the small scale structure
has a large local curvature which is far from the center and may even be out of the range of previous research [11,12,16]. Thus, the small scale structure with large curvature was not properly presented by using the PDF of curvature in previous research. This problem is even more severe at high pressure and high temperature as the flame front is more wrinkled with a much smaller scale structure which corresponds to a much larger curvature. By using the local radius of curvature in the present study, the small scale structure and its distribution can be well presented and a higher probability of positive structure which is convex to unburned mixtures is clearly indicated. It is also seen from Fig. 3 that the ADF distribution of the local radius of curvature at both positive and negative radius obviously decreased with CO2 dilution. The flame curvature has a weak effect on local burning velocity because the Lewis number of all the mixtures in this study is close to unity, as shown in Table 1 [13,26]. This means that the variation of ADF distribution of the local radius of curvature mainly influences the turbulent flame front area by varying the wrinkled scale. The decrease of ADF distribution of the local radius of curvature indicated that the wrinkled scale of the flame front tends to become larger and less convoluted with CO2 dilution. This seems to be contradictory to the direct observation of OHPLIF images as shown in Fig. 1, that the flame front is more wrinkled with CO2 dilution. This is discussed in a later section based on the comparison of characteristic length scales of the flame front. In the case of H2O dilution, the ADF distribution of the local radius of curvature is almost the same as that of CH4/air mixtures without dilution. Figure 4 shows variations of ADF distribution of the local radius of curvature with u0 /SL, where u0 is turbulence intensity. It shows that the ADF distribution of the local radius of curvature increased for both positive and negative radius with
Convex and concave precentages (%)
a
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ZCO2,H2O=0.05
CH4/air CH4/air/CO2 CH4/air/H2O
56
solid: convex open: concave
52 48 44 40 36 0.5
1.0
1.5
2.0
2.5
3.0
u'/SL
Fig. 5. Variations of convex and concave percentages on the flame front with u0 /SL and CO2/H2O dilution.
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the increase of u0 /SL in the case of CO2 dilution (Fig. 4b), while the effect is much weaker for CH4/air flame without dilution (Fig. 4a) in the experimental range. This means that the effect of turbulence flow on flame front wrinkling is much more obvious and that the flame response against turbulence flow is passive with CO2 dilution. The Lewis number of the mixtures decreased with CO2 dilution, as shown in Table 1. The characteristic flame instability scale, li, indicated the combined effect of the hydrodynamic instability and the diffusive-thermal instability, which corresponds to the wavelength at the maximum growth rate of flame front disturbance calculated using Sivashinsky’s formulation [27]. The li decreased from li = 0.600 mm at Z = 0 to li = 0.467 mm at Z CO2 ¼ 0:05. This means that the flame wrinkling caused by turbulence is amplified due to the promotion of the intrinsic flame instability with CO2 dilution. Figure 5 shows variations of convex and concave percentages on the flame front with u0 /SL. The convex and concave percentages correspond to the ADF value at the boundary of the calculation range of ±100 mm, which indicates the proportion of convex and concave structure on the flame front. As discussed in Fig. 3, the convex structure on the flame front is much more frequent than concave ones, which is well indicated in Fig. 5 for all the conditions in this study. The difference between convex and concave percentages is about 6% in the case of CH4/air flames. It is noted that the sum of the convex and concave percentages is less than unity for all the conditions which would be due to the existence of the flat patterns on the flame front. It is seen that both the convex and concave percentages show a very slight increase with the increase of u0 /SL in the experimental range for CH4/air flames. In the case of CO2 dilution, both the convex and concave percentages show a much more remarkable increase with the increase of u0 / SL compared to that of CH4/air flames without dilution, which indicates the passive flame response against flow turbulence as discussed in Fig. 4. The convex percentage decreases slightly while the concave percentage is much less (about 4% in this study) for CO2 dilution compared with that of CH4/air flames. This indicates that the concave structure propagating to the burned mixture was suppressed significantly with CO2 dilution. This would be due to the relatively larger heat capacity of CO2, which suppresses the propagation of the concave structure to the burned mixture. It is observed from Fig. 1b that deep, sharp concave cusps were formed due to the suppression of concave branch propagation to the burned mixture, leading to the breakage of the concave cusps; this effect also increases the smooth flame front structure because the propagation of the concave branch cannot be well developed. This indicates that the separation of convex and concave structures is very
important for understanding the flame response against turbulence flow. It is also seen that the convex and concave percentages with H2O dilution are almost the same as those of CH4/air flames without dilution, indicating that the flame response against turbulent flow is not passive in the case of H2O dilution. As mentioned Section 1, our previous research [6,7] showed a complicated effect of CO2 dilution on the flame front structure, and this will now be discussed in detail based on various characteristic flame front wrinkled scales combined with the scale of turbulence flow. The characteristic radius of curvature, R0.5, which is a characteristic length scale of the wrinkled flame front, was defined as the absolute local radius of curvature at ADF = 0.5 as shown in Fig. 6a, where only the scale was considered regardless of the direction. This means that 50% of the wrinkled structure was located at a scale smaller than R0.5, which can be considered as the average radius of curvature on the flame front. Figure 6b shows variations of the characteristic radius of