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air premixed flame
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Proceedings of the Combustion Institute 35 (2015) 989–997
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Flame dynamics of equivalence ratio oscillations in a laminar stagnating lean methane/air premixed flame Hisashi Tomita a,⇑, Mohd Rosdzimin Abdul Rahman b, Sotaro Miyamae a, Takeshi Yokomori c, Toshihisa Ueda c b
a Graduate School of Science and Technology, Keio University, Japan Department of Mechanical Engineering, Faculty of Engineering, Universiti Pertahanan Nasional Malaysia, Malaysia c Department of Mechanical Engineering, Keio University, Japan
Available online 12 August 2014
Abstract This study investigates the effect of fuel concentration oscillation on laminar stagnating premixed flames by both experiment and numerical simulation. The numerical analysis is conducted using ANSYS Fluent 14.5. The equivalence ratio oscillation in the experiments is formed by a novel oscillator with two cylinder piston units that can produce alternating ejections of leaner and richer pre-mixtures. Velocity fluctuation is well suppressed by installing screens on the burner exit. The fuel concentration oscillation between the stagnation plate and the burner exit is visualized and analyzed by acetone ultraviolet light-induced fluorescence in the isothermal condition. The oscillator frequency is varied in the range 2–20 Hz, and the oscillation wavelength is much longer than the flame thickness. The flame oscillates with the fuel concentration, and in the experiment, the amplitude of the flame oscillation attenuates as the frequency of fuel concentration oscillation increases above 5 Hz, which corresponds to a Strouhal number of unity. This indicates that the Strouhal number distinguishes quasi-steadiness for St < 1 and unsteadiness for St > 1. The flame oscillation pattern is a closed loop, which might be attributable to variation of the back support effect on the flame. The numerical results show a similar trend for the flame response to oscillations in fuel concentration. This study finds the flame motion is significantly affected by fuel concentration oscillations, even at low frequencies; in other words, the oscillation wavelength is much longer than the flame thickness, as a result of the back support effect. Ó 2014 The Combustion Institute. Published by Elsevier Inc. All rights reserved. Keywords: Flame dynamics; Equivalence ratio oscillation; Laminar premixed flame; Stagnating flame; Acetone ultraviolet light-induced fluorescence
⇑ Corresponding author. Address: Department of
Mechanical Engineering, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan. Fax: +81 45 566 1495. E-mail address: [email protected] (H. Tomita).
1. Introduction Lean premixed combustion is used in power plants and factories to reduce NOx emissions [1,2]. However, operating a lean premixed combustor has led to some technical problems that have not been solved yet, such as combustion
http://dx.doi.org/10.1016/j.proci.2014.07.061 1540-7489/Ó 2014 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
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H. Tomita et al. / Proceedings of the Combustion Institute 35 (2015) 989–997
Nomenclature A d f L qleaner qprimary qricher qtotal r R St t T Tper vexit
area of piston cross section distance between burner exit and stagnation plate frequency of piston movement half stroke of piston movement flow rate of leaner secondary flow flow rate of primary flow flow rate of richer secondary flow total flow rate coordinate axis of burner diameter direction universal gas constant Strouhal number time temperature period burner exit velocity
instability. Resolving combustion instability is a key goal of current combustion science. The relations of combustion instability to combustor geometry, operating conditions, and fuel have not been clarified yet [3]. Lieuwen et al. [4] showed that reactant unmixedness could drive strong instabilities in a lean premixed combustor. Combustion instability occurs when the pressure oscillation is in phase with the heat release oscillation; this induces poor performance of the combustor to the extent that it may be damaged. Richard et al. [5] and Yu and Wilson [6] show that combustion instability in gas turbine combustors can be reduced by adjusting the fuel injection into the combustion chamber. Candel [7] has studied relevant topics in combustion dynamics, including the mechanisms that drive instabilities in premixed combustion and the coupling between pressure waves and combustion. Those results are applicable in solving the problem of instability in modern low NOx heavy-duty gas turbine combustors. Less effort has been devoted to studying the dynamics of the flame response to the equivalence ratio oscillation [7] because the characteristic timescale of the equivalence ratio oscillation can couple with the timescale of convection, diffusion, and reaction [8]. Most studies on laminar premixed flame dynamics regarding equivalence ratio oscillation have been conducted numerically [9– 18], allowing an ideal equivalence ratio variation to be obtained. In contrast, few studies on this topic have been conducted experimentally [19– 22] because of difficulties in suppressing velocity perturbation in an actual flow field. A quasisteady state and attenuation of the flame response at low and very high oscillation frequencies were observed numerically, as was extension of the lean flammability limit [9,10]. Birbaud et al. [12]
y u uA ui uleaner um uprimary uricher x
coordinate axis along burner center line equivalence ratio amplitude of equivalence ratio at the burner exit initial difference between the equivalence ratios of secondary flows leaner (ui ¼ uricher u ) 2 initial equivalence ratio of leaner secondary flow mean equivalence ratio initial equivalence ratio of primary flow initial equivalence ratio of richer secondary flow angular velocity
numerically demonstrated that the non-linear heat release response related to equivalence ratio oscillation is due to rapid burning of the fresh mixture in the pocket. Moreover, Rahman et al. [16] numerically observed deviation of the flame motion from a steady state and hysteresis of the laminar premixed flames in response to equivalence ratio oscillations. Unfortunately, no reliable experimental results are available to support these numerical findings. Movement of a conical flame tip in response to equivalence ratio oscillations was experimentally observed by Kutne et al. [20] in a low-pressure environment (69 mbar) and found to be significantly influenced by velocity perturbation by Schwarz et al. [22]. Moreover, Takahashi et al. [19] and Suenaga et al. [21] experimentally observed variations in the oscillation amplitudes of burning velocity, burnt gas temperature, burnt gas composition, and flame luminosity with increasing equivalence ratio oscillation frequency in the cylindrical flame configuration. Those observations are difficult to replicate in numerical work because of the complexity of the geometry. In contrast to the case with equivalence ratio oscillation, the stratified case has been investigated experimentally by in many studies, and the back support effect has been found to influence the deviation of flame propagation speed from uniformity [23]. To address the dearth of experimental work on the response of laminar premixed flame to equivalence ratio oscillations, more reliable and simple flame configurations, such as a stagnating flame configuration, are required for validation of numerical work. Accordingly, the aim of this study was to quantify the dynamics of laminar lean methane/air premixed flame subject to equivalence ratio oscillation by suppressing velocity perturbation at ambient pressure.
