air in micro-scale planar combustors

Energy 179 (2019) 558e570

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Energy journal homepage: www.elsevier.com/locate/energy

Experimental study on flame structure transitions of premixed propane/air in micro-scale planar combustors Aikun Tang a, *, Tao Cai a, Jiang Deng a, Dan Zhao b, Qiuhan Huang a, Chen Zhou a a b

School of Energy and Power Engineering, Jiangsu University, Zhenjiang, 212013, China Department of Mechanical Engineering, College of Engineering, University of Canterbury, Private Bag 4800, Christchurch, 8140, New Zealand

a r t i c l e i n f o

a b s t r a c t

Article history: Received 12 October 2018 Received in revised form 16 April 2019 Accepted 1 May 2019 Available online 9 May 2019

Flame structure transitions of propane/air premixed combustion in a micro-planar quartz glass combustor in the present study have been extensively investigated using a high-speed digital camera. Six different flame propagation modes, namely, flame repetitive extinction and ignition (FREI), cellular flame, planar flame, U-shaped, inclined and spinning flames, are observed with varying inlet velocity and equivalence ratio. The FREI and cellular flames as well as spinning flames, for the first time, are experimentally discovered in micro-planar straight channel combustor. Based on the upper and lower limit of each flame mode, the flame structure regime diagram is constructed. Experimental observations indicate that different flame propagation modes may coexist during transition due to the hysteresis phenomenon. The effects of equivalence ratio, channel length and channel height on flame characteristics are analyzed. Results show that channel length has minimal effect on flame structure, whereas channel height significantly affects flame propagation behavior. It is also found that the flame dynamics is much more complicated with wider channel height. Furthermore, it is worthy of pointing out that the inclined flames disappears as the channel height is decreased from 3 to 2.5 and 2.0 mm, and the ultimate flame is Ushaped at large inlet velocity. © 2019 Elsevier Ltd. All rights reserved.

Keywords: Micro-combustion Premixed flame structures Flame instability Cellular flames Spinning flames

1. Introduction The micro power systems fuelled with hydrocarbon fuel have received extensive interest due to the higher energy density as compared to the conventional electrochemical batteries [1]. Despite the opportunity brought by micro-combustion, some challenges emerge as the combustion chamber is scaled down [2]. The flame suffers from thermal/radical quenching easily because of the large surface-to-volume ratio under micro/meso scale conditions [3]. To overcome the negative influence of large heat loss on flame stability, various effective measures have been put forward. The concept of excess enthalpy combustion was firstly proposed by Lloyd and Weinberg [4]. Vijayan and Gupta [5] explored the combustion and heat transfer at meso-scale with thermal recuperation, and they found that flames could be obtained well below the normal quenching distance. Chou et al. [6] reviewed the heat recirculation for micro power generator. Yang et al. [7] investigated the development of micro-thermophotovoltaic power generator

* Corresponding author. E-mail address: [email protected] (A. Tang). https://doi.org/10.1016/j.energy.2019.05.005 0360-5442/© 2019 Elsevier Ltd. All rights reserved.

with heat recuperation. They concluded that the electrical power of the system was significantly increased for the micro-TPV system with a heat recuperator. Tang et al. [8] found that heat recirculation can profoundly promote the flame flammable range. Meanwhile, porous combustion is another feasible approach of recirculating heat. Li et al. [9] experimentally investigated the effect of inserting porous media into a micro-combustor. It was found that the flame location can be effectively anchored in the presence of porous media. Chou et al. [10] analyzed the porous media combustion on energy output performance for micro thermophotovoltaic system applications. Wang et al. [11] experimentally tested the temperature variation in porous combustor. Moreover, blended combustion exhibits great advantage in flame stability. Zarvandi et al. [12] studied the effect of hydrogen addition on methane/air combustion. Tang et al. [13] examined the enhancement of hydrogen on propane/air combustion. Both of their works indicated that the flammable range of pure fuel combustion can be boosted significantly with the addition of hydrogen. Dogwiler et al. [14] established two-dimensional modeling to explore the stabilized mechanism in a catalytic combustor. Numerical study on catalytic combustion and extinction characteristics of premixed methane-air

