air in a micro-combustor with a hollow hemispherical bluff body

Energy Conversion and Management 94 (2015) 293–299

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Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman

Numerical investigation on the combustion characteristics of methane/ air in a micro-combustor with a hollow hemispherical bluff body Li Zhang a,b,⇑, Junchen Zhu b, Yunfei Yan a,b, Hongliang Guo b, Zhongqing Yang a,b a b

Key Laboratory of Low-grade Energy Utilization Technologies and Systems, Chongqing University, Ministry of Education, Chongqing 400030, PR China College of Power Engineering, Chongqing University, Chongqing 400030, PR China

a r t i c l e

i n f o

Article history: Received 26 September 2014 Accepted 6 January 2015

Keywords: Micro-combustor Hollow hemispherical bluff body Numerical investigation Combustion characteristics Blow-off limit

a b s t r a c t The combustion characteristics of methane in a cube micro-combustor with a hollow hemispherical bluff body were numerically investigated. The blow-off limit, recirculation zone length and methane conversion rate were examined. The results illustrate that the blow-off limit of the micro-combustor with a hollow hemispherical bluff body is 2.5 times higher than that without bluff body, which are 24.5 m/s and 9.5 m/s at the same equivalence ratio (/ = 1), respectively. With the use of hollow hemispherical bluff body, methane conversion sharply increases from 0.24% to 17.95% at 3 mm along the inlet-flow direction, where is the location of bluff-body, which is not affected by equivalence ratio and inlet velocity. The recirculation zone size has determined influence on residence time of the mixture gas, which increases with the increase of inlet velocity. Methane conversion rate is determined by equivalence ratio and inlet velocity. Methane conversion rate firstly increases and then decreases when the equivalence ratio and inlet velocity increase, reaching the maximum value (97.84%) at / = 1 and 0.02 m/s. Methane conversion rate sharply increases from 45% to 97.84% when the inlet velocity increases from 0.008 m/s to 0.02 m/s. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction With the fast development and intensive applications of Micro Electro-Mechanical Systems (MEMS), it is urgent to develop new micro combustors with small volume, light weight and high power density. Micro devices have been playing a key role on human life for aviation, spaceflight, automobile, biomedicine, environmental regulation, military affair and so on, such as sensors, micro medical devices, micro-pumps, micro-motors [1]. The combustion stability and efficiency directly affect the performance of MEMS. The availability of efficient micro-combustors could significantly enhance the functionality of MEMS for portable equipment, because they require high energy density and low recharge time. Therefore, many scholars try to develop some efficient systems in micro-scale in which the combustion energy is used to meet the need of highpower density applications [2–4]. Aiming at solving the existing problem of the micro-scale combustor, Kang et al. [5] studied on the flow, mixing and combustion characteristics of methane. Heat release from combustion has a significant impact on micro-scale combustor. In order to decrease ⇑ Corresponding author at: Key Laboratory of Low-grade Energy Utilization Technologies and Systems, Chongqing University, Ministry of Education, Chongqing 400030, PR China. Tel.: +86 23 65103114; fax: +86 23 65111832. E-mail address: [email protected] (L. Zhang). http://dx.doi.org/10.1016/j.enconman.2015.01.014 0196-8904/Ó 2015 Elsevier Ltd. All rights reserved.

heat release of the micro-combustor, many scholars put forward many solutions. Cao et al. [6] experimented on the low combustion efficiency of the micro-combustor, a micro heat-recirculating combustor using porous media plates is designed to study its diffusion combustion characteristics, including variations of combustion efficiency. It is confirmed that by adopting the special structure of regenerative jacket and opposite direction inlet mode of reaction gas through porous media plates, flow direction of reaction gas is opposite to that of heat loss. Three-dimensional numerical simulations of spiral counter flow Swiss roll heat-recirculating combustors were performed including gas-phase conduction, convection and chemical reaction of propane-air mixtures, solid-phase conduction and surface-to-surface radiation. These simulations showed that in 3D model, results are surprisingly similar to the experimental date [7]. Zhong and Hong [8] numerically investigated the micro-scale combustor with counter current heat transfer with Computational Fluid Dynamics (CFD), the results revealed that methane conversion rate could be increased and the combustion stability could be enhanced with catalyst, but high wall temperature of the micro-combustor would lead to catalyst deactivation. In order to solve the shortage of the micro-combustor, Zhang et al. [9] studied the characteristics of catalytic combustion, flow and heat transfer in micro-combustor, using laminar finite-rate and second-order upwind discretization model. The results demonstrated that wall temperature has determined influence on the rate of methane

