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air flames in a micro-combustor
Energy 179 (2019) 315e322
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Effects of flow rate and fuel/air ratio on propagation behaviors of diffusion H2/air flames in a micro-combustor Ying Xiang, Zili Yuan, Shixuan Wang, Aiwu Fan* State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology, Wuhan, 430074, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 25 October 2018 Received in revised form 21 April 2019 Accepted 7 May 2019 Available online 8 May 2019
A Y-shaped micro combustor consisting of three quartz tubes with an identical inner diameter of 2.0 mm was designed. Effects of total flow rate and fuel/air ratio on the propagation behaviors of non-premixed hydrogen-air flames were investigated experimentally. Six distinct modes were observed, including flames with repetitive extinction and ignition. Noise emission was detected during flame propagation towards upstream under relatively high mixture velocities. The mean flame propagation speed decreased monotonically with an increasing mixture velocity, however, it exhibited a non-monotonic variation with the nominal equivalence ratio and peaked at f ¼ 0.9. Stable edge flames were obtained at the intersection under suitable conditions, and its length increased as the mixture velocity was increased, but varied non-monotonically with the nominal equivalence ratio and had a maximum value at f ¼ 0.9. The regime diagram of the six flame propagation modes was drawn, which can provide an overview of the relationship between the inlet condition and flame propagation mode. Qualitative analysis showed that flame dynamics of non-premixed micro-combustion was closely associated with the total flow rate and fuel/air ratio by influencing the mixing effectiveness, residence time, heat release rate and heat loss rate. © 2019 Elsevier Ltd. All rights reserved.
Keywords: Micro combustor Non-premixed combustion Flame propagation Edge flame Noise emission Flame dynamics
1. Introduction In recent decades, the emerging of various portable devices and miniature space vehicle has led to vast demands for light, compact and long duration power sources. Current electrochemical batteries might not satisfy these requirements due to their low specific energy, long recharging time and other shortcomings compared to combustion-based micro energy conversion and power generation systems, such as micro thermophotovoltaic (TPV) and thermoelectric (TE) devices, micro engines, and micro propulsion systems [1]. For this reason, combustion of hydrogen and hydrocarbon fuels in micro- or meso-scale combustors has attracted extensive attentions [2]. However, flames stability and combustion efficiency are two major challenges for micro-combustion owing to augmented heat loss ratio, shortened residence time, and the tight coupling between flame and wall [3]. Daou et al. studied the effects of flow velocity [4], Lewis number [5] and channel width [6] on the flame propagation limit in micro-channels, which demonstrated that
* Corresponding author. 1037 Luoyu Road, Wuhan 430074, China. E-mail address: [email protected] (A. Fan). https://doi.org/10.1016/j.energy.2019.05.052 0360-5442/© 2019 Elsevier Ltd. All rights reserved.
near-wall quenching and flame curvature were the major factors leading to flame quenching. Ju and Xu [7] found that flame-wall thermal coupling extended the quenching limit significantly and resulted in a slow flame regime. Maruta et al. observed flames with repetitive extinction and ignition (FREI) in heated U-shaped microchannel [8] and straight micro-channel [9]. Such flame dynamics was also observed in curved mesoscale ducts [10] and mesoscale reactors with a backward facing step in CH4eO2 [11] and CH4eO2eCO2 flames [12]. Fan et al. [13] studied the formation mechanism of FREI based on OH-PLIF technique. Minaev et al. [14] theoretically interpreted the FREI phenomenon. Jackson et al. [15] numerically investigated the FREI in a semi-infinite, thermally active channel. Pizza et al. [16] studied the detailed dynamics of FREI in planar micro-channels with different heights through direct numerical simulation. Miyata et al. [17] scrutinized the influence of wall temperature and incoming mixture temperature on FREI in cylindrical narrow channel. Alipoor et al. numerically explored the detailed behaviors of FREI [18] and the impacts of channel width, equivalence ratio and inlet velocity [19] on H2-air flames in a heated micro-channel. Nair et al. [20] numerically examined the effects of channel diameter and equivalence ratio on the occurrence of FREI in micro-channels with a wall temperature gradient. Stazio et al. [21] experimentally investigated the impact of external heating
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method on the formation of FREI. Kishore et al. [22] numerically studied the effect of wall temperature gradient on FREI dynamics. Kang et al. [23] also numerically demonstrated that flame behaviors are controlled by wall temperature gradients. Various technologies have been proposed to improve flame stability and combustion efficiency in micro- and meso-scale combustors. Kim et al. [24] studied the scale effect of small “Swiss roll” combustors. Zhong et al. [25] compared flame stability in three different types of “Swiss roll” combustors with double spiral-shaped channels. Fan et al. [26] investigated the flame blowoff limit in meso-scale Swiss-roll combustors with/without a bluffbody. Wang et al. [27] numerically demonstrated that the FREI instability can be suppressed by a platinum segment coated on the channel wall at a suitable location. They [28] also revealed that the flame position exhibited an S-shaped curve versus the inlet mixture velocity. Yang et al. [29] found that slight oxygen enrichment can significantly increase the combustion efficiency of a microcombustor with cavity flame holders. Fan et al. [30] designed a micro cavity combustor with double-layered walls to alleviate flame tip opening and reduce fuel leakage. Peng et al. [31] proposed new inlet shapes to enhance the thermal performance of a microcombustor for micro TPV applications. Bani et al. [32] investigated the effects of key parameters on the performance of porous media combustion based micro-TPV system. Li et al. [33] developed a small combustor with a percolated platinum emitter tube for micro-TPV devices. Compared to premixed combustion, non-premixed combustion (diffusion combustion) seems to be more suitable for practical applications from the viewpoint of safety because flame flashback can be inhibited. However, the effective mixing of fuel and oxidizer is an additional challenge for non-premixed combustion. Multiple isolated flame cells were observed by Miesse et al. [34] in a Yshaped non-premixed micro-burner. They found that this phenomenon only occurred for heavier fuels and were not observed in H2eO2 mixtures. For non-premixed CH4eO2 combustion within the same configuration made of alumina, Prakash et al. [35] captured transient flame dynamics leading to the formation of a stable edge flame and distinct cellular structures. Xu and Ju [36] observed isolated flamelets and edge flames of non-premixed CH4eair combustion in a heated meso-scale planar channel. Li et al. [37] numerically investigated the mixing performance and diffusion combustion characteristics of H2 and air in planar microcombustor. Lee and Kim [38] classified the non-premixed CH4eair flames into premixed flame mode and edge flame mode based on their experimental observation. Pham et al. [39] investigated the flame structure of liquid fuel and they found double flames, one anchored at the exit rim and another triple flame located within the combustor chamber. Ning et al. [40] experimentally studied nonpremixed CH4/air flames in Y-shaped meso-scale combustors with different diameters (i.e., 4, 5 and 6 mm). It was shown that all the flames were inclined and propagated downstream after ignition. Wang et al. [41] designed a meso-scale Swiss-roll combustor for non-premixed combustion for the first time. Their experimental investigation showed that flame can be sustained in this novel combustor at very small fuel flow rate and very low equivalence ratio. From the above literature review, it can be known that compared to premixed combustion, the investigation on diffusion flame dynamics in micro- and meso-scale channels are insufficient, especially for the mostly used hydrogen (H2) fuel. As is well known that H2 has much larger diffusion coefficient and heat value compared to CH4, whereas its Lewis number is much less. Therefore, it is speculated that rich diffusion flame dynamics might occur in micro-channels. Motivated by those considerations, in this work we focus our concentration on propagation characteristics of non-
premixed H2/air flames in a Y-shaped microscale combustor with an inner diameter of 2 mm. The significance of the present study lies in two aspects. On the one hand, the results can enrich the fundamental knowledge of diffusion flame propagation in narrow channels. On the other hand, the Y-shaped combustor can be applied to micro TPV [31e33] and TE [42] systems.