A. Tang et al. / Energy 179 (2019) 558e570

was performed by Yan et al. [15]. Combustion of propane with Pt and Rh catalysts in a meso-scale heat recirculating combustor was compared by Wierzbicki et al. [16]. Furthermore, some unique structures have been fabricated to anchor flame and enhance thermal performance. Wan et al. [17] carried out experimental and numerical analysis on blow-off limits in a meso-scale combustor with bluff-body and subsequently the dynamics of methane/air flames in combustors with a flame holder and preheating channels were extensively examined by them [18]. The effect of wire insertion on premixed flame is numerically investigated by Baigmohammadi et al. [19]. Zuo et al. [20] compared the counterflow and coflow double-channel micro combustors for microthermophotovoltaic system. At the same time, they [21] developed a micro elliptical tube combustor and found that the power output can be increased effectively. In addition, flame stability issues in micro/meso channel have attracted wide interest all around the world. Veeraragavan and Cadou [22] established a two-dimensional conjugate heat transfer model for flame stabilization, and it was concluded that heat recirculation from the post-flame to the pre-flame is shown to be the primary mechanism for flame stabilization and burning rate enhancement in micro-channels. Norton and Vlachos [23] numerically studied the premixed methane/air combustion characteristics and flame stability in a micro-channel. It was found that the wall thermal conductivity have significantly effect on flame stabilization inside the micro-combustor. Too low thermal conductivity would inhibit the streamwise heat transfer which is not favor of stabilizing flame, while too large conductivity would bring large heat losses leading to flame quenching. Subsequently, they [24] analyzed the combustion characteristics and the steady-state, self-sustained flame stability propane/air using two-dimensional elliptic computational fluid dynamics model. They found that the wall thermal conductivity was vital in determining the flame stability of the system, as the walls were responsible for the majority of the upstream heat transfer as well as the external heat losses. Veeraragavan [25] demonstrated that total heat recirculation was shown to be the primary parameter to control the flame speed. Furthermore, a large number of experimental and numerical works has been conducted to understand and reveal the mechanism of flame structures and dynamics. Kurdyumov et al. [26] investigated the dynamics of premixed flames in a narrow channel, and they found that thermal flame/wall interaction is the mechanism leading to the observed flame instabilities. Wan et al. [27] experimentally and numerically analyzed the effect of wall cavity on flame stabilization, while Kang et al. [28] studied the effect of flame stability limits in a mesoscale combustor with thermally orthotropic walls. Generally, the flame can be classified into two types according to the flame behavior. The first one is the unsteady flames including flame repetitive extinction and ignition (FREI), oscillating flames, rotating flames, spinning flames and cellular flames. Another is the steady flames which always refer to weak flames, planar flames, U-shaped flames as well as inclined flames. Regarding numerical and analytical works, Kang et al. [29] studied the suppression mechanism of instabilities of methane-air flame in a narrow channel. Pizza et al. [30] investigated the effect of channel height on the flame dynamics in micro-scale channels using direct numerical simulation (DNS) with detailed kinetic mechanism, and it was concluded that the flame dynamics become more complex with an increase in channel height and different flame propagation modes coexist in transition due to the hysteresis phenomenon. Subsequently, they [31] analyzed the flame dynamics under meso-scale condition and found that the maximum number of cellular structures of chaotic oscillating flame behavior in mesoscale channel increases with increasing the channel height. Numerical works on the mechanism of flame bifurcation with regard

559

to repetitive extinction-ignition dynamics were carried out by Alipoor and Mazaher [32] in a heated micro channel. Results indicated that the flame propagation process can be divided into five phases (i.e., initiation phase, ignition phase, propagation phase, weak reaction phase, and flowing phase) and flame bifurcation takes place at the beginning of ignition phase. Alipoor and Mazaher [33] investigated the flame bifurcation in repetitive extinctionignition dynamics for premixed hydrogen-air combustion in a heated micro channel. The theoretical and numerical analysis on oscillating flame near flashback conditions was carried out by Kurdyumov et al. [34]. They concluded that the thermal flame-wall interaction was the principle mechanism for flame oscillating. Minaev et al. [35] investigated the nonlinear dynamics of flame in a narrow channel with a temperature gradient. In the follow-up work, they [36] adopted thermal-diffusive model and confirmed the presence of formation of oscillating or rotating spatial flame structures. Pizza et al. [37] used a three-dimensional simulation to investigate the flame dynamics of premixed hydrogen/air. Considering three-dimensional perturbations in a cylindrical domain, a linear stability analysis was presented by Sivashinsky et al. [38], and they demonstrated that as a critical parameter was exceeded, the uniformly flame front become unstable to perturbations corresponding to spinning modes of propagation. Spinning flame propagation has been theoretically analyzed by numerous authors, including Matkowsky and Olagunju [39], Kwon et al. [40] and Pearlman [41]. The cellular flame was analyzed by Matkowsky et al. [42], Sivashinsky [43] and Joulin [44]. Kadowaki et al. [45] numerically investigated the influence of hydrodynamic instability on the structure of two-dimensional and three-dimensional cellular flames. They found that the stationary cellular flame was obtained when the inlet-flow velocity was set to the flame velocity of the cellular flame. Ciccarelli and Dorofeev [46] reviewed the underlying mechanism for cellular flame. Simulation was performed by Dogwiler et al. [47] to elucidate the characteristics of inclined flame. Kurdyumov et al. [48] numerically and experimentally confirmed the existence of inclined flame stabilized in a tube close to the flashback condition. Kim and Maruta [49] compared the effects of thermal boundary conditions and flow fields on flame propagation, it was found that different velocity profiles induced significant variations in propagation velocities and the flame structures coupled with thermal conditions and tube sizes. Aside from numerical studies, a wide variety of experimental investigations also have been done to enhance the understanding of flame propagation characteristics. During the experiment, a method that an external heat source is imposed on the channel to maintain a positive wall temperature gradient along the direction of fluid flow is widely adopted by some scholars to suppress the flame-wall coupling effect. The propagation characteristics of premixed methane-air in meso-scale diverging channels are experimentally studied by Kumar and Akram [50]. They claimed that planar flames are observed near flash back limits. Negatively stretched flames were observed for moderate flow rates and rich mixtures and for high flow rates, flames were positively stretched. These flames were either symmetric or asymmetric in nature. Stazio et al. [51] experimentally demonstrated the existence of FREI and weak flame. Characteristics of premixed combustion in a heated channel with an inner diameter smaller than the conventional quenching distance of the employed mixture were investigated experimentally by Maruta et al. [52]. The lower limit of weak flame regime was first experimentally identified by Tsuboi et al. [53]. Fan et al. [54] presented the flame propagation mode of premixed CH4/air in quartz combustors and revealed the quenching mechanism of oscillating flame using phase-locked OH-PLIF. Stazio et al. [55] experimentally observed the existence of oscillating

