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air in a micro-combustor with a regular triangular pyramid bluff body
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 8 ) 1 e1 0
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Numerical investigation on combustion characteristics of methane/air in a micro-combustor with a regular triangular pyramid bluff body Yunfei Yan a,*, Hongyu Yan a, Li Zhang a, Lixian Li b,**, Junchen Zhu a, Zhien Zhang a,c a Key Laboratory of Low-grade Energy Utilization Technologies and Systems, Chongqing University, Ministry of Education, Chongqing, 400030, PR China b Chongqing University Cancer Hospital, Chongqing Cancer Institute, Chongqing Cancer Hospital, Chongqing, 400030, PR China c College of Chemistry and Chemical Engineering, Chongqing University of Technology, Chongqing, 400054, PR China
article info
abstract
Article history:
Combustion characteristics of methane/air in a micro-combustor with a regular triangular
Received 4 December 2017
pyramid bluff body were numerically investigated. Results reveal that the blow-off limit of
Received in revised form
the micro-combustor with a regular triangular pyramid bluff body is 2.4 times of that in the
3 February 2018
micro-combustor without bluff body. With the increase of inlet velocity, the recirculation
Accepted 26 February 2018
zone expands and preferential transport effect behind the bluff body is intensified.
Available online xxx
Therefore, the local equivalence ratio in the recirculation zone increases when F ¼ 0.8, but the growth trend of local equivalence ratio is not obvious when the inlet velocity exceeds
Keywords:
10 m/s. When F < 1.0, adding small amount of hydrogen into gas mixture can speed up the
Micro-combustor
significant elementary reaction, leading to an increase of methane conversion. It's found
Regular triangular pyramid bluff
that both the methane conversion rate and the temperature behind the bluff body reaches
body
the highest when blockage ratio increase to 0.22.
Catalytic combustion
© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Hydrogen addition Preferential transport effect
Introduction With the fast development and intensive applications of Micro Electro-Mechanical Systems (MEMS), lightweight and compact energy resources have been gradually stepped into aviation, military, healthcare and even our daily life [1]. Applications of
compact energy are expected to increase due to the development of electronic devices, which mainly depends on the development of micro-powers [2,3]. The efficiency and stability of the combustion have a decisive effect on the performance of MEMS. Since the energy density of hydrocarbon fuels is much greater than the traditional batteries, the
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (Y. Yan), [email protected] (L. Li). https://doi.org/10.1016/j.ijhydene.2018.02.168 0360-3199/© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Yan Y, et al., Numerical investigation on combustion characteristics of methane/air in a microcombustor with a regular triangular pyramid bluff body, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.02.168
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Nomenclature Ys Ds Rs Vsk
mass fraction of species diffusion coefficient of species consumption or decomposition rate species stoichiometric coefficient in forward direction of surface reaction total number of elementary surface reactions ks 00 stoichiometric coefficient in negative direction V ik of reaction forward rate coefficient of reaction ksk pre-exponential factor Ak Ea activation energy of reaction temperature exponent bk surface coverage rate of species Qi surface coverage parameter εik surface coverage parameter mik number of surface species Ns number of gas phase species Ng [Xi] molar concentration of surface species G surface site density of the catalyst body force fi m viscosity of the mixture h enthalpy q heat of reaction l thermal conductivity R universal gas constant F equivalence ratio of the inlet mixture MCH4·in mass flow rate of methane at the entrance of the combustor MCH4·out mass flow rate of methane at the exit of the combustor A/F actual ratio of air/fuel in the inlet mixture gas stoichiometric air/fuel ratio A0/F0 equivalence ratio of the rear of the bluff body Flocal
combustion of hydrocarbons has been extensively studied in the field of micro-power generation [4e7]. Fernandez-Pello pointed out that the energy density of typical hydrocarbon fuels was about 45 MJ/kg, which is several tens of times than that of a lithium battery [8]. Many studies have proved that hydrogen and methane are the most promising fuels in micro combustors. The tiny size that shortens residence time of fuels and the large surface area-to-volume ratio that multiplies the heat loss of micro-combustors are two prominent characteristics of micro-combustor [8]. It has been proved that a short residence time of fuels and heat loss from exterior surface of combustor can significantly weaken the performance of microcombustor [9]. Scholars have made tremendous contributions to solve these problems. In order to reduce heat loss, Tang et al. [10]. proposed a new planar combustor for micro thermo photovoltaic system, which had multi-mode heat transfer passages in the combustion channel. It was found that the new combustor could achieve a higher temperature of the radiation surface due to the enhancement of heat transfer. Cao et al. [11] studied the diffusion combustion in a microcombustor with porous media. It revealed that porous media
can improve the enthalpy of reactant and reduce the heat loss. Furthermore, Pan et al. [12] investigated the hydrogen/oxygen premixed combustion characteristics in micro porous media combustor. They figured out that porous media material with low Cp and high thermal conductivity led to better temperature distribution on the wall, which can improve the efficiency of the thermophotovoltaic (TPV) system. In order to prolong the residence time of inlet gas, some researchers proposed to add a bluff body into micro-combustor to optimize combustion characteristics [13e16]. Stability of the flame and combustion efficiency were considerably improved by using bluff body. Bagheri et al. [13] investigated effects of bluff body shape on the flame stability in premixed micro-combustion of hydrogen/air mixture via 2D CFD simulations, it's concluded that the wall temperature and combustion efficiency was improved in all cases compared with the conventional model, and they also suggested that wall-blade is better than other shape of bluff body at high inlet velocities. Fan et al. [15,16] compared combustion characteristics of gas mixture in a 2D planar micro-combustor model with and without a bluff body. The results showed that the blow-off limit of flame can be extended to several times in the micro-combustor with a bluff body than that without bluff body. Hosseini et al. [17] studied the effect of bluff-body on flameless combustion in a cylinder micro-combustor, they pointed out that micro-flameless mode with bluff-body illustrated better performance than other cases. Ran et al. [18] compared the flow field and combustion efficiency of methane in a micro-channel with concave cavity and with convex cavity, they reported that concave cavity can lead to the reflux of methane at a high velocity and convex cavity can enhance the contact between gas mixture and catalyst. Zhang et al. [19] investigated the cavity structure, position, and number of convex cavities on the catalytic combustion characteristics of methane/air in a micro-channel, they reported that increasing the normalized cavity diameter is an effective way to expand the blow-off limit. E et al. [20] proposed a cavity and backward-facing step in micro-combustor. It was found that the cavity or backward-facing step contributed to the formation of the recirculation zone, enhanced the heat recirculation and the flame stabilization. Niu et al. [21] have made a comparative study between streamline-type bluff body and nonstreamline-type bluff body. The results showed that the micro-combustor with non-streamline-type bluff body performs better in the formation of recirculation zone and stability of the flame. In addition, catalytic combustion is also an effective way to improve the performance of micro-combustors. It has been proved that catalyst can decrease the activation energy of the reactions and improve conversion rate of fuels [22e24]. Suzanne et al. [25] experimentally investigated the surface oxidation on small-scale catalytic combustion for methane/air and propane/air, and they found a sharp temperature increase occurred on the catalytic surface with simultaneous fast depletion of the combustible gas. Ran et al. [26] employed a 2D model to numerically investigate wall heat transfer characteristics of CH4/air in the microtube with Pt catalyst. It's concluded that both the heat transfer along the solid structure and the total heat loss increased with increasing wall thickness. Lee et al. [27]
Please cite this article in press as: Yan Y, et al., Numerical investigation on combustion characteristics of methane/air in a microcombustor with a regular triangular pyramid bluff body, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.02.168
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experimentally studied the CH4/air combustion characteristics in a micro heat-regenerative combustor with and without catalytic platinum wires. They reported that the Pt wire located within the recirculation area promoted the catalytic reaction and extended the self-sustainable reaction conditions. Yan et al. [28,29] have taken tremendous efforts in studying the catalytic combustion characteristics of methane/air in micro-combustors, they mainly investigated the effect of operating conditions, materials of the solid wall, the amount of catalyst and obtained the optimum conditions for catalytic combustion in micro-combustors. Meanwhile, they discussed the thermal and chemical effects of hydrogen addition on catalytic micro-combustion of CH4/air, concluding that adding a small amount of hydrogen into methane/air mixture can significantly improve the combustion efficiency of methane [30,31]. Although the effects of structure of the micro-combustor and bluff body on combustion characteristics of micro combustion have been extensively observed, the impacts of a regular triangular pyramid bluff body in a microcombustor have not been studied. literatures mentioned above have mostly adopted a 2D model, efforts need to be made in the study of bluff body which employ the 3D model. Furthermore, many literatures about the addition of hydrogen into methane/air mixture reported the positive effect of hydrogen addition, but too much hydrogen will scramble for radicals with methane, the negative effect have not been properly studied. In the present work, a 3D microcombustor with a regular triangular pyramid bluff body is developed, and the effect of regular triangular pyramid bluff body, preferential transport effect, hydrogen addition and blockage ratio on the combustion characteristics of methane/air are investigated.