2. Design of experimental methodology Fig. 1 schematically shows the experimental system for the present study. A Y-shaped micro-combustor was designed, which was also applied in previous studies by Miesse et al. [34] and Prakash et al. [35] on diffusion combustion in micro-combustors. An included angle of 90 between the inlet ports was adopted because our previous study [43] showed that a relatively larger included angle could intensify the mixing of fuel and oxidant in the Y-shaped micro-combustor. A 200-mm long horizontal tube was used to ensure an ideal mixing such that the mixture can be successfully ignited under all the considered conditions at the end of the horizontal channel. Meanwhile, a sufficient long channel enables us to capture the details of flame propagation dynamics, especially in fast flame regimes. Quartz glass was selected to make the combustor since it can facilitate a direct observation of flame behaviors. The channel diameter is 2.0 mm which is smaller than those used in our previous study [40] for non-premixed CH4-air combustion. This is because H2 has a smaller quenching diameter than CH4 and meanwhile a smaller diameter can enhance the mixing process. The wall thickness of the combustor is 1.0 mm to ensure the mechanical strength of the suspended horizontal channel. H2 of 99.99% purity and dry air under high pressure were stored in two gas tanks. After the pressure reducing valves, the gas pressures were reduced to 1.0 atm. The flow rates of H2 and air were controlled by two digital mass-flow meters (Alicat Scientific Inc.) with an accuracy of 0.2%. The measurement ranges of the mass-flow meters are 0.005e1.0 SLM for H2 side and 0.015e3.0 SLM for air side, respectively. The mixture was ignited with a butane torch by heating the 2.0-cm long external wall near the combustor exit for 5 s. Right after the ignition, a still digital single lens reflex camera (Canon EOS6D) was used to take flame images and make movie recordings of flame propagation dynamics in dark background for better quality of photographs. Each case was started from cold state to ensure all the results were obtained under the same initial condition. A digital noise meter (WS1361) with an accuracy of ±1.5 dB was employed to record the noise emission during the combustion process. In the data processing, the background noise was eliminated. A stainless steel ruler was applied to determine the flame position and a digital timer was used to record the flame propagation time, based on
Fig. 1. Schematic diagram of the experimental system: 1-manual valve; 2-pressure gauge; 3-pressure reducing valve; 4-mass flow controller; 5-DSLR camera; 6-micro combustor; 7-digital noise meter; 8-laptop computer.
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which the mean flame propagation speed could be calculated. In this paper, a term “nominal equivalence ratio (f)” was used to characterize the relative amount of fuel and air, whose value is identical to the equivalence ratio of the perfect fuel/air mixture. Meanwhile, the word “mixture velocity (V)” was adopted to represent the total flow rate of cold gaseous mixture. Since the inner diameters of the three channels are identical, the mixing velocity (V) is equal to the sum of inlet velocities of air (Vair) and hydrogen (VH2). The range of nominal equivalence ratio was f ¼ 0.5e1.6 with an interval of 0.1. Meanwhile, the range of mixture velocity was 0.5e13 m/s with an interval of 1.0 m/s, except for the very low velocity case, i.e., V ¼ 0.5 m/s. Within these ranges of operational parameters, the diffusion flame propagation behaviors that occur in the Y-shaped micro-combustor can be fully revealed. 3. Results and discussion Six flame propagation modes were observed in our experiment, which will be introduced in section 3.1, accompanied with specific flame characteristics such as noise intensity, mean flame propagation speed and edge flame length. In section 3.2, the regime diagram of these flame propagation modes was presented in a graph of “mixture velocity (V) - nominal equivalence ratio (f)”. Brief discussion about this flame phenomenon map was given to analyze the factors that affect the flame propagation process. Note that in all the pictures shown below, the inlet port of hydrogen is on the upper side while that of air is on the lower side. 3.1. Flame propagation modes and characteristics 3.1.1. Mode I: upstream propagating ends with a stable edge flame at the intersection It was found that in most cases, the flame propagated upstream after being ignited at the end of horizontal channel, and finally a stable edge flame could be established at the intersection. In the following, we will exemplify the dynamic process by two cases with a low and high mixture velocity (i.e., V ¼ 2 m/s and 6 m/s), respectively. Fig. 2 shows the flame propagation process under V ¼ 2 m/s, f ¼ 1.0, which were taken from the movie recording. A red line was drawn to indicate flame positions at different instants. Since the external heating for ignition lasts for 5 s, the flame position at t ¼ 0 s in Fig. 2 is a little far from the combustor exit. It can be seen from Fig. 2 that the flame length is very short with fairly weak chemiluminescence. Moreover, it is noted that the flame was quenched in the near-wall region. These phenomena are caused by the limited heat release amount due to the low energy input and also a result of the large heat loss ratio. In the last 1 s, the flame propagation accelerates and shifts to a relatively short edge flame stabilized at the intersection. The total propagation process only takes up 17 s. Fig. 3a shows the upstream propagating process under f ¼ 0.9,
Fig. 2. Upstream propagating ends with a stable edge flame at the intersection under f ¼ 1.0, V ¼ 2 m/s.
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Fig. 3. Upstream propagating ends with a stable edge flame at the intersection under f ¼ 0.9, V ¼ 6 m/s: (a) upstream propagating process, (b) funnel flame, (c) oscillating edge flame.
V ¼ 6 m/s. This figure demonstrates that, compared to the low velocity case (Fig. 2), the flame becomes brighter at V ¼ 6 m/s. Meanwhile, the heat release rate also increases significantly, which is reflected by the hot wall behind the flame. It is observed that within t ¼ 0e250 s, the flame exhibits a funnel shape, as shown in the enlarged Fig. 3b. At t ¼ 125 s, whistle-like noise was heard and disappeared at t ¼ 200 s, however, the flame shape almost remained unchanged, indicating that only very small flame pulsating appeared. At t ¼ 250 s, the flame shifts to an oscillating edge flame (Fig. 3c) and emits buzzing sound. It is worth mentioning that these kinds of noises were also observed by Prakash et al. [35]. Nevertheless, the edge flame remains propagating towards upstream and finally transits to a steady state at the intersection. The whole process lasted for ~340 s in this case, which is around 20 times that of V ¼ 2 m/s. A summary of all the experimental cases demonstrates that there are three scenarios for noise emission: a) no sound; b) only once occurs during flame pulsating (Fig. 4b); and c) twice appear in the first and second stages (Fig. 4a), respectively. Fig. 4 presents two examples of the variation of noise intensity during flame propagation, where the first and second stages are indicated by “1” and “2”, respectively. Since the exact mechanisms for these noise emissions are unclear, only qualitative discussion can be given herein. During the first pulsating stage shown in Fig. 4a (t ¼ 125e200 s), the flame is located at the downstream of the horizontal channel, where a relatively good mixing of H2 and air can be obtained. Thereby, the first flame pulsating is expected to be mainly caused by the gradually increasing heat loss owing to the enhanced axial heat conduction in the solid wall, and meanwhile, the gradually worsen mixing performance might play a secondary role. This hypothesis can be confirmed by the case of Fig. 4b (f ¼ 1.0, V ¼ 5 m/s). Compared to the case of Fig. 4a, the mixture velocity is lower and nominal equivalence ratio is larger, which means that a better mixing can be expected and the equivalence ratio is more suitable. Therefore, the combustion process becomes more intense and the heat release rate is large enough to suppress the flame instability resulting from excessive heat loss rate. The pulsating flame can warm up the upstream wall with the time going. As a result, the flame becomes more robust and the noise intensity decreases gradually (Fig. 4a). However, as the flame reaches a critical location, oscillating edge flame is triggered by the deteriorated mixing of fuel and oxidizer. The turbulent reacting flow can enhance the mixing of H2 and air, which leads to the formation of a stoichiometric plane reflected by a long edge flame. Once the stationary edge flame is established, the noise intensity drops down. Fig. 5 shows the variation of maximum noise intensity of each stage versus the mixture velocity under f ¼ 0.9 and 1.0. As shown in Fig. 5, noise emission occurs only once under V ¼ 5 m/s while occurs twice when V > 5 m/s. In addition, the maximum noise intensity of each stage increases with the increase of mixture velocity.