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flames and weak flames in a micro reactor with a controlled temperature profile. Kumar et al. [56] analyzed the flame dynamics in a radial microchannel. Fan et al. [57] also employed experiment in a radial microchannel to further understanding the multiple rotating Pelton-like flame. However, Xu and Ju [58] carried out an experimental study in a mesoscale divergent channel without imposing an external heat source which is focused on the impact of variable cross-section area and the flame-wall coupling on the flame transition regimes and flame instability. Spinning flames, for the first time, were experimentally reported in mesoscale combustion for both lean and rich methane and propane-air mixtures in a broad range of equivalence ratios. Despite some numerical, experimental and theoretical studies have revealed the flame propagation characteristics and dynamics, flame structure transitions are not fully understood and huge efforts are still needed to experimentally understand the flame behavior in micro-scale planar combustors. In view of the simple structure and easy modular assembly, the micro-planar straight combustor, a core component to release combustion chemical energy, is widely used for micro-thermophotovoltaic systems [59,60]. Therefore, a deep and comprehensive analysis on flame propagation characteristics in micro-channel contributes to the development of micro power systems. The main purpose of the present paper aims at experimentally analyzing the flame structure transitions of premixed propane/air and its underlying mechanism due to the flame-wall coupling effect.

high-speed digital camera (OLYMPUS I-SPEED 3) at 500 frames per second. The camera does not have an optical band-path filter tuned to free radical (OH or CH), but it can collect all the flame luminosity representing the flame shape. When recording the flame dynamics, the exposure time is 500ms and the pixel size are 256
384. The typical configuration of the micro planar combustor is shown in Fig. 2. The overall sizes of the combustor are 40 mm in length (L), 12 mm in width (W) and 5 mm in height (H), respectively. Since the thickness of all walls including quartz and stainless steel 316 are 1 mm, the dimensional size of reaction zone is 40
10
3 mm (L
W
h). Meanwhile, we also have fabricated other size combustors, namely, channel length from 30 to 50 mm and 2e3 mm for channel height (h), for the purpose of comparison. According to the previous study [61], the premixed propane/air cannot maintain stable combustion when the channel height is below 2 mm. Due to the fact that the characteristic length scales closely related to quenching distance, the term of micro-scale combustion is used in the present paper. The micro-combustor is vertically mounted on the connector.

2. Experimental method and setup Fig. 1 depicted the schematic diagram of experimental setup used in this work, which mainly consists of a gas supply system, a micro-combustor and an image acquisition. The fuel/oxidizer is decompressed by the reducing value before flowing into the microcombustor. Propane gas (purity: 99.0%) is adopted as fuel during the present investigations. Mass flow controllers (DSN-2000B) within 0.5% measurement accuracy at full scale are used to precisely monitor the mass flow rates of propane (0e0.5 SLM) and air (0e5 SLM). The flame mode and transition process are recorded by

Fig. 2. Design scheme of visual combustor.

High-speed digital camera

Microcombustor

Flow controller

Flash-back arrestor

Connector Mixing chamber

Micro flowmeter Reducing valve

Propane

Computer

Air Fig. 1. Schematic diagram of experimental setup.

A. Tang et al. / Energy 179 (2019) 558e570

This placement method of micro-combustor is similar to the study carried out by Xu and Ju [58] who confirmed that buoyancy has little effect on flame structure. At the entrance of the microcombustor, fine stainless steel mesh is placed to form a uniform velocity field. For the exit of the combustor, an electric spark with 10 kV direct-current discharge is applied to ignite the fresh mixture. During the experiment, no external heat source is used to obtain a constant, well-characterized wall temperature gradient in the streamwise direction. The fresh mixture is ignited at a suitable low velocity at which the flame can maintain stable combustion. After being ignited, the flames are sustained by heat recirculation through wall heat conduction and they do not need to be reignited. Subsequently, we gradually increase or decrease the inlet velocity at the interval of 1 cm/s until other flame propagation mode occurs. All the flame structures are recorded in one time rather than leave the channel wall temperature to cool down and return to its original temperature. 3. Six flame structures In this section, we illustrate the flame propagation modes experimentally observed in micro planar combustor. Unless especially pointed out, the L, W, h of the micro-channel are respectively 40, 12 and 3 mm and the equivalence ratio (f) is kept as 1.0 in this section. Five flame propagation modes including FREI, cellular flame, planar flame, U-shaped flame and inclined flame are observed with increasing inlet velocity at a fixed equivalence ratio. Furthermore, it should also be pointed out that a special propagation mode, namely spinning flame, can only occur when the equivalence ratio is 1.2. 3.1. FREI flame (20e22 cm/s) For the lowest inlet velocity, a flame is hard to stabilize in the channel due to the large heat losses. Therefore, a continual ignition

561

is imposed at the exit of the combustor until a steady flame, like planar flame (discussed in the following section), appears near the channel inlet. Fig. 3 depicts trajectory history of FREI in one cycle when the inlet velocity is 20 cm/s. Once the mixture is ignited near the channel exit, a flame forms and propagates into the channel. During moving upstream in the micro-channel, the flame continuously losses heat to the wall. The flame quenches as it reaches to a critical location where the heat generated by combustion cannot balance the heat loss. Meanwhile, it is found that the location varies for different inlet velocities due to the change in heat release rate. Subsequently, the fresh mixture recharges, fills up the whole channel and is ignited again which means the start of another half cycle. Different from the first half cycle, the flame travels in the opposite direction during second half cycle. After re-ignition, the flame propagates and experiencing quenching again until the flame is formed at the outlet similar to the one under initial condition. The whole process of the FREI lasts nearly 468 ms. This type of FREI, for the first time, is experimentally observed in the micro-planar combustor, and is not identical to those FREI demonstrated by Stazio et al. [51] and Maruta et al. [52]. 3.2. Cellular flame (23 cm/s) As the inlet velocity slightly increases to 23 cm/s, the FREI disappears and a new flame mode occurs. The luminous region of the flame is continuous at first, and then the flame front breaks up into two cells, and eventually merges. This mode, previously reported by Ronney et al. [44], is called as cellular flame. The temporal variation of cellular flame in one cycle is displayed in Fig. 4 when the inlet velocity is set as 23 cm/s. It can be seen from the figure that the flame is continuous at 0 ms, starts to break at 2 ms and the gap between the two small flames reaches largest at 4 ms. Then the two flames merge and become a line at 12 ms. The variation frequency of cellular flame is approximately 83.3 Hz. Moreover, it should be noted that the upper and lower limit of the cellular flame is

Outlet

Progragation

Recharge

Ingnition

Inlet 0 ms

26 ms

52 ms

78 ms

104 ms

Progragation

234 ms 260 ms

286 ms

130 ms

156 ms

182 ms

208 ms

Recharge

312 ms

338 ms

364 ms

390 ms

416 ms

Fig. 3. Trajectory history of FREI in one cycle (Vin ¼ 20 cm/s).