Numerical methods Physical model The model of cube micro combustor with a regular triangular pyramid bluff body is depicted in Fig. 1 using Fluent. The total length of the micro-combustor is 20 mm, the width and the height of the combustor are 5 mm and 8 mm, respectively. The distance from vertex of the regular triangular pyramid bluff body to combustor inlet is 3 mm. Edge length of the regular triangular pyramid bluff body is 2 mm. The bluff body was symmetrically located with respect to the micro-combustor. Combustion of methane/air was examined in the micro
combustor, which was coated with Rh catalyst on the inner surface of the combustor.
Mathematical model Both surface catalytic reactions and gas phrase reaction are considered in the simulation. The heat dissipation effect and gas radiation in the process of reaction have not been taken into account, and a similar application was adopted in the previous publication of Zhang et al. [32]. Nonetheless, the equations for conservation of continuity, momentum, and energy are used in control volume. The governing equations in Cartesian coordinates include: Continuity equation: v ruj ¼0 vxj
(1)
where r is the density of the mixture, and u is the velocity. Composition equation: ruj
vYs v vYs ¼ Ds r þ Rs vXj vXj vXj
(2)
where YS corresponds to mass fraction of species s in microcombustor, Ds is the diffusion coefficient and Rs refers to the consumption or decomposition rate. Both surface species and gas phase species can be generated and consumed by surface reactions. The consumption or decomposition rate RS is defined as below: Rs ¼
Ks X
vsk ksk
k¼1
NgþNs Y
½Xiv}ik
(3)
i¼1
where vsk is stoichiometric coefficient in forward direction of surface reaction k, Ks is the total number of elementary surface reactions, v}ik is stoichiometric coefficient in negative direction of reaction k, ksk is forward rate coefficient of reaction k, Ns is the number of surface species, and Ng is the number of gas phase species. ½Xi is the molar concentration of surface species i, and ksk is calculated by Arrhenius reaction source of the reaction k as below: Ksk ¼ Ak Tbk exp
Ns Ea Y εik Qi m Qi jk exp RT i¼1 RT
(4)
where Ak is the pre-exponential factor, Ea is the activation energy of reaction k, bk is the temperature exponent, Qi is the surface coverage rate of species i, εik and mik are surface coverage parameter. In addition, ½Xi is written as below: ½Xi ¼ GQi
(5)
here, G is surface site density of the catalyst. The value of 2:72 109 mol$cm2 is used for the Rh catalyst in this paper. The surface of the catalytic is described by its coverage with adsorbed species and temperature. The temperature is based on the governing equation for enthalpy as below: Momentum equation: v vp v2 ui þm ruj ui ¼ fi vXj vXj vXj vXj Fig. 1 e Structural diagram of the micro-combustor with a regular triangular pyramid bluff body.
(6)
where fi is body force, and m is viscosity of gaseous mixture.
Please cite this article in press as: Yan Y, et al., Numerical investigation on combustion characteristics of methane/air in a microcombustor with a regular triangular pyramid bluff body, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.02.168
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Model validation
Energy equation: ruj
vh v vT v ¼ l þ vXj vXj vXj vXj
Ng X
hs rDs
s¼1
vYs vXj
! þq
(7)
where h is the enthalpy, q is heat of reaction, l is thermal conductivity, and Ds is the diffusion coefficient of species s. Closure equation: Ng X
Ys ¼ 1
(8)
s¼1
State of ideal gas: P ¼ rRT
Ng X Ys s¼1
Ms
(9)
here, R is universal gas constant, Ys corresponds to mass fraction of the species s and Ms is molar mass. Methane conversion rate could be calculated by Eq. (10): h¼
mCH4 $in mCH4 $out mCH4 $in
(10)
where MCH4 $in is the mass flow rate of methane at the entrance of the combustor and MCH4 $out is the mass flow rate of methane at the exit of the combustor. The equivalence ratio (F) is defined by Eq. (11): F¼
A0 =F0 A=F
(11)
where A=F is the actual ratio of air/fuel in the inlet mixture gas and AO =FO is the stoichiometric air/fuel ratio.
The mesh scale has a significant impact on reliability of simulation results. In order to get the optimum mesh scale, we investigate methane conversion rate under various mesh scale, which decreases from 0.6 mm to 0.1 mm. Fig. 2 shows methane conversion rates and the exhaust gas temperature at various mesh size. In this study, the exhaust gas temperature is defined as the average surface temperature of the outlet. As shown in Fig. 2(a), the methane conversion rate increases when the mesh sizes decrease from 0.6 mm to 0.3 mm. However, with further decrease of the mesh size, both the methane conversion rate and exhaust gas temperature remain nearly unchanged. It means that the simulation results are independent from mesh scale when the mesh size is less than or equal to 0.3 mm. Therefore, in order to achieve reliable results and save the calculation time, mesh size is set at 0.3 mm in this paper. The accuracy of the numerical model is validated by comparing the methane conversion rate between the experimental and numerical results. In the experiment [37], the reactor is a hollow cylinder, which has an inside diameter of 4 mm coated with Rh, an outside diameter of 5 mm, 20 mm length of the channel coated with Rh. The volume fraction of CH4/O2 is 0.56, and the equivalent velocity is 0.15 m/s. Fig. 2(b) shows the methane conversion rate of experimental and modeling results at different temperatures, the trends of two profiles are basically the same, and the maximum relative error is 6.25%. Therefore, the reaction mechanisms and mathematical model applied in this paper are feasible and reasonable.
Chemical reaction mechanism
Results and discussion For the rhodium surface, the catalytic reaction mechanism suggested by Olaf Deutschmann et al. [21] is applied. There are 11 surface species (H(s), O(s), OH(s), H2O(s), C(s), CO(s), CO2 (s), CH3(s), CH2(s), CH(s), Rh(s)) which describe the coverage of the surface with adsorbed species. The accuracy of the mechanisms has been well verified with literature [33]. For the gas phrase reaction, C1 mechanism [34] which includes 18 species was applied, this mechanism is usually adopted in simulations [35,36] and was also verified by literatures.
Calculation method A 3D model for catalytic combustion of methane was simulated, the inlet was specified as velocity boundary condition and outlet was specified as pressure boundary condition. Methane/air mixture of 500 K flowed slowly to the Rh catalyst. In the present study, the flow transforms into turbulence when the inlet velocity is greater than 13 m/s. Hence, we adopted the k ~ epsilon turbulence model for cases with inlet velocity greater than 13 m/s, and laminar model was employed for cases with lower inlet velocity. The governing equations are solved with the finite volume method, and pressure and velocity are coupled with the SIMPLEC algorithm.
Comparisons between combustor with and without the regular triangular pyramid bluff body In order to figure out whether the regular triangular pyramid bluff body has a positive effect on methane combustion and how it affects the combustion characteristics, this paper firstly compares blow-off limit, velocity field and exhaust gas temperature of the combustor with and without a regular triangular pyramid bluff body. The impacts of equivalence ratio on the blow-of limit of two kinds of combustors are shown in Fig. 3. When F increases from 0.6 to 1.0, the blow-off limit in the combustor without bluff body increases from 6.2 m/s to 9.5 m/s, while in the combustor with a bluff body, it increases from 13.8 m/s to 23.6 m/s. When F is 1.0, both curves reach their peaks at 9.5 m/ s and 23.6 m/s, respectively, the maximum blow-off limit in combustor with a regular triangular pyramid bluff body is 2.4 times of that in the combustor without bluff body. It means that a regular triangular pyramid bluff body can extent the velocity limit of the combustion dramatically, and release much more heat in a constant time, which can expand the application of MEMS. Fig. 4 illustrates the longitudinal velocity field of combustors with and without a regular triangular pyramid bluff body
Please cite this article in press as: Yan Y, et al., Numerical investigation on combustion characteristics of methane/air in a microcombustor with a regular triangular pyramid bluff body, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.02.168
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 8 ) 1 e1 0
1800
100
96
1750
94
1700
92 1650
90 88
1600
86 84
1550
82 80
Menthane conversion rate (%)
100
Methane conversion rate Exhaust gas temperature
Exhaust gas temperature（K）
Methane conversion rate（%）
98
0.2
0.3
0.4
0.5
80
60
40
20
0
1500 0.1
Experiment [22] Simulation
700
0.6
800
900
1000
1100
1200
Temperature (K)
Mesh size (mm)
(a)
(b)
Fig. 2 e Model validation. (a) Methane conversion rates versus mesh scale. (b) Verification of the simulation results.