H. Tomita et al. / Proceedings of the Combustion Institute 35 (2015) 989–997
2. Experimental setup and procedure 2.1. Characteristics of burner and oscillator Figure 1 shows the flow system used in the present study. The flow system consists of a primary flow with constant concentration and two secondary flows with fuel concentration oscillations produced by a novel oscillator. The equivalence ratios of the primary flow, richer secondary flow, and leaner secondary flow are uprimary, uricher, and uleaner, respectively. The flows are carried by individual flow lines. The primary flow is supplied directly to the burner. The richer and leaner flows are supplied to the oscillator. In the oscillator, one piston for the richer mixture and another piston for the leaner mixture move in antiphase, resulting in a mixture with sinusoidally varying equivalence ratio. This mixture is supplied to the burner. A laminar methane/air premixed flame in stagnation flow configuration is created by the burner as shown in Fig. 2. The primary flow passes through a converging nozzle. The secondary flows with sinusoidal equivalence ratio
Secondary flow
MFC (Richer air)
Needle valve
Burner
MFC (Richer CH4)
Air compressor
MFC (Leaner air)
CH4 cylinder
MFC (Leaner CH4)
richer
Oscillator
leaner
MFC (Air)
MFC: Mass Flow Controller
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oscillations are supplied to the primary flow via 16 exhaust holes, as shown in detail in Fig. 2(b), and mix with the primary flow. As a result, a mixture with sinusoidal equivalence ratio oscillation, formulized in Eq. (1), is created. u ¼ uprimary þ ui ðqricher;Air qleaner;Air Þ=qtotal;Air n þ 2pf ðAL sinð2pftÞÞricher;Air o i ðAL sin ð2pft pÞÞleaner;Air =qtotal;Air : ð1Þ Because the two pistons are in antiphase, the total flow rate of the primary and secondary flows is given by qtotal ¼ qprimary þ ½qricher þ 2pfAL sin ð2pftÞ þ ½qleaner þ 2pfAL sinð2pft pÞ:
ð2Þ
This indicates that the total flow rate from the burner is kept theoretically constant, but the equivalence ratio oscillates sinusoidally according to Eq. (1). Additionally, Eq. (2) indicates that the amplitude of flow rate fluctuation increases with frequency. In the actual system, however, a small velocity fluctuation is observed because the pistons do not move exactly sinusoidally due to the crank mechanism. The influence of velocity fluctuation on the flame oscillation is well suppressed relative to the fuel concentration oscillation by inserting two 100-mesh screens on the burner exit. Two honeycombs are also installed, one at the upstream side of the nozzle and the other at burner exit, to obtain a uniform profile flow at the burner exit. The coordinate system is defined as shown in Fig. 2(a). 2.2. Experimental conditions
Primary flow
MFC (CH4)
Table 1 shows the experimental conditions used in the present study. The primary equivalence
Fig. 1. Flow channel.
Stagnation plate Co-flow (N2)
y r
21
Flame
20
Screens
Richer premixed flow
Leaner premixed flow
Honeycomb Concentration oscillation
Secondary flow
Honeycomb Primary flow
(a)
(b)
Fig. 2. Stagnation flame burner: (a) cross section and (b) fuel concentration supply system.
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Table 1 Experimental conditions of equivalence ratio. Case No-oscil. (ui = 0) Oscil. (ui = 0.1) Oscil. (ui = 0.2) Oscil. (ui = 0.3)
Primary
Secondary
uprimary
uricher
uleaner
uA (5 Hz)
0.70 0.70 0.70 0.70
0.70 0.80 0.90 1.0
0.70 0.60 0.50 0.40
0.70 0.69–0.71 0.68–0.72 0.67–0.73
ratio is set to 0.70. In the case with no oscillation (“no-oscil.” in the table), a mixture with equivalence ratio u = 0.70 is issued from both the primary and secondary flows with the oscillator in operation. The influence of the velocity fluctuation induced by the oscillator is estimated from this operating condition. In the case with oscillation (“oscil.” in the table), the fuel concentration oscillates as a function of the initial difference between the equivalence ratio of the secondary flows ui and the frequency f, as mentioned in Eq. (1). The amplitude of fuel concentration oscillation at the burner exit in the case of f = 5 Hz is estimated as uA in Table 1. The mean velocity at the burner exit is fixed at 0.8 m/s. The distance between the stagnation plate and the burner exit is 21 mm. The oscillator frequency is varied in the range 2–20 Hz. In the present study, we focus on very low frequencies to investigate the flame dynamics of equivalence ratio oscillation without changing the flame structure. 2.3. Experimental procedure Fuel concentration oscillation between the burner exit and stagnation plate was visualized by acetone ultraviolet light-induced fluorescence (UVIF) in an isothermal condition, using air for the primary and secondary flows at the same flow rates as in the combustion case. Acetone is mixed on one side of the secondary flow by an acetone seeder. The light sheet from the ultraviolet lamp (Hamamatsu Photonics K.K., LC8), which has a peak intensity at 320 nm, is irradiated to the region between the burner and the stagnation plate to excite the acetone. The intensity of luminescence from the acetone is detected by a high-speed video camera (Keyence Corp., VW-6000) with an image intensifier. Because the concentration oscillation is produced by alternately mixing leaner and richer flows from the oscillator with the primary flow, the distribution of the luminescence intensity shows the relative fuel concentration. The flame position movement is measured to elucidate the effect of fuel concentration oscillation on the flame dynamics. The flame position is measured by the high-speed video camera. We define the flame position as the minimum y position of the blue flame zone along the centerline
Estimated
axis. Mathematica 9 (Wolfram, Inc.) is used to determine the flame region (to a certain tolerance) and to measure flame positions by analyzing the optical image. 3. Numerical analysis 3.1. Numerical method and chemical reaction mechanism Numerical analysis was performed with ANSYS Fluent 14.5. The flow field is modeled after the experimental geometry shown in Fig. 2. Details of the numerical method, computational domain, boundary conditions, and reaction models are given in Miyamae et al. [24]. The equivalence ratio with variation is given by u = um + uA sin xt. The condition on the equivalence ratio is set to um = 0.70 and uA = 0.10. The inlet velocity is kept constant, while the inlet density is varied as a function of equivalence ratio. The density variation is itself a source of equivalence ratio oscillation. Two chemical reaction models are applied; a one-step overall reaction model and a four-step reaction model that includes CO and H2. These models and the associated reaction rates are shown below. The one-step overall reaction model is CH4 þ 2O2 ! CO2 þ 2H2 O:
ð3Þ
The following Arrhenius-type reaction rate model of a one-step overall reaction mechanism is used: xCH4 ¼ 5:6 1011 2:027 108 J=kmol exp RT ½CH4 0:2 ½O2 1:3 :
ð4Þ
The four-step reaction model is CH4 þ 1=2O2 ! CO þ 2H2 ;
ð5Þ
CH4 þ H2 O ! CO þ 3H2 ;
ð6Þ
H2 þ 1=2O2 $ H2 O;
ð7Þ
CO þ H2 O $ CO2 þ H2 :
ð8Þ
H. Tomita et al. / Proceedings of the Combustion Institute 35 (2015) 989–997
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The following Arrhenius-type reaction rate models are used: xCH4 ;12O2 ¼ 7:824 1013 1:2561 108 J=kmol exp RT ½CH4 0:5 ½O2 1:25 ;
ð9Þ
xCH4 ;H2 O ¼ 3:0 1011 1:2561 108 J=kmol exp RT
5 mm
Fig. 3. Acetone fluorescence between burner exit and stagnation plate.