442 ms 468 ms

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Outlet

Inlet

0 ms

2 ms

4 ms

6 ms

8 ms

10 ms

12 ms

Fig. 4. Temporal variation of cellular flame in one cycle (Vin ¼ 23 cm/s).

coincident. For example, a cellular flame could only be observed as inlet velocity is 23 cm/s. With increasing inlet velocity (decreasing) by just 1 cm/s, the flame becomes a planar (FREI). 3.3. Planar flame (21e68 cm/s) Starting from a cellular flame with equal to 23 cm/s, a stabilized planar flame will appear near the inlet of the combustor as the inlet

Outlet

velocity reaches 24 cm/s. The flame propagation mode persists over a significantly range of inlet velocity from 21 to 68 cm/s. Fig. 5 (a) displays the planar flame at various inlet velocities. One can notice that the propagating flame is symmetric in the spanwise direction. Further increasing inlet velocity up to 65 cm/s in steps of 1 cm/s, the planar flame will fluctuate slightly and be away from the inlet temporarily, as shown in Fig. 5 (b). The reason over this phenomenon is that once the inlet velocity increases to some critical points,

(a)

Inlet 24 cm/s

Outlet

44 cm/s

58 cm/s

(b)

Inlet

0 ms

180 ms

360 ms

540 ms

720 ms

Fig. 5. Structures of planar flames: (a) variation as a function of inlet velocity; (b) Slight fluctuate for high inlet velocity (Vin ¼ 65 cm/s).

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a slight perturbation of inlet velocity will result in the change of flame location and shape. However, the planar flame ceases once the inlet velocity reaches 69 cm/s and a suspended U-shaped flame (discussed in the next section) is observed. On the other hand, reducing the inlet velocity from the planar flame (Vin ¼ 24 cm/s) in steps of 1 cm/s, it is observed that they can be sustained for Vin as low as 21 cm/s (i.e. over a range where FREI are observed). Both FREI and planar flames occur in the velocity range from 21 to 22 cm/s, indicating that there exists a hysteretic phenomenon in transition from planar flame to FREI. The occurrence for this phenomenon is mainly due to the flame-wall coupling effect. The flame-wall coupling will significantly increase the effective Lewis number and lead to a new mechanism to promote the thermal diffusion instability, thereby changing the nature of flame propagation and yielding multiple flame regimes. Meanwhile, Xu and Ju [1] pointed out that hysteresis phenomenon reflected the thermal historic effect of the wall and further indicated that the wall temperature for flame-wall coupling was the key parameter. Similar phenomena were also observed by Pizza et al. [30,31], Xu and Ju [58], that is to say, the gradual increase or decrease in the inlet velocity will lead to the non-coincidence of transition point from one flame propagation mode to another. 3.4. U-shaped flame (60e75 cm/s) An increase in inlet velocity from 68 to 69 cm/s, the planar flame disappears and a new propagation mode characterized by U-shaped flame is constructed, as shown in Fig. 6. The dominant factor for flame transition from planar flame to U-shaped flame is flow field, namely, the imbalance between the inlet velocity and flame propagation velocity. However, once a new flame structure is formed, both the flame-wall coupling effect and flow field play the significant role. It is the combined effect that makes the flame anchor in

Outlet

(a) l2 = 15 mm

l2 = 30 mm l = 2.5 mm

l2 = 37 mm

l2 = 38 mm

l = 1.1 mm

l = 0 mm

Outlet

70 cm/s

the micro-channel instead of oscillating. The flame termed as negatively stretched flames is also found by Akram and Kumar [50] who pointed out that once the flame stabilizes inside the channel, the unburned mixture gets heated which intern pushes the flame to upstream direction and flame gets a concave shape. This flame suspends at the channel and has a distance from the inlet. The Ushaped flame is symmetrical relative to the spanwise direction at most cases and becomes slightly tilted at large inlet velocity. Before analyzing the basic features of U-shaped flame, some nomenclatures are defined. Here, △l is defined as the height difference in the streamwise direction between the front and back edges of the flame, which is referred to as the flame thickness, l2 represents the distance from the point of flame centerline to the outlet, and R is the curvature radius. It can be acknowledged from Fig. 6(a) that the flame thickness and curvature radius of U-shaped flame becomes larger with an increase in inlet velocity. During the experiment the flame thickness of the U-shaped is very thin at a relatively small inlet velocity of 66 cm/s. However, when the inlet velocity is added to 75 cm/s, the value reaches 2.5 mm. This finding is consistent with the results carried out by Kim et al. [49] who claimed the flame thickness is related to the tube diameter and flow rates. Moreover, as the inlet velocity increases, the flame propagates downstream and thus results in the deduction of l2. Fig. 6(b) displays the variation of curvature radius versus inlet velocity. One can notice that the higher the inlet velocity, the larger the curvature radius becomes. The reason for this can be attributed to that the high temperature region expands to the entire cross section due to the increased chemical energy input and thus resulting in the large R. In addition, it is interesting to note that once the inlet velocity of U-shaped flame reaches a critical point (e.g., 75 cm/s), two peaks in the middle of the flame will occur, as shown in Fig. 6(a). Experimental observation shows that the U-shaped flame with two peaks always occurs before the flame become tilted (discussed in the next section). Similar to planar flame, the hysteresis phenomenon also exists in transition from U-shaped flame to other propagation mode. 3.5. Inclined flame (55e83 cm/s) Based on the critical value of the U-shaped flame, the flame shape will become tilted if the inlet velocity continues to increase. As shown in Fig. 7, the distinctive characteristic of this type flame is asymmetry relative to spanwise direction. As the inlet velocity increases, the flame location approaches the outlet. When the value

l = 1.8 mm

Inlet 66 cm/s

563

73 cm/s

75 cm/s

Outlet

(b) R = 26 mm

R = 15 mm R = 11 mm

Inlet 70 cm/s

73 cm/s

75 cm/s

Fig. 6. The features of U-shaped flames under different inlet velocities: (a) Variation of flame thickness and location; (b) Variation of curvature radius.