24 1800
Exhaust gas temperature (K)
Blow-off limit (m/s)
22 20 18 16 Micro-combustor with bluff body Micro-combustor without bluff body
14 12 10 8 6 0.8
0.9
1.0
1.1
1.2
1.3
1.4
Φ
Fig. 3 e Effect of equivalence ratio on the blow-off limit. when F is 1.0. It's evident that in the combustor without a regular triangular pyramid bluff body, the velocity significantly increases along the axial direction, and the growth trend gets more drastic with the increase of inlet velocity, which means that gas mixture of methane/air is easy to blow out of the micro-combustor. However, the velocity is much
With a regualr triangular pyramid bluff body Without bluff body
1600 1400 1200 1000 800 600 0.0
0.5
1.0
1.5
2.0
2.5
Inlet velocity (m/s)
Fig. 5 e Exhaust gas temperature in micro-combustor with/ without a bluff body (F ¼ 1.0). lower in combustor with a bluff body, and when the inlet velocity increases to 1 m/s, a counter flow appears in the microcombustor. The recirculation zone gradually forms behind the bluff body and becomes fiercer with the increase of inlet velocity. It means that the reaction of methane/air is more
Fig. 4 e Longitudinal velocity isogram in the micro-combustor with/without a bluff body (F ¼ 1.0). (a) Without a bluff body. (b) With a regular triangular pyramid bluff body. Please cite this article in press as: Yan Y, et al., Numerical investigation on combustion characteristics of methane/air in a microcombustor with a regular triangular pyramid bluff body, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.02.168
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complete in the combustor with a bluff body at large inlet velocity. Thus, the bluff body combustor has a much higher blow-off limit as shown in Fig. 4. A high exhaust gas temperature is beneficial to some steam-driven machine, for example, the micro turbine. The work efficiency of such machinery can be significantly improved by improving the temperature of its driving gas. Therefore, we compared the exhaust gas temperature of the micro-combustor with and without a regular triangular pyramid bluff body at various inlet velocities. As shown in Fig. 6, it is noted that the exhaust gas temperature increases firstly and then it decreases with further increase of inlet velocity both in micro-combustor with and without a regular triangular pyramid bluff body. The maximum of exhaust gas temperature for combustor with and without bluff body are 1746 K and 1674 K, respectively. When the inlet velocity is less than 0.1 m/ s, the temperatures of exhaust gas in two combustors are almost the same. However, the exhaust gas temperature in combustor without a bluff body declines sharply when the inlet velocity increases from 0.1 m/s to 0.2 m/s, while it keeps increasing in combustor with a bluff body. Even though the temperature of exhaust gas in both combustor decreases when inlet velocity increases from 0.2 m/s to 2.5 m/s, it can be observed that the temperature of exhaust gas in combustor with a bluff body is about 200 K higher than that of combustor without a bluff body. The reason is that the impact of bluff body is relatively weak at a low inlet velocity, which can also be observed in Fig. 4. With the increase of inlet velocity, the recirculation zone forms and the advantage of combustor with a regular triangular pyramid bluff body becomes visible.
Effects of inlet velocity and equivalence ratio Effects of inlet velocity and equivalence ratio on the methane conversion rate and preferential transport in the microcombustor with a regular triangular pyramid bluff body are investigated. Fig. 6 shows the mass fraction of methane along the axial direction at different inlet velocities when F ¼ 1.0. It's observed that the concentration of methane drops sharply at 4 mm-6mm, which is exactly the position of recirculation zone. Furthermore, the downtrend in the recirculation zone
Flocal ¼
(12)
0.900
V=5.0 m/s V=2.5 m/s V=1.0 m/s
5.0
0:5 XH2 þ XH2 O þ XCO2 þ XCO þ 2XCH4 XO2 þ XCO2 þ 0:5 XCO þ XH2 O
where Xi is the mole fraction of the species. Fig. 7 depicts the local equivalence ratio (Flocal) in the recirculation zone at different inlet velocities, keeping the inlet equivalence ratio at 0.8. The location of Flocal is shown in Fig. 1, which is 4.7 mm away from the inlet, tangent to the rear of the bluff body. We can see that Flocal is equal to the inlet equivalence ratio at inlet velocity of 0.2 m/s. However, it increases sharply when the inlet velocity increase from 1 m/s to 10 m/s, the local equivalence ratio for velocity of 10 m/s is 0.863, which is 8% higher than the inlet equivalence ratio, but we can see that the growth trend of Flocal is not obvious with further increase of inlet velocity. The reason for the change of Flocal is that there is no recirculation zone behind the bluff body under a low inlet velocity, with the increase of velocity, the recirculation zone intensifies, which can be seen from Fig. 5(b), amplifying the preferential transport effect, leading to the increase of Flocal. However, when the inlet velocity
0.875
Φ in Φ local
0.863
0.800
3.5
0.867
15
20
0.823
0.825
4.0
0.867
0.848
0.850
4.5 Φ
Mss fraction of methane (%)
5.5
becomes more drastic with the inlet velocity increasing from 1 m/s to 5 m/s, that's because the area of recirculation zone is bigger and its effect is stronger under a higher inlet velocity which can be seen from Fig. 4(b). We can also see that methane consumes more quickly when the inlet velocity is 1 m/s, especially in the front of the micro-combustor, hence, lowering the inlet velocity is beneficial to cut down the size of the combustor. Barlow et al. [38,39] experimentally investigated the anchoring mechanisms of bluff-body-stabilized methane/air flames. They reported that a strong preferential transport effect exists in the recirculation zone for the different diffusion coefficient of each species and the sharp velocity gradients near the bluff body when F < 1.0. Wan et al. [36] has found out similar phenomenon when they investigated the blow-off limits of premixed CH4/air flames in a mesoscale bluff-body combustor. Consequently, the preferential transport effect lead to an increase of local equivalence ratio which is beneficial for flame stabilization. In order to quantitatively evaluate this effect, Barlow et al. [38] proposed a local equivalence ratio based on local fuel/oxygen atom balance of major species, which is defined as follows:
0.8
0.808
0.775
3.0 0
2
4
6
8
10
12
14
16
18
20
Axial position (mm)
Fig. 6 e Mass fraction of methane along the axial direction (F ¼ 1.0).
0.750 0.2
1
2.5
5
10
V (m/s)
Fig. 7 e Local equivalence ratio in the recirculation zone versus inlet velocities.
Please cite this article in press as: Yan Y, et al., Numerical investigation on combustion characteristics of methane/air in a microcombustor with a regular triangular pyramid bluff body, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.02.168
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exceeds 10 m/s, the entrainment of the recirculation zone reaches its saturation, thus Flocal remains unchanged when inlet velocity increase to 15 m/s and 20 m/s. Fig. 8 depicts the methane conversion rate with various F at an inlet velocity of 0.2 m/s. The methane conversion rate increases with increasing F, reaching the peak of 72.6% when F increases to 1.0, and then drops to 68.0% when F reaches 1.2. One reason is that the oxygen is excessive for the combustion of methane when F is less than 1.0, heat comes from methane combustion is carried away by the cold air instead of preheating unburned gas. Another important reason is that the combustion of methane/air is largely affected by catalyst and oxygen atoms. When F is less than 1.0, the oxygen is excessive for the combustion of methane, noticing that the oxygen atoms are much easier to be absorbed by the catalyst, so the oxygen atoms take the majority of the catalytic surfaces, therefore, methane conversion rate declines. With the increase of F, the amount of methane increases, increasing the chance of adsorption reaction between methane and the catalyst, leading to an increase of methane conversion. However, when F is bigger than 1.0, the methane is rich and there is no enough oxygen to support methane combustion, therefore, methane conversion declines. In addition, it can be seen that when the fuel is rich, methane conversion rate is relatively higher compared with the fuel lean condition, which means the competition of catalytic surface between methane and oxygen plays a vital role in catalytic combustion. As it shows in Fig. 8, the equivalence ratio not only exerts a significant effect on methane conversion rate, but also dominates the emission performance of the micro-combustor. It is observed that the mass fraction of CO increases with increasing F, increasing from 0.13% to 0.62% with F rising from 0.8 to 1.3. But it can be seen that the growth trend of the curve is comparatively gentle when F increases from 0.8 to 1.0, while it grows drastically when F is bigger than 1.0. According to the quantitative calculation, 10% increase of methane brings about 23% increase of CO when F is less than 1.0, but when F is bigger than 1.0, 10% increase of methane brings about 75% increase of CO. Therefore, in order to reduce the pollutant emission of the micro-combustor, the equivalence ratio smaller than 1.0 should be applied.