½CH4 1:0 ½H2 O1:0 ; ð10Þ xH2 ;1=2O2 ¼ 8:0 1017 T1 1:6748 108 J=kmol exp RT ½H2 0:25 ½O2 1:5 ; ð11Þ xH2 O ¼ 8:0 1017 T1 1:6748 108 J=kmol exp RT ½H2 O1:0 ½O2 1:0 ½H2 0:75 ;
4.2. Flame position movement with fuel concentration oscillation ð12Þ
xCO;H2 O ¼ 2:75 1012 0:8374 108 J=kmol exp RT ½CO1:0 ½H2 O1:0 ;
ð13Þ
xCO2 ;H2 ¼ 2:75 1012 0:8374 108 J=kmol exp RT ½CO2 1:0 ½H2 1:0 :
increases linearly. This means that dynamic effects appear in the fuel concentration oscillation when f > 15 Hz. Figure 4(b) shows the amplitude of the equivalence ratio oscillation upstream of the preheated zone as calculated by numerical simulation. A region of constant equivalence ratio amplitude indicates quasi-steadiness in each reaction mechanism. Above 15 Hz, the equivalence ratio amplitude attenuates, which indicates the effect of the unsteadiness on the fuel concentration oscillation.
ð14Þ
4. Results and discussion 4.1. Fuel concentration oscillation in the isothermal condition Figure 3 shows a typical image of the acetone UVIF. The intensity of the luminescent green region varies with fuel concentration. Figure 4(a) shows the variation in the amplitude of the intensity in the acetone UVIF at (r, y) = (0, 5). The green-level intensity amplitude increases following the oscillation characteristics shown in Eq. (2) up to 15 Hz. However, it decreases when f > 15 Hz, even though the theoretical oscillation of Eq. (2)
Figure 5 shows the flame position is a function of ui for f = 5 Hz. Since the flame position is slightly different for each cycle, the phase-averaged value across five periods is shown in Fig. 5. The flame position amplitude in the oscillating cases shows clear sinusoidal movement and increases with an increase in ui, while the flame position variation without concentration oscillation is negligibly small. This suggests that the effect of fuel concentration oscillation is clearly distinguished from that of velocity fluctuation. In Fig. 5, the dotted line shows the flame position at the lean flammability limit in a steady state; the variation when ui = 0.7 is also shown. When ui = 0.7, the estimated uA is 0.62–0.76 which exceeds the equivalence ratio of lean flammability in the steady condition, u = 0.65. It is interesting to note that the flame position in the ui = 0.7 case can momentarily exceed that of the lean flammability limit. This shows that dynamical motion of the flame extends the flammability limit, which agrees with findings by Sankaran and Im [10]. Figure 6 shows images of the flame shape at the minimum and maximum positions with ui = 0.7. These extrema show that the flame intensity at a location beyond the flammability limit is less than that far from the flammability limit. Flame beyond the flammability limit is weakened as occurs just before flame extinction, but it can be recovered as a result of flame oscillation. Flame beyond the flammability
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(a)
(b)
Fig. 4. Frequency characteristics of equivalence ratio: (a) green-level intensity amplitude by acetone UVIF and (b) numerical simulation using one-step and four-step reactions.
no-oscil A = 0.1 A = 0.2 A = 0.3 A = 0.7
Fig. 5. Phase average of flame position variation (5 Hz).
Fig. 6. Flame shape at two different equivalence ratios at 5 Hz (ui = 0.7).
limit is very weak and unstable and it is seemingly random whether such a flame will survive at ui = 0.7. The flame location returns much more quickly than suggested by the standard sinusoidal trajectory, as shown in Fig. 5. This sharp drop in flame position as it returns in the ui = 0.70 case agrees with the profile shown in Birbaud et al. [12], which indicates that the sharp drop is due to heat release from fuel concentration oscillation. A recovery from significant decrease in flame luminosity shown in Fig. 6 in the sharp drop of the flame location supports the significant heat release there. The temperature at the surface of
the stagnation plate on the burner axis is measured by a thermocouple. The surface temperature is 683 ± 3 K for all experimental conditions. This suggests some role of heat loss in the flammability limit. Surface temperature measurement also showed that the nearly constant temperature is probably due to the large heat capacity of the stagnation wall. This indicates that the effect of heat loss to the stagnation wall can be neglected when we discuss the dynamic motion of the flame, despite steady heat loss to the stagnation wall. Figure 7 shows the flame position amplitude as a function of frequency. The amplitude in the “oscil” case increases monotonically with an increase in f up to f = 5 Hz, but it decreases as f is further increased under all values of ui. The increase in the amplitude is due to the increase in the fuel concentration oscillation amplitude according to the oscillator characteristics (Fig. 4(a)). However, the attenuation of the amplitude for f > 5 Hz ought to exhibit effects of unsteadiness. Here, we introduce the Strouhal number, a dimensionless number related to the unsteadiness of flow. The Strouhal number is defined as the ratio between the characteristic time of flow d/vexit and the oscillation 1/(2pf), as shown in Eq. (15). d 1 2pfd St ¼ : ð15Þ ¼ vexit 2pf vexit The condition St = 1 corresponds to f = 6.06 Hz in the present experiment; this is shown as a dotted line in Fig. 7(a). This suggests that the variation in the flame position amplitude changes significantly according to whether the Strouhal number is less than or greater than unity. Figure 7(b) shows the same results from numerical simulation. In this case, it is interesting to note that the amplitude attenuates when St > 1, which is similar to experimental results. The flame position amplitude in the experiment is much less than that in the simulation because the actual equivalence ratio variation in the experiment is less than ui (Table 1).
H. Tomita et al. / Proceedings of the Combustion Institute 35 (2015) 989–997
(a)
A = 0.3
995
(b)
A = 0.2 A = 0.1
no-oscil
Fig. 7. Frequency characteristics of flame position amplitude: (a) experiment and (b) numerical simulation.