Inlet 64 cm/s

76 cm/s

79 cm/s

Fig. 7. Inclined flames under different inlet velocities.

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reaches 83 cm/s, the high side of the flame is blown out of the combustor. As time goes on, the entire flame will be gradually pushed out. It should be pointed out that the inclined direction of this flame mode is randomly selected with equal probability. For example, the inclined direction of flame at 64 cm/s is left and right for 76 cm/s. However, the flame become left-tilted again as the inlet velocity is added to 79 cm/s in steps of 1 cm/s. Meanwhile, we also have found an interesting phenomenon that decreasing 79 cm/s in steps of 1 cm/s, the tilted direction of the flame keeps consistent until 55 cm/s. This also shows that the wall temperature does not remain unchanged but presents a dynamic fluctuation. As the inlet velocity is beyond a critical point, the inclined flame is blown out of the micro-channel. Similar asymmetric results have been reported by Kurdyumov et al. [48], where an inclined flame structure was experimentally observed for lean propane/air flames in tubes at conditions close to flashback. Furthermore, hysteresis effect is also observed in the transition process from inclined flame to U-shaped flame. 3.6. Spinning flame (26e37 cm/s) Along with the five flame modes observed at the equivalence ratio of 1.0, a unique flame behavior takes place when the equivalence ratio is set as 1.2. The flame behavior can persist in the inlet velocity range of 26e37 cm/s. Similar to the FREI discussed in the previous section, this flame is unsteady and resembles an X-shape in bare eyes. The flame propagation mode, named as spinning flame, has also been found by Kwon et al. [40] and Xu and Ju [58], which is due to the thermal-wall coupling. Xu and Ju [58] pointed out that for a fixed equivalence ratio, there existed a critical flow rate, above which flame starts to spin. It was also found that the

spinning flame only occurred after the transition from fast flame regime to slow flame regime. Fig. 8 shows the evolution of the spinning flame in a period at inlet velocity of 36 cm/s. At first half period, the flame is right-leaning and propagates. However, the tilted direction changes at next half period. The evolution of spinning flame within a cycle lasts approximately 100 ms. 4. Results and discussion In order to systematically study the flame propagation modes and the transition process in the micro-planar straight combustor, the effects of equivalence ratio, channel height, and channel length are analyzed in this section so as to provide an experimental reference for the development of the micro-combustion. 4.1. Effect of equivalence ratio The flame structure regime diagram over a wide range of equivalence ratios of 0.5e1.2 and inlet velocities of 10e89 cm/s is constructed, as shown in Fig. 9. Here, the minimum inlet velocity at which a flame behavior remains unchanged inside the channel is considered as lower limit of flame. Similarly, the upper limit is defined as the maximum inlet velocity under which condition the flame is about to become another propagation mode or moves out of the channel. First, we discuss three unsteady flame types at low inlet velocity: FREI, cellular flame and spinning flame. With respect to FREI, it exists for a wide range of equivalence ratio from 0.6 to 1.1 and has lowest inlet velocity which does not exceed 26 cm/s. When it comes to cellular flame, it is observed at a narrow equivalence ratio range of 0.9e1.1. As for spinning flame, it only takes place when the equivalence ratio is 1.2 and its range of inlet velocity is

Outlet

Inlet 0 ms

6 ms

12 ms

18 ms

24 ms

30 ms

36 ms

42 ms

44 ms

52 ms

60 ms

68 ms

76 ms

84 ms

92 ms

100 ms

Fig. 8. Evolution of spinning flame in a period (Vin ¼ 36 cm/s).

A. Tang et al. / Energy 179 (2019) 558e570

Inclined flame (lower) Inclined flame (upper) U-shaped flame (lower) U-shaped flame (upper)

100

Cellular flame (lower) Cellular flame (upper) Flat flame (lower) Flat flame (upper)

80 70 60 50 Spinning flame (lower) Spinning flame (upper) FREI (lower) FREI (upper)

40 30 20

(a) 0.99

10 0.6

0.7

0.8 0.9 1.0 Equivalence ratio

1.1

Fig. 9. Flame structure regime diagram under various equivalence ratios and inlet velocities.