75
It has been proved that hydrogen can lower the ignition temperature and initial reaction temperature of methane/air mixture. A small amount of hydrogen has thermal effect on combustion of methane, heat comes from combustion of hydrogen can warm methane/air mixture. With the increase of hydrogen, thermal effect turns into chemical effect, which means hydrogen involves in elementary reactions of methane/air. Both thermal and chemical effects are positive to the ignition of methane. However, as we can see from Fig. 10, the addition of hydrogen is not always helpful for the conversion of methane. Fig. 9 depicts methane conversion rates in different amount of hydrogen at the equivalence ratio of 0.8, 1.0 and 1.2, keeping the inlet velocity at 0.4 m/s. When F ¼ 0.8, methane conversion rate first increase with increasing hydrogen, reaching the peak of 55.3% at 1.25% mass fraction of hydrogen, and then it slightly declines to 54.5% when mass fraction of hydrogen increases to 2%. The curve shows similar trend when F ¼ 1.0, the difference is that methane conversion rate peaks at 1% mass fraction of hydrogen. However, when F ¼ 1.2, methane conversion rate decreases from 51.2% to 49.9% with increasing hydrogen, which is completely different from the other two curves. The reason is that when the oxygen is sufficient (F ¼ 0.8, F ¼ 1.0), a small amount of hydrogen exerts positive effect on combustion of methane, thus methane conversion rate increases. However, note that hydrogen is easier to be absorbed by Rh(s) and O(s) compared with methane, which means with the increase of hydrogen, it scrambles for radicals with methane, therefore, methane conversion rate declines with further increase of hydrogen. When F ¼ 1.2, the oxygen is insufficient, radical competition between methane and hydrogen is drastic enough to cover the positive effect of hydrogen, thus methane conversion rate drops slightly with increasing hydrogen. It has been proved that O þ CH4dOH þ CH3 (R31) plays a significant role in the combustion of methane [33,35]. In this reaction CH4 is oxidized to CH3, which is the essential step of methane combustion, and the radical change from O to CH3 and OH, increasing the concentration of radicals. The reaction rate of R31 directly impacts the conversion of methane. Fig. 10
58
0.5
72 71
0.4
70 0.3
69 68
0.2
67
Methane conversion rate Mass fraction of CO
66
Mass fraction of CO (%)
0.6
73
0.8
0.9
1.0
1.1
1.2
56
54 Φ =0.8 Φ =1.0 Φ =1.2
52
0.1
50 0.0
65
Methane conversion rate (%)
0.7
74
Mehtane conversion rate (%)
Effects of hydrogen addition
1.3
Φ
Fig. 8 e Methane conversion rate & mass fraction of CO at the outlet versus F.
0.0
0.5
1.0
1.5
2.0
Mass fraction of H2(%)
Fig. 9 e Methane conversion rate versus mass fraction of H2.
Please cite this article in press as: Yan Y, et al., Numerical investigation on combustion characteristics of methane/air in a microcombustor with a regular triangular pyramid bluff body, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.02.168
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Methane conversion rate（%）
56
52
48
44
40
0.0
0.1
0.2
0.3
Blockage ratio
Fig. 10 e The maximal reaction rate of R31 versus mass fraction of H2 (F ¼ 1.0).
Fig. 11 e Methane conversion rate versus blockage ratio (F ¼ 1.0).
depicts the maximal reaction rate of R31when F ¼ 1.0. It can be observed that the maximal reaction rate of R31 increases with increasing hydrogen firstly, reaching the peak of 2.9 104 at 1% mass fraction of hydrogen, then it decreases to 2.79 104 when mass reaction of hydrogen increases to 2%, which is coincident with the trend of methane conversion rate in Fig. 10. It can be concluded that the effect of radical competition is dominated by the amount of hydrogen and oxygen, when the oxygen is abundant, adding a small amount of hydrogen can promote the conversion of methane, while the negative effect of hydrogen becomes obvious with increasing hydrogen, and the value of turning point is bigger when the equivalence ratio is smaller.
Effects of blockage ratio The size of bluff body has a great influence on methane combustion in the micro-combustor. In order to achieve a better flow field and a higher methane conversion rate, the effects of bluff body size are studied. In this paper, the blockage ratio is defined as the ratio of maximum cross sectional area of regular triangular pyramid bluff body and cross sectional area of micro-combustor. Methane conversion rate versus blockage ratio is shown in Fig. 11, where F keeps 1.0, inlet velocity keeps 1 m/s, and blockage ratio increases from 0.04 to 0.27. Methane conversion rate increases from 38.5% to 54.9% when the blockage ratio increases from 0.04 to 0.22. However, it declines with further increase of blockage ratio, dropping to 50% when blockage ratio increases to 0.27. Recirculation zone cannot form when the bluff body is too small, thus cannot increase the residence time of gas mixture. With a bigger bluff body, the recirculation zone gradually forms and enlarges. However, a bigger bluff body leads to a strong flow resistance to incoming gas mixture, therefore, the feeding rate of mixture can't meet its consumption rate, leading to the decline of methane conversion rate. It is found that the optimum blockage in the present research is 0.22. The size of bluff body exerts a dominate effect on flame stabilization. Fig. 12 shows the temperature flieds in microcombustors with blockage ratio of 0.1, 0.22 and 0.27. It can be seen that the temperature behind the bluff body is quite low when the blockage ratio is 0.1, and the high-temperature
Fig. 12 e Temperature fields in micro-combustors with different size of bluff body (F ¼ 1.0).
zone lies in the wall of exit. When the blockage ratio increases to 0.22, the second high-temperature zone appears behind the bluff body, where the maximum temperature is about 1300 K, which is beneficial to anchor the flame behind the bluff body. However, the high-temperature zone behind the bluff body shrinks when the blockage ratio increase to 0.27, and the maximum temperature in this zone declines to 1250 K, which means the stability of the flame is not as good as blockage ratio of 0.22. The change of temperature fields is consistent with Fig. 10. It proves that the size of bluff body has a great influence on the temperature field behind the bluff body and flame stabilization. The optimum temperature field is achieved at blockage ratio of 0.22.
Conclusions The blow-off limit is significantly improved with the use of regular triangular pyramid bluff body. The maximum blow-off limit is 23.6 m/s in combustor with a bluff body, which is 2.4 times of that in a combustor without a bluff body. And the
Please cite this article in press as: Yan Y, et al., Numerical investigation on combustion characteristics of methane/air in a microcombustor with a regular triangular pyramid bluff body, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.02.168
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exhaust gas temperature in combustor with a bluff body is also improved in micro-combustor with a regular triangular pyramid bluff body. A recirculation zone behind the bluff body gradually forms and expands with increasing inlet velocity in the combustor with a regular triangular pyramid bluff body. The preferential transport effect is intensified due to the expansion of recirculation zone, leading to an increase of local equivalence ratio behind the bluff body when F ¼ 0.8. However, the growth trend of local equivalence ratio is not obvious when the inlet velocity exceeds 10 m/s for the saturation of entrainment effect in the recirculation zone. When the oxygen is sufficient, methane conversion rate and the reaction rate of O þ CH4dOH þ CH3 first increases with increasing hydrogen addition, then it declines with further increase of hydrogen addition. The turning point of methane conversion rate for F ¼ 1.0 and F ¼ 0.8 is 1% and 1.25% mass fraction of hydrogen, respectively. While when F ¼ 1.2, adding hydrogen into mixtures of methane/air hinders the combustion of methane. Blockage ratio exerts significant impacts on methane conversion rate and flame stabilization. Methane conversion rate increases with increasing blockage ratio firstly, reaching the peak at blockage ratio of 0.22, then it declines with further increase of blockage ratio. The temperature fields behind the bluff body show similar change, it's found that the temperature behind the bluff body reaches the highest when blockage ratio increase to 0.22, meaning that it has a better flame stability when blockage is 0.22.
