air catalytic combustion characteristic of micro–combustors with a conventional, slotted or controllable slotted bluff body

Energy 189 (2019) 116242

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

Numerical comparison of H2/air catalytic combustion characteristic of microecombustors with a conventional, slotted or controllable slotted bluff body Yunfei Yan a, *, Ying Liu a, Lixian Li b, **, Yu Cui a, Li Zhang a, Zhongqing Yang a, Zhien Zhang a a b

Key Laboratory of Lowegrade Energy Utilization Technologies and Systems, Chongqing University, Chongqing, 400030, PR China Chongqing University Cancer Hospital & Chongqing Cancer Institute & Chongqing Cancer Hospital, Chongqing, 400030, PR China

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 23 September 2019 Accepted 28 September 2019 Available online 30 September 2019

The flame instability and low combustion efficiency are still two vital problems to be solved in micro ecombustion power field. In this work, the detailed chemical reaction mechanism of H2/O2 is used to numerical study the combustion characteristics of microecombustors under three kinds of bluff body structures. The results show that a recirculation zone is formed in the conventional bluff body (CBB) microecombustor, while two symmetric recirculation zones are formed in the slotted bluff body (SBB) microecombustor and controllable slotted bluff body (CSBB) microecombustor. Notably, the recirculation zones in the CSBB microecombustor is significantly prolonged with the increase of controllable flow ratios. Meanwhile, the corresponding equivalence ratio reaching a maximum combustion efficiency is 0.8 in the CSBB microecombustor, while the CBB and SBB microecombustors reach the highest combustion efficiency at 4 ¼ 1.0. This means that the CSBB microecombustor exhibits superior combustion performance in the fuel lean condition (4 ¼ 0.8). Additionally, the adjustment of the controllable flow ratios can significantly affect the combustion efficiency of the CSBB microecombustor. Moreover, the bloweoff limits of CBB, SBB and CSBB microecombustors achieve the maximum at 4 ¼ 1.0, which are about 540, 456 and 600 cm3/s, respectively. Thus, the proposed CSBB microecombustor is of notable advantages compared with the other two microecombustors in terms of combustion efficiency, temperature distribution, bloweoff limit, and so on. Another interesting finding is that the CSBB microecombustor with a gap width of 0.6 mm has better combustion performance. In conclusion, the controllable slotted bluff body is a good way to stabilize the flame in microecombustors. This design offers us another way to design such kind of microecombustors. © 2019 Elsevier Ltd. All rights reserved.

Keywords: Controllable slotted bluff body Recirculation zone Micro combustion Catalytic combustion stabilization

1. Introduction With the rapid development of micro-electro-mechanical system (MEMS), microecombustion devices have drawn extensive concern due to hydrocarbon fuels have merits of high energy density, small volume and long work time [1]. Meanwhile, there are two challenges that need to be addressed in developing microecombustors compared with the conventional scale combustor. One is that the flame is hard to attain stable combustion

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (Y. Yan), [email protected] (L. Li). https://doi.org/10.1016/j.energy.2019.116242 0360-5442/© 2019 Elsevier Ltd. All rights reserved.

[2], since the residence time of the premixed gas under reduced dimension is shortened. Another critical issue is the high heat losses rate due to the large surface areaetoevolume ratio in microecombustors [3]. For addressing these issues, many scholars have made tremendous contributions to improve the flame stability of premixed combustion in micro combustors. Ten years ago, Kim et al. [4] studied all kinds of the propane-air mixture Swiss Roll combustors experimentally in order to improve flames stability. In later years, Pan et al. [5] and Li et al. [6] used the porous media to strengthen heat transfer and improve flame stabilities in micro channels. Due to hydrogen has highest energy density values compared with the conventional hydrocarbon fuels [7]. Hence many scholars began to study the combustion characteristic of hydrogen fuel in