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conversion than other factors. Catalytic and regenerative structure can increase the efficiency of micro-scale combustor, but catalytic is easy to lose activity under higher temperature and regenerative structure increases complexity of the micro-scale combustor. By applying bluff body in the micro-combustors, the stability of the flame and combustion efficiency was considerably improved. Equilateral triangle shape and V-shape bluff bodies have been commonly developed, which are simulated in 2D model. The shortage of the 2D model simulation is that the data of the simulation cannot accurately reflect the real situation due to the simplify of 3D model. Fan et al. [10,11] studied on the shape and size of bluff body, results showed that there was the recirculation zone behind the bluff body due to the interaction of the mixture gas, which prolonged the residence time of mixture gas to make the reaction carried out adequately. Yan et al. [12–14] studied on the combustion characteristics of methane/air, the results revealed that the combustion of methane/air could be improved by added hydrogen into the mixture gas. Hydrogen is easy to burn, to the heat release of which can contribute to the combustion of methane. Although the effects of micro-combustor structure and operating condition on combustion characteristics of micro combustion have been noted, the impacts of a cube micro-combustor with a hollow hemispherical bluff body have not been properly studied. Spherical surface can reduce the gas flow resistance and hollow hemisphere can prolong the recirculation zone. The paper focuses on the combustion characteristics of methane/air in a 3D micro-combustor model and compares the methane conversion rate and blow-off limit between micro-combustor with and without bluff body, and then investigates the combustion characteristics in order to optimize the structure and the combustion characteristics of the burner. 2. Physical model and mathematical model 2.1. Physical model The 3D numerical models of cube micro combustor (20 mm length, 8 mm width, 5 mm height) with and without a hollow hemispherical bluff body are depicted in Fig. 1, which were constructed using FLUENT. Numerical calculation of the present problem included solutions of the momentum, energy, species available in FLUENT [15]. The cross-sectional view of the studied bluff body located at the entrance of micro-combustor (3 mm from the entrance). The radius and thickness of the hollow hemispherical bluff body are 2 mm and 0.1 mm, respectively. The bluff body was symmetrically located with respect to the micro-combustor. The combustion of methane/air was carried out in a micro combustor, which was packed with Rh catalyst on the inner wall of the combustor.

mixture gas is low, the fluid volume is neglected and heat dissipation effect and gas radiation in the process of reaction have not been taken into account either. A similar application was adopted in the literature of Zhang et al. [16]. Nonetheless, equations for conservation of continuity, momentum, and energy are used in control volume. The mathematical model could be described by the following equations, which has been used in another literature [17] The governing equations in Cartesian coordinates include: Continuity equation:

@ðquj Þ ¼0 @xj

ð1Þ

where q is the density of the gas mixture, u is the velocity. Composition equation:

quj

  @Ys @ @Ys þ Rs ¼ Dq @X j @X j @X j

ð2Þ

where Yi corresponds to the mass fraction of the ith species in micro premixing chamber, D is 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

NgþNs Y

k¼1

i¼1

v sk ksk

½X i v 00ik

ð3Þ

where vsk is stoichiometric coefficient in forward direction of the combustion k, Ks is the total number of elementary surface reactions, v 00ik is stoichiometric coefficient in negative direction of the combustion k, ksk is forward rate coefficient of the combustion k. Ns is the number of surface species and Ng is the number of gas phase species. [Xi] is the molar concentration of surface species i. ksk is calculated by Arrhenius reaction source of the reaction k as below:

  Ns   Ea Y e H ksk ¼ Ak T bk exp  Hli ik exp ik i RT i¼1 RT

ð4Þ

where Ak is the pre-exponential factor, Ea is the activation energy of reaction k, bk is the temperature exponent, Hi is the surface coverage rate of species i, eik and lik are surface coverage parameter. In addition, [Xi] is written as below:

½X i  ¼ CHi

ð5Þ

2.2. Mathematical model

Here, C 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:

Surface catalytic reactions are merely considered in the calculation. As the size of the reactor is small and the flow velocity of the

@ @p ðquj ui Þ ¼  @X j @X j

 

l

@ui 1 @uj þ @X j 3 @X i



Fig. 1. Structural diagram of the micro-combustor: (a) without a hollow hemispherical bluff body, (b) with a hollow hemispherical bluff body.

ð6Þ

L. Zhang et al. / Energy Conversion and Management 94 (2015) 293–299

Energy equation:

!   Ng @h @ @T @ X @Y s þ quj ¼ k hs qDs þq @X j @X j @X j @X j s¼1 @X j

ð7Þ

With l as viscosity. h is the enthalpy, q is heat of reaction and k is thermal conductivity. The equation system is closed by the ideal gas law. Closure equation: Ng X Ys ¼ 1

ð8Þ

s¼1

State of ideal gas:

P ¼ qRT

Ng X Ys s¼1

ð9Þ

Ms

Here, R is universal gas constant and Ms is molar mass. Methane conversion rate of methane could be calculated by Eq. (10):

g¼

mCH4 in  mCH4 out mCH4 in

ð10Þ

where mCH4 in is the mass flow rate at the entrance of the combustor and mCH4 out is the mass flow rate of methane at the exit of the combustor. 3. Calculation method and mechanism 3.1. Refined grid method and calculation method The mesh quality and partial difference equation solved are inseparable, and related to the degree of twisting. Mesh scale is varied from the small size to the larger size at about 0.1 mm, 0.2 mm, 0.25 mm, 0.3 mm and 0.45 mm when the combustor without a hollow hemispherical. For the combustor with a hollow hemispherical, the mesh interval is varied from 0.3 mm to 0.7 mm. Methane conversion rates under different meshes are showed in Fig. 2. As shown in Fig. 2, until the iterative computation convergence is achieved, it is found that methane conversion rate reaches the maximum value when the mesh size is 0.25 mm about the combustor without a hollow hemisphere, which is 75.16%. Similarly, methane conversion rate reaches the maximum value when the mesh size is 0.5 mm about the combustor with a hollow hemisphere, which is 94.47%. In conclusion, we setup mesh sizes are 0.25 mm about the combustor without a bluff body and 0.5 mm

Methane conversion rate (%)