To clarify the relation between the fuel concentration oscillation and flame position oscillation, we normalize the flame position amplitude, dividing the flame position amplitude by the green-level intensity amplitude to remove the characteristic of the oscillator. The result is shown in Fig. 8. The normalized amplitude is constant at St < 1, indicating quasi-steadiness, and attenuates at St > 1, indicating unsteadiness. If the gradient of attenuation of the flame position amplitude is the same as that of the equivalence ratio attenuation, the calculated value in Fig. 8 should be unity, even when St > 1. However, the normalized amplitude in the present study decreases when St > 1. This indicates that the flame motion itself is influenced by the unsteadiness as well as attenuation of the fuel concentration oscillation shown in Fig. 4. A qualitatively similar attenuation is numerically predicted in the one-step and four-step models, although some quantitative differences are observed. The quantitative differences are due to uncertainty in the experiments and numerical simulations. In fact, we could not obtain the absolute value of the UVIF intensity and the flame motion was only a few millimeters in our experiments.
Fig. 8. Normalized flame position amplitudes from experiment and numerical simulation using one-step and four-step models.
For numerical simulations, it is not clear how applicable the reaction model is, and some boundary conditions do not completely agree with the experimental conditions. Further study is needed to obtain quantitative agreement between experiments and numerical simulations. 4.3. Dynamic response of flame position compared with steady case The dynamic response of the flame position at various oscillation frequencies is shown in Fig. 9. The horizontal axis “modified equivalence ratio” in Fig. 9(a) shows the relative value of the equivalence ratio with respect to the green level intensity of UVIF to remove the characteristic of the oscillator. Fig. 9(a) shows that the movement of the flame position forms a closed loop near the steady flame position in the clockwise direction. The gradient of closed loops at St < 1 is parallel to that of the steady flame position line in the experiment and in the numerical simulation using four-step models as shown in Fig. 9 (a) and (b), respectively. This loop is mainly due to the back support effect [25]. A schematic image of the back support effect is shown in Fig. 10. When the fuel concentration varies to leaner, the burnt gas temperature is higher for gas burned further downstream, as shown in Fig. 10(a). Thus, the heat is supplied from the burnt gas, and the flame is intensified. In contrast, the flame is weakened when the fuel concentration varies toward the stoichiometric condition, as shown in Fig. 10(b). As a result, the flame is located closer to the wall when the fuel concentration varies toward the stoichiometric condition. The closed loop is tilted toward the anti-clockwise direction when the Strouhal number exceeds unity. Similar tilting is reported in Ref. [10]. These results propose that the flame motion is affected by the temporal fuel concentration oscillation mainly thorough transport phenomena, such as a back support effect, even though the frequency is too low to change the flame structure.
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(a)
(b)
Fig. 9. Dynamic response of flame position compared with steady flame position: (a) experiment and (b) numerical simulation using a four-step model.
Fig. 10. Flame strengthening and weakening by the back support effect: (a) from stoichiometric to lean and (b) from lean to stoichiometric.
5. Conclusions
Acknowledgement
Laminar stagnating premixed flame with temporal equivalence ratio oscillation, that is, with temporal fuel concentration oscillation, was studied by both experiment and numerical simulation. The equivalence ratio oscillation was produced experimentally by using the novel oscillator that well suppresses velocity fluctuation. The fuel concentration oscillation was measured by acetone UVIF in the isothermal condition. The normalized amplitude of the flame location oscillation remained constant when St < 1 and attenuated when St > 1. This indicates that flame motion is influenced by the unsteadiness as well as the attenuation of the fuel concentration oscillation when St > 1 but achieves a quasi-steady state condition when St < 1. The flame motion became a closed loop by the effect of transport phenomena, such as a back support effect. Numerical simulations showed qualitatively similar behavior. These results suggest that flame motion is affected by the temporal equivalence ratio oscillation through transport phenomena, such as a back support effect, even though the temporal equivalence ratio oscillation does not change the flame structure.
A. R. M. R was supported by the Government of Malaysia.
References [1] K. Ishida, available at. [2] R. Matsuyama, M. Kobayashi, H. Ogata, A. Horikawa, Y. Kinoshita, ASME Turbo Expo 2012: Turbine Technical Conference and Exposition, (2): Combustion, Fuels and Emissions, Parts A and B, 2012 (No. GT2012-68272) 211–218. [3] Alan H. Epstein, Combust. Flame 159 (2012) 1791– 1792. [4] T. Lieuwen, Y. Neumeier, B.T. Zinn, Combust. Sci. Technol. 135 (1998) 193–211. [5] G.A. Richards, M. Janus, E.H. Robey, J. Propul. Power 15 (2) (1999) 232–240. [6] K.H. Yu, K.J. Wilson, J. Propul. Power 18 (1) (2002) 53–60. [7] S. Candel, Proc. Combust. Inst. 29 (2002) 1–28. [8] F.N. Egolfopoulos, C.S. Campbell, J. Fluid Mech. 318 (1996) 1–29. [9] R. Lauvergne, F.N. Egolfopoulos, Proc. Combust. Inst. 28 (2000) 1841–1850.
H. Tomita et al. / Proceedings of the Combustion Institute 35 (2015) 989–997 [10] R. Sankaran, H.G. Im, Proc. Combust. Inst. 29 (2002) 77–84. [11] J.H. Cho, T. Lieuwen, Combust. Flame 140 (2005) 116–129. [12] A.L. Birbaud, S. Ducruix, D. Durox, S. Candel, Combust. Flame 154 (2008) 356–367. [13] E.S. Richardson, V.E. Granet, A. Eyssartier, J.H. Chen, Combust. Theory Model. 14 (6) (2010) 775– 792. [14] Shreekrishna, S. Hemchandra, T. Lieuwen, Combust. Theory Model. 14 (5) (2010) 681–714. [15] S. Hemchandra, Combust. Flame 159 (2012) 3530– 3543. [16] M.R.A. Rahman, T. Yokomori, T. Ueda, J. Therm. Sci. Technol. 7 (1) (2012) 16–30. [17] A.R.M. Rosdzimin, T. Yokomori, T. Ueda, J. Therm. Sci. Technol. 8 (1) (2013) 28–43.
997
[18] R. Zhou, S. Hochgreb, Combust. Flame 160 (6) (2013) 1070–1082. [19] Y. Takahashi, Y. Suenaga, M. Kitano, M. Kudo, JSME Int. J. Ser. B 49 (4) (2006) 1307–1315. [20] H. Ax, P. Kutne, W. Meier, et al., Appl. Phys. B Lasers Optics 94 (4) (2009) 705–714. [21] Y. Suenaga, M. Kitano, Y. Takahashi, J. Therm. Sci. Technol. 5 (1) (2010) 124–134. [22] H. Schwarz, L. Zimmer, D. Durox, S. Candel, Exp. Fluids 49 (2010) 809–821. [23] T. Kang, D.C. Kyritsis, Proc. Combust. Inst. 31 (2007) 1075–1083. [24] S. Miyamae, A.R.M. Rosdzimin, H. Tomita, T. Yokomori, T. Ueda, Therm. Sci. Technol. (submitted for publication). [25] Y.M. Marzouk, A.F. Ghoniem, H.N. Najm, Proc. Combust. Inst. 28 (2000) 1859–1866.