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microecombustors. Wan et al. [8] came up with a micro cavity combustor and revealed the flameeanchoring mechanism for premixed H2/air flame. Yilmaz et al. [9] investigated the microecombustor geometry by adding backward facing step, cavity and multi-channels. The results showed that narrow channel height has more important effect on temperature distribution and emanated NOx levels than backward facing step arrangement in premixed hydrogen/air mixture microecombustors. The combustion performance of H2/air premixed combustion in a micro elliptical tube combustor was studied by Zuo et al. [10] via numerical study. They found that the micro elliptical tube combustor has higher emitter efficiency and combustion efficiency, compared with the micro circular tube combustor. Besides, the catalytic combustion characteristics of the combustor with/without preheating channels are numerically studied at steady conditions by Yan et al. [11]. In their study, preheating channels facilitated heat recirculation effect, and heat recirculation rate exceeded 10% for all cases. The high surface area to volume ratio is also ideal for use with a catalyst, as increased surface area favors catalytic combustion by providing more sites upon which the fuel and oxidizer can adsorb [12]. Hence, the catalytic combustion has been put forward to improve the flame stability and conversion efficiency in microecombustors. Boyarko et al. [13] confirmed that the catalytic combustion was well fitted for subemillimeter scale combustors. Qazi Zade et al. [14] showed the importance of surface reactions in the stability of hydrogeneair combustion. Brambilla et al. [15] took the influence of wall material, inlet velocity and inlet temperature into account. They investigated lean hydrogeneair combustion in a planar channel coated with platinum through numerical simulation. Recirculation zone is another classic strategy of flame stability by utilizing a bluff body. The combustion characteristics of premixed hydrogen/air flame in a microecombustor with a bluff body was studied by Wan et al. [16] and Fan et al. [17]. They found that the bloweoff limit was greatly extended as compared with the microecombustor without a bluff body. Then, Fan et al. [18] investigated the effects of the bluff body size, blockage ratio, equivalence ratio and solid materials on the bloweoff limit. After that Fan et al. [19] demonstrated that bluff body shapes play an important role in flame stability. Meanwhile, nine different shapes of bluff body were numerical studied by Bagheri et al. [20] with a twoedimensional axisymmetric model. The effects of bluff body shapes, bloweoff limit, combustion efficiency and wall temperature of the microetube combustor were analyzed. Yan et al. [21] have done a systematic numerical simulation work of recirculation zones in micro combustors. With the increase of inlet velocity, the recirculation zone expands and preferential transport effect behind the bluff body is intensified. The blow-off limit is significantly improved with the use of regular triangular pyramid bluff body. Sun et al. [22] studied the differences between the conventional bluff body (CBB) microecombustor and the slotted bluff body (SBB) microecombustor. The results showed that the flame temperature distribution in the SBB microecombustor is more uniform and the combustion efficiency was improved compared to the CBB microecombustor. In addition, the controllable slotted bluff body technology was applied to pulverized coal combustors at the first time. Zeng et al. [23] developed a new type of controllable vortex pulverized coal combustor, which can not only control the length of the recirculation zone, but also enhance the turbulence intensity of airflow and raise the flame temperature. Although a variety of flame stabilization and combustion efficiency approaches have been studied, simple yet effective methods are still desirable to widen the stable operating range of micro combustors. As is known to all, bluff bodies are widely used in

various industrial combustion and propulsion systems for flame stability [24]. Especially the controllable slotted bluff body (CSBB) microecombustor has better combustion characteristics. Yan et al. [25] mainly investigated the effect of controllable slit width in a micro combustor with a sides-slitted bluff body. And they found that narrow controllable slit width can significantly improve the combustion efficiency. Besides, Baigmohammadi et al. [26] showed that a catalytic segmented bluffebody had significant effects on flame stability and flame location in the micro reactor. Similarly, Wierzbicki et al. [27] found the use of catalyst greatly expanded the range of extinction limits and flow rates in a singleeturn swiss roll combustor. However, there are relatively few studies devoted to the catalytic combustion characteristic of the CSBB microecombustor. Motivated by the aforementioned works, we developed a catalytic microecombustor with a controllable slotted bluff body to prolong the stable combustion region of premixed H2/air flame in microechannels in the present work. Further work was focus on comparing the combustion characteristics among the CBB microecombustor, the SBB microecombustor and the developed CSBB microecombustor via establishing threeedimensional models of bluff body microecombustors numerically. The bluff body structures, bloweoff limits, exhaust gas temperature, equivalence ratios and combustion efficiency were investigated. In this work, the developed CSBB microecombustor is of notable advantages compared with the other two microecombustors. More importantly, we explored effects of gap width on combustion characteristics in the CSBB microecombustor. The present work provides an important guideline to design such kind of microecombustors. 2. Numerical methods 2.1. Geometric and mathematical models In order to study the effect of different bluff bodies on the combustion characteristics of microecombustors, three bluff body microecombustors are schematically illustrated in Fig. 1. All the values of geometrical parameters are summarized in Table 1. All of the three combustors have the same dimensions (Table 1). As depicted in Fig. 1, the width (W) and length (L) of the three combustors are 6.0 mm and 20.0 mm, respectively, while the height (H) is 2.0 mm. The wall thickness (d) is 0.5 mm. The crossesection of bluffebody is an approximately equilateral triangle with a sideelength of 3.0 mm, symmetrically placed at the entrance of combustion chamber. And the bluff body obstruction ratio is 0.5 [28]. The combustion process involves the interaction of flow, chemical reactions and heat transfer. Hence, the continuity, momentum, energy and species equations are used to solve the problem. The governing equations for the gaseous mixture are shown in the literature [29]. 2.2. Computation scheme In this paper, we select hydrogen as the fuel due to it has much higher burning velocity than other hydrocarbon fuels [30]. The reaction mechanism reported by Li et al. [31] is applied to the combustion of H2/air mixtures. It consists of 19 reversible elementary reactions and 13 species. Additionally, component transport model is utilized. The thermal conductivity, heat capacity and viscosity of gas mixture are calculated as a mass fractioneweighted mean of all species. Some assumptions are shown in the literature [32]. Owing to width to height ratio of the microecombustors is three, the effect of width direction on the combustion characteristics cannot be ignored. Therefore,

Y. Yan et al. / Energy 189 (2019) 116242

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Fig. 1. Schematic diagram of microecombustors.