also narrow. At low inlet velocities, the flame can easily suffer large heat losses due to the increased surface to volume ratio of microcombustor and is more likely to become unstable. As the inlet velocity reaches a critical point, the large heat during combustion can remedy the weakness and then a planar flame stabilize near the inlet. The flame structure has the largest velocity range among the six flame types. The stable flame lasts until a new propagation mode appears at high inlet velocity at which condition the flame moves downstream and the residence time is significantly important. Once the residence time is too short, the flame will be pushed out of the combustor and eventually extinguishes. It can be seen from the figure that the upper limit of inlet velocity among the inclined flame, U-shaped flame and planar flame presents the similar tendency with varying the equivalence ratio, indicating that these exists a close connection during the transition of these flame propagation modes. Generally, the reason for the flame transition can be attributed to the imbalance between the flow velocity and the local flame propagation velocity [50]. Moreover, it is worth pointing out that most flame structures observed under the condition of fuel-lean have a wider range of inlet velocity than that under fuel-rich condition. This phenomenon reveals that in comparison to fuel-rich condition, the premixed propane/air combustion under micro-scale condition is more stable at fuel-lean under which condition the air is excessive and can boost the completeness degree of combustion. Since a slight increase or reduction in equivalence ratio has a significant effect on flame behavior for relatively low inlet velocity, the unsteady flames including FREI and cellular flame are selected for further investigation in this section. Generally, the occurrence of these unsteady flame behaviors is a result of the combined effect of flame-wall coupling effect and flow field since the variation of flow field leads to the oscillation of flame and the change of flame location could alter the heat transfer between flame and wall. Fig. 10 depicts the variation of flame behavior with varying equivalence ratio. The initial flame propagation mode is FREI when the equivalence ratio is 1.0 and the inlet velocity is set as 20 cm/s. With regard to operating method, the volume flow rate of the fuel is constant while the change of equivalence ratio is obtained with varying the volume flow rate of the air. From the figure, the flame can mainly be divided into two types. One is that the flame only propagates to some place of the combustion channel where the heat generated along axial conduction equals the heat loss of the

(c) 0.93

(d) 0.90

(e) 0.87

Fig. 10. Variation of flame behavior with varying equivalence ratio.

1.2

wall. However, when the equivalence ratio is decreased to a critical point (0.9), the flame can reach the inlet of the combustor implying that the heat through conduction can be transferred to the whole combustor. At the same time, a new flame behavior, termed as cellular flame, occurs. As further reducing the equivalence ratio to 0.87, the planar flame is observed and stabilizes near the inlet of the combustor. Therefore, it is concluded that at low inlet velocities, flame heats the unburned mixture and moved upstream with decreasing equivalence ratio and further indicates the flame is more stable under fuel-lean condition. To further investigate the basic characteristics of flame structure, we compare the variation of flame frequency and flame position as a function of equivalence ratio as depicted in Fig. 11. For the present cases, zero frequency means that the flame is stable. It can be seen from the figure that a decrease in the flame frequency is observed with a decrease in equivalence ratio, thus demonstrating that for fuel-lean condition decreasing the equivalence ratio is more suitable for stable combustion which is consistent with the analysis in Fig. 10. For instance, the frequency of the flame at equivalence ratio of 0.99 is 4.2 Hz and the value becomes 0 for 0.87. Meanwhile, it is worth mentioning that although the flame is near inlet under the equivalence ratio of 0.87 and 0.90, the former flame is steady while the other fluctuates. Furthermore, we also investigate the variation of cellular flames at different equivalence ratios, as shown in Fig. 12. As previously mentioned, the cellular flame may exist under the condition of

The maximun distance from outlet (mm)

0.5

(b) 0.96

32

5

28 4

24 20

3

16 2

12 8

Frequency (Hz)

Inlet velocity (cm/s)

90

565

1

4 0

0.87

0.90

0.93 0.96 Equivalence ratio

0.99

0

Fig. 11. Variation of flame frequency and position as a function of equivalence ratio.

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A. Tang et al. / Energy 179 (2019) 558e570

30

20 15

26 24

10

22 5

Time (ms)

Inlet velocity (cm/s)

28

20 18

0

16 0.9

1.0 Equivalence ratio

1.1

-5

Fig. 12. Variation of cellular flames at different equivalence ratios.

equivalence ratio from 0.9 to 1.1. As the equivalence ratio increases, the inlet velocity under which circumstance the cellular flame emerges becomes smaller. For instance, the cellular flame occurs at equivalence ratio of 0.9 for an inlet velocity of 19 cm/s. Nevertheless, for equivalence ratio 1.0 and 1.1, these values are 23 cm/s and 27 cm/s respectively. It can be also seen from Fig. 12 that fluctuating time of cellular flame in a period decreases with increasing equivalence ratio. This phenomenon indicates that the altering frequency of cellular flame is low under fuel-rich condition, which is mostly correlated with parameters in flame-wall thermal coupling. 4.2. Effect of channel length To study the effect of channel length on the flame structures, three types of micro-combustors with different channel lengths are adopted. The lengths of the combustors are 30, 40 and 50 mm, while the channel heights are 3 mm. Three typical equivalence ratios of 0.9, 1.0, and 1.1 are selected which can represent fuel-lean, stoichiometry ratio and fuel-rich, respectively. The flame structure distribution as a function of channel length is depicted in Fig. 13. Since the spinning flame only appears at the equivalence ratio of 1.2, it will not be discussed in this section. From Fig. 13(a), (c) and (d), it can be known that regardless of equivalence ratio, the velocity ranges of the FREI, planar flame and U-shaped flame in micro-combustor with 30 mm channel length are widest among the investigated channel lengths, while the ranges with regard to 40 mm case are always the narrowest. Taking the FREI for example, when the equivalence ratio is 1.0, the velocity ranges of channel length varying from 30 to 50 mm are 19e24 cm/s, 21e22 cm/s and 20e23 cm/s, respectively. The velocity difference is more obvious under fuel-lean conditions. When it comes to planar flame, the velocity discrepancy is remarkable under fuel-rich condition. However, for the inclined flame shown in Fig. 13(e), the variation tendency is totally different from previous flame modes. That is, the increase in channel length will enlarge the velocity range of inclined flame at different equivalence ratios. For instance, when the equivalence ratio is set as 1.0, the interval between the upper and lower limits of the inclined flame are 40, 29 and 27 cm/s for the channel length of 50, 40 and 30 mm, respectively. Furthermore, the upper limit of inclined flame in a combustor with channel length equal to 50 mm is significantly extended, which benefits from the longer residence time of mixture in longer channel. As a result, it can be concluded that the channel length has a remarkable effect on the upper limit of inclined flame.