94.5 94.4 94.3 94.2 94.1 75 74 73 72 71 70 69 68

295

about the combustor with a bluff body, fine mesh size will be able to provide good spatial resolution for the distribution of most variables within the combustion chamber. The geometry model is created in 3D with the smallest cells in the center where the reaction zone is located, the combustor is meshed using about 6400 cells without hollow hemisphere, and a total of 40,489 cells are generated for the combustor with hollow hemisphere. A 3D model for catalytic combustion of methane is simulated using a Computational Fluid Dynamics (CFD) method. Inlet is specified as velocity boundary condition and outlet is specified as pressure boundary condition. Methane/air mixture of 500 K flows slowly onto the Rh catalyst. Gas dynamic process is according to incompressible laminar and no-slip boundary condition is applied for surface conditions. The governing equations are solved with the finite volume method. The pressure and velocity are coupled with the SIMPLEC algorithm. 3.2. Chemical reaction mechanism For the rhodium surface, the catalytic reaction mechanism suggested by Deutschmann et al. [18] is used, the chemical processes on the surface are described by a set of elementary reactions, which include 6 elementary adsorption reactions, 6 elementary dissociation reactions and 26 surface chemical reactions. Eleven surface species (H(s), O(s), OH(s), H2O(s), C(s), CO(s), CO2 (s), CH3(s), CH2(s), CH(s), Rh(s)) describe the coverage of the surface with adsorbed species. The specific reaction mechanism is showed in Table 1, which is used in the literature [17]. This reaction mechanism well explained the reflection of gas molecules on the catalyst surface, and played a vital role in promoting in-depth research on catalytic combustion of methane under micro reactor. 4. Results and discussion In order to verify the correctness of the model, the comparison between the simulation data and experimental data [20] is developed in Fig. 3. Both simulation data and experimental data are collected when equivalence ratio is 0.6. The combustion of methane is simulated in a micro scale combustor with a hollow hemisphere bluff body. An equilateral triangle bluff body was used in the experiment. The inlet gases are the mixture of methane/air in the simulation and hydrogen/air in the experiment. As shown in Fig. 3, the effects of inlet velocity on the exhaust gas temperature between simulation data and experimental data are same. It is noted that exhaust gas temperature increases first and then decreases with the increasing inlet velocity. This means that the exhaust gas temperature reaches a peak at a moderate inlet velocity. The corresponding inlet velocities for simulation and experiment are 0.05 m/s and 15 m/s, respectively. Obviously, the exhaust gas temperature depends on many factors, including the combustor model, combustor size, inlet temperature, bluff body and inlet gas, which are the reasons of the differences values between simulation data and experimental data. However, the same effect of velocity on the exhaust gas temperature can prove the correctness of the simulation model. 4.1. A comparison of combustion characteristics between microcombustor with and without a hollow hemisphere bluff body

Micro-combustor with bluff body Micro-combustor without bluff body

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Size (mm) Fig. 2. Methane conversion rates in the different divide mesh.

In order to investigate the combustion characteristics of the combustor with a hollow hemisphere, the paper firstly compares the blow-off limit of the combustor with and without a hollow bluff body. And then the paper studies the methane conversion rates of combustors when the equivalence ratio (/) is 1.0. The equivalence ratio is defined by Eq. (11):

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L. Zhang et al. / Energy Conversion and Management 94 (2015) 293–299

Table 1 Catalytic reaction mechanism of methane (A: pre-exponential factor; b: temperature exponent; Ea: activation energy of the reaction). Case

Reaction

A (s1)

b

Ea (kJ mol1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38

H2 + Rh(s) + Rh(s) ) H(s) + H(s) O2 + Rh(s) + Rh(s) ) O(s) + O(s) CH4 + Rh(s) ) CH4(s) H2O + Rh(s) ) H2O(s) CO2 + Rh(s) ) CO2(s) CO + Rh(s) ) CO(s) H(s) + H(s) ) Rh(s) + Rh(s) + H2 O(s) + O(s) ) Rh(s) + Rh(s) + O2 H2O(s) ) H2O + Rh(s) CO(s) ) CO + Rh(s) CO2(s) ) CO2 + Rh(s) CH4(s) ) CH4 + Rh(s) H(s) + O(s) ) OH(s) + Rh(s) OH(s) + Rh(s) ) H(s) + O(s) H(s) + OH(s) ) H2O(s) + Rh(s) H2O(s) + Rh(s) ) H(s) + OH(s) OH(s) + OH(s) ) H2O(s) + O(s) H2O(s) + O(s) ) OH(s) + OH(s) C(s) + O(s) ) CO(s) + Rh(s) CO(s) + Rh(s) ) C(s) + O(s) CO(s) + O(s) ) CO2(s) + Rh(s) CO2(s) + Rh(s) ) CO(s) + O(s) CH4(s) + Rh(s) ) CH3(s) + H(s) CH3(s) + H(s) ) CH4(s) + Rh(s) CH3(s) + Rh(s) ) CH2(s) + H(s) CH2(s) + H(s) ) CH3(s) + Rh(s) CH2(s) + Rh(s) ) CH(s) + H(s) CH(s) + H(s) ) CH2(s) + Rh(s) CH(s) + Rh(s) ) C(s) + H(s) C(s) + H(s) ) CH(s) + Rh(s) CH4(s) + O(s) ) CH3(s) + OH(s) CH3(s) + OH(s) ) CH4(s) + O(s) CH3(s) + O(s) ) CH2(s) + OH(s) CH2(s) + OH(s) ) CH3(s) + O(s) CH2(s) + O(s) ) CH(s) + OH(s) CH(s) + OH(s) ) CH2(s) + O(s) CH(s) + O(s) ) C(s) + OH(s) C(s) + OH(s) ) CH(s) + O(s)