Table 1 Values of the geometrical parameters. Type of microecombustors Parameter (mm)

CBB microecombustor

SBB microecombustor

CSBB microecombustor

Length from inlet of combustors to the front wall of bluff body (La1, Lb1, Lc1) Front wall width of bluff body (wa1, wb1, wc1) Sideelength of bluff body (wa2, wb2, wc2) Gap width (db, dc)

1.0 0.2 3.0 0

1.0 0.2 3.0 0.3

1.0 0.2 3.0 0.3

threeedimensional bluff body combustor models are established in this work. The materials of the micro combustor wall and bluff body are quartz. Its density, heat capacity and thermal conductivity are 2650 kg/m3, 750 J/(kg$K), 1.05 W/(m$K), respectively. At the exterior surfaces, heat losses via both natural convection and thermal radiation are considered, which can be calculated through Eq. (1):

  q ¼ hðTw  T∞ Þ þ as T 4w  T 4∞

(1)

where h is the heat transfer coefficient of natural convection, Tw is the outer wall temperature and T∞ is the ambient temperature with a constant value of 300 K, a is the emissivity of combustor wall of 0.92 and s is the StephaneBoltzmann constant (5.67  108 W/ (m2$K4)). In order to maintain a high temperature level of the surfaces, the natural convection heat transfer coefficient is about 20 W/(m2$K) [33]. In this work, Pl radiation model is adopted in calculating radiation heat transfer. Kuo and Ronney [34] reported that it is more appropriate to predict the combustion characteristics by using a turbulence model when the Reynolds number is above 500 in microecombustors. In our case, the Reynolds number is greater than 500 for all premixed gas inlet velocity. Therefore, the Realizable keε turbulence model is adopted here. Pressure outlet and adiabatic wall are set as boundary conditions. For convenience of description, the concept of equivalence ratio and controllable flow ratio are introduced. The controllable flow ratio (a) is defined as the ratio of the gas flow rate in the controllable inlet channel to the main inlet channel of the CSBB microecombustor. The equivalence ratio (4) is a commonly used index to indicate quantitatively whether the fueleair mixture for a chemical reaction is rich (4 > 1), lean (4 < 1), or stoichiometric (4 ¼ 1). The equivalence ratio is defined as [35]:

4¼

ðF=AÞ ðF=AÞstoic

2.3. Grid independence Fig. 2 presents three mesh models (coarse, medium and fine) at different mesh densities. The mesh numbers of coarse, medium and fine models are 193457 (mesh 1), 458589 (mesh 2) and 576584 (mesh 3), respectively. In order to determine a proper mesh model, Fig. 2 shows the effect of mesh numbers on the outer wall temperature. It can be observed in Fig. 2 that the three mesh models have the same trends. The curve of 193457 cells has lower quality of grid and it affects the simulation results, while the gas temperature differences between the other two meshes which have higher grid quality are small at the same location. Moreover, most part of the temperature lines of 458589 and 576584 cells is overlapped. In order to save computational efforts and improve accuracy the intermediate mesh with 458589 cells is adopted. Similarly, the mesh numbers of the SBB microecombustor and the CSBB microecombustor are 471581 and 480983, respectively.

(2)

where (F/A) and (F/A)stoic are the real and stoichiometric mass ratios of the fuel (F) to the air (A).

Fig. 2. Outer wall temperature distribution of the CBB microecombustor at different mesh numbers.

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2.4. Model verification In order to verify the accuracy of the numerical model and calculation method, a validation of the computational method is performed by comparing the numerical results with the experimental results of Wan et al. [28] under the same conditions. Fig. 3 presents the outer wall temperature under the numerical models and the experimental results in the literature [28]. It can be obviously observed in Fig. 3 that the numerical and experimental data have a similar variation trend. The maximum and the mean relative error of the two profiles are 4.3% and 2.8%, respectively. It should also be noted that the difference between the numerical and experimental results is large at the inlet, and then it becomes smaller with the increase of inlet velocity. The main reason is that the adiabatic wall boundary condition is adopted in the computational model while there is still some heat loss in experiment in spite of insulation measures. Thereby, it is reasonable that the numerical results are higher and the computational result matched well with the experimental outcome. Regarding all these above, the predicted results of the proposed model of this study are effective and acceptable. 3. Results and discussions

3.2. Temperature distributions of three bluff body microecombustors

3.1. Velocity distribution of three bluff body microecombustors Velocity fields of the middle crossesection (z ¼ 0 mm) under different bluffebody structures are shown in Fig. 4. The equivalence ratio (4) and premixed gas flow rate (Vin) are kept at 0.6 and 120 cm3/s, while the controllable flow rate ratio (a) are 0.1, 0.2, 0.3 and 0.4, respectively. Fig. 4 illustrates that the recirculation zones behind the bluff body of the SBB microecombustor and CSBB microecombustor are significantly prolonged compared with the CBB microecombustor. Negative values indicate that the direction of gas flow velocity is opposite to the direction of the inlet velocity of premixed gas. The average outlet velocity of the CSBB microecombustor is 45.65 m/s, which is 1.33 and 1.14 times higher than that in the CBB microecombustor and SBB microecombustor, respectively. In addition, two special waist flares are formed on both sides of the slotted bluff body and controllable slotted bluff body, which is contribute to forming recirculation zone. Another interesting finding is that there is only one in the CBB microecombustor. Due to the influence of the controllable gap, two recirculation zones are formed behind the bluff body of the SBB and CSBB microecombustor. Namely, the recirculation zones are prolonged with 1000

Mean outer wall temperature (K)