Since the cellular flame can only occur at some particular points, there do not exist in velocity discrepancy, as shown in Fig. 13(b). Nevertheless, it is interesting to notice that the value under which inlet velocity cellular flame appears increases with an increase in equivalence ratio, irrespective of channel length. From the above discussion, it is found that the upper limits of Ushaped flame and inclined flame have a significantly difference. Therefore, it is meaningful to investigate the flame position under a certain inlet velocity. Fig. 14(a) displays the flame position of Ushaped flame with varying channel length from 30 to 50 mm when the inlet velocity is 75 cm/s and the equivalence ratio is set as 1.0. One can expect that the upper limit of U-shaped flame in channel length of 30 mm is largest, followed by 50 mm, and last is 40 mm. Because the U-shaped flames for 40 mm and 50 cases have two peaks, a transition to inclined flame is about to take place which has been pointed out in section 3. Moreover, this phenomenon can be confirmed in Fig. 13(d). Fig. 14(b) exhibits the flame position of inclined flame when the inlet velocity and the equivalence ratio are set as 66 cm/s and 1.0 respectively. To better reveal the above phenomenon, qualitative analysis about flame position is carried out for the two flame modes. Here, l1 represents the standoff distance, i.e., how far the flame is from the combustor entrance and equals L - l2 (L ¼ channel length). Moreover, the ratio of l1 to L is termed as dimensionless flame position. There is no doubt that the smaller the dimensionless flame position is, the less possibility the flame propagation mode will change. First, we calculate the value of dimensionless flame position of Ushaped flame. These values are 0.27, 0.625 and 0.4 for channel length of 30, 40 and 50 mm, indicating that the flame behavior with channel length 30 mm is most stable and 40 mm case is extremely unstable which is in accordance with the upper limit in Fig. 13(d). As for inclined flame, the values of dimensionless flame position are 0.633, 0.625 and 0.3 with channel length varying from 30 to 50 mm. These figures present a remarkably similar to the phenomenon shown in Fig. 13(e). Therefore, it is not hard to understand the reason why the upper limit of channel length with 50 mm is highest. 4.3. Effect of channel height As is universally acknowledged that the deduction in channel height of micro-combustor will result in the increase in surface-tovolume ratio and thus the flame will encounter with large heat loss under micro-scale condition. Meanwhile, the residence time of mixture in channels is shortened for the same chemical energy input. Accordingly, the flame is more likely to become instability due to the presence of the above comprehensive effects. To reveal the channel height how to influence the flame propagation mode, three kinds of micro-combustors with different channel heights (2.0, 2.5 and 3.0 mm) are fabricated, while the channel lengths are set at 40 mm. Fig. 15 depicts the effect of channel height on the velocity range of various flame structures. From Fig. 15 (a) to (d), Experimental observation shows that the flame propagation modes reduce as the channel height decreases. This phenomenon is similar to the observation reported by Pizza et al. [30,31] who found that for narrower channels some of the dynamics are suppressed using direct numerical simulation. As mentioned above, there are six flame patterns in the micro-combustor with height channel of 3.0 mm; however, only four types of flame can be observed when the channel height is reduced to 2.0 mm including spinning, planar, U-shaped and inclined flame. Regarding the FREI, the flame propagation mode tends to occur under fuel-lean condition as decreasing channel height from 3.0 to 2.0 mm, as shown in Fig. 15(a). When the channel height is 2.0 mm, the spinning flame

A. Tang et al. / Energy 179 (2019) 558e570

26

(a) FREI flame

26

567

(b) Cellular flame

Inlet velocity (cm/s)

Inlet velocity (cm/s)

24 22 20 18

L = 30 mm (lower) L = 30 mm (upper) L = 40 mm (lower) L = 40 mm (upper) L = 50 mm (lower) L = 50 mm (upper)

16 14 12 10

0.9

1.0 Equivalence ratio

0.9

1.1

70 60 50

L = 30 mm (lower) L = 30 mm (upper) L = 40 mm (lower) L = 40 mm (upper) L = 50 mm (lower) L = 50 mm (upper)

40 30

20

20 0.9

1.0 Equivalence ratio

(d) U-shaped flame

Inlet velocity (cm/s)

Inlet velocity (cm/s)

30

L = 30 mm L = 40 mm L = 50 mm

18

80

L = 30 mm (lower) L = 30 mm (upper) L = 40 mm (lower) L = 40 mm (upper) L = 50 mm (lower) L = 50 mm (upper)

40

20

14

1.1

(b) Planar flame

50

22

16

70 60

24

1.0 Equivalence ratio 100

1.1

0.9

1.0 Equivalence ratio

1.1

(e) Inclined flame

Inlet velocity (cm/s)

90 L = 30 mm (lower) L = 30 mm (upper) L = 40 mm (lower) L = 40 mm (upper) L = 50 mm (lower) L = 50 mm (upper)

80 70 60 50 0.9

1.0 Equivalence ratio

1.1

Fig. 13. Flame structure distribution as a function of channel length.

can appear under the equivalence ratios of 0.8, 0.9, and 1.0. As for 2.5 mm case, these values are 1.0 and 1.1. When it comes to 3 mm, the flame only emerges at the equivalence ratio of 1.2. The phenomenon reveals that the spinning flame is more likely to occur for smaller channel heights. Fig. 15(b) exhibits the effect of channel height on the velocity range of planar flame. One can see that the flame behavior does not occur at equivalence ratio of 1.1 and is observed over a wide range of inlet velocities under fuel-lean condition. Meanwhile, it is worthy of noting that the lower limit of planar flame becomes larger as decreasing channel height implying that more combustion heat is needed to address the large heat losses. As the channel height is further reduced to 2.5 and