Proceedings
ScienceDirect
of the
Combustion Institute
Proceedings of the Combustion Institute 35 (2015) 989–997
www.elsevier.com/locate/proci
Flame dynamics of equivalence ratio oscillations in a laminar stagnating lean methane/air premixed flame Hisashi Tomita a,⇑, Mohd Rosdzimin Abdul Rahman b, Sotaro Miyamae a, Takeshi Yokomori c, Toshihisa Ueda c b
a Graduate School of Science and Technology, Keio University, Japan Department of Mechanical Engineering, Faculty of Engineering, Universiti Pertahanan Nasional Malaysia, Malaysia c Department of Mechanical Engineering, Keio University, Japan
Available online 12 August 2014
Abstract This study investigates the effect of fuel concentration oscillation on laminar stagnating premixed flames by both experiment and numerical simulation. The numerical analysis is conducted using ANSYS Fluent 14.5. The equivalence ratio oscillation in the experiments is formed by a novel oscillator with two cylinder piston units that can produce alternating ejections of leaner and richer pre-mixtures. Velocity fluctuation is well suppressed by installing screens on the burner exit. The fuel concentration oscillation between the stagnation plate and the burner exit is visualized and analyzed by acetone ultraviolet light-induced fluorescence in the isothermal condition. The oscillator frequency is varied in the range 2–20 Hz, and the oscillation wavelength is much longer than the flame thickness. The flame oscillates with the fuel concentration, and in the experiment, the amplitude of the flame oscillation attenuates as the frequency of fuel concentration oscillation increases above 5 Hz, which corresponds to a Strouhal number of unity. This indicates that the Strouhal number distinguishes quasi-steadiness for St < 1 and unsteadiness for St > 1. The flame oscillation pattern is a closed loop, which might be attributable to variation of the back support effect on the flame. The numerical results show a similar trend for the flame response to oscillations in fuel concentration. This study finds the flame motion is significantly affected by fuel concentration oscillations, even at low frequencies; in other words, the oscillation wavelength is much longer than the flame thickness, as a result of the back support effect. Ó 2014 The Combustion Institute. Published by Elsevier Inc. All rights reserved. Keywords: Flame dynamics; Equivalence ratio oscillation; Laminar premixed flame; Stagnating flame; Acetone ultraviolet light-induced fluorescence
⇑ Corresponding author. Address: Department of
Mechanical Engineering, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan. Fax: +81 45 566 1495. E-mail address: [email protected] (H. Tomita).
1. Introduction Lean premixed combustion is used in power plants and factories to reduce NOx emissions [1,2]. However, operating a lean premixed combustor has led to some technical problems that have not been solved yet, such as combustion
http://dx.doi.org/10.1016/j.proci.2014.07.061 1540-7489/Ó 2014 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
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H. Tomita et al. / Proceedings of the Combustion Institute 35 (2015) 989–997
Nomenclature A d f L qleaner qprimary qricher qtotal r R St t T Tper vexit
area of piston cross section distance between burner exit and stagnation plate frequency of piston movement half stroke of piston movement flow rate of leaner secondary flow flow rate of primary flow flow rate of richer secondary flow total flow rate coordinate axis of burner diameter direction universal gas constant Strouhal number time temperature period burner exit velocity
instability. Resolving combustion instability is a key goal of current combustion science. The relations of combustion instability to combustor geometry, operating conditions, and fuel have not been clarified yet [3]. Lieuwen et al. [4] showed that reactant unmixedness could drive strong instabilities in a lean premixed combustor. Combustion instability occurs when the pressure oscillation is in phase with the heat release oscillation; this induces poor performance of the combustor to the extent that it may be damaged. Richard et al. [5] and Yu and Wilson [6] show that combustion instability in gas turbine combustors can be reduced by adjusting the fuel injection into the combustion chamber. Candel [7] has studied relevant topics in combustion dynamics, including the mechanisms that drive instabilities in premixed combustion and the coupling between pressure waves and combustion. Those results are applicable in solving the problem of instability in modern low NOx heavy-duty gas turbine combustors. Less effort has been devoted to studying the dynamics of the flame response to the equivalence ratio oscillation [7] because the characteristic timescale of the equivalence ratio oscillation can couple with the timescale of convection, diffusion, and reaction [8]. Most studies on laminar premixed flame dynamics regarding equivalence ratio oscillation have been conducted numerically [9– 18], allowing an ideal equivalence ratio variation to be obtained. In contrast, few studies on this topic have been conducted experimentally [19– 22] because of difficulties in suppressing velocity perturbation in an actual flow field. A quasisteady state and attenuation of the flame response at low and very high oscillation frequencies were observed numerically, as was extension of the lean flammability limit [9,10]. Birbaud et al. [12]
y u uA ui uleaner um uprimary uricher x
coordinate axis along burner center line equivalence ratio amplitude of equivalence ratio at the burner exit initial difference between the equivalence ratios of secondary flows leaner (ui ¼ uricher u ) 2 initial equivalence ratio of leaner secondary flow mean equivalence ratio initial equivalence ratio of primary flow initial equivalence ratio of richer secondary flow angular velocity
numerically demonstrated that the non-linear heat release response related to equivalence ratio oscillation is due to rapid burning of the fresh mixture in the pocket. Moreover, Rahman et al. [16] numerically observed deviation of the flame motion from a steady state and hysteresis of the laminar premixed flames in response to equivalence ratio oscillations. Unfortunately, no reliable experimental results are available to support these numerical findings. Movement of a conical flame tip in response to equivalence ratio oscillations was experimentally observed by Kutne et al. [20] in a low-pressure environment (69 mbar) and found to be significantly influenced by velocity perturbation by Schwarz et al. [22]. Moreover, Takahashi et al. [19] and Suenaga et al. [21] experimentally observed variations in the oscillation amplitudes of burning velocity, burnt gas temperature, burnt gas composition, and flame luminosity with increasing equivalence ratio oscillation frequency in the cylindrical flame configuration. Those observations are difficult to replicate in numerical work because of the complexity of the geometry. In contrast to the case with equivalence ratio oscillation, the stratified case has been investigated experimentally by in many studies, and the back support effect has been found to influence the deviation of flame propagation speed from uniformity [23]. To address the dearth of experimental work on the response of laminar premixed flame to equivalence ratio oscillations, more reliable and simple flame configurations, such as a stagnating flame configuration, are required for validation of numerical work. Accordingly, the aim of this study was to quantify the dynamics of laminar lean methane/air premixed flame subject to equivalence ratio oscillation by suppressing velocity perturbation at ambient pressure.