1000

55

950

Numerical simulation 950 Experiment[28] Difference

900

900

850

850

800

800

750

750

25

700

700

20

650

650

600

600

50 45 40

5

7

9

11

13

15

17

the increment of the controllable flow ratio. A black dashed line is drawn to indicate the end of the recirculation zone. In the H2/air premixed combustion, the combustors use lowespeed and highetemperature characteristics of the recirculation zone to ignite the fuel. Thus, the recirculation zone can broaden the bloweoff limit and increase the temperature, which means that the increase of the length and number of recirculation zone are vital for flame stabilization. Hence, the CSBB microecombustor has better stable combustion performance. As depicted in Fig. 5, the difference of the radial velocity of the SBB and CSBB microecombustor is more obvious than the CBB microecombustor. It is worthwhile mentioning that the velocity difference becomes larger with the increase of the controllable flow ratio. The velocity difference of the CBB microecombustor, SBB microecombustor and CSBB microecombustor (a ¼ 0.1, a ¼ 0.2, a ¼ 0.3, a ¼ 0.4) are 19.62 m/s, 23.59 m/s, 23.25 m/s, 33.96 m/s, 48.91 m/s and 56.78 m/s, respectively. The main reason is that the flow rate in the controllable channel increases and the flow rate in the main channel decreases, which enlarge the velocity difference. Owing to the increase of the velocity difference is beneficial to increase the length of the CSBB microecombustor and turbulent intensity. Therefore, it is easier to form recirculation zones in the CSBB microecombustor.

35 30

15 10

19

Inlet velociy (m/s) Fig. 3. Comparison between the outlet temperature of the experimental [28] and numerical results under different inlet velocities.

Temperature fields of the middle crossesection (z ¼ 0 mm) for the CBB microecombustor, SBB microecombustor and CSBB microecombustor are presented in Fig. 6. The equivalence ratio and premixed gas flow rate (Vin) are 0.6 and 120 cm3/s, while the controllable flow rate ratio (a) are 0.1, 0.2, 0.3 and 0.4, respectively. As shown in Fig. 6, the red zone is high temperature zone, in which the flame is able to burn steadily, while temperature of the blue zone is lowest without flame burning. Besides, there is large temperature gradient and concentration gradient in the green ribbon zone. And high temperature fuel and low temperature fresh premixed fuel exchange heat and mass here. Notably, the high temperature zone is formed behind the three kind of bluff bodies. At the same time, the temperature range is extended. Two small recirculation zones are formed behind the bluff bodies in the SBB microecombustor and CSBB microecombustor result from gas mixture is injected into the bluff body gap. But there is only one recirculation zone behind the bluff body in the CBB microecombustor (refer to Fig. 4). As is known to us, the recirculation zone not only provides a radical pool for stable combustion, but also extends the residence time in favor of increasing conversion rate. Consequently, the SBB microecombustor and CSBB microecombustor have better combustion stability and temperature uniformity relative to the CBB microecombustor. With the increase of controllable flow ratio, the recirculation zone becomes wider and part of the flame root is located in the recirculation zone, which means that the flame is anchored by both the recirculation zone and lowevelocity zone behind the bluff body (refer to Fig. 4). As a result, the CSBB microecombustor obtain a more uniform temperature fields and wider high temperature zone with the increase of controllable flow ratio. From the above demonstration of temperature fields, it can be roughly deduced that the CSBB microecombustor is more propitious to stable combustion. One can reasonably expect that the flame shape and location are determined by the recirculation zone and the local flow field at the entrance of the combustion chamber. Of course, these stem from the differences of bluff body structures. To further analyze the underlying mechanisms which lead to the difference of temperature distribution in the three

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Fig. 4. Velocity fields in the middle crossesection (z ¼ 0 mm) under different bluffebody structures at 4 ¼ 0.6 and Vin ¼ 120 cm3/s.

in the gaps of the CSBB microecombustor, the shear force of the surrounding gas on the flame is weakened and the flame splitting tendency is not obvious. Fig. 7(b) shows that combustion mainly occurs behind the bluff body (about 5 mm) in the SBB microecombustor and CSBB microecombustor (refer to Figs. 4 and 6). Furthermore, there is a peak in the temperature profiles of the internal wall (see Fig. 7(b)). The reason for these two phenomena is because that the flame front directly attaches to the internal wall. The maximum outer wall temperature among the CBB microecombustor, the SBB microecombustor and the CSBB microecombustor are 1001 K, 1007.26 K and 1121.98 K, respectively, which means that the CSBB microecombustor has better temperature capability. 3.3. Combustion efficiency of three bluff body microecombustors

Fig. 5. Radial velocity distribution of three microecombustors (x ¼ 5 mm).

microecombustors, radial temperature distribution and outer wall temperature distribution under different bluffebody structures are revealed in Fig. 7(a) and Fig. 7(b), respectively. The radial temperature of the CSBB microecombustor is higher than the other two combustors and the temperature distribution is more homogeneous (see Fig. 7(a)). Due to the disturbance of supplementary gas

Combustion efficiency is an important criterion to evaluate the combustion performance. Since only hydrogen is used as the fuel, the combustion efficiency and the conversion rate of the hydrogen are the same. It can be calculated through Eq. (3).

h¼

mH2 ,in  mH2 ,out mH2 ,in

(3)

where mH2 ,in and mH2 ,out are inlet and outlet hydrogen mass fraction of microecombustors. Fig. 8(a) and Fig. 8(b) depict the combustion efficiency, which is

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Fig. 6. Temperature fields in the middle crossesection (z ¼ 0 mm) of different combustors under 4 ¼ 0.6 and Vin ¼ 120 cm3/s.