0.010E00 0.010E00 8.000E03 1.000E01 1.000E05 5.000E01 3.000E+21 1.300E+22 3.000E+13 3.500E+13 1.000E+13 1.000E+13 5.000E+22 3.000E+20 3.000E+20 5.000E+22 3.000E+21 3.000E+21 3.000E+22 2.500E+21 1.400E+20 3.000E+21 3.700E+21 3.700E+21 3.700E+24 3.700E+21 3.700E+24 3.700E+21 3.700E+21 3.700E+21 1.700E+24 3.700E+21 3.700E+24 3.700E+21 3.700E+24 3.700E+21 3.700E+21 3.700E+21

0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

0.0 0.0 0.0 0.0 0.0 0.0 77,800 355,200 45,000 133,400 21,700 25,100 83,700 37,700 33,500 106,400 100,800 224,200 97,900 169,000 121,600 115,300 61,000 51,000 103,000 44,000 100,000 68,000 21,000 172,800 80,300 24,300 120,300 15,100 158,400 36,800 30,100 145,500

Exhaust gas temperature (K)

1400

1350

1350

1300

1300

1250

0.4

0.6

5

10

15

20

25

30

35

Fig. 3. Simulation data and experimental data of exhaust gas temperature versus inlet velocity.

A=F A0 =F 0

8.849 6.787 5.504 4.63

21.2039 21.719 22.018 22.22

Blow-off limit (m/s) 45

Velocity (m/s)

/¼

500 500 500 500

20 18 16 Micro-combustor with bluff body Micro-combustor without bluff body

14 12 10

0.7

0.8

0.9

1.0

1.1

1.2

Fig. 4. Blow-off limits of the micro-combustor with and without a hollow hemisphere bluff body.

0.8

40

0.6 0.8 1.0 1.2

Φ

1150 0

1 2 3 4

0.6

1200 0.2

Mass fraction of O2 (%)

6

Simulation data

0.0

Mass fraction of CH4 (%)

8

1250 1200

Inlet temperature (K)

22

1500 1400

Equivalence ratio

24

Experimental data

1450

Case

26

1600 1550

Table 2 Operational parameters of each case.

ð11Þ

where A/F is the actual ratio of air/fuel in the inlet mixture gas and A0/F0 is the stoichiometric air/fuel ratio. The operational parameters of each case is shown in Table 2. As shown in Fig. 4, blow-off limits of the micro-combustor with and without a hollow hemisphere bluff body versus the equivalence ratio when the velocity keeps 1.0 m/s is discussed. The