curvature, R0.5, and the inner cutoff scale, ei, with u0 /SL. The scale of turbulence flow, lv, which is the average scale of the vortex tubes in isotropic turbulence and is about 10gk (gk is Kolmogorov scale) as reported by Tanahashi et al. [28], is also given. It is seen that R0.5 of CO2 dilution is much larger than that of CH4/air flames without dilution, indicating a larger average wrinkled scale with CO2 dilution. As discussed in Fig. 5, CO2 dilution suppresses the flame front propagation to the burned mixture leading to the breakage of the concave cusps and the formation of sharp, deep flame cusps. Meanwhile, the suppression of CO2 dilution also smoothes the flame front because the concave branch cannot be well developed and this effect is predominant and leads to an increase of average scale on the flame front with CO2 dilution. Both ei and R0.5 are the characteristic scales of the wrinkled flame front of turbulent premixed flames. However, the physical meanings are quite different. ei represents the smallest scale corresponding to the smallest wrinkled cusps on the flame front, while R0.5 means the average scale of the flame front wrinkled structure. It can be seen that both ei and R0.5 decreased with the increase of u0 /SL mainly caused by the small scale turbulence interaction because the turbulence scale, lv, decreased with the increase of u0 /SL. It was noted that the decrease of ei is more obvious compared to the R0.5 with the increase of u0 /SL, which means that the small scale cusps tend to be finer and sharper. However, the overall wrinkled scale decreased slightly with the increase of u0 / SL. It is seen that ei decreased but R0.5 obviously increased with CO2 dilution, which revealed the complicated effects of CO2 dilution on the flame front structure. It is seen that the small scale cusps are sharp and propagate deep into the burned mixture, as shown in Fig. 1b and e; meanwhile, the overall flame front is smoother due to the suppres-
J. Wang et al. / Proceedings of the Combustion Institute 34 (2013) 1429–1436
a
0.040
1.0
u'/SL 1.2 ZCO2,H2O=0.05
CH4/air CH4/air/CO2 CH4/air/H2O
0.035
0.8
ADF
0.6
0.030
CH4/air CH4/air/CO2 CH4/air/H2O
u'/SL≅1.2 ZCO2,H2O=0.05
0.025 0.020 0.015
0.4
burned
0.010 0.2
0.000
0.0 0
2
4
6
8
10
-150 -100 -50
50
100 150
Fig. 7. Variations of PDF distribution of the local flame angle with CO2 and H2O dilution.
R0.5
ZCO2,H2O=0.05
0
Local flame angle,α (°)
abs(R) (mm) 1.4
α
0.005
R0.5
b
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R0.5, , lv (mm)
1.2 1.0
CH4/air/H2O
0.8
CH4/air/CO2
CH4/air
0.6
lv
0.4 0.5
1.0
1.5
2.0
2.5
3.0
u'/SL Fig. 6. Definition of R0.5, variations of characteristic length scales of turbulent flame and flow turbulence with u0 /SL and CO2/H2O dilution: (a) definition of R0.5; (b) R0.5, ei and lv.
sion of the propagation of the concave structure with CO2 dilution, as discussed in Fig. 5. The sharp and deep small scale cusps lead to the decrease of inner cutoff scale, formation of a thick flame brush, and enlargement of the mean flame volume. The overall wrinkled scale becomes larger, leading to a decrease of the turbulent flame front area, and subsequently to the decrease of ST/SL with CO2 dilution. It is also found that the R0.5 of H2O dilution slightly increases and almost remains the same as that of the CH4/air flames without dilution. This would be the reason that only H2O dilution had a weak effect on ST/SL in our previous research [7]. 3.2. Effect of CO2 and H2O dilution on local flame angle Local flame angle indicates the distribution of flame front normal vectors and is an important flame front geometric property as well as having potential application in turbulent flame modeling [29]. As defined above, the local flame angle is measured from the flame front normal vector to the
upward vertical axis in the counterclockwise direction as shown in Fig. 7. The positive value indicates that the flame front is concave to the unburned mixture and that the larger angle corresponds to the flamelet propagating upward. The PDF distribution of the local flame angle is given in Fig. 7. It is seen that the PDF distribution of the local flame angle of CH4/air flame shows a relatively symmetrical shape and has almost the same distribution as that of H2O dilution. This means that the local flame propagation is relatively isotropic for CH4/ air flames and that there is almost no influence from H2O dilution. The local flame angle shows higher probability at a positive degree and lower probability at a negative degree; the peak of PDF distribution is located at about 120° with CO2 dilution. This indicates that the local flame tends to propagate upward and that the most frequent structure is the concave cusps propagating upward due to the breakage of the concave branches with CO2 dilution. 4. Conclusions 1. The flame front of turbulent premixed flames at high pressure and high temperature is a wrinkled flame front with small scale convex and concave cusps superimposed on large scale branches, while the whole flame front tends to be convex to unburned mixture. 2. The statistical local radius of curvature and local flame angle with H2O dilution were almost the same as that of CH4/air flames, while ADF and PDF distribution of the local radius of curvature and characteristic radius of curvature showed an obvious increase of average radius of curvature on the flame front with CO2 dilution. This indicated that the
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effect of EGR gas dilution on the structure of turbulent premixed flames is dominated by CO2 rather than H2O dilution. 3. The quantitative flame front characteristics reveal the complicated effects of CO2 dilution on turbulent premixed flames characteristics observed in our previous research [6,7]. In the case of CO2 dilution, some local wrinkled structures become sharp and propagate deep into the burned mixture, resulting in a decrease of fractal inner cutoff scale, formation of a thick flame brush and enlargement of the mean flame volume, while the overall wrinkled scale increases significantly due to the suppression of concave structure with CO2 dilution, leading to a decrease of the turbulent flame front area, and subsequently to a decrease of ST/SL.
Acknowledgement Jinhua Wang acknowledges the Japan Society for the Promotion of Science for a JSPS Postdoctoral Fellowship grant.
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