Acknowledgments We gratefully acknowledge financial support from Joint Fund of the National Natural Science Foundation of China and China Aerospace Science and Technology Corporation on Advanced Manufacturing Technology for Aerospace Industry (No. U1737113) and Chongqing Research Program of Basic Research and Frontier Technology (No. cstc2016jcyjA0106).
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Numerical investigation on combustion characteristics of methane/air in a micro-combustor with a regular triangular pyramid bluff body Yunfei Yan a,*, Hongyu Yan a, Li Zhang a, Lixian Li b,**, Junchen Zhu a, Zhien Zhang a,c a Key Laboratory of Low-grade Energy Utilization Technologies and Systems, Chongqing University, Ministry of Education, Chongqing, 400030, PR China b Chongqing University Cancer Hospital, Chongqing Cancer Institute, Chongqing Cancer Hospital, Chongqing, 400030, PR China c College of Chemistry and Chemical Engineering, Chongqing University of Technology, Chongqing, 400054, PR China
article info
abstract
Article history:
Combustion characteristics of methane/air in a micro-combustor with a regular triangular
Received 4 December 2017
pyramid bluff body were numerically investigated. Results reveal that the blow-off limit of
Received in revised form
the micro-combustor with a regular triangular pyramid bluff body is 2.4 times of that in the
3 February 2018
micro-combustor without bluff body. With the increase of inlet velocity, the recirculation
Accepted 26 February 2018
zone expands and preferential transport effect behind the bluff body is intensified.
Available online xxx
Therefore, the local equivalence ratio in the recirculation zone increases when F ¼ 0.8, but the growth trend of local equivalence ratio is not obvious when the inlet velocity exceeds
Keywords:
10 m/s. When F < 1.0, adding small amount of hydrogen into gas mixture can speed up the
Micro-combustor
significant elementary reaction, leading to an increase of methane conversion. It's found
Regular triangular pyramid bluff
that both the methane conversion rate and the temperature behind the bluff body reaches
body
the highest when blockage ratio increase to 0.22.
Catalytic combustion
© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Hydrogen addition Preferential transport effect
Introduction With the fast development and intensive applications of Micro Electro-Mechanical Systems (MEMS), lightweight and compact energy resources have been gradually stepped into aviation, military, healthcare and even our daily life [1]. Applications of
compact energy are expected to increase due to the development of electronic devices, which mainly depends on the development of micro-powers [2,3]. The efficiency and stability of the combustion have a decisive effect on the performance of MEMS. Since the energy density of hydrocarbon fuels is much greater than the traditional batteries, the
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (Y. Yan), [email protected] (L. Li). https://doi.org/10.1016/j.ijhydene.2018.02.168 0360-3199/© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Yan Y, et al., Numerical investigation on combustion characteristics of methane/air in a microcombustor with a regular triangular pyramid bluff body, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.02.168
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Nomenclature Ys Ds Rs Vsk
mass fraction of species diffusion coefficient of species consumption or decomposition rate species stoichiometric coefficient in forward direction of surface reaction total number of elementary surface reactions ks 00 stoichiometric coefficient in negative direction V ik of reaction forward rate coefficient of reaction ksk pre-exponential factor Ak Ea activation energy of reaction temperature exponent bk surface coverage rate of species Qi surface coverage parameter εik surface coverage parameter mik number of surface species Ns number of gas phase species Ng [Xi] molar concentration of surface species G surface site density of the catalyst body force fi m viscosity of the mixture h enthalpy q heat of reaction l thermal conductivity R universal gas constant F equivalence ratio of the inlet mixture MCH4·in mass flow rate of methane at the entrance of the combustor MCH4·out mass flow rate of methane at the exit of the combustor A/F actual ratio of air/fuel in the inlet mixture gas stoichiometric air/fuel ratio A0/F0 equivalence ratio of the rear of the bluff body Flocal
combustion of hydrocarbons has been extensively studied in the field of micro-power generation [4e7]. Fernandez-Pello pointed out that the energy density of typical hydrocarbon fuels was about 45 MJ/kg, which is several tens of times than that of a lithium battery [8]. Many studies have proved that hydrogen and methane are the most promising fuels in micro combustors. The tiny size that shortens residence time of fuels and the large surface area-to-volume ratio that multiplies the heat loss of micro-combustors are two prominent characteristics of micro-combustor [8]. It has been proved that a short residence time of fuels and heat loss from exterior surface of combustor can significantly weaken the performance of microcombustor [9]. Scholars have made tremendous contributions to solve these problems. In order to reduce heat loss, Tang et al. [10]. proposed a new planar combustor for micro thermo photovoltaic system, which had multi-mode heat transfer passages in the combustion channel. It was found that the new combustor could achieve a higher temperature of the radiation surface due to the enhancement of heat transfer. Cao et al. [11] studied the diffusion combustion in a microcombustor with porous media. It revealed that porous media
can improve the enthalpy of reactant and reduce the heat loss. Furthermore, Pan et al. [12] investigated the hydrogen/oxygen premixed combustion characteristics in micro porous media combustor. They figured out that porous media material with low Cp and high thermal conductivity led to better temperature distribution on the wall, which can improve the efficiency of the thermophotovoltaic (TPV) system. In order to prolong the residence time of inlet gas, some researchers proposed to add a bluff body into micro-combustor to optimize combustion characteristics [13e16]. Stability of the flame and combustion efficiency were considerably improved by using bluff body. Bagheri et al. [13] investigated effects of bluff body shape on the flame stability in premixed micro-combustion of hydrogen/air mixture via 2D CFD simulations, it's concluded that the wall temperature and combustion efficiency was improved in all cases compared with the conventional model, and they also suggested that wall-blade is better than other shape of bluff body at high inlet velocities. Fan et al. [15,16] compared combustion characteristics of gas mixture in a 2D planar micro-combustor model with and without a bluff body. The results showed that the blow-off limit of flame can be extended to several times in the micro-combustor with a bluff body than that without bluff body. Hosseini et al. [17] studied the effect of bluff-body on flameless combustion in a cylinder micro-combustor, they pointed out that micro-flameless mode with bluff-body illustrated better performance than other cases. Ran et al. [18] compared the flow field and combustion efficiency of methane in a micro-channel with concave cavity and with convex cavity, they reported that concave cavity can lead to the reflux of methane at a high velocity and convex cavity can enhance the contact between gas mixture and catalyst. Zhang et al. [19] investigated the cavity structure, position, and number of convex cavities on the catalytic combustion characteristics of methane/air in a micro-channel, they reported that increasing the normalized cavity diameter is an effective way to expand the blow-off limit. E et al. [20] proposed a cavity and backward-facing step in micro-combustor. It was found that the cavity or backward-facing step contributed to the formation of the recirculation zone, enhanced the heat recirculation and the flame stabilization. Niu et al. [21] have made a comparative study between streamline-type bluff body and nonstreamline-type bluff body. The results showed that the micro-combustor with non-streamline-type bluff body performs better in the formation of recirculation zone and stability of the flame. In addition, catalytic combustion is also an effective way to improve the performance of micro-combustors. It has been proved that catalyst can decrease the activation energy of the reactions and improve conversion rate of fuels [22e24]. Suzanne et al. [25] experimentally investigated the surface oxidation on small-scale catalytic combustion for methane/air and propane/air, and they found a sharp temperature increase occurred on the catalytic surface with simultaneous fast depletion of the combustible gas. Ran et al. [26] employed a 2D model to numerically investigate wall heat transfer characteristics of CH4/air in the microtube with Pt catalyst. It's concluded that both the heat transfer along the solid structure and the total heat loss increased with increasing wall thickness. Lee et al. [27]
Please cite this article in press as: Yan Y, et al., Numerical investigation on combustion characteristics of methane/air in a microcombustor with a regular triangular pyramid bluff body, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.02.168
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experimentally studied the CH4/air combustion characteristics in a micro heat-regenerative combustor with and without catalytic platinum wires. They reported that the Pt wire located within the recirculation area promoted the catalytic reaction and extended the self-sustainable reaction conditions. Yan et al. [28,29] have taken tremendous efforts in studying the catalytic combustion characteristics of methane/air in micro-combustors, they mainly investigated the effect of operating conditions, materials of the solid wall, the amount of catalyst and obtained the optimum conditions for catalytic combustion in micro-combustors. Meanwhile, they discussed the thermal and chemical effects of hydrogen addition on catalytic micro-combustion of CH4/air, concluding that adding a small amount of hydrogen into methane/air mixture can significantly improve the combustion efficiency of methane [30,31]. Although the effects of structure of the micro-combustor and bluff body on combustion characteristics of micro combustion have been extensively observed, the impacts of a regular triangular pyramid bluff body in a microcombustor have not been studied. literatures mentioned above have mostly adopted a 2D model, efforts need to be made in the study of bluff body which employ the 3D model. Furthermore, many literatures about the addition of hydrogen into methane/air mixture reported the positive effect of hydrogen addition, but too much hydrogen will scramble for radicals with methane, the negative effect have not been properly studied. In the present work, a 3D microcombustor with a regular triangular pyramid bluff body is developed, and the effect of regular triangular pyramid bluff body, preferential transport effect, hydrogen addition and blockage ratio on the combustion characteristics of methane/air are investigated.