blow-off limits of the micro-combustor without a bluff body increases from 6.2 m/s to 9.5 m/s and the blow-off limits of the combustor with a bluff body increases from 14.6 m/s to 24.5 m/s when / increases from 0.6 to 1.0. The blow-off limits of the two micro-combustors reach the maximum values when / is 1.0, which are 9.5 m/s and 24.5 m/s, respectively. In other words, the blow-off limits of the micro-combustor with a hollow hemisphere is 2.5 times higher than the combustor without a bluff body. The combustor with a bluff body has a better flame stabilization ability than the combustor without a bluff body. Hosseini et al. also proved that in the premixed conventional micro-combustion the stability of the flame is increased when a triangular bluff body is applied [19]. It can be also seen from Fig. 4, the blow-off limits decrease when / exceeds 1.0. The below-off limit of the combustor with a hollow hemisphere bluff body decreases from 24.5 m/s to 22.8 m/s when / increases from 1.0 to 1.1. The same phenomenon occurs in the combustor without a bluff body, blow-off limit decreases from 9.5 m/s to 8.5 m/s. Combustion of methane is affected by catalytic and oxygen atoms. Adsorption ability of catalytic is better than the ability of methane, which makes the catalytic wall covered by excess oxygen atoms instead of methane. A small amount of catalyst activity space reduces the blow-off limit of the combustor with and without a hollow hemisphere bluff body. Methane conversion rate is a crucial index to evaluate the combustor. Fig. 5 reveals the methane conversion rates of the micro-combustor with and without a hollow hemisphere bluff body versus flow velocities under constant / (1.0). It can be seen from Fig. 5, methane conversion rates of the combustor with a bluff body is higher than that without a bluff body under same inlet conditions. Fig. 5 also shows that the difference values of methane conversion rates between the combustor with and without a hollow hemisphere bluff body increase with the increase of inlet flow velocity. Methane conversion rate of the combustor with a bluff body is 81%, which is 5.8% higher than that without a bluff body when the velocity is 0.1 m/s. The difference value is 8.8% when

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L. Zhang et al. / Energy Conversion and Management 94 (2015) 293–299

Micro-combustor with bluff body Micro-combustor without bluff body

70 60 50 40

Mass fraction of methane

Menthane conversion rate (%)

0.056 80

0.054 0.052 Micro-combustor with bluff body Micro-combustor without bluff body

0.050 0.048 0.046 0.044

30 0.0

0.2

0.4

0.6

0.8

1.0

0

Velocity (m/s) Fig. 5. Methane conversion rates of the micro-combustor with and without a hollow hemisphere bluff body.

15

20

0.09 Φ=0.6 Φ=0.8 Φ=1.0

0.08

0.07

0.06

0.05

0.04 0

5

10

15

20

Distance from inlet boundary (mm) Fig. 7. Effects of / on mass fraction of methane along axial direction.

4.2. Mass fraction of methane along axial direction

0.060

Mass fraction of methane

In order to study the effects of the hollow hemisphere bluff body, the paper compares the mass fraction of methane along axial direction in the two micro-combustors. Fig. 6 shows the mass fraction of methane along axial direction when the velocity is 1.0 m/s and / is 1.0. It can be seen from Fig. 6, mass fraction of methane in the combustor without a bluff body decreases slowly along the axial direction. Mass fraction of methane in the combustor with a bluff body significantly drops at 3 mm from the entrance, where is the position of bluff body. In order to study the reason of this phenomenon, we firstly investigate the effects of / (Fig. 7) and mixture gas velocity (Fig. 8) on mass fraction of methane along the axial direction. Fig. 7 shows mass fraction of methane along axial direction versus / when the velocity keeps constant (1.0 m/s). Significant drop of mass fraction of methane keeps at 3 mm from entrance, where is the top of the bluff body. It can be seen from Fig. 7 that / does not have influences on the phenomenon. The significant drops of mass fraction of methane lead to difference values, which are 13%, 16% and 18% when / increases from 0.6 to 1.0. The reason is with the increase of /, more and more oxygen atoms involved in the combustion of methane, which makes the reaction severely to consume more methane. Fig. 8 shows mass fraction of methane along axial direction versus velocity when / keeps constant value (1.0). It can be obviously seen from Fig. 8, velocity of mixture gas has nothing influence on the decrease site of the mass fraction of methane. Sudden decreas-

10

Fig. 6. Mass fraction of methane along axial direction.