1200

1800

b

a Outer wall temperature (K)

Radial temperature (K)

1600 1400 1200 1000 CBB SBB CSBB(D CSBB(D CSBB(D CSBB(D

800 600 400 200

-4

-3

-2

-1

0

) ) ) )

1

2

3

Radial distance (mm)

4

1000

800 CBB SBB CSBB(D CSBB(D CSBB(D CSBB(D

600

400

200

0

5

10

) ) ) )

15

20

Axial distance (mm)

Fig. 7. (a) The outlet radial temperature profiles of microecombustors under. Different bluff body structures, and (b) Outer wall temperature under different bluff body structures.

induced by equivalence ratio and inlet velocities, respectively. It is seen from Fig. 8(a) that the combustion efficiency increases first and then decreases with the increase of equivalence ratios. Notably, combustion efficiency under different equivalence ratios in the CSBB microecombustors are obviously higher than the other two microecombustors. Even the lowest combustion efficiency is larger than 0.84, which demonstrates that relatively complete conversion can be achieved in the CSBB microecombustors. Another

phenomenon is that the corresponding equivalence ratio reaching a maximum efficiency is 0.8 in the CSBB microecombustor, while the CBB and SBB microecombustors reach the highest efficiency at 4 ¼ 1.0. This means that the CSBB microecombustor exhibits superior combustion performance when the chemical reaction is lean (4 ¼ 0.8). As illustrated in Fig. 8 (b), the combustion efficiency decrease monotonously and the differences of the combustion efficiency increase among the three microecombustors with the

Y. Yan et al. / Energy 189 (2019) 116242

90 80 70 60

CBB SBB CSBB (D CSBB (D CSBB (D CSBB (D

50 40 30

105

a

0.6

0.7

0.8

0.9

) ) ) )

1.0

b

100

Combustion efficiency (%)

Combustion efficiency (%)

100

7

95 90 85 80 75 70

CBB SBB CSBB (D )

65 1.1

1.2

Equivalence ratio (I)

20 40 60 80 100 120 140 160 180 200

Inlet velocity (cm3/s)

Fig. 8. Effects of (a) equivalence ratios (Vin ¼ 144 cm3/s), and(b) inlet velocities (4 ¼ 0.8, a ¼ 0.1) on combustion efficiency.

increase of the premixed gas flow. To be specific, the combustion efficiency of the CSBB microecombustor is 98.73% at Vin ¼ 96 cm3/s, which is higher than the CBB microecombustor and the SBB microecombustor about 6.42% and 0.61%, respectively. The difference expands to 8.28% and 0.82% at Vin ¼ 192 cm3/s. On the one hand, the residence time of premixed gas in the combustor is negatively correlated with the inlet velocity. On the other hand, more unburned low temperature premixed gas enters the combustion channels with the increase of the inlet velocity. Overall temperature and combustion efficiency has a downward trend as a result of more heat absorbed by cold fuel. Under the coupling effects of the two reasons, the combustion efficiency decreases with the increase of inlet velocity. However, due to the reasonable flow distribution in the CSBB microecombustor, a large amount of fresh gas has little effect on it. This indicates that the CSBB microecombustor can achieve a relative completely combustion. The controllable flow ratio plays critical roles in combustion characteristics of the CSBB microecombustor. Fig. 9 mentions the effect of controllable flow ratio on combustion efficiency at 4 ¼ 0.8. Obviously, the adjustment of the controllable flow ratio can significantly affect the combustion efficiency of hydrogen and the optimal value of the controllable ratio is different under various

Fig. 9. The effect of controllable flow ratio on combustion efficiency at 4 ¼ 0.8.

inlet velocities. Specially, when the premixed gas flow rates are 144 cm3/s and 192 cm3/s, the optimum controllable flow ratio values are 0.35 and 0.5, respectively. It is worthwhile mentioning that the combustion efficiency under the premixed gas flow rate of 144 cm3/s is greater than the corresponding combustion efficiency at Vin ¼ 192 cm3/s. The main reason is that the inlet velocity is so large that part of the flame is blown out.