Numerical methods Physical model The model of cube micro combustor with a regular triangular pyramid bluff body is depicted in Fig. 1 using Fluent. The total length of the micro-combustor is 20 mm, the width and the height of the combustor are 5 mm and 8 mm, respectively. The distance from vertex of the regular triangular pyramid bluff body to combustor inlet is 3 mm. Edge length of the regular triangular pyramid bluff body is 2 mm. The bluff body was symmetrically located with respect to the micro-combustor. Combustion of methane/air was examined in the micro
combustor, which was coated with Rh catalyst on the inner surface of the combustor.
Mathematical model Both surface catalytic reactions and gas phrase reaction are considered in the simulation. The heat dissipation effect and gas radiation in the process of reaction have not been taken into account, and a similar application was adopted in the previous publication of Zhang et al. [32]. Nonetheless, the equations for conservation of continuity, momentum, and energy are used in control volume. The governing equations in Cartesian coordinates include: Continuity equation: v ruj ¼0 vxj
(1)
where r is the density of the mixture, and u is the velocity. Composition equation: ruj
vYs v vYs ¼ Ds r þ Rs vXj vXj vXj
(2)
where YS corresponds to mass fraction of species s in microcombustor, Ds is the diffusion coefficient and Rs refers to the consumption or decomposition rate. Both surface species and gas phase species can be generated and consumed by surface reactions. The consumption or decomposition rate RS is defined as below: Rs ¼
Ks X
vsk ksk
k¼1
NgþNs Y
½Xiv}ik
(3)
i¼1
where vsk is stoichiometric coefficient in forward direction of surface reaction k, Ks is the total number of elementary surface reactions, v}ik is stoichiometric coefficient in negative direction of reaction k, ksk is forward rate coefficient of reaction k, Ns is the number of surface species, and Ng is the number of gas phase species. ½Xi is the molar concentration of surface species i, and ksk is calculated by Arrhenius reaction source of the reaction k as below: Ksk ¼ Ak Tbk exp
Ns Ea Y εik Qi m Qi jk exp RT i¼1 RT
(4)
where Ak is the pre-exponential factor, Ea is the activation energy of reaction k, bk is the temperature exponent, Qi is the surface coverage rate of species i, εik and mik are surface coverage parameter. In addition, ½Xi is written as below: ½Xi ¼ GQi
(5)
here, G is surface site density of the catalyst. The value of 2:72 109 mol$cm2 is used for the Rh catalyst in this paper. The surface of the catalytic is described by its coverage with adsorbed species and temperature. The temperature is based on the governing equation for enthalpy as below: Momentum equation: v vp v2 ui þm ruj ui ¼ fi vXj vXj vXj vXj Fig. 1 e Structural diagram of the micro-combustor with a regular triangular pyramid bluff body.
(6)
where fi is body force, and m is viscosity of gaseous mixture.
Please cite this article in press as: Yan Y, et al., Numerical investigation on combustion characteristics of methane/air in a microcombustor with a regular triangular pyramid bluff body, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.02.168
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Model validation
Energy equation: ruj
vh v vT v ¼ l þ vXj vXj vXj vXj
Ng X
hs rDs
s¼1
vYs vXj
! þq
(7)
where h is the enthalpy, q is heat of reaction, l is thermal conductivity, and Ds is the diffusion coefficient of species s. Closure equation: Ng X
Ys ¼ 1
(8)
s¼1
State of ideal gas: P ¼ rRT
Ng X Ys s¼1
Ms
(9)
here, R is universal gas constant, Ys corresponds to mass fraction of the species s and Ms is molar mass. Methane conversion rate could be calculated by Eq. (10): h¼
mCH4 $in mCH4 $out mCH4 $in
(10)
where MCH4 $in is the mass flow rate of methane at the entrance of the combustor and MCH4 $out is the mass flow rate of methane at the exit of the combustor. The equivalence ratio (F) is defined by Eq. (11): F¼
A0 =F0 A=F
(11)
where A=F is the actual ratio of air/fuel in the inlet mixture gas and AO =FO is the stoichiometric air/fuel ratio.
The mesh scale has a significant impact on reliability of simulation results. In order to get the optimum mesh scale, we investigate methane conversion rate under various mesh scale, which decreases from 0.6 mm to 0.1 mm. Fig. 2 shows methane conversion rates and the exhaust gas temperature at various mesh size. In this study, the exhaust gas temperature is defined as the average surface temperature of the outlet. As shown in Fig. 2(a), the methane conversion rate increases when the mesh sizes decrease from 0.6 mm to 0.3 mm. However, with further decrease of the mesh size, both the methane conversion rate and exhaust gas temperature remain nearly unchanged. It means that the simulation results are independent from mesh scale when the mesh size is less than or equal to 0.3 mm. Therefore, in order to achieve reliable results and save the calculation time, mesh size is set at 0.3 mm in this paper. The accuracy of the numerical model is validated by comparing the methane conversion rate between the experimental and numerical results. In the experiment [37], the reactor is a hollow cylinder, which has an inside diameter of 4 mm coated with Rh, an outside diameter of 5 mm, 20 mm length of the channel coated with Rh. The volume fraction of CH4/O2 is 0.56, and the equivalent velocity is 0.15 m/s. Fig. 2(b) shows the methane conversion rate of experimental and modeling results at different temperatures, the trends of two profiles are basically the same, and the maximum relative error is 6.25%. Therefore, the reaction mechanisms and mathematical model applied in this paper are feasible and reasonable.
Chemical reaction mechanism
Results and discussion For the rhodium surface, the catalytic reaction mechanism suggested by Olaf Deutschmann et al. [21] is applied. There are 11 surface species (H(s), O(s), OH(s), H2O(s), C(s), CO(s), CO2 (s), CH3(s), CH2(s), CH(s), Rh(s)) which describe the coverage of the surface with adsorbed species. The accuracy of the mechanisms has been well verified with literature [33]. For the gas phrase reaction, C1 mechanism [34] which includes 18 species was applied, this mechanism is usually adopted in simulations [35,36] and was also verified by literatures.
Calculation method A 3D model for catalytic combustion of methane was simulated, the inlet was specified as velocity boundary condition and outlet was specified as pressure boundary condition. Methane/air mixture of 500 K flowed slowly to the Rh catalyst. In the present study, the flow transforms into turbulence when the inlet velocity is greater than 13 m/s. Hence, we adopted the k ~ epsilon turbulence model for cases with inlet velocity greater than 13 m/s, and laminar model was employed for cases with lower inlet velocity. The governing equations are solved with the finite volume method, and pressure and velocity are coupled with the SIMPLEC algorithm.
Comparisons between combustor with and without the regular triangular pyramid bluff body In order to figure out whether the regular triangular pyramid bluff body has a positive effect on methane combustion and how it affects the combustion characteristics, this paper firstly compares blow-off limit, velocity field and exhaust gas temperature of the combustor with and without a regular triangular pyramid bluff body. The impacts of equivalence ratio on the blow-of limit of two kinds of combustors are shown in Fig. 3. When F increases from 0.6 to 1.0, the blow-off limit in the combustor without bluff body increases from 6.2 m/s to 9.5 m/s, while in the combustor with a bluff body, it increases from 13.8 m/s to 23.6 m/s. When F is 1.0, both curves reach their peaks at 9.5 m/ s and 23.6 m/s, respectively, the maximum blow-off limit in combustor with a regular triangular pyramid bluff body is 2.4 times of that in the combustor without bluff body. It means that a regular triangular pyramid bluff body can extent the velocity limit of the combustion dramatically, and release much more heat in a constant time, which can expand the application of MEMS. Fig. 4 illustrates the longitudinal velocity field of combustors with and without a regular triangular pyramid bluff body
Please cite this article in press as: Yan Y, et al., Numerical investigation on combustion characteristics of methane/air in a microcombustor with a regular triangular pyramid bluff body, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.02.168
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92 1650
90 88
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82 80
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Methane conversion rate Exhaust gas temperature
Exhaust gas temperature（K）
Methane conversion rate（%）
98
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0.4
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80
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1500 0.1
Experiment [22] Simulation
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Mesh size (mm)
(a)
(b)
Fig. 2 e Model validation. (a) Methane conversion rates versus mesh scale. (b) Verification of the simulation results.