Mass fraction of methane

the velocity increases to 1.0 m/s. The reason is that higher velocity leads to larger recirculation zone, which prolongs the residence time of methane for complete reaction. It is expected that the flame stabilization in the micro-combustor is affected by flow field, flow velocity, etc. As a result of the hollow hemisphere bluff body, recirculation zone is formed behind the bluff body, which can prolong the residence time of methane for complete combustion. It is well known that the recirculation zone has a dominated effect on the flame stabilization in the bluff body combustor. This is because the flow velocity in the recirculation zone is relatively low and a pool of key radicals (reaction zone) is formed. The result is similar to that in the literature [9], which studied the effects of the semicircular bluff body and the triangular bluff body. In conclusion, the blow-off limits and methane conversion rates of the combustor significantly increase, combustion characteristics of the micro-combustor improved as the exit of hollow hemisphere bluff body.

5

Distance from inlet boundary (mm)

V=0.1 m/s V=0.5 m/s V= 1.0m/s V=1.5 m/s

0.055 0.050 0.045 0.040 0.035 0.030 0.025 0.020 0

5

10

15

20

Distance from inlet boundary (mm) Fig. 8. Effects of velocity on mass fraction of methane along axial direction.

ing location of methane keeps at the top of bluff body. Fig. 8 also shows that decreasing range of mass fraction of methane decreases with the increase of velocity. The reason is that exist of bluff body can prolong the residence time of the mixture gas, but the effect of the bluff body is limited by the velocity. Mixture gas will be blow out of the combustor before the reaction due to faster velocity. Fig. 8 shows that the combustion of methane is most completed when the velocity is 0.1 m/s, which is benefited from slower velocity of mixture gas.

L. Zhang et al. / Energy Conversion and Management 94 (2015) 293–299

4.3. Blow-off limit and methane conversion rate of micro-combustor with hollow hemisphere bluff body The effects of / and inlet velocity on the combustion characteristics of methane in the micro-combustor with a hollow hemisphere bluff body are numerical investigated using Fluent. The stability range of methane is shown in Fig. 9. As shown in Fig. 9, the limit of methane combustion rate can be expanded with the increase of / from 0.6 to 1.0 under the proper inlet velocity. The blow-off limit of methane combustion increases from 14.6 m/s to 24.5 m/s with the increase of / from 0.6 to 1.0. This behavior is the similar to that found in the work of Wan et al. [20]. The experimental results of blow-off limits under different equivalence ratios have been obtained through both experimental investigation and numerical simulation. The blow-off limit increases with the increase of the equivalence ratio. Due to the hollow hemisphere bluff body, flame can be kept stable and efficient combustion, which is benefited from the recirculation zone. Recirculation zone is full of higher temperature mixture gas, which wills entrainment more and more fresh gas and form continuous ignition, prolong the residence time of the mixture gas to make the combustion of methane more completely. Effects of / and velocity on methane conversion rate of the micro-combustor with a hollow hemisphere bluff body is shown in Fig. 10. The methane conversion rate of the micro-combustor increases with the increase of velocity from 0.008 m/s to 0.02 m/ s and then decreases with the further increase of velocity from 0.02 m/s to 1.0 m/s under the same /. Methane conversion rate increases from 45% to 97.8% when the velocity increases from 0.008 m/s to 0.02 m/s, and then decreases from 97.8% to 45.9 m/s when the velocity of mixture gas increases to 1.0 m/s. Methane conversion rate of methane is lower when the inlet velocity is slow, the reason is that the inlet gas cannot meet the need of methane combustion, the inlet velocity of fuel is lower than the consume velocity of methane. With the increase of inlet velocity of mixture gas, methane conversion rate decreases because the mixture gas is blow out of the micro-combustor before combustion. Methane conversion rate increases with the increase of / from 0.8 to 1.0 and then decreases with the increase of / from 1.0 to 1.2 under the same inlet velocity. When the velocity keeps at 0.1 m/s and / increases from 0.8, 1.0 to 1.2, the methane conversion rate is 75.3%, 81% and 80.8%, respectively. The methane conversion rate reaches the maximum value when / reaches 1.0 at the same inlet condition. More oxygen in the mixture gas will participate in the methane combustion with the increase of / until / is 1.0, which will increase methane conversion rate. When / is higher than 1.0, the combustor is full of exceed oxygen, which will cover the

Methane conversion rate (%)

298

100 90 Φ = 0.8 Φ = 1.0 Φ = 1.2

80 70 60 50 40 30 0.0

0.2

0.4

0.6

0.8

1.0

Velocity (m/s) Fig. 10. Effects of / and velocity on methane conversion rate of methane.