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(a) =0.9, V=6 m/s
(b) =1.0, V=5 m/s
Fig. 4. Noise intensity during flame propagation under different conditions.
(a) =0.9
(b) =1.0
Fig. 5. Variation of the maximum noise intensity versus mixture velocity.
This indicates that the fuel/air mixing becomes worse and the flame-wall thermal coupling is strengthened at a large mixture velocity. Furthermore, it can be seen that when f ¼ 1.0 (Fig. 5b), the maximum noise intensity of the first stage is higher than that of the second stage, whereas for f ¼ 0.9 (Fig. 5a), the maximum noise intensity of the first stage is lower at V ¼ 6 m/s and then becomes higher when V > 6 m/s. These facts demonstrate that the relative magnitude of noise intensity in the two stages depends on both the mixture velocity and the nominal equivalence ratio. Fig. 6 shows the variation of mean flame propagation speed versus the mixture velocity and nominal equivalence ratio. Overall, the mean flame propagation speed decreases with the increase of mixture velocity and three different zones are marked in Fig. 6a according to the mean flame propagation speed. It should be pointed out that when the mixture velocity is low (V ¼ 0.5 m/s and 1 m/s), the flame flashes back to the intersection very quickly (<1.0 s) without any oscillation. These two cases are classified into the high speed zone and the points are not shown in Fig. 6a because they would be located in much higher positions. As the mixture velocity ranges from 2.0 to 3.0 m/s, the mean flame propagation speed lies within 0.2e1.0 cm/s and is classified into the moderate speed zone. Once the mixture velocity is > 3.0 m/s, the mean propagation speed is < 0.1 cm/s and this mixture velocity range is called low speed zone. Actually, the small mean propagation speed
is because the long duration of flame oscillations (refer to Figs. 3 and 4). Ju and Xu [7] found that flame-wall thermal coupling extended the quenching limit significantly and resulted in a slow flame regime. However, in their study, the flame speed of the slow flame regime is still >10 cm/s, two orders of magnitude larger than that of our cases. Fig. 6b indicates that when the mixture velocity keeps constant (V ¼ 6 m/s), the mean flame propagation speed exhibits a nonmonotonic variation with the increase of nominal equivalence ratio, and it peaks at f ¼ 0.9. This demonstrates that the nonpremixed combustion in this micro-combustor is very complicated because it is affected by many factors, such as fuel/air mixing performance and heat loss rate. Fig. 7 illustrates the images of edge flames under different mixture velocities (f ¼ 1.0) and nominal equivalence ratios (V ¼ 6 m/s), while Fig. 8 presents the corresponding quantitative variation of edge flame length. It is clearly seen from Figs. 7a and 8a that for a fixed nominal equivalence ratio, the edge flame grows longer as the mixture velocity is increased. This is because when the mixture velocity is increased, the fuel supply is increased whereas the mixing grows worse, which means that more time and longer distance are needed for the mixing and combustion processes. Figs. 7b and 8b demonstrate that for a fixed mixture velocity, the
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(a) =1.0
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(b) V=6 m/s
Fig. 6. Variation of mean flame propagation speed with mixture velocity and nominal equivalence ratio.
Fig. 7. Photographs of stable edge flames under different mixture velocities and nominal equivalence ratios: (a) f ¼ 1.0, (b) V ¼ 6 m/s.
(a) =1.0
(b) V=6 m/s
Fig. 8. Variation of edge flame length with mixture velocity and nominal equivalence ratio.
edge flame length varies non-monotonically with an increasing nominal equivalence ratio, and it obtains a peak value at f ¼ 0.9. Obviously, this variation trend is consistent with that of the flame propagation speed (see Fig. 6b), which shows that the combustion
process is most intense around f ¼ 0.9. In other words, the edge flame under f ¼ 0.9 can sustain till a farther downstream region with a lower wall temperature level.
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3.1.2. Mode II: upstream propagates and finally extinguishes In some cases, the flame can propagate upstream after ignition but will be extinguished as it approaches the intersection. This mode mainly occurs under the conditions with a nominal equivalence ratio below unity or very large, and a medium or small mixture velocity. In these cases, as the flame approaches the intersection, the heat release rate will reduce significantly and extinction will occur due to the deteriorated mixing and insufficient fuel supply. 3.1.3. Mode III: propagates upstream and transits to an oscillating edge flame Under conditions of large mixture velocity, the flame will propagate upstream after ignition. However, the edge flame cannot reach the intersection and become stationary in the end, but transits to an oscillating edge flame with noise emission, as exemplified in Fig. 9. This indicates that although the heat release rate is sufficient to compensate the heat loss rate, the local burning velocity cannot exceed the mixture velocity when the flame approaches a certain location. 3.1.4. Mode IV: downstream stabilized flame with a stable diffusion flame at the exit In the cases with both a large mixture velocity and a very high nominal equivalence ratio, the flame cannot travel upstream for a long distance after ignition. Instead, it can be stabilized at a certain location in the downstream channel, as exemplified in Fig. 10. Meanwhile, another blue flame is observed to be anchored at the combustor exit. This is because the redundant fuel is sufficient to sustain a stable diffusion flame at the exhaust port. Similar phenomenon was reported by Aravind and coworkers [42]. 3.1.5. Mode V: internal flame with repetitive extinction and ignition, accompanied by an external flame stabilized at the exit As has been mentioned in the introduction, the flame with repetitive extinction and ignition (FREI) has been extensively investigated since it was observed in a heated micro-channel by Maruta et al. [8,9]. However, to the authors’ knowledge, this flame phenomenon has not been detected in non-premixed micro-combustion to date. Fortunately, in two cases (V ¼ 3 m/s, f ¼ 1.5 and 1.6) of the present investigation, the FREI dynamics was observed. Fig. 11 demonstrates two FREI cycles under V ¼ 3 m/s, f ¼ 1.6, which will be described briefly herein. At first, an internal flame and an external flame appeared simultaneously after the gas mixture was ignited at the end of the channel. It is noted that the internal flame is small and weak due to the very large equivalence ratio and low mixture velocity, and it propagates upstream during t ¼ 0e360 s. The fuel consumption by the internal flame and the gradually increased heat loss (because the distance between the two flames increases) darken the external flame without being extinguished. Here, it should be mentioned that we have raised the brightness of these images to make the external flame visible at its weakest instant, but this makes the flame look red when it is strong. At
Fig. 9. Oscillating edge flame with noise emission near the intersection under f ¼ 1.3, V ¼ 8 m/s.