24 1800
Exhaust gas temperature (K)
Blow-off limit (m/s)
22 20 18 16 Micro-combustor with bluff body Micro-combustor without bluff body
14 12 10 8 6 0.8
0.9
1.0
1.1
1.2
1.3
1.4
Φ
Fig. 3 e Effect of equivalence ratio on the blow-off limit. when F is 1.0. It's evident that in the combustor without a regular triangular pyramid bluff body, the velocity significantly increases along the axial direction, and the growth trend gets more drastic with the increase of inlet velocity, which means that gas mixture of methane/air is easy to blow out of the micro-combustor. However, the velocity is much
With a regualr triangular pyramid bluff body Without bluff body
1600 1400 1200 1000 800 600 0.0
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1.0
1.5
2.0
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Inlet velocity (m/s)
Fig. 5 e Exhaust gas temperature in micro-combustor with/ without a bluff body (F ¼ 1.0). lower in combustor with a bluff body, and when the inlet velocity increases to 1 m/s, a counter flow appears in the microcombustor. The recirculation zone gradually forms behind the bluff body and becomes fiercer with the increase of inlet velocity. It means that the reaction of methane/air is more
Fig. 4 e Longitudinal velocity isogram in the micro-combustor with/without a bluff body (F ¼ 1.0). (a) Without a bluff body. (b) With a regular triangular pyramid bluff body. Please cite this article in press as: Yan Y, et al., Numerical investigation on combustion characteristics of methane/air in a microcombustor with a regular triangular pyramid bluff body, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.02.168
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complete in the combustor with a bluff body at large inlet velocity. Thus, the bluff body combustor has a much higher blow-off limit as shown in Fig. 4. A high exhaust gas temperature is beneficial to some steam-driven machine, for example, the micro turbine. The work efficiency of such machinery can be significantly improved by improving the temperature of its driving gas. Therefore, we compared the exhaust gas temperature of the micro-combustor with and without a regular triangular pyramid bluff body at various inlet velocities. As shown in Fig. 6, it is noted that the exhaust gas temperature increases firstly and then it decreases with further increase of inlet velocity both in micro-combustor with and without a regular triangular pyramid bluff body. The maximum of exhaust gas temperature for combustor with and without bluff body are 1746 K and 1674 K, respectively. When the inlet velocity is less than 0.1 m/ s, the temperatures of exhaust gas in two combustors are almost the same. However, the exhaust gas temperature in combustor without a bluff body declines sharply when the inlet velocity increases from 0.1 m/s to 0.2 m/s, while it keeps increasing in combustor with a bluff body. Even though the temperature of exhaust gas in both combustor decreases when inlet velocity increases from 0.2 m/s to 2.5 m/s, it can be observed that the temperature of exhaust gas in combustor with a bluff body is about 200 K higher than that of combustor without a bluff body. The reason is that the impact of bluff body is relatively weak at a low inlet velocity, which can also be observed in Fig. 4. With the increase of inlet velocity, the recirculation zone forms and the advantage of combustor with a regular triangular pyramid bluff body becomes visible.
Effects of inlet velocity and equivalence ratio Effects of inlet velocity and equivalence ratio on the methane conversion rate and preferential transport in the microcombustor with a regular triangular pyramid bluff body are investigated. Fig. 6 shows the mass fraction of methane along the axial direction at different inlet velocities when F ¼ 1.0. It's observed that the concentration of methane drops sharply at 4 mm-6mm, which is exactly the position of recirculation zone. Furthermore, the downtrend in the recirculation zone
Flocal ¼
(12)
0.900
V=5.0 m/s V=2.5 m/s V=1.0 m/s
5.0
0:5 XH2 þ XH2 O þ XCO2 þ XCO þ 2XCH4 XO2 þ XCO2 þ 0:5 XCO þ XH2 O
where Xi is the mole fraction of the species. Fig. 7 depicts the local equivalence ratio (Flocal) in the recirculation zone at different inlet velocities, keeping the inlet equivalence ratio at 0.8. The location of Flocal is shown in Fig. 1, which is 4.7 mm away from the inlet, tangent to the rear of the bluff body. We can see that Flocal is equal to the inlet equivalence ratio at inlet velocity of 0.2 m/s. However, it increases sharply when the inlet velocity increase from 1 m/s to 10 m/s, the local equivalence ratio for velocity of 10 m/s is 0.863, which is 8% higher than the inlet equivalence ratio, but we can see that the growth trend of Flocal is not obvious with further increase of inlet velocity. The reason for the change of Flocal is that there is no recirculation zone behind the bluff body under a low inlet velocity, with the increase of velocity, the recirculation zone intensifies, which can be seen from Fig. 5(b), amplifying the preferential transport effect, leading to the increase of Flocal. However, when the inlet velocity
0.875
Φ in Φ local
0.863
0.800
3.5
0.867
15
20
0.823
0.825
4.0
0.867
0.848
0.850
4.5 Φ
Mss fraction of methane (%)
5.5
becomes more drastic with the inlet velocity increasing from 1 m/s to 5 m/s, that's because the area of recirculation zone is bigger and its effect is stronger under a higher inlet velocity which can be seen from Fig. 4(b). We can also see that methane consumes more quickly when the inlet velocity is 1 m/s, especially in the front of the micro-combustor, hence, lowering the inlet velocity is beneficial to cut down the size of the combustor. Barlow et al. [38,39] experimentally investigated the anchoring mechanisms of bluff-body-stabilized methane/air flames. They reported that a strong preferential transport effect exists in the recirculation zone for the different diffusion coefficient of each species and the sharp velocity gradients near the bluff body when F < 1.0. Wan et al. [36] has found out similar phenomenon when they investigated the blow-off limits of premixed CH4/air flames in a mesoscale bluff-body combustor. Consequently, the preferential transport effect lead to an increase of local equivalence ratio which is beneficial for flame stabilization. In order to quantitatively evaluate this effect, Barlow et al. [38] proposed a local equivalence ratio based on local fuel/oxygen atom balance of major species, which is defined as follows:
0.8
0.808
0.775
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Fig. 6 e Mass fraction of methane along the axial direction (F ¼ 1.0).
0.750 0.2
1
2.5
5
10
V (m/s)
Fig. 7 e Local equivalence ratio in the recirculation zone versus inlet velocities.
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exceeds 10 m/s, the entrainment of the recirculation zone reaches its saturation, thus Flocal remains unchanged when inlet velocity increase to 15 m/s and 20 m/s. Fig. 8 depicts the methane conversion rate with various F at an inlet velocity of 0.2 m/s. The methane conversion rate increases with increasing F, reaching the peak of 72.6% when F increases to 1.0, and then drops to 68.0% when F reaches 1.2. One reason is that the oxygen is excessive for the combustion of methane when F is less than 1.0, heat comes from methane combustion is carried away by the cold air instead of preheating unburned gas. Another important reason is that the combustion of methane/air is largely affected by catalyst and oxygen atoms. When F is less than 1.0, the oxygen is excessive for the combustion of methane, noticing that the oxygen atoms are much easier to be absorbed by the catalyst, so the oxygen atoms take the majority of the catalytic surfaces, therefore, methane conversion rate declines. With the increase of F, the amount of methane increases, increasing the chance of adsorption reaction between methane and the catalyst, leading to an increase of methane conversion. However, when F is bigger than 1.0, the methane is rich and there is no enough oxygen to support methane combustion, therefore, methane conversion declines. In addition, it can be seen that when the fuel is rich, methane conversion rate is relatively higher compared with the fuel lean condition, which means the competition of catalytic surface between methane and oxygen plays a vital role in catalytic combustion. As it shows in Fig. 8, the equivalence ratio not only exerts a significant effect on methane conversion rate, but also dominates the emission performance of the micro-combustor. It is observed that the mass fraction of CO increases with increasing F, increasing from 0.13% to 0.62% with F rising from 0.8 to 1.3. But it can be seen that the growth trend of the curve is comparatively gentle when F increases from 0.8 to 1.0, while it grows drastically when F is bigger than 1.0. According to the quantitative calculation, 10% increase of methane brings about 23% increase of CO when F is less than 1.0, but when F is bigger than 1.0, 10% increase of methane brings about 75% increase of CO. Therefore, in order to reduce the pollutant emission of the micro-combustor, the equivalence ratio smaller than 1.0 should be applied.