H. Tomita et al. / Proceedings of the Combustion Institute 35 (2015) 989–997
2. Experimental setup and procedure 2.1. Characteristics of burner and oscillator Figure 1 shows the flow system used in the present study. The flow system consists of a primary flow with constant concentration and two secondary flows with fuel concentration oscillations produced by a novel oscillator. The equivalence ratios of the primary flow, richer secondary flow, and leaner secondary flow are uprimary, uricher, and uleaner, respectively. The flows are carried by individual flow lines. The primary flow is supplied directly to the burner. The richer and leaner flows are supplied to the oscillator. In the oscillator, one piston for the richer mixture and another piston for the leaner mixture move in antiphase, resulting in a mixture with sinusoidally varying equivalence ratio. This mixture is supplied to the burner. A laminar methane/air premixed flame in stagnation flow configuration is created by the burner as shown in Fig. 2. The primary flow passes through a converging nozzle. The secondary flows with sinusoidal equivalence ratio
Secondary flow
MFC (Richer air)
Needle valve
Burner
MFC (Richer CH4)
Air compressor
MFC (Leaner air)
CH4 cylinder
MFC (Leaner CH4)
richer
Oscillator
leaner
MFC (Air)
MFC: Mass Flow Controller
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oscillations are supplied to the primary flow via 16 exhaust holes, as shown in detail in Fig. 2(b), and mix with the primary flow. As a result, a mixture with sinusoidal equivalence ratio oscillation, formulized in Eq. (1), is created. u ¼ uprimary þ ui ðqricher;Air qleaner;Air Þ=qtotal;Air n þ 2pf ðAL sinð2pftÞÞricher;Air o i ðAL sin ð2pft pÞÞleaner;Air =qtotal;Air : ð1Þ Because the two pistons are in antiphase, the total flow rate of the primary and secondary flows is given by qtotal ¼ qprimary þ ½qricher þ 2pfAL sin ð2pftÞ þ ½qleaner þ 2pfAL sinð2pft pÞ:
ð2Þ
This indicates that the total flow rate from the burner is kept theoretically constant, but the equivalence ratio oscillates sinusoidally according to Eq. (1). Additionally, Eq. (2) indicates that the amplitude of flow rate fluctuation increases with frequency. In the actual system, however, a small velocity fluctuation is observed because the pistons do not move exactly sinusoidally due to the crank mechanism. The influence of velocity fluctuation on the flame oscillation is well suppressed relative to the fuel concentration oscillation by inserting two 100-mesh screens on the burner exit. Two honeycombs are also installed, one at the upstream side of the nozzle and the other at burner exit, to obtain a uniform profile flow at the burner exit. The coordinate system is defined as shown in Fig. 2(a). 2.2. Experimental conditions
Primary flow
MFC (CH4)
Table 1 shows the experimental conditions used in the present study. The primary equivalence
Fig. 1. Flow channel.
Stagnation plate Co-flow (N2)
y r
21
Flame
20
Screens
Richer premixed flow
Leaner premixed flow
Honeycomb Concentration oscillation
Secondary flow
Honeycomb Primary flow
(a)
(b)
Fig. 2. Stagnation flame burner: (a) cross section and (b) fuel concentration supply system.
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Table 1 Experimental conditions of equivalence ratio. Case No-oscil. (ui = 0) Oscil. (ui = 0.1) Oscil. (ui = 0.2) Oscil. (ui = 0.3)
Primary
Secondary
uprimary
uricher
uleaner
uA (5 Hz)
0.70 0.70 0.70 0.70
0.70 0.80 0.90 1.0
0.70 0.60 0.50 0.40
0.70 0.69–0.71 0.68–0.72 0.67–0.73
ratio is set to 0.70. In the case with no oscillation (“no-oscil.” in the table), a mixture with equivalence ratio u = 0.70 is issued from both the primary and secondary flows with the oscillator in operation. The influence of the velocity fluctuation induced by the oscillator is estimated from this operating condition. In the case with oscillation (“oscil.” in the table), the fuel concentration oscillates as a function of the initial difference between the equivalence ratio of the secondary flows ui and the frequency f, as mentioned in Eq. (1). The amplitude of fuel concentration oscillation at the burner exit in the case of f = 5 Hz is estimated as uA in Table 1. The mean velocity at the burner exit is fixed at 0.8 m/s. The distance between the stagnation plate and the burner exit is 21 mm. The oscillator frequency is varied in the range 2–20 Hz. In the present study, we focus on very low frequencies to investigate the flame dynamics of equivalence ratio oscillation without changing the flame structure. 2.3. Experimental procedure Fuel concentration oscillation between the burner exit and stagnation plate was visualized by acetone ultraviolet light-induced fluorescence (UVIF) in an isothermal condition, using air for the primary and secondary flows at the same flow rates as in the combustion case. Acetone is mixed on one side of the secondary flow by an acetone seeder. The light sheet from the ultraviolet lamp (Hamamatsu Photonics K.K., LC8), which has a peak intensity at 320 nm, is irradiated to the region between the burner and the stagnation plate to excite the acetone. The intensity of luminescence from the acetone is detected by a high-speed video camera (Keyence Corp., VW-6000) with an image intensifier. Because the concentration oscillation is produced by alternately mixing leaner and richer flows from the oscillator with the primary flow, the distribution of the luminescence intensity shows the relative fuel concentration. The flame position movement is measured to elucidate the effect of fuel concentration oscillation on the flame dynamics. The flame position is measured by the high-speed video camera. We define the flame position as the minimum y position of the blue flame zone along the centerline
Estimated
axis. Mathematica 9 (Wolfram, Inc.) is used to determine the flame region (to a certain tolerance) and to measure flame positions by analyzing the optical image. 3. Numerical analysis 3.1. Numerical method and chemical reaction mechanism Numerical analysis was performed with ANSYS Fluent 14.5. The flow field is modeled after the experimental geometry shown in Fig. 2. Details of the numerical method, computational domain, boundary conditions, and reaction models are given in Miyamae et al. [24]. The equivalence ratio with variation is given by u = um + uA sin xt. The condition on the equivalence ratio is set to um = 0.70 and uA = 0.10. The inlet velocity is kept constant, while the inlet density is varied as a function of equivalence ratio. The density variation is itself a source of equivalence ratio oscillation. Two chemical reaction models are applied; a one-step overall reaction model and a four-step reaction model that includes CO and H2. These models and the associated reaction rates are shown below. The one-step overall reaction model is CH4 þ 2O2 ! CO2 þ 2H2 O:
ð3Þ
The following Arrhenius-type reaction rate model of a one-step overall reaction mechanism is used: xCH4 ¼ 5:6 1011 2:027 108 J=kmol exp RT ½CH4 0:2 ½O2 1:3 :
ð4Þ
The four-step reaction model is CH4 þ 1=2O2 ! CO þ 2H2 ;
ð5Þ
CH4 þ H2 O ! CO þ 3H2 ;
ð6Þ
H2 þ 1=2O2 $ H2 O;
ð7Þ
CO þ H2 O $ CO2 þ H2 :
ð8Þ
H. Tomita et al. / Proceedings of the Combustion Institute 35 (2015) 989–997
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The following Arrhenius-type reaction rate models are used: xCH4 ;12O2 ¼ 7:824 1013 1:2561 108 J=kmol exp RT ½CH4 0:5 ½O2 1:25 ;
ð9Þ
xCH4 ;H2 O ¼ 3:0 1011 1:2561 108 J=kmol exp RT
5 mm
Fig. 3. Acetone fluorescence between burner exit and stagnation plate.