3.4. Effects of controllable gap widths on the CSBB microecombustor The width of the controllable gap has significate effects on combustion characteristics of microecombustors. Therefore, future work focuses on study the effect of controllable gap width. According to the previous work, the combustion performance of the CSBB microecombustor is the worst at a ¼ 0.1. We aim to study the controllable gap width in improvement the combustion characteristics. Hence, we still maintain the controllable flow ratio is 0.1. Fig. 10 and Fig. 11 depict the velocity fields and outlet radial velocity distribution under different gap widths where the gap widths are 0.3, 0.6, and 0.9 mm respectively (Vin ¼ 144 cm3/s and 240 cm3/s). The recirculation zone behind the bluff body is continuously expanded at Vin ¼ 144 cm3/s with the increase of the gap width (see Fig. 10), which is conducive to the stable combustion of the microecombustor. Thus, the CSBB microecombustor has the best combustion performance at Vin ¼ 144 cm3/s and dc ¼ 0.9 mm. Another interesting finding is that the width of the gap has little effect on the recirculation zone at Vin ¼ 240 cm3/s. This is because premixed gas flow rate is too large that the range of highespeed areas increase. And it is hard to extend the recirculation zone. Generally, the bloweoff limit is used to reflect the flame stabilization ability of combustors, which is defined as the largest inlet velocity exceeding which the flame will be blown out. Fig. 11 displays the effect of equivalence ratios on the bloweoff limits at a ¼ 0.1. When the flame blow-off occurs, the combustion is completely extinguished and the temperature field becomes uniform with a same value as the incoming mixture. It is seen from Fig. 12 that the blow-off limits of the three micro-combustors increases first and then decrease with the increase of the equivalence ratios. And this figure demonstrates that the largest flame blow-off limit can be achieved at 4 ¼ 1.0. For lean H2/air mixtures, the amount of heat release and chemical reaction rate increase with the equivalence

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Fig. 10. Velocity fields with different gap widths at a ¼ 0.1.

Fig. 12. Effects of equivalence ratio on the bloweoff limit at a ¼ 0.1.

Fig. 11. Outlet radial velocity distribution under different gap widths.

ratio. On the other hand, it is difficult to form a stable flame in the microecombustor due to the insufficient oxygen at 4 > 1. Moreover, the bloweoff limits of CBB, SBB and CSBB microecombustors are

about 540, 456 and 600 cm3/s respectively at 4 ¼ 1.0. Comparing these values, one can see that the CSBB microecombustor greatly extends the stable combustion range of H2/air mixture. Notably, the bloweoff limit of the CBB microecombustor is higher than the SBB microecombustor’s. The bloweoff limit of the CBB microecombustor is mainly due to the flame is blown out by the

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shearing action of the gas mixture. While flame is instability in the SBB microecombustor for a large amount of unburned premixed gas will reduce the temperature behind the bluff body and even blowout the flame on duty. Thus, the gap width has a great influence on bloweoff limit. In the CSBB micro-combustor, the flow rate in the controllable gap can be controlled. A suitable amount of premixed gas is injected into the rear of the bluff body via adjusting the controllable flow ratio. Then, the premixed gas and the lowspeed, high-temperature gas is mixed and burned, which increases the temperature behind the bluff body and widens the blow-off limits. It is expected that the flame stabilization ability of this specific configuration depends on several aspects, such as heat recirculation effect and local flow field. Moreover, these factors interact closely with each other. To gain more insight, we conduct a systematic analysis in the following sections. To take a closer look of the mechanism of flame stabilization by the bluff body, the effect of gap width on bloweoff limit of microecombustor is illustrated in Fig. 13. The gap widths are 10% (0.30 mm), 15% (0.45 mm), 20% (0.60 mm), 25% (0.75 mm) and 30% (0.90 mm), respectively at a ¼ 0.1, 4 ¼ 0.6. As depicted in mini graph of Fig. 13, the flame bloweoff limit increases first and then decreases with an increasing width of the gap. The CSBB microecombustor of dc ¼ 0.6 mm has the largest flame bloweoff limit (564 cm3/s). When the controllable gap is too narrow, the flow rate of the premixed gas in the controllable gap is relatively fast, which easily destroys the recirculation zone behind the bluff body and even leads to flame is blowout. When the controllable gap is too wide, the main air inlet becomes very narrow and causes the flow velocity on both sides of the bluff body is too fast. Moreover most of the fuel is directly blown out. Therefore, the proper gap width is extremely important for the stability of the flame. A sufficient preheating on the cold fresh fuel can improve the flame speed [29] and extend the bloweoff limit. As can be clearly seen in the Fig. 14, combustion efficiency is much higher at Vin ¼ 144 cm3/s than that inlet velocity is 240 cm3/s. It is worthwhile mentioning that the flame is blown out at dc ¼ 0.3 mm, a ¼ 0.2 and Vin ¼ 240 cm3/s. One reason is that the flame is hard to be stabilized and shortened residence time of the gaseous mixture under reduced dimension. Another reason is that the velocity of the premixed air flow in the controllable gap is so fast that the combustion is instability. In addition, the combustor becomes more efficient with increasingly width of the gap at a < 0.4,

Fig. 13. Effect of gap widths on the bloweoff limit at a ¼ 0.1 and 4 ¼ 0.6.

9

Fig. 14. Effect of gap widths on combustion efficiency under different controllable flow ratios.

Vin ¼ 240 cm3/s. On the contrary, combustion efficiency has a slightly decrease with the increase of the gap at a > 0.4, Vin ¼ 240 cm3/s. This is because more and more cold fresh fuel into the CSBB microecombustors via controllable gaps, which causes the flame speed is reduced and stable combustion zone is destroyed. Another important fact is combustion efficiency of dc ¼ 0.6 mm and Vin ¼ 240 cm3/s exceeds the CSBB microecombustor with dc ¼ 0.9 mm and Vin ¼ 240 cm3/s where the controllable flow ratio is about 0.45. It is mainly because of too much cold fresh fuel into the CSBB microecombustors so that it can’t be preheated well. In summary, the CSBB microecombustor of dc ¼ 0.6 mm has the largest flame bloweoff limit (564 cm3/s). The combustion performance is the best at dc ¼ 0.6 mm when taking the bloweoff limit, combustion efficiency and temperature distribution into account.