75
It has been proved that hydrogen can lower the ignition temperature and initial reaction temperature of methane/air mixture. A small amount of hydrogen has thermal effect on combustion of methane, heat comes from combustion of hydrogen can warm methane/air mixture. With the increase of hydrogen, thermal effect turns into chemical effect, which means hydrogen involves in elementary reactions of methane/air. Both thermal and chemical effects are positive to the ignition of methane. However, as we can see from Fig. 10, the addition of hydrogen is not always helpful for the conversion of methane. Fig. 9 depicts methane conversion rates in different amount of hydrogen at the equivalence ratio of 0.8, 1.0 and 1.2, keeping the inlet velocity at 0.4 m/s. When F ¼ 0.8, methane conversion rate first increase with increasing hydrogen, reaching the peak of 55.3% at 1.25% mass fraction of hydrogen, and then it slightly declines to 54.5% when mass fraction of hydrogen increases to 2%. The curve shows similar trend when F ¼ 1.0, the difference is that methane conversion rate peaks at 1% mass fraction of hydrogen. However, when F ¼ 1.2, methane conversion rate decreases from 51.2% to 49.9% with increasing hydrogen, which is completely different from the other two curves. The reason is that when the oxygen is sufficient (F ¼ 0.8, F ¼ 1.0), a small amount of hydrogen exerts positive effect on combustion of methane, thus methane conversion rate increases. However, note that hydrogen is easier to be absorbed by Rh(s) and O(s) compared with methane, which means with the increase of hydrogen, it scrambles for radicals with methane, therefore, methane conversion rate declines with further increase of hydrogen. When F ¼ 1.2, the oxygen is insufficient, radical competition between methane and hydrogen is drastic enough to cover the positive effect of hydrogen, thus methane conversion rate drops slightly with increasing hydrogen. It has been proved that O þ CH4dOH þ CH3 (R31) plays a significant role in the combustion of methane [33,35]. In this reaction CH4 is oxidized to CH3, which is the essential step of methane combustion, and the radical change from O to CH3 and OH, increasing the concentration of radicals. The reaction rate of R31 directly impacts the conversion of methane. Fig. 10
58
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Methane conversion rate Mass fraction of CO
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54 Φ =0.8 Φ =1.0 Φ =1.2
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50 0.0
65
Methane conversion rate (%)
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74
Mehtane conversion rate (%)
Effects of hydrogen addition
1.3
Φ
Fig. 8 e Methane conversion rate & mass fraction of CO at the outlet versus F.
0.0
0.5
1.0
1.5
2.0
Mass fraction of H2(%)
Fig. 9 e Methane conversion rate versus mass fraction of H2.
Please cite this article in press as: Yan Y, et al., Numerical investigation on combustion characteristics of methane/air in a microcombustor with a regular triangular pyramid bluff body, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.02.168
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Methane conversion rate（%）
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48
44
40
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Blockage ratio
Fig. 10 e The maximal reaction rate of R31 versus mass fraction of H2 (F ¼ 1.0).
Fig. 11 e Methane conversion rate versus blockage ratio (F ¼ 1.0).
depicts the maximal reaction rate of R31when F ¼ 1.0. It can be observed that the maximal reaction rate of R31 increases with increasing hydrogen firstly, reaching the peak of 2.9 104 at 1% mass fraction of hydrogen, then it decreases to 2.79 104 when mass reaction of hydrogen increases to 2%, which is coincident with the trend of methane conversion rate in Fig. 10. It can be concluded that the effect of radical competition is dominated by the amount of hydrogen and oxygen, when the oxygen is abundant, adding a small amount of hydrogen can promote the conversion of methane, while the negative effect of hydrogen becomes obvious with increasing hydrogen, and the value of turning point is bigger when the equivalence ratio is smaller.
Effects of blockage ratio The size of bluff body has a great influence on methane combustion in the micro-combustor. In order to achieve a better flow field and a higher methane conversion rate, the effects of bluff body size are studied. In this paper, the blockage ratio is defined as the ratio of maximum cross sectional area of regular triangular pyramid bluff body and cross sectional area of micro-combustor. Methane conversion rate versus blockage ratio is shown in Fig. 11, where F keeps 1.0, inlet velocity keeps 1 m/s, and blockage ratio increases from 0.04 to 0.27. Methane conversion rate increases from 38.5% to 54.9% when the blockage ratio increases from 0.04 to 0.22. However, it declines with further increase of blockage ratio, dropping to 50% when blockage ratio increases to 0.27. Recirculation zone cannot form when the bluff body is too small, thus cannot increase the residence time of gas mixture. With a bigger bluff body, the recirculation zone gradually forms and enlarges. However, a bigger bluff body leads to a strong flow resistance to incoming gas mixture, therefore, the feeding rate of mixture can't meet its consumption rate, leading to the decline of methane conversion rate. It is found that the optimum blockage in the present research is 0.22. The size of bluff body exerts a dominate effect on flame stabilization. Fig. 12 shows the temperature flieds in microcombustors with blockage ratio of 0.1, 0.22 and 0.27. It can be seen that the temperature behind the bluff body is quite low when the blockage ratio is 0.1, and the high-temperature
Fig. 12 e Temperature fields in micro-combustors with different size of bluff body (F ¼ 1.0).
zone lies in the wall of exit. When the blockage ratio increases to 0.22, the second high-temperature zone appears behind the bluff body, where the maximum temperature is about 1300 K, which is beneficial to anchor the flame behind the bluff body. However, the high-temperature zone behind the bluff body shrinks when the blockage ratio increase to 0.27, and the maximum temperature in this zone declines to 1250 K, which means the stability of the flame is not as good as blockage ratio of 0.22. The change of temperature fields is consistent with Fig. 10. It proves that the size of bluff body has a great influence on the temperature field behind the bluff body and flame stabilization. The optimum temperature field is achieved at blockage ratio of 0.22.
Conclusions The blow-off limit is significantly improved with the use of regular triangular pyramid bluff body. The maximum blow-off limit is 23.6 m/s in combustor with a bluff body, which is 2.4 times of that in a combustor without a bluff body. And the
Please cite this article in press as: Yan Y, et al., Numerical investigation on combustion characteristics of methane/air in a microcombustor with a regular triangular pyramid bluff body, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.02.168
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 8 ) 1 e1 0
exhaust gas temperature in combustor with a bluff body is also improved in micro-combustor with a regular triangular pyramid bluff body. A recirculation zone behind the bluff body gradually forms and expands with increasing inlet velocity in the combustor with a regular triangular pyramid bluff body. The preferential transport effect is intensified due to the expansion of recirculation zone, leading to an increase of local equivalence ratio behind the bluff body when F ¼ 0.8. However, the growth trend of local equivalence ratio is not obvious when the inlet velocity exceeds 10 m/s for the saturation of entrainment effect in the recirculation zone. When the oxygen is sufficient, methane conversion rate and the reaction rate of O þ CH4dOH þ CH3 first increases with increasing hydrogen addition, then it declines with further increase of hydrogen addition. The turning point of methane conversion rate for F ¼ 1.0 and F ¼ 0.8 is 1% and 1.25% mass fraction of hydrogen, respectively. While when F ¼ 1.2, adding hydrogen into mixtures of methane/air hinders the combustion of methane. Blockage ratio exerts significant impacts on methane conversion rate and flame stabilization. Methane conversion rate increases with increasing blockage ratio firstly, reaching the peak at blockage ratio of 0.22, then it declines with further increase of blockage ratio. The temperature fields behind the bluff body show similar change, it's found that the temperature behind the bluff body reaches the highest when blockage ratio increase to 0.22, meaning that it has a better flame stability when blockage is 0.22.
Acknowledgments We gratefully acknowledge financial support from Joint Fund of the National Natural Science Foundation of China and China Aerospace Science and Technology Corporation on Advanced Manufacturing Technology for Aerospace Industry (No. U1737113) and Chongqing Research Program of Basic Research and Frontier Technology (No. cstc2016jcyjA0106).
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Please cite this article in press as: Yan Y, et al., Numerical investigation on combustion characteristics of methane/air in a microcombustor with a regular triangular pyramid bluff body, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.02.168