½CH4 1:0 ½H2 O1:0 ; ð10Þ xH2 ;1=2O2 ¼ 8:0 1017 T1 1:6748 108 J=kmol exp RT ½H2 0:25 ½O2 1:5 ; ð11Þ xH2 O ¼ 8:0 1017 T1 1:6748 108 J=kmol exp RT ½H2 O1:0 ½O2 1:0 ½H2 0:75 ;
4.2. Flame position movement with fuel concentration oscillation ð12Þ
xCO;H2 O ¼ 2:75 1012 0:8374 108 J=kmol exp RT ½CO1:0 ½H2 O1:0 ;
ð13Þ
xCO2 ;H2 ¼ 2:75 1012 0:8374 108 J=kmol exp RT ½CO2 1:0 ½H2 1:0 :
increases linearly. This means that dynamic effects appear in the fuel concentration oscillation when f > 15 Hz. Figure 4(b) shows the amplitude of the equivalence ratio oscillation upstream of the preheated zone as calculated by numerical simulation. A region of constant equivalence ratio amplitude indicates quasi-steadiness in each reaction mechanism. Above 15 Hz, the equivalence ratio amplitude attenuates, which indicates the effect of the unsteadiness on the fuel concentration oscillation.
ð14Þ
4. Results and discussion 4.1. Fuel concentration oscillation in the isothermal condition Figure 3 shows a typical image of the acetone UVIF. The intensity of the luminescent green region varies with fuel concentration. Figure 4(a) shows the variation in the amplitude of the intensity in the acetone UVIF at (r, y) = (0, 5). The green-level intensity amplitude increases following the oscillation characteristics shown in Eq. (2) up to 15 Hz. However, it decreases when f > 15 Hz, even though the theoretical oscillation of Eq. (2)
Figure 5 shows the flame position is a function of ui for f = 5 Hz. Since the flame position is slightly different for each cycle, the phase-averaged value across five periods is shown in Fig. 5. The flame position amplitude in the oscillating cases shows clear sinusoidal movement and increases with an increase in ui, while the flame position variation without concentration oscillation is negligibly small. This suggests that the effect of fuel concentration oscillation is clearly distinguished from that of velocity fluctuation. In Fig. 5, the dotted line shows the flame position at the lean flammability limit in a steady state; the variation when ui = 0.7 is also shown. When ui = 0.7, the estimated uA is 0.62–0.76 which exceeds the equivalence ratio of lean flammability in the steady condition, u = 0.65. It is interesting to note that the flame position in the ui = 0.7 case can momentarily exceed that of the lean flammability limit. This shows that dynamical motion of the flame extends the flammability limit, which agrees with findings by Sankaran and Im [10]. Figure 6 shows images of the flame shape at the minimum and maximum positions with ui = 0.7. These extrema show that the flame intensity at a location beyond the flammability limit is less than that far from the flammability limit. Flame beyond the flammability limit is weakened as occurs just before flame extinction, but it can be recovered as a result of flame oscillation. Flame beyond the flammability
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H. Tomita et al. / Proceedings of the Combustion Institute 35 (2015) 989–997
(a)
(b)
Fig. 4. Frequency characteristics of equivalence ratio: (a) green-level intensity amplitude by acetone UVIF and (b) numerical simulation using one-step and four-step reactions.
no-oscil A = 0.1 A = 0.2 A = 0.3 A = 0.7
Fig. 5. Phase average of flame position variation (5 Hz).
Fig. 6. Flame shape at two different equivalence ratios at 5 Hz (ui = 0.7).
limit is very weak and unstable and it is seemingly random whether such a flame will survive at ui = 0.7. The flame location returns much more quickly than suggested by the standard sinusoidal trajectory, as shown in Fig. 5. This sharp drop in flame position as it returns in the ui = 0.70 case agrees with the profile shown in Birbaud et al. [12], which indicates that the sharp drop is due to heat release from fuel concentration oscillation. A recovery from significant decrease in flame luminosity shown in Fig. 6 in the sharp drop of the flame location supports the significant heat release there. The temperature at the surface of
the stagnation plate on the burner axis is measured by a thermocouple. The surface temperature is 683 ± 3 K for all experimental conditions. This suggests some role of heat loss in the flammability limit. Surface temperature measurement also showed that the nearly constant temperature is probably due to the large heat capacity of the stagnation wall. This indicates that the effect of heat loss to the stagnation wall can be neglected when we discuss the dynamic motion of the flame, despite steady heat loss to the stagnation wall. Figure 7 shows the flame position amplitude as a function of frequency. The amplitude in the “oscil” case increases monotonically with an increase in f up to f = 5 Hz, but it decreases as f is further increased under all values of ui. The increase in the amplitude is due to the increase in the fuel concentration oscillation amplitude according to the oscillator characteristics (Fig. 4(a)). However, the attenuation of the amplitude for f > 5 Hz ought to exhibit effects of unsteadiness. Here, we introduce the Strouhal number, a dimensionless number related to the unsteadiness of flow. The Strouhal number is defined as the ratio between the characteristic time of flow d/vexit and the oscillation 1/(2pf), as shown in Eq. (15). d 1 2pfd St ¼ : ð15Þ ¼ vexit 2pf vexit The condition St = 1 corresponds to f = 6.06 Hz in the present experiment; this is shown as a dotted line in Fig. 7(a). This suggests that the variation in the flame position amplitude changes significantly according to whether the Strouhal number is less than or greater than unity. Figure 7(b) shows the same results from numerical simulation. In this case, it is interesting to note that the amplitude attenuates when St > 1, which is similar to experimental results. The flame position amplitude in the experiment is much less than that in the simulation because the actual equivalence ratio variation in the experiment is less than ui (Table 1).
H. Tomita et al. / Proceedings of the Combustion Institute 35 (2015) 989–997
(a)
A = 0.3
995
(b)
A = 0.2 A = 0.1
no-oscil
Fig. 7. Frequency characteristics of flame position amplitude: (a) experiment and (b) numerical simulation.
To clarify the relation between the fuel concentration oscillation and flame position oscillation, we normalize the flame position amplitude, dividing the flame position amplitude by the green-level intensity amplitude to remove the characteristic of the oscillator. The result is shown in Fig. 8. The normalized amplitude is constant at St < 1, indicating quasi-steadiness, and attenuates at St > 1, indicating unsteadiness. If the gradient of attenuation of the flame position amplitude is the same as that of the equivalence ratio attenuation, the calculated value in Fig. 8 should be unity, even when St > 1. However, the normalized amplitude in the present study decreases when St > 1. This indicates that the flame motion itself is influenced by the unsteadiness as well as attenuation of the fuel concentration oscillation shown in Fig. 4. A qualitatively similar attenuation is numerically predicted in the one-step and four-step models, although some quantitative differences are observed. The quantitative differences are due to uncertainty in the experiments and numerical simulations. In fact, we could not obtain the absolute value of the UVIF intensity and the flame motion was only a few millimeters in our experiments.
Fig. 8. Normalized flame position amplitudes from experiment and numerical simulation using one-step and four-step models.
For numerical simulations, it is not clear how applicable the reaction model is, and some boundary conditions do not completely agree with the experimental conditions. Further study is needed to obtain quantitative agreement between experiments and numerical simulations. 4.3. Dynamic response of flame position compared with steady case The dynamic response of the flame position at various oscillation frequencies is shown in Fig. 9. The horizontal axis “modified equivalence ratio” in Fig. 9(a) shows the relative value of the equivalence ratio with respect to the green level intensity of UVIF to remove the characteristic of the oscillator. Fig. 9(a) shows that the movement of the flame position forms a closed loop near the steady flame position in the clockwise direction. The gradient of closed loops at St < 1 is parallel to that of the steady flame position line in the experiment and in the numerical simulation using four-step models as shown in Fig. 9 (a) and (b), respectively. This loop is mainly due to the back support effect [25]. A schematic image of the back support effect is shown in Fig. 10. When the fuel concentration varies to leaner, the burnt gas temperature is higher for gas burned further downstream, as shown in Fig. 10(a). Thus, the heat is supplied from the burnt gas, and the flame is intensified. In contrast, the flame is weakened when the fuel concentration varies toward the stoichiometric condition, as shown in Fig. 10(b). As a result, the flame is located closer to the wall when the fuel concentration varies toward the stoichiometric condition. The closed loop is tilted toward the anti-clockwise direction when the Strouhal number exceeds unity. Similar tilting is reported in Ref. [10]. These results propose that the flame motion is affected by the temporal fuel concentration oscillation mainly thorough transport phenomena, such as a back support effect, even though the frequency is too low to change the flame structure.
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H. Tomita et al. / Proceedings of the Combustion Institute 35 (2015) 989–997
(a)
(b)
Fig. 9. Dynamic response of flame position compared with steady flame position: (a) experiment and (b) numerical simulation using a four-step model.
Fig. 10. Flame strengthening and weakening by the back support effect: (a) from stoichiometric to lean and (b) from lean to stoichiometric.
5. Conclusions
Acknowledgement
Laminar stagnating premixed flame with temporal equivalence ratio oscillation, that is, with temporal fuel concentration oscillation, was studied by both experiment and numerical simulation. The equivalence ratio oscillation was produced experimentally by using the novel oscillator that well suppresses velocity fluctuation. The fuel concentration oscillation was measured by acetone UVIF in the isothermal condition. The normalized amplitude of the flame location oscillation remained constant when St < 1 and attenuated when St > 1. This indicates that flame motion is influenced by the unsteadiness as well as the attenuation of the fuel concentration oscillation when St > 1 but achieves a quasi-steady state condition when St < 1. The flame motion became a closed loop by the effect of transport phenomena, such as a back support effect. Numerical simulations showed qualitatively similar behavior. These results suggest that flame motion is affected by the temporal equivalence ratio oscillation through transport phenomena, such as a back support effect, even though the temporal equivalence ratio oscillation does not change the flame structure.
A. R. M. R was supported by the Government of Malaysia.
References [1] K. Ishida, available at
H. Tomita et al. / Proceedings of the Combustion Institute 35 (2015) 989–997 [10] R. Sankaran, H.G. Im, Proc. Combust. Inst. 29 (2002) 77–84. [11] J.H. Cho, T. Lieuwen, Combust. Flame 140 (2005) 116–129. [12] A.L. Birbaud, S. Ducruix, D. Durox, S. Candel, Combust. Flame 154 (2008) 356–367. [13] E.S. Richardson, V.E. Granet, A. Eyssartier, J.H. Chen, Combust. Theory Model. 14 (6) (2010) 775– 792. [14] Shreekrishna, S. Hemchandra, T. Lieuwen, Combust. Theory Model. 14 (5) (2010) 681–714. [15] S. Hemchandra, Combust. Flame 159 (2012) 3530– 3543. [16] M.R.A. Rahman, T. Yokomori, T. Ueda, J. Therm. Sci. Technol. 7 (1) (2012) 16–30. [17] A.R.M. Rosdzimin, T. Yokomori, T. Ueda, J. Therm. Sci. Technol. 8 (1) (2013) 28–43.
997
[18] R. Zhou, S. Hochgreb, Combust. Flame 160 (6) (2013) 1070–1082. [19] Y. Takahashi, Y. Suenaga, M. Kitano, M. Kudo, JSME Int. J. Ser. B 49 (4) (2006) 1307–1315. [20] H. Ax, P. Kutne, W. Meier, et al., Appl. Phys. B Lasers Optics 94 (4) (2009) 705–714. [21] Y. Suenaga, M. Kitano, Y. Takahashi, J. Therm. Sci. Technol. 5 (1) (2010) 124–134. [22] H. Schwarz, L. Zimmer, D. Durox, S. Candel, Exp. Fluids 49 (2010) 809–821. [23] T. Kang, D.C. Kyritsis, Proc. Combust. Inst. 31 (2007) 1075–1083. [24] S. Miyamae, A.R.M. Rosdzimin, H. Tomita, T. Yokomori, T. Ueda, Therm. Sci. Technol. (submitted for publication). [25] Y.M. Marzouk, A.F. Ghoniem, H.N. Najm, Proc. Combust. Inst. 28 (2000) 1859–1866.