2 mm, the inclined flame does not appear and the flame is pushed out of the channel in the form of U-shaped, as shown in Fig. 15(c) and (d). It can be concluded that for narrower channels the stable U-shaped flame do not have enough time to become unsteady and is blown out of the channel for large inlet velocity, thus demonstrating that the flame is more unstable under smaller scale. To gain a better understanding of the effect of channel height on spinning flames, we investigate the frequency variation for different inlet velocities and equivalence ratios, as displayed in Fig. 16. It is interesting to note that the spinning frequency of the flame increases drastically with decreasing channel height, indicating that the unsteady degree become serious for smaller channel

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A. Tang et al. / Energy 179 (2019) 558e570

l2= 30 mm

l2= 15 mm

l2= 15 mm

(a)

l2= 35 mm

l2= 11 mm

l2= 22 mm

(b)

30 mm

40 mm

30 mm

50 mm

40 mm

50 mm

Fig. 14. Flame position versus channel length: (a) U-shaped flames (75 cm/s, f ¼ 1.0); (b) Inclined flames (66 cm/s, f ¼ 1.0).

42

(a) Spinning flame

(b) Planar flame

70

38

Inlet velocity (cm/s)

Inlet velocity (cm/s)

40 36 34 32

Lower (h = 3 mm) Upper (h = 3 mm) Lower (h = 2.5 mm) Upper (h = 2.5 mm) Lower (h = 2 mm) Upper (h = 2 mm)

30 28 26 24

0.8

0.9

1.0 1.1 Equivalence ratio

Lower (h = 3 mm) Upper (h = 3 mm) Lower (h = 2.5 mm) Upper (h = 2.5 mm) Lower (h = 2 mm) Upper (h = 2 mm)

40 30

0.8

1.2 90

80

0.9 1.0 Equivalence ratio

1.1

(d) Inclined flame

80

70 60

Lower (h = 3 mm) Upper (h = 3 mm) Lower (h = 2.5 mm) Upper (h = 2.5 mm)

50 40

Lower (h = 2 mm) Upper (h = 2 mm)

30 0.8

0.9 1.0 Equivalence ratio

1.1

Inlet velocity (cm/s)

Inlet velocity (cm/s)

50

20

(c) U-shaped flame

20

60

70 60

Lower (h = 3 mm) Upper (h = 3 mm)

50

Lower (h = 2.5 mm) Upper (h = 2.5 mm)

40 30

Lower (h = 2 mm) Upper (h = 2 mm)

0.8

0.9 1.0 Equivalence ratio

1.1

Fig. 15. Velocity range of various flame structures as a function of channel height.

height combustors. For example, the maximum frequency of flame is almost 10 Hz in 3.0 mm channel height combustor. When the channel height is reduced to 2.5 mm, this value is nearly three

times as that of 3.0 mm channel height combustor. When it comes to the 2.0 mm channel height case, the maximum frequency can reach 167 Hz, and the lowest flame frequency is nearly 90 Hz.

A. Tang et al. / Energy 179 (2019) 558e570

24

(a) h = 2 mm

= 0.9

130

= 0.8

120 110

18 15

= 1.0

100

80

= 1.1

12

90 32

34

36 38 Inlet velocity (cm/s)

40

42

28

(c) h = 3 mm

8.5

21

= 1.0

140

Frequency (Hz)

Frequency (Hz)

150

9.0

(b) h = 2.5 mm

Frenquency (Hz)

160

569

8.0 = 1.2

7.5 7.0 6.5

30

32

34 36 38 Inlet velocity (cm/s)

40

42

27

28 29 Inlet velocity (cm/s)

30

Fig. 16. Variation of frequency of spinning flames versus channel height.

Furthermore, it should be also pointed out that regardless of Fig. 16(a), (b) and (c), an increase in inlet velocity will result in the increase of the flame frequency. This phenomenon is qualitatively similar to the results reported by Xu and Ju [58] who pointed out that the spin frequency was approximately proportional to the flame speed. 5. Conclusions The flame propagation mode of premixed propane/air mixture in the micro-planar combustor is experimentally investigated adopting a high-speed digital camera. Six flame structures, namely, FREI, cellular flame, planar, U-shaped, inclined, and spinning flames, are observed with varying inlet velocity and equivalence ratio. First, the fundamental characteristics and transition process of these flame behaviors are illustrated in detail. Then, the effects of equivalence ratio, channel length and channel height on flame structures are explored. The flame structure regime diagram in the range of equivalence ratios of 0.5e1.2 and inlet velocities of 10e89 cm/s is constructed. Experimental results show that most of flame structures observed under the condition of fuel-lean own a wider range of inlet velocity than that under fuel-rich condition. For lowest inlet velocity, a slight variation in equivalence ratio can change flame behavior dramatically. Moreover, it is worthy of pointing out that channel length has minimal effect on flame structures, but has a remarkable effect on the upper limit of inclined flame. In contrast, channel height can significantly affect flame propagation behavior. Qualitative analysis with respect to dimensionless flame position presents a well consistency with the upper limit of U-shaped flame and blow off limit. Furthermore, it is found that the flame dynamics is much more complicated with an increase in channel height. When increasing the inlet velocity in steps of 1 cm/s till the flame is blown off the channel, the inclined flames occur in 3 mm channel height channel. As the channel height is further decreased to 2.5 and 2.0 mm, the inclined flame disappears and the ultimate flame type is U-shaped. Acknowledgements This work is supported by National Natural Science Foundation of China (No.51676088), China Postdoctoral Science Foundation (No. 2018M632243), and Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). References [1] Fern andez-Pello AC. Micropower generation using combustion: issues and approaches. Proc Combust Inst 2002;29(1):883e99. [2] Walther DC, Ahn J. Advances and challenges in the development of powergeneration systems at small scales. Prog Energy Combust Sci 2011;37:

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