Fig. 10. Downstream stabilized flame with a stable diffusion flame at the exit under f ¼ 1.5, V ¼ 6 m/s.
Fig. 11. Internal flame with repetitive extinction and ignition, accompanied by an external flame stabilized at the exit: V ¼ 3 m/s, f ¼ 1.6.
Fig. 12. Internal flame with repetitive extinction and ignition, ending with a stable edge flame at the intersection: V ¼ 3 m/s, f ¼ 1.4.
t ¼ ~400 s, extinction of the internal flame occurs as it approaches the intersection. As a result, the external flame turns brighter again with increased fuel leakage. This dynamic process repeats periodically. It is worth mentioning that the external flame acts as a heat source for the re-ignition of the internal flame. Without the external flame, the FREI dynamics cannot be formed, which can be confirmed by Mode II and by the absence of FREI in the cases with a relatively low equivalence ratio.
Fig. 13. Regime diagram of various flame propagation modes.
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Table 1 Correspondence between the abbreviation and flame propagation mode. Abbreviation
Description of flame propagation mode
UP þ SEF UP þ EX UP þ OEF DSF(i)þSDF(o) FREI(i)þSDF(o) FREI þ SEF
Mode Mode Mode Mode Mode Mode
I: Upstream Propagating ends with a Stable Edge Flame at the intersection II: Upstream Propagating ends with an EXtinction III: Upstream Propagating and transits to an Oscillating Edge Flame IV: Downstream Stabilized Flame (inner) with a Stable Diffusion Flame (outer) at the combustor exit V: Flame with Repetitive Extinction and Ignition (inner), accompanied by a Stabilized Diffusion Flame (outer) VI: Flame with Repetitive Extinction and Ignition, and ends with a Stable Edge Flame at the intersection
3.1.6. Mode VI: internal flame with repetitive extinction and ignition, and ends with a stable edge flame at the intersection It is interesting to find that under V ¼ 3 m/s, f ¼ 1.4, the FREI dynamics also takes place, however, the internal flame shifts to a stable edge flame after two cycles, as shown in Fig. 12. Other disparate features are also observed for this case compared to those with f ¼ 1.5 and 1.6. First, the external flame is much weaker under f ¼ 1.4 due to the reduced fuel leakage, and it will be extinguished once the internal flame grows stable. Second, the next period becomes much shorter than the previous one. Specifically, the first and second periods are 189 s and 59 s, respectively, and the final transition to a stable edge flame takes only 3 s. This acceleration trend could be attributed to the heating effect on the wall during flame propagation. 3.2. Flame propagation regime diagram Based on systematic experimental investigation, the regime diagram of aforementioned six flame propagation modes was drawn, as shown in Fig. 13. The variables for the horizontal and vertical axes are mixture velocity (V) and nominal equivalence ratio (f), respectively. For the sake of brevity, abbreviations are used to represent different flame propagation modes. The correspondences between the abbreviation and description of the flame propagation mode are provided in Table 1. Fig. 13 provides an overview of the conditions for the six flame propagation modes. It can be seen that in most cases within f ¼ 0.9e1.4, the flame propagation mode is UP þ SEF, suggesting that the combustion is relatively intense under those conditions and the flames are able to transit to a stable edge flame at last. If the nominal equivalence ratio is higher than 1.4 or lower than 0.9, the region of UP þ EX expands, which can be attributed to the weaker combustion process under larger or smaller equivalence ratios. Moreover, it is interesting to note that for an identical equivalence ratio, the mixture velocity of UP þ EX always lies between those of UP þ SEFs, suggesting that the mixture velocity affects the combustion process from various aspects. For example, with the increase of mixture velocity, the fuel supply is increased whereas the mixing time is reduced (and vice versa). The former is positive to flame stability while the latter is negative. Thereby, the stability of diffusion flames has been demonstrated to be a non-monotonic function of the mixture velocity. Furthermore, Fig. 13 shows that the flame is more likely to be stabilized in downstream channel at relatively large mixture velocity if the nominal equivalence ratio is very high. For example, under f ¼ 1.5 and 1.6, the flames are downstream stabilized ones when V 6 m/s. The explanation of underlying mechanism of FREI could be found in previous section.
4. Conclusions Propagation characteristics of diffusion H2-air flames in a Yshaped micro-combustor were investigated experimentally. The major findings are summarized below.
(1) Six different flame propagation modes were observed, especially, flames with repetitive extinction and ignition (FREI) was identified in non-premixed micro combustion for the first time. (2) Under relatively high mixture velocities, noise emission was noticed during flame propagation towards upstream. (3) For a fixed nominal equivalence ratio, the mean flame propagation speed decreases monotonically with an increasing mixture velocity. However, it shows a nonmonotonic variation with the nominal equivalence ratio and peaks at f ¼ 0.9 when the mixture velocity remains constant. (4) Stable edge flame can be established at the intersection under suitable conditions. The edge flame length increases as the mixture velocity is increased, however, it exhibits a nonmonotonic change with the nominal equivalence ratio, and peaks at f ¼ 0.9. (5) The regime diagram of the six flame propagation modes was drawn, and an overview of the relationship between the inlet condition and flame propagation mode can be obtained. In conclusion, flame propagation dynamics of non-premixed combustion in micro-channels is affected by several key aspects, including fuel/air mixing effectiveness, residence time, heat release rate, and heat loss rate, which depend on two inlet variables, i.e., mixture velocity and nominal equivalence ratio. The tight interplays between these aspects lead to rich flame propagation dynamics. To quantitatively reveal the underlying physics, unsteady three dimensional numerical simulation with detailed chemistry is necessary, which will be our future work. Acknowledgments This work was supported by the National Natural Science Foundation of China under the Grant Number: 51576084. References [1] Fernandez-Pello AC. Micropower generation using combustion: issues and approaches. Proc Combust Inst 2002;29:883e99. [2] Maruta K. Micro and mesoscale combustion. Proc Combust Inst 2011;33: 125e50. [3] Ju Y, Maruta K. Microscale combustion: technology development and fundamental research. Prog Energy Combust Sci 2011;37:669e715. [4] Daou J, Matalon M. Flame propagation in Poiseuille flow under adiabatic conditions. Combust Flame 2001;124:337e49. [5] Daou J, Matalon M. Influence of conductive heat-losses on the propagation of premixed flames in channels. Combust Flame 2002;128:321e39. €hler’s [6] Daou J, Dold J, Matalon M. The thick flame asymptotic limit and Damko hypothesis. Combust Theor Model 2002;6:141e53. [7] Ju Y, Xu B. Effects of channel width and Lewis number on the multiple flame regimes and propagation limits in mesoscale. Combust Sci Technol 2006;178: 1723e53. [8] Maruta K, Park JK, Oh KC, Fujimori T, Minaev S, Fursenko RV. Characteristics of microscale combustion in a narrow heated channel. Combust Explos Shock Waves 2004;40:516e23. [9] Maruta K, Kataoka T, Kim NI, Minaev S, Fursenko R. Characteristics of combustion in a narrow channel with a temperature gradient. Proc Combust Inst 2005;30:2429e36.
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Effects of flow rate and fuel/air ratio on propagation behaviors of diffusion H2/air flames in a micro-combustor Ying Xiang, Zili Yuan, Shixuan Wang, Aiwu Fan* State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology, Wuhan, 430074, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 25 October 2018 Received in revised form 21 April 2019 Accepted 7 May 2019 Available online 8 May 2019
A Y-shaped micro combustor consisting of three quartz tubes with an identical inner diameter of 2.0 mm was designed. Effects of total flow rate and fuel/air ratio on the propagation behaviors of non-premixed hydrogen-air flames were investigated experimentally. Six distinct modes were observed, including flames with repetitive extinction and ignition. Noise emission was detected during flame propagation towards upstream under relatively high mixture velocities. The mean flame propagation speed decreased monotonically with an increasing mixture velocity, however, it exhibited a non-monotonic variation with the nominal equivalence ratio and peaked at f ¼ 0.9. Stable edge flames were obtained at the intersection under suitable conditions, and its length increased as the mixture velocity was increased, but varied non-monotonically with the nominal equivalence ratio and had a maximum value at f ¼ 0.9. The regime diagram of the six flame propagation modes was drawn, which can provide an overview of the relationship between the inlet condition and flame propagation mode. Qualitative analysis showed that flame dynamics of non-premixed micro-combustion was closely associated with the total flow rate and fuel/air ratio by influencing the mixing effectiveness, residence time, heat release rate and heat loss rate. © 2019 Elsevier Ltd. All rights reserved.
Keywords: Micro combustor Non-premixed combustion Flame propagation Edge flame Noise emission Flame dynamics
1. Introduction In recent decades, the emerging of various portable devices and miniature space vehicle has led to vast demands for light, compact and long duration power sources. Current electrochemical batteries might not satisfy these requirements due to their low specific energy, long recharging time and other shortcomings compared to combustion-based micro energy conversion and power generation systems, such as micro thermophotovoltaic (TPV) and thermoelectric (TE) devices, micro engines, and micro propulsion systems [1]. For this reason, combustion of hydrogen and hydrocarbon fuels in micro- or meso-scale combustors has attracted extensive attentions [2]. However, flames stability and combustion efficiency are two major challenges for micro-combustion owing to augmented heat loss ratio, shortened residence time, and the tight coupling between flame and wall [3]. Daou et al. studied the effects of flow velocity [4], Lewis number [5] and channel width [6] on the flame propagation limit in micro-channels, which demonstrated that
* Corresponding author. 1037 Luoyu Road, Wuhan 430074, China. E-mail address: [email protected] (A. Fan). https://doi.org/10.1016/j.energy.2019.05.052 0360-5442/© 2019 Elsevier Ltd. All rights reserved.
near-wall quenching and flame curvature were the major factors leading to flame quenching. Ju and Xu [7] found that flame-wall thermal coupling extended the quenching limit significantly and resulted in a slow flame regime. Maruta et al. observed flames with repetitive extinction and ignition (FREI) in heated U-shaped microchannel [8] and straight micro-channel [9]. Such flame dynamics was also observed in curved mesoscale ducts [10] and mesoscale reactors with a backward facing step in CH4eO2 [11] and CH4eO2eCO2 flames [12]. Fan et al. [13] studied the formation mechanism of FREI based on OH-PLIF technique. Minaev et al. [14] theoretically interpreted the FREI phenomenon. Jackson et al. [15] numerically investigated the FREI in a semi-infinite, thermally active channel. Pizza et al. [16] studied the detailed dynamics of FREI in planar micro-channels with different heights through direct numerical simulation. Miyata et al. [17] scrutinized the influence of wall temperature and incoming mixture temperature on FREI in cylindrical narrow channel. Alipoor et al. numerically explored the detailed behaviors of FREI [18] and the impacts of channel width, equivalence ratio and inlet velocity [19] on H2-air flames in a heated micro-channel. Nair et al. [20] numerically examined the effects of channel diameter and equivalence ratio on the occurrence of FREI in micro-channels with a wall temperature gradient. Stazio et al. [21] experimentally investigated the impact of external heating
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method on the formation of FREI. Kishore et al. [22] numerically studied the effect of wall temperature gradient on FREI dynamics. Kang et al. [23] also numerically demonstrated that flame behaviors are controlled by wall temperature gradients. Various technologies have been proposed to improve flame stability and combustion efficiency in micro- and meso-scale combustors. Kim et al. [24] studied the scale effect of small “Swiss roll” combustors. Zhong et al. [25] compared flame stability in three different types of “Swiss roll” combustors with double spiral-shaped channels. Fan et al. [26] investigated the flame blowoff limit in meso-scale Swiss-roll combustors with/without a bluffbody. Wang et al. [27] numerically demonstrated that the FREI instability can be suppressed by a platinum segment coated on the channel wall at a suitable location. They [28] also revealed that the flame position exhibited an S-shaped curve versus the inlet mixture velocity. Yang et al. [29] found that slight oxygen enrichment can significantly increase the combustion efficiency of a microcombustor with cavity flame holders. Fan et al. [30] designed a micro cavity combustor with double-layered walls to alleviate flame tip opening and reduce fuel leakage. Peng et al. [31] proposed new inlet shapes to enhance the thermal performance of a microcombustor for micro TPV applications. Bani et al. [32] investigated the effects of key parameters on the performance of porous media combustion based micro-TPV system. Li et al. [33] developed a small combustor with a percolated platinum emitter tube for micro-TPV devices. Compared to premixed combustion, non-premixed combustion (diffusion combustion) seems to be more suitable for practical applications from the viewpoint of safety because flame flashback can be inhibited. However, the effective mixing of fuel and oxidizer is an additional challenge for non-premixed combustion. Multiple isolated flame cells were observed by Miesse et al. [34] in a Yshaped non-premixed micro-burner. They found that this phenomenon only occurred for heavier fuels and were not observed in H2eO2 mixtures. For non-premixed CH4eO2 combustion within the same configuration made of alumina, Prakash et al. [35] captured transient flame dynamics leading to the formation of a stable edge flame and distinct cellular structures. Xu and Ju [36] observed isolated flamelets and edge flames of non-premixed CH4eair combustion in a heated meso-scale planar channel. Li et al. [37] numerically investigated the mixing performance and diffusion combustion characteristics of H2 and air in planar microcombustor. Lee and Kim [38] classified the non-premixed CH4eair flames into premixed flame mode and edge flame mode based on their experimental observation. Pham et al. [39] investigated the flame structure of liquid fuel and they found double flames, one anchored at the exit rim and another triple flame located within the combustor chamber. Ning et al. [40] experimentally studied nonpremixed CH4/air flames in Y-shaped meso-scale combustors with different diameters (i.e., 4, 5 and 6 mm). It was shown that all the flames were inclined and propagated downstream after ignition. Wang et al. [41] designed a meso-scale Swiss-roll combustor for non-premixed combustion for the first time. Their experimental investigation showed that flame can be sustained in this novel combustor at very small fuel flow rate and very low equivalence ratio. From the above literature review, it can be known that compared to premixed combustion, the investigation on diffusion flame dynamics in micro- and meso-scale channels are insufficient, especially for the mostly used hydrogen (H2) fuel. As is well known that H2 has much larger diffusion coefficient and heat value compared to CH4, whereas its Lewis number is much less. Therefore, it is speculated that rich diffusion flame dynamics might occur in micro-channels. Motivated by those considerations, in this work we focus our concentration on propagation characteristics of non-
premixed H2/air flames in a Y-shaped microscale combustor with an inner diameter of 2 mm. The significance of the present study lies in two aspects. On the one hand, the results can enrich the fundamental knowledge of diffusion flame propagation in narrow channels. On the other hand, the Y-shaped combustor can be applied to micro TPV [31e33] and TE [42] systems.
2. Design of experimental methodology Fig. 1 schematically shows the experimental system for the present study. A Y-shaped micro-combustor was designed, which was also applied in previous studies by Miesse et al. [34] and Prakash et al. [35] on diffusion combustion in micro-combustors. An included angle of 90 between the inlet ports was adopted because our previous study [43] showed that a relatively larger included angle could intensify the mixing of fuel and oxidant in the Y-shaped micro-combustor. A 200-mm long horizontal tube was used to ensure an ideal mixing such that the mixture can be successfully ignited under all the considered conditions at the end of the horizontal channel. Meanwhile, a sufficient long channel enables us to capture the details of flame propagation dynamics, especially in fast flame regimes. Quartz glass was selected to make the combustor since it can facilitate a direct observation of flame behaviors. The channel diameter is 2.0 mm which is smaller than those used in our previous study [40] for non-premixed CH4-air combustion. This is because H2 has a smaller quenching diameter than CH4 and meanwhile a smaller diameter can enhance the mixing process. The wall thickness of the combustor is 1.0 mm to ensure the mechanical strength of the suspended horizontal channel. H2 of 99.99% purity and dry air under high pressure were stored in two gas tanks. After the pressure reducing valves, the gas pressures were reduced to 1.0 atm. The flow rates of H2 and air were controlled by two digital mass-flow meters (Alicat Scientific Inc.) with an accuracy of 0.2%. The measurement ranges of the mass-flow meters are 0.005e1.0 SLM for H2 side and 0.015e3.0 SLM for air side, respectively. The mixture was ignited with a butane torch by heating the 2.0-cm long external wall near the combustor exit for 5 s. Right after the ignition, a still digital single lens reflex camera (Canon EOS6D) was used to take flame images and make movie recordings of flame propagation dynamics in dark background for better quality of photographs. Each case was started from cold state to ensure all the results were obtained under the same initial condition. A digital noise meter (WS1361) with an accuracy of ±1.5 dB was employed to record the noise emission during the combustion process. In the data processing, the background noise was eliminated. A stainless steel ruler was applied to determine the flame position and a digital timer was used to record the flame propagation time, based on
Fig. 1. Schematic diagram of the experimental system: 1-manual valve; 2-pressure gauge; 3-pressure reducing valve; 4-mass flow controller; 5-DSLR camera; 6-micro combustor; 7-digital noise meter; 8-laptop computer.
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which the mean flame propagation speed could be calculated. In this paper, a term “nominal equivalence ratio (f)” was used to characterize the relative amount of fuel and air, whose value is identical to the equivalence ratio of the perfect fuel/air mixture. Meanwhile, the word “mixture velocity (V)” was adopted to represent the total flow rate of cold gaseous mixture. Since the inner diameters of the three channels are identical, the mixing velocity (V) is equal to the sum of inlet velocities of air (Vair) and hydrogen (VH2). The range of nominal equivalence ratio was f ¼ 0.5e1.6 with an interval of 0.1. Meanwhile, the range of mixture velocity was 0.5e13 m/s with an interval of 1.0 m/s, except for the very low velocity case, i.e., V ¼ 0.5 m/s. Within these ranges of operational parameters, the diffusion flame propagation behaviors that occur in the Y-shaped micro-combustor can be fully revealed. 3. Results and discussion Six flame propagation modes were observed in our experiment, which will be introduced in section 3.1, accompanied with specific flame characteristics such as noise intensity, mean flame propagation speed and edge flame length. In section 3.2, the regime diagram of these flame propagation modes was presented in a graph of “mixture velocity (V) - nominal equivalence ratio (f)”. Brief discussion about this flame phenomenon map was given to analyze the factors that affect the flame propagation process. Note that in all the pictures shown below, the inlet port of hydrogen is on the upper side while that of air is on the lower side. 3.1. Flame propagation modes and characteristics 3.1.1. Mode I: upstream propagating ends with a stable edge flame at the intersection It was found that in most cases, the flame propagated upstream after being ignited at the end of horizontal channel, and finally a stable edge flame could be established at the intersection. In the following, we will exemplify the dynamic process by two cases with a low and high mixture velocity (i.e., V ¼ 2 m/s and 6 m/s), respectively. Fig. 2 shows the flame propagation process under V ¼ 2 m/s, f ¼ 1.0, which were taken from the movie recording. A red line was drawn to indicate flame positions at different instants. Since the external heating for ignition lasts for 5 s, the flame position at t ¼ 0 s in Fig. 2 is a little far from the combustor exit. It can be seen from Fig. 2 that the flame length is very short with fairly weak chemiluminescence. Moreover, it is noted that the flame was quenched in the near-wall region. These phenomena are caused by the limited heat release amount due to the low energy input and also a result of the large heat loss ratio. In the last 1 s, the flame propagation accelerates and shifts to a relatively short edge flame stabilized at the intersection. The total propagation process only takes up 17 s. Fig. 3a shows the upstream propagating process under f ¼ 0.9,
Fig. 2. Upstream propagating ends with a stable edge flame at the intersection under f ¼ 1.0, V ¼ 2 m/s.
317
Fig. 3. Upstream propagating ends with a stable edge flame at the intersection under f ¼ 0.9, V ¼ 6 m/s: (a) upstream propagating process, (b) funnel flame, (c) oscillating edge flame.
V ¼ 6 m/s. This figure demonstrates that, compared to the low velocity case (Fig. 2), the flame becomes brighter at V ¼ 6 m/s. Meanwhile, the heat release rate also increases significantly, which is reflected by the hot wall behind the flame. It is observed that within t ¼ 0e250 s, the flame exhibits a funnel shape, as shown in the enlarged Fig. 3b. At t ¼ 125 s, whistle-like noise was heard and disappeared at t ¼ 200 s, however, the flame shape almost remained unchanged, indicating that only very small flame pulsating appeared. At t ¼ 250 s, the flame shifts to an oscillating edge flame (Fig. 3c) and emits buzzing sound. It is worth mentioning that these kinds of noises were also observed by Prakash et al. [35]. Nevertheless, the edge flame remains propagating towards upstream and finally transits to a steady state at the intersection. The whole process lasted for ~340 s in this case, which is around 20 times that of V ¼ 2 m/s. A summary of all the experimental cases demonstrates that there are three scenarios for noise emission: a) no sound; b) only once occurs during flame pulsating (Fig. 4b); and c) twice appear in the first and second stages (Fig. 4a), respectively. Fig. 4 presents two examples of the variation of noise intensity during flame propagation, where the first and second stages are indicated by “1” and “2”, respectively. Since the exact mechanisms for these noise emissions are unclear, only qualitative discussion can be given herein. During the first pulsating stage shown in Fig. 4a (t ¼ 125e200 s), the flame is located at the downstream of the horizontal channel, where a relatively good mixing of H2 and air can be obtained. Thereby, the first flame pulsating is expected to be mainly caused by the gradually increasing heat loss owing to the enhanced axial heat conduction in the solid wall, and meanwhile, the gradually worsen mixing performance might play a secondary role. This hypothesis can be confirmed by the case of Fig. 4b (f ¼ 1.0, V ¼ 5 m/s). Compared to the case of Fig. 4a, the mixture velocity is lower and nominal equivalence ratio is larger, which means that a better mixing can be expected and the equivalence ratio is more suitable. Therefore, the combustion process becomes more intense and the heat release rate is large enough to suppress the flame instability resulting from excessive heat loss rate. The pulsating flame can warm up the upstream wall with the time going. As a result, the flame becomes more robust and the noise intensity decreases gradually (Fig. 4a). However, as the flame reaches a critical location, oscillating edge flame is triggered by the deteriorated mixing of fuel and oxidizer. The turbulent reacting flow can enhance the mixing of H2 and air, which leads to the formation of a stoichiometric plane reflected by a long edge flame. Once the stationary edge flame is established, the noise intensity drops down. Fig. 5 shows the variation of maximum noise intensity of each stage versus the mixture velocity under f ¼ 0.9 and 1.0. As shown in Fig. 5, noise emission occurs only once under V ¼ 5 m/s while occurs twice when V > 5 m/s. In addition, the maximum noise intensity of each stage increases with the increase of mixture velocity.
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(a) =0.9, V=6 m/s
(b) =1.0, V=5 m/s
Fig. 4. Noise intensity during flame propagation under different conditions.
(a) =0.9
(b) =1.0
Fig. 5. Variation of the maximum noise intensity versus mixture velocity.
This indicates that the fuel/air mixing becomes worse and the flame-wall thermal coupling is strengthened at a large mixture velocity. Furthermore, it can be seen that when f ¼ 1.0 (Fig. 5b), the maximum noise intensity of the first stage is higher than that of the second stage, whereas for f ¼ 0.9 (Fig. 5a), the maximum noise intensity of the first stage is lower at V ¼ 6 m/s and then becomes higher when V > 6 m/s. These facts demonstrate that the relative magnitude of noise intensity in the two stages depends on both the mixture velocity and the nominal equivalence ratio. Fig. 6 shows the variation of mean flame propagation speed versus the mixture velocity and nominal equivalence ratio. Overall, the mean flame propagation speed decreases with the increase of mixture velocity and three different zones are marked in Fig. 6a according to the mean flame propagation speed. It should be pointed out that when the mixture velocity is low (V ¼ 0.5 m/s and 1 m/s), the flame flashes back to the intersection very quickly (<1.0 s) without any oscillation. These two cases are classified into the high speed zone and the points are not shown in Fig. 6a because they would be located in much higher positions. As the mixture velocity ranges from 2.0 to 3.0 m/s, the mean flame propagation speed lies within 0.2e1.0 cm/s and is classified into the moderate speed zone. Once the mixture velocity is > 3.0 m/s, the mean propagation speed is < 0.1 cm/s and this mixture velocity range is called low speed zone. Actually, the small mean propagation speed
is because the long duration of flame oscillations (refer to Figs. 3 and 4). Ju and Xu [7] found that flame-wall thermal coupling extended the quenching limit significantly and resulted in a slow flame regime. However, in their study, the flame speed of the slow flame regime is still >10 cm/s, two orders of magnitude larger than that of our cases. Fig. 6b indicates that when the mixture velocity keeps constant (V ¼ 6 m/s), the mean flame propagation speed exhibits a nonmonotonic variation with the increase of nominal equivalence ratio, and it peaks at f ¼ 0.9. This demonstrates that the nonpremixed combustion in this micro-combustor is very complicated because it is affected by many factors, such as fuel/air mixing performance and heat loss rate. Fig. 7 illustrates the images of edge flames under different mixture velocities (f ¼ 1.0) and nominal equivalence ratios (V ¼ 6 m/s), while Fig. 8 presents the corresponding quantitative variation of edge flame length. It is clearly seen from Figs. 7a and 8a that for a fixed nominal equivalence ratio, the edge flame grows longer as the mixture velocity is increased. This is because when the mixture velocity is increased, the fuel supply is increased whereas the mixing grows worse, which means that more time and longer distance are needed for the mixing and combustion processes. Figs. 7b and 8b demonstrate that for a fixed mixture velocity, the
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(a) =1.0
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(b) V=6 m/s
Fig. 6. Variation of mean flame propagation speed with mixture velocity and nominal equivalence ratio.
Fig. 7. Photographs of stable edge flames under different mixture velocities and nominal equivalence ratios: (a) f ¼ 1.0, (b) V ¼ 6 m/s.
(a) =1.0
(b) V=6 m/s
Fig. 8. Variation of edge flame length with mixture velocity and nominal equivalence ratio.
edge flame length varies non-monotonically with an increasing nominal equivalence ratio, and it obtains a peak value at f ¼ 0.9. Obviously, this variation trend is consistent with that of the flame propagation speed (see Fig. 6b), which shows that the combustion
process is most intense around f ¼ 0.9. In other words, the edge flame under f ¼ 0.9 can sustain till a farther downstream region with a lower wall temperature level.
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3.1.2. Mode II: upstream propagates and finally extinguishes In some cases, the flame can propagate upstream after ignition but will be extinguished as it approaches the intersection. This mode mainly occurs under the conditions with a nominal equivalence ratio below unity or very large, and a medium or small mixture velocity. In these cases, as the flame approaches the intersection, the heat release rate will reduce significantly and extinction will occur due to the deteriorated mixing and insufficient fuel supply. 3.1.3. Mode III: propagates upstream and transits to an oscillating edge flame Under conditions of large mixture velocity, the flame will propagate upstream after ignition. However, the edge flame cannot reach the intersection and become stationary in the end, but transits to an oscillating edge flame with noise emission, as exemplified in Fig. 9. This indicates that although the heat release rate is sufficient to compensate the heat loss rate, the local burning velocity cannot exceed the mixture velocity when the flame approaches a certain location. 3.1.4. Mode IV: downstream stabilized flame with a stable diffusion flame at the exit In the cases with both a large mixture velocity and a very high nominal equivalence ratio, the flame cannot travel upstream for a long distance after ignition. Instead, it can be stabilized at a certain location in the downstream channel, as exemplified in Fig. 10. Meanwhile, another blue flame is observed to be anchored at the combustor exit. This is because the redundant fuel is sufficient to sustain a stable diffusion flame at the exhaust port. Similar phenomenon was reported by Aravind and coworkers [42]. 3.1.5. Mode V: internal flame with repetitive extinction and ignition, accompanied by an external flame stabilized at the exit As has been mentioned in the introduction, the flame with repetitive extinction and ignition (FREI) has been extensively investigated since it was observed in a heated micro-channel by Maruta et al. [8,9]. However, to the authors’ knowledge, this flame phenomenon has not been detected in non-premixed micro-combustion to date. Fortunately, in two cases (V ¼ 3 m/s, f ¼ 1.5 and 1.6) of the present investigation, the FREI dynamics was observed. Fig. 11 demonstrates two FREI cycles under V ¼ 3 m/s, f ¼ 1.6, which will be described briefly herein. At first, an internal flame and an external flame appeared simultaneously after the gas mixture was ignited at the end of the channel. It is noted that the internal flame is small and weak due to the very large equivalence ratio and low mixture velocity, and it propagates upstream during t ¼ 0e360 s. The fuel consumption by the internal flame and the gradually increased heat loss (because the distance between the two flames increases) darken the external flame without being extinguished. Here, it should be mentioned that we have raised the brightness of these images to make the external flame visible at its weakest instant, but this makes the flame look red when it is strong. At
Fig. 9. Oscillating edge flame with noise emission near the intersection under f ¼ 1.3, V ¼ 8 m/s.