4. Conclusion A catalytic microecombustor with a controllable slotted bluff body is proposed to improve the combustion efficiency and combustion stability in microscale combustors. The combustion performance of the CSBB microecombustor is best among the three combustors. Therefore, the potential future works could pay attention to improving combustion characteristics of the CSBB microecombustor by optimizing its structure. In this paper, the bluff body structures, bloweoff limits, exhaust gas temperature, equivalence ratios and combustion efficiency of the CBB microecombustor, the SBB microecombustor and the CSBB microecombustor are examined and compared numerically. More importantly, effects of gap width on combustion characteristics on the CSBB microecombustor is explored. The following conclusions can be realized. 1) The newly proposed CSBB microecombustor shows superiority in improving combustion efficiency at fuel lean condition (4 ¼ 0.8) and extending the blow-off limits for stable combustion. The highest outer wall temperature are increased by 120.98 K and 114.72 K compared with the CBB and SBB microecombustor, respectively, which indicates that the CSBB microecombustor has better temperature capability. Besides, there are two recirculation zones behind the bluff body of the SBB and CSBB microecombustors, while only ones is formed in

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the CBB microecombustor. Thereby the increase of number of recirculation zone are vital for flame stability. 2) The controllable flow ratio play important roles on combustion performance. With the increase of controllable flow ratio, the CSBB microecombustor obtain a more uniform temperature fields and wider high temperature zone. More importantly, the recirculation zone becomes longer, which enhances the flame stability. And the increment of the controllable flow ratio can significantly extend the combustion efficiency. From the above demonstration, the CSBB microecombustor has best combustion characteristics at a ¼ 0.4. 3) The proper gap width is extremely important for the stability of the flame. The recirculation zone behind the bluff body is continuously expanded at low inlet velocity cases with the increase of the gap width. In addition, the CSBB microecombustor has the largest flame bloweoff limit under dc ¼ 0.6 mm and combustion efficiency is much higher under high inlet velocity. Another important fact is the CSBB microecombustor performs better in combustion efficiency at dc ¼ 0.6 mm and a ¼ 0.45 cases. Thus, the CSBB microecombustor of dc ¼ 0.6 mm shows excellent combustion performance. Declaration of competing interest We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of, the manuscript entitled, “Numerical comparison of H2/ air catalytic combustion characteristic of micro-combustors with a conventional, slotted or controllable slotted bluff body”. Acknowledgments The authors gratefully acknowledge financial support from Joint Fund of the National Natural Science Foundation of China-China Aerospace Science and Technology Corporation on Advanced Manufacturing Technology for Aerospace Industry (No. U1737113), Natural Science Foundation of Chongqing (cstc2019jcyjmsxmX0223), Fundamental Research Funds for the Central Universities (No. 2018CDXYDL0001), Graduate Scientific Research and Innovation Foundation of Chongqing (No. CYB19061). References [1] Peng Q, JQ E, Yang WM, Xu HP, Chen JW, Meng T, Qiu RZ. Effects analysis on combustion and thermal performance enhancement of a nozzle-inlet micro tube fueled by the premixed hydrogen/air. Energy 2018;160:349e60. [2] Pizza G, Frouzakis CE, Mantzaras J, Tomboulides AG, Boulouchos K. Dynamics of premixed hydrogen/air flames in microchannels. Combust Flame 2008;152(3):433e50. [3] Alireza A, Mohammad H. Numerical study of hydrogeneair combustion characteristics in a novel microethermophotovoltaic power generator. Appl Energy 2017;199:382e99. [4] Kim NI, Alzumi S, Yokomori T, Kato S, Fujimori T, Maruta K. Development and scale effects of small Swisseroll combustors. Proc Combust Inst 2007;31: 3243e50. [5] Pan JF, Wu D, Liu YX, Zhang HF, Tang AK, Xue H. Hydrogen/oxygen premixed combustion characteristics in micro porous media combustor. Appl Energy 2015;160:802e7. [6] Li J, Wang YT, Chen JX, Shi JR, Liu XL. Experimental study on standing wave regimes of premixed H2eair combustion in planar microecombustors partially filled with porous medium. Fuel 2016;167:98e105. [7] Li HJ, Yan YF, Feng S, Chen YR, Li LX, Zhang L, Yang ZQ. Hydrogen release mechanism and performance of ammonia borane catalyzed by transition metal catalysts Cu-Co/MgO (100). Int J Energy Res 2019;43(2):921e30. [8] Wan JL, Fan AW, Yao H, Liu W. Flame-anchoring mechanisms of a micro cavity