(a) V=0.1 m/s

(b) V=1.0 m/s

1.1 Burn

Unburn

1.0

Φ

0.9 0.8 0.7

(c) V=3.0 m/s Fig. 11. Axial velocity isogram of the micro-combustor.

0.6 0

5

10

15

20

25

Velocity (m/s) Fig. 9. Effects of / and velocity on stable combustion of methane.

catalytic wall and decrease the reaction extend of methane. The results are the similar to that found in the study of Wan et al. [20]. Effects of the equivalence ratio and inlet velocity on

L. Zhang et al. / Energy Conversion and Management 94 (2015) 293–299

combustion efficiency were studied. The combustion efficiency increases first and then decreases with the increasing inlet velocity when the equivalence ratio is fixed. Effect of inlet velocity on methane conversion rate in a fixed bed reactor system had been experimental studied in our another literature [21]. Methane conversion rate decreases from 79% to 52% with the increasing inlet velocity from 0.02 m/s to 0.05 m/s under the fixed temperature and equivalence ratio in an empty fixed bed. The reason is that the residence time increases with the decreases of inlet velocity, which makes methane combustion more fully at the same temperature. The result in the experimental study is same to that found in our simulation investigation. In order to explain the effect of hollow hemisphere bluff body, the paper studies the axial velocity isogram of the micro-combustor with a hollow hemisphere bluff body when / is 1.0, which is shown in Fig. 11. The paper defines that velocity of the mixture gas below 0 m/s is recirculation zone. Fig. 11 reveals that with the increase of inlet velocity, the area of recirculation zone is increasing. Recirculation zone plays a positive role in the combustion of methane, which gathers the combustion reaction intermediate component. The velocity of mixture gas in recirculation zone is slower, which is conducive to prolonging the residence time of the mixture gas. The combustion of methane can react more completely due to the effect of hollow hemisphere bluff body.

5. Conclusions (1) Blow-off limit of the micro-combustor with a hollow hemisphere bluff body is 2.5 times than the micro-combustor without a bluff body. Due to the exist of the hollow hemisphere bluff body, the two micro-combustors with and without a hollow hemisphere bluff body reach the maximum value when / is 1.0, the combustion of methane is more completely. (2) Methane conversion rate of the micro-combustor with a hollow hemisphere bluff body sharply increases at the location of bluff body, which is not affected by / and inlet velocity. (3) Methane conversion rate is mainly affected by / and inlet velocity, which increases firstly and then decreases with the increase of velocity. The maximum methane conversion rate is 97.8% when / is 1.0 and inlet velocity is 0.02 m/s. Methane conversion rate sharply increases when the velocity increases from 0.08 m/s to 0.02 m/s and then the growth trend of methane conversion rate gradually decreases. Methane conversion rate increases with the increase of / when / is below 1.0, which reaches the maximum value at 1.0 when inlet velocity of mixture gas keeps constant state. (4) Exist of hollow hemisphere bluff body forms recirculation zone. Recirculation zone is formed back the bluff body, which can effectively prolong the residence time of the mixture gas and increase the methane conversion rate. Recirculation zone is mainly affected by inlet velocity, which is increasing with the increase of inlet velocity.

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Acknowledgments The authors gratefully acknowledge financial support from Fundamental Research Funds for the Central Universities (NO. CDJZR14145501) and Chongqing Science and Technology Talent Training Plