Behavior of partially premixed flame fronts excited with a fuel tube resonance frequency

Behavior of partially premixed flame fronts excited with a fuel tube resonance frequency

Combustion and Flame 139 (2004) 351–357 www.elsevier.com/locate/jnlabr/cnf Short Communication Behavior of partially premixed flame fronts excited w...

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Combustion and Flame 139 (2004) 351–357 www.elsevier.com/locate/jnlabr/cnf

Short Communication

Behavior of partially premixed flame fronts excited with a fuel tube resonance frequency Seung-Gon Kim a , Kang-Tae Kim b , Jeong Park b,∗ , Chang Bo Oh c a Institute of Fluid Science, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan b School of Mechanical & Automotive Engineering, Sunchon National University, 315 Meagok-dong,

Sunchon, Jeonnam 540-742, South Korea c School of Mechanical Engineering, Inha University, 253, Yonghyun-dong, Nam-gu, Incheon 402-751, South Korea

Received 9 July 2003; received in revised form 15 September 2004; accepted 23 September 2004 Available online 28 October 2004

1. Introduction In laminar lifted nonpremixed flames the flame base has been verified to be anchored through the characteristics of a triple flame [1–6]. Müller et al. [7] combined the formulation on laminar stretched flamelets of premixed and nonpremixed flames to treat partially premixed combustion. With this background more recent research showed from simultaneous visualization of CH-PLIF and PIV that the flame base, even in turbulent nonpremixed lifted flames, should be stabilized through the characteristics of edge or triple flames [8]. Meanwhile, the displayed photographs in the previous experimental study [3] showed clearly that the flame base was anchored via the characteristics of a triple flame at moderate nozzle exit velocities. The trailing diffusion flame became gradually smaller with increasing nozzle exit velocity, and finally near the flame blow-out limit the whole flame was changed into a flat flame, which would not presume a triple flame but is typical of a premixed flame. Hence, considerable concern may be given to the flat flame near flame blow-out, even if the authors did not address the flat flame itself. In nonpremixed lifted jet flames [9–11], the anchoring mechanism of the flame base was well described as an edge flame near the nozzle exit, a triple flame at the moderate axial * Corresponding author. Fax: +82-61-750-3530.

E-mail address: [email protected] (J. Park).

position, and then a weakly varying partially premixed flame near flame blow-out. There may be a room to argue whether weakly varying partially premixed flames, which may be intuitively grouped into a distributed reaction regime or a well-stirred reactor regime, belong to the flamelet regime. It would be, in the present state of understanding, very difficult to clarify this point. Studies on flame holes [12] might suggest the modification of extinction strain rate obtained in steady counterflow diffusion flame, but it seems likely that partially premixed flames appearing from a flame hole cannot explain the behavior of weakly varying partially premixed flame near flame blow-out. Thus much more experimental research is needed to understand the characteristics of weakly varying partially premixed flame. From this point of view, research efforts may be directed toward lifted nonpremixed jet flames excited with a fuel tube resonance frequency because the application of these techniques has been shown to dramatically change concentration fields, a phenomenon termed “collapsible mixing” [13–16]. It was also noted in an extended study [14] that acoustic forcing with a fuel tube resonance frequency did not make a jet azimuthal structure form far downstream but just at the fuel tube exit, and this could affect the mixing of fuel and oxidizer considerably with increasing forcing strength. In the present study various laminar lifted jet flame base topologies are experimentally examined through the introduction of acoustic forcing with a

0010-2180/$ – see front matter  2004 The Combustion Institute. Published by Elsevier Inc. All rights reserved. doi:10.1016/j.combustflame.2004.09.007

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Fig. 1. Schematic of burner and flow system.

fuel tube resonance frequency. Flame lift-off stability curves are displayed in terms of fuel tube exit velocity and forcing strength for various N2 -diluted flames. Representative flame base topologies in categorized regimes are described. Special concern is focused on the characteristics of weakly varying partially premixed flame that deviate from those of a typical triple flame.

2. Experimental setup The experimental setup shown in Fig. 1 can be found elsewhere in detail [16]. The gases are commercial-grade propane of 99.5% purity as fuel and nitrogen of 99.95% purity as diluent. The forcing frequency is tuned to a resonant frequency of the fuel tube (159 Hz) to maximize the forcing strength. A microphone is attached at the top of the plenum chamber to check the level of acoustic pressure for reproducible experiments irrespective of ambient condition. The forcing amplitude (Uf ), which is defined by the peak-to-peak magnitude of the periodic velocity component and measured at the nozzle exit, is well correlated with the acoustic pressure level according to flow velocity, as was shown in previous research [14]. Hence a nondimensional forcing strength (Uf /Uo ) is expressed as a ratio of the peakto-peak magnitude of the periodic velocity component (Uf ) to the mean exit velocity (Uo ). Axial velocity data, in which the resonant frequency of the fuel tube is 172 Hz for air, are obtained using a single hotwire probe and a constant-temperature anemometer with some assumptions on cold air flow [14]. Turbulent lifted flames diluted with nitrogen are also taken for the comparison between a typical triple flame and a weakly varying partially premixed flame. The Reynolds numbers based on fuel tube diameter

are 5337 and 9073, respectively, and the latter corresponds to that of flame blow-out. The concentration just upstream of the lifted flame front is measured using gas chromatography.

3. Results and discussion Fig. 2 shows representative lifted laminar jet flames excited with a tube resonant frequency taken at a shutter speed of 1/1000 s. The flame base for a small forcing strength, as shown in Fig. 2a, is regarded as an edge flame. The flame base for a relatively large forcing strength in Fig. 2b is characterized by dramatic reduction of flame length, wide jet width, and a highly distorted flame base caused by complicated interactions with inner vortex motions. This phenomenon, termed “collapsible mixing” [13–16], was attributed to the periodic repeat of the instantaneous velocity, which was already amplified and thus negative at a certain phase. It was, meanwhile, recognized even in laminar jet flames that the introduction of acoustic excitation with a fuel tube resonance frequency did not cause azimuthal structure to be formed far downstream but just at the fuel tube exit [14], and this could affect the mixing of fuel and oxidizer considerably, even near the fuel tube exit [13,15,16]. As a result, the lifted flame base is affected by a variety of forcing strengths from strong levels of collapsible mixing to mild levels of well-organized vortex motion. These flame lift-off behaviors are shown according to nozzle exit velocity and forcing strength for various N2 -diluted flames in Fig. 3. Flame lift-off behavior can be grouped into three categories; a regime in which a large forcing strength is required for flame lift-off (regime I), a regime in which the flame lifts without forcing (regime III), and a regime in which

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Fig. 2. Representative lifted flames excited with a tube resonant frequency taken at a shutter speed of 1/1000 s: (a) a typical edge flame and (b) a collapsible-mixing flame.

Fig. 3. Flame lift-off behavior according to nozzle exit velocity and forcing strength for various N2 -diluted flames.

the flame lifts at a moderate forcing and appears to have transient characteristics between the other two regimes (regime II). For the case of pure propane the flames with nozzle exit velocities of more than 2.1 m/s were excluded since they belong to transition or turbulent flames, and the flames with nozzle exit velocities less than 0.34 m/s were not lifted within the measurable range of microphone. It is also shown in Fig. 3 that the nozzle exit velocity, at which flame lifts without forcing, decreases with increasing nitrogen volume fraction. This is natural because the concentration of reactive species decreases with increasing nitrogen volume fraction. In cases of nitrogen volume fraction of more than 80% all the flames lift without forcing. The indicated blow-out in Fig. 3 means that

the flame blows out as soon as it lifts at the condition of nozzle exit velocity of 0.25 m/s and nitrogen volume fraction of 60%. In general, lifted flame base may be regarded as an edge flame, a triple flame, and finally a weakly varying partially premixed flame depending on the lift-off height [9–11]. A comparison between the present result and the results of Oh and Shin [14] may raise a couple of strong questions. Is the flame base, which is blown out near the nozzle exit through strong forcing with the fuel tube resonance frequency, an edge flame or a weakly varying partially premixed flame? This may be seen through comparison of partially premixed flames acoustically excited with a fuel tube resonance frequency.

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(a)

(b)

Fig. 4. (a) Representative direct photo in regime 3 and (b) variation of mixture fraction just below the lifted flame base with radial distance.

It was shown in the previous research [11] that the tip flames were well described in terms of an edge-flame aspect ratio which is expressed by the ratio between a diffusion length measured along the stoichiometric surface (premixed kernel) and a diffusion length across the stoichiometric surface (diffusion flame). For simplicity, with the assumption of constant thermal thickness of the premixed kernel, the edge-flame aspect ratio, f is directly related to the thickness of the mixing layer, δM [6,11]:   ∂ξ f ∼ 1/δM ∼ (1) . ∂r atξst In the case of no acoustic forcing, the concentration just upstream of the lifted flame front is measured using gas chromatography, and thereby the radial location of the stoichiometric mixture fraction can be attained. In the case of acoustic forcing, the position of the stoichiometric mixture fraction, which was defined by an instantaneous visible flame location, was obtained with 2000 images taken at a shutter time of 1/1000 s with a ICCD camera. Then the difference between both the locations, rd = rforcing,st − rnonforcing,st , is attributed by the enhanced mixing due to acoustic forcing. The increase in rd physically implies the enhancement of mixing and thereby the reduction of the radial gradient of mixture fraction. Then according to Eq. (1) the edge-flame aspect ratio can be expressed as f ∼ 1/rd . This implies physically that extreme increase of rd with a large forcing using a fuel tube resonance may induce f  1. The previous results [11] clarified that for f  1 a weakly varying partially premixed flame is appeared and strongly burning trailing diffusion flames cannot be developed. The following mixture fraction [9] was used for the

evaluation of the mixture fraction gradient: ξ=

γ YF − YO + YO,∞ . γ YF,j − YO,j + YO,∞

(2)

Fig. 4 shows (a) a direct photo of a typical partially premixed flame in regime 3 and (b) the variation of mixture fraction just below the lifted flame base with radial distance. The direct photos in Figs. 4 to 7 were taken at a shutter time of 1/30 s with F5.6. The condition in Fig. 4a was Uo = 0.75 m/s without forcing for 25% C3 H8 /75% N2 flame. The mixture fraction was measured using gas chromatography on the basis of cold flow. The flame locations with acoustic forcing in Figs. 4 to 7 were measured with 2000 images taken at a shutter time of 1/1000 s with an ICCD camera (Princeton Instrument) and the maximum deviated error was less than 4.7%. Here the effect of axial movement was neglected in the estimation of the measurement error. In Fig. 4b the flame location with acoustic forcing approximately corresponds to the measured stoichiometric line, and this implies that the flame base behaves as a triple flame. Fig. 5 shows (a) the direct photo at the condition of Uo = 0.4 m/s and Uf /Uo = 3.904 for 40% C3 H8 /60% N2 flame in regime 1 and (b) the variation of mixture fraction just below the lifted flame base with radial distance for various conditions. As shown in Fig. 5b, the flame locations in the flames of regime 1 are quite different from those of stoichiometric mixture fractions without acoustic forcing. Furthermore, the rd increases with forcing strength, and thereby the gradient of mixture fraction may decrease. Then extreme reduction of the mixture fraction gradient may lead to the reduction of flame strength due to

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Fig. 5. (a) Representative direct photo in regime 1 and (b) variation of mixture fraction just below the lifted flame base with radial distance.

(a)

(b)

(c) Fig. 6. Representative direct and Schlieren images in regime 2: 40% C3 H8 /60% N2 , Uo = 1.5 m/s, (a) Uf /Uo = 0.88, (b) Uf /Uo = 1.88. (c) Variation of the mixture fraction just below the lifted flame base in regime 2: 40% C3 H8 /60% N2 , Uo = 1.5 m/s.

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Fig. 7. Representative direct photos in turbulent flames of 40% C3 H8 /60% N2 : (a) Uo = 10 m/s and (b) Uo = 17 m/s. (c) Variation of mixture fraction just below the lifted flame base with radial distance.

the reduction of reactive species available to participate in the reactions so that flame blow-out occurs even near the nozzle exit as indicated in Fig. 3. It is seen that large forcing, including the blown-out flame in regime 1, causes partially premixed flame fronts to change into weakly varying partially premixed flames. Intermediate results may be expected in regime 2. Fig. 6 displays representative direct and Schlieren images in regime 2; 40% C3 H8 /60% N2 , Uo = 1.5 m/s, (a) Uf /Uo = 0.88, (b) Uf /Uo = 1.88, and (c) variation of the mixture fraction at the base of lifted flame in regime 2; 40% C3 H8 /60% N2 , Uo = 1.5 m/s. In Fig. 6 the twist motion, the cause of which has been attributed to an axially positive pressure gradient due to acoustic forcing, is related to the flame rotation [16]. In Fig. 6a the motion corresponds to the case of relatively weak forcing. As the forcing strength increases in regime 2, the shape takes a kind of double jet appearance as shown in Fig. 6b. As the forcing strength increases, well-organized vortices in the inner fuel structure are formed from the laminar fuel stem of nonforced flames. At Uf /Uo = 0.88 a twisted motion [16], which is caused by flame rotation, appears in the inner fuel structure. This flame rotation, irregularly turning clockwise and counterclockwise, causes the advancing fuel jet to decelerate while the upstream flow continuously catch up the advancing fuel jet. It is also shown that flame rotation affects the upstream fuel jet at Uf /Uo = 1.15 and thus even the upstream fuel jet is twisted, although we do not address this in the present paper. As a result, complicated interactions among them forced the fuel jet be highly distorted and stagnant, while the downstream fuel jet is changed to small scale eddies at Uf /Uo = 1.28. The double jet flame behavior be-

comes transiently coexistent at Uf /Uo = 1.15–1.88 such that the downstream flame behaves like a wellmixed flame and the upstream flame acts like an edge flame as described at Uf /Uo = 1.88. Therefore, in Fig. 6b, the lifted height of the left flame seldom changes while the lifted height of the right flame intermittently varies according to its interaction with the upstream vortex. In Fig. 6c the relatively small forcing in regime 2 makes the flame behave like a triple flame. The mixture fraction gradient decreases with forcing strength and it is then believed that the flame is being changed to a weakly varying partially premixed flame. An examination of turbulent flames without forcing may be also useful for a comparison. Fig. 7 displays representative direct photos of turbulent flames of 40% C3 H8 /60% N2 , (a) Uo = 10 m/s and (b) Uo = 17 m/s, and (c) variation of mixture fraction with radial distance at the lifted flame base. In Figs. 7a and 7b the Reynolds numbers based on fuel tube diameter are 5337 and 9073, respectively. The flame in Fig. 7b was approaching flame blow-out. As shown in Fig. 7c, the lifted flame near flame blow-out shows a large value of rd and thus a mild gradient of mixture fraction. Acknowledgment This work was supported by the Brain Korea 21 Project in 2004.

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