combustor for premixed H2/air flame. Chem Eng J 2015;275:17e26. [9] Yilmaz H, Cam O, Yilmaz I. Effect of micro combustor geometry on combustion and emission behavior of premixed hydrogen/air flames. Energy 2017;135: 585e97. [10] Zuo W, JQ E, Hu WY, Jin Y, Han DD. Numerical investigations on combustion characteristics of H2/air premixed combustion in a micro elliptical tube combustor. Energy 2017;126:1e12. [11] Yan YF, Feng S, Huang ZZ, Zhang Li, Pan WL, Li LX, Yang ZZ. Thermal management and catalytic combustion stability characteristics of premixed methane/air in heat recirculation meso-combustors. Int J Energy Res 2018;42(5):999e1012. [12] Fanaee SA, Esfahani JA. Two-dimensional analytical model of flame characteristic in catalytic micro-combustors for a hydrogeneair mixture. Int J Hydrogen Energy 2014;39(9):4600e10. [13] Boyarko GA, Sung CJ, Schneider SJ. Catalyzed combustion of hydrogeneoxygen in platinum tubes for microepropulsion applications. Proc Combust Inst 2005;30:2481e8. [14] Qazi Zade A, Renksizbulut M, Friedman J. Contribution of homogeneous reactions to hydrogen oxidation in catalytic microchannels. Combust Flame 2012;159(2):784e92. [15] Brambilla A, Frouzakis CE, Mantzaras J, Tomboulides A, Kerkemeier S, Boulouchos K. Detailed transient numerical simulation of H2/air heteroehomogeneous combustion in platinumecoated channels with conjugate heat transfer. Combust Flame 2014;161(10):2692e707. [16] Wan JL, Fan AW, Maruta K, Yao H, Liu W. Experimental and numerical investigation on combustion characteristics of premixed hydrogen/air flame in a microecombustor with a bluff body. Int J Hydrogen Energy 2012;37(24): 19190e7. [17] Fan AW, Zhang H, Wan JL. Numerical investigation on flame blow-off limit of a novel microscale Swiss-roll combustor with a bluff-body. Energy 2017;123: 252e9. [18] Fan AW, Wan JL, Liu Y, Pi BM, Yao H, Maruta K, Liu W. The effect of the blockage ratio on the bloweoff limit of a hydrogen/air flame in a planar microecombustor with a bluff body. Int J Hydrogen Energy 2013;38(26): 11438e45. [19] Fan AW, Wan JL, Liu Y, Pi BM, Yao H, Liu W. Effect of bluff body shape on the bloweoff limit of hydrogen/air flame in a planar microecombustor. Appl Therm Eng 2014;62(1):13e9. [20] Bagheri G, Hosseini SE, Wahid MA. Effects of bluff body shape on the flame stability in premixed microecombustion of hydrogeneair mixture. Appl Therm Eng 2014;67(1e2):266e72. [21] Yan YF, Yan HY, Zhang L, Li LX, Zhu JC, Zhang ZE. Numerical investigation on combustion characteristics of methane/air in a micro-combustor with a regular triangular pyramid bluff body. Int J Hydrogen Energy 2018;43(2018): 7581e90. [22] Sun QY, Guo XY. Numerical analysis on combustion characteristics in microcombustor with a slotted bluff-body. Energy Eng 2016;5:8e13. [23] Zeng ZX, Guo SS, Chen CJ, Wei JJ. Numerical simulation of turbulent combustion characteristics in the vortex controlled diffuser. J Eng Therm Energy Power 2016;6:42e7. [24] Nair S, Lieuwen TC. Neareblow off dynamics of a bluffebody stabilized flame. J Propuls Power 2012;23(2):421e7. [25] Yan YF, Xu FL, Xu QY, Zhang L, Yang ZQ, Ran JY. Influence of controllable slit width and angle of controllable flow on hydrogen/air premixed combustion characteristics in micro combustor with both sides-slitted bluff body. Int J Hydrogen Energy 2019;44(36):20482e92. [26] Baigmohammadi M, Tabejamaat S, Zarvandi J. Numerical study of the behavior of methaneehydrogen/air preemixed flame in a micro reactor equipped with catalytic segmented bluff body. Energy 2015;85:117e44. [27] Wierzbicki TA, Lee IC, Gupta AK. Combustion of propane with Pt and Rh catalysts in a mesoescale heat recirculating combustor. Appl Energy 2014;130:350e6. [28] Wan JL, Fan AW, Maruta K, Yao H, Liu W. Experimental and numerical investigation on combustion characteristics of premixed hydrogen/air flame in a microecombustor with a bluff body. Int J Hydrogen Energy 2012;37(24): 19190e7. [29] Alipoor A, Saidi MH. Numerical study of hydrogen-air combustion characteristics in a novel micro-thermophotovoltaic power generator. Appl Energy 2017;199:382e99. [30] Biesheuvel PM, Kramer GJ. Two-section reactor model for autothermal reforming of methane to synthesis gas. AlChE J 2003;49(7):1827e37. [31] Li J, Zhao ZW, Kazakov A, Dryer FL. An updated comprehensive kinetic model of hydrogen combustion. Int J Chem Kinet 2004;36(10):566e75. [32] Zuo W, JQ E, Liu XL, Peng QG, Deng YW, Zhu H. Orthogonal experimental design and fuzzy grey relational analysis for emitter efficiency of the microecylindrical combustor with a step. Appl Therm Eng 2016;103:945e51. [33] Holman JP. Heat transfer: instructor’s solutions manual to accompany. ninth ed. London: McGraw Hill; 2002. [34] Kuo CH, Ronney PD. Numerical modeling of noneadiabatic heaterecirculating combustors. Proc Combust Inst 2007;31(2):3277e84. [35] Turns SR. An introduction to combustion. second ed. New York: McGRAWeHill; 2000.