Fig. 10. Downstream stabilized flame with a stable diffusion flame at the exit under f ¼ 1.5, V ¼ 6 m/s.
Fig. 11. Internal flame with repetitive extinction and ignition, accompanied by an external flame stabilized at the exit: V ¼ 3 m/s, f ¼ 1.6.
Fig. 12. Internal flame with repetitive extinction and ignition, ending with a stable edge flame at the intersection: V ¼ 3 m/s, f ¼ 1.4.
t ¼ ~400 s, extinction of the internal flame occurs as it approaches the intersection. As a result, the external flame turns brighter again with increased fuel leakage. This dynamic process repeats periodically. It is worth mentioning that the external flame acts as a heat source for the re-ignition of the internal flame. Without the external flame, the FREI dynamics cannot be formed, which can be confirmed by Mode II and by the absence of FREI in the cases with a relatively low equivalence ratio.
Fig. 13. Regime diagram of various flame propagation modes.
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Table 1 Correspondence between the abbreviation and flame propagation mode. Abbreviation
Description of flame propagation mode
UP þ SEF UP þ EX UP þ OEF DSF(i)þSDF(o) FREI(i)þSDF(o) FREI þ SEF
Mode Mode Mode Mode Mode Mode
I: Upstream Propagating ends with a Stable Edge Flame at the intersection II: Upstream Propagating ends with an EXtinction III: Upstream Propagating and transits to an Oscillating Edge Flame IV: Downstream Stabilized Flame (inner) with a Stable Diffusion Flame (outer) at the combustor exit V: Flame with Repetitive Extinction and Ignition (inner), accompanied by a Stabilized Diffusion Flame (outer) VI: Flame with Repetitive Extinction and Ignition, and ends with a Stable Edge Flame at the intersection
3.1.6. Mode VI: internal flame with repetitive extinction and ignition, and ends with a stable edge flame at the intersection It is interesting to find that under V ¼ 3 m/s, f ¼ 1.4, the FREI dynamics also takes place, however, the internal flame shifts to a stable edge flame after two cycles, as shown in Fig. 12. Other disparate features are also observed for this case compared to those with f ¼ 1.5 and 1.6. First, the external flame is much weaker under f ¼ 1.4 due to the reduced fuel leakage, and it will be extinguished once the internal flame grows stable. Second, the next period becomes much shorter than the previous one. Specifically, the first and second periods are 189 s and 59 s, respectively, and the final transition to a stable edge flame takes only 3 s. This acceleration trend could be attributed to the heating effect on the wall during flame propagation. 3.2. Flame propagation regime diagram Based on systematic experimental investigation, the regime diagram of aforementioned six flame propagation modes was drawn, as shown in Fig. 13. The variables for the horizontal and vertical axes are mixture velocity (V) and nominal equivalence ratio (f), respectively. For the sake of brevity, abbreviations are used to represent different flame propagation modes. The correspondences between the abbreviation and description of the flame propagation mode are provided in Table 1. Fig. 13 provides an overview of the conditions for the six flame propagation modes. It can be seen that in most cases within f ¼ 0.9e1.4, the flame propagation mode is UP þ SEF, suggesting that the combustion is relatively intense under those conditions and the flames are able to transit to a stable edge flame at last. If the nominal equivalence ratio is higher than 1.4 or lower than 0.9, the region of UP þ EX expands, which can be attributed to the weaker combustion process under larger or smaller equivalence ratios. Moreover, it is interesting to note that for an identical equivalence ratio, the mixture velocity of UP þ EX always lies between those of UP þ SEFs, suggesting that the mixture velocity affects the combustion process from various aspects. For example, with the increase of mixture velocity, the fuel supply is increased whereas the mixing time is reduced (and vice versa). The former is positive to flame stability while the latter is negative. Thereby, the stability of diffusion flames has been demonstrated to be a non-monotonic function of the mixture velocity. Furthermore, Fig. 13 shows that the flame is more likely to be stabilized in downstream channel at relatively large mixture velocity if the nominal equivalence ratio is very high. For example, under f ¼ 1.5 and 1.6, the flames are downstream stabilized ones when V 6 m/s. The explanation of underlying mechanism of FREI could be found in previous section.
4. Conclusions Propagation characteristics of diffusion H2-air flames in a Yshaped micro-combustor were investigated experimentally. The major findings are summarized below.
(1) Six different flame propagation modes were observed, especially, flames with repetitive extinction and ignition (FREI) was identified in non-premixed micro combustion for the first time. (2) Under relatively high mixture velocities, noise emission was noticed during flame propagation towards upstream. (3) For a fixed nominal equivalence ratio, the mean flame propagation speed decreases monotonically with an increasing mixture velocity. However, it shows a nonmonotonic variation with the nominal equivalence ratio and peaks at f ¼ 0.9 when the mixture velocity remains constant. (4) Stable edge flame can be established at the intersection under suitable conditions. The edge flame length increases as the mixture velocity is increased, however, it exhibits a nonmonotonic change with the nominal equivalence ratio, and peaks at f ¼ 0.9. (5) The regime diagram of the six flame propagation modes was drawn, and an overview of the relationship between the inlet condition and flame propagation mode can be obtained. In conclusion, flame propagation dynamics of non-premixed combustion in micro-channels is affected by several key aspects, including fuel/air mixing effectiveness, residence time, heat release rate, and heat loss rate, which depend on two inlet variables, i.e., mixture velocity and nominal equivalence ratio. The tight interplays between these aspects lead to rich flame propagation dynamics. To quantitatively reveal the underlying physics, unsteady three dimensional numerical simulation with detailed chemistry is necessary, which will be our future work. Acknowledgments This work was supported by the National Natural Science Foundation of China under the Grant Number: 51576084. References [1] Fernandez-Pello AC. Micropower generation using combustion: issues and approaches. Proc Combust Inst 2002;29:883e99. [2] Maruta K. Micro and mesoscale combustion. Proc Combust Inst 2011;33: 125e50. [3] Ju Y, Maruta K. Microscale combustion: technology development and fundamental research. Prog Energy Combust Sci 2011;37:669e715. [4] Daou J, Matalon M. Flame propagation in Poiseuille flow under adiabatic conditions. Combust Flame 2001;124:337e49. [5] Daou J, Matalon M. Influence of conductive heat-losses on the propagation of premixed flames in channels. Combust Flame 2002;128:321e39. €hler’s [6] Daou J, Dold J, Matalon M. The thick flame asymptotic limit and Damko hypothesis. Combust Theor Model 2002;6:141e53. [7] Ju Y, Xu B. Effects of channel width and Lewis number on the multiple flame regimes and propagation limits in mesoscale. Combust Sci Technol 2006;178: 1723e53. [8] Maruta K, Park JK, Oh KC, Fujimori T, Minaev S, Fursenko RV. Characteristics of microscale combustion in a narrow heated channel. Combust Explos Shock Waves 2004;40:516e23. [9] Maruta K, Kataoka T, Kim NI, Minaev S, Fursenko R. Characteristics of combustion in a narrow channel with a temperature gradient. Proc Combust Inst 2005;30:2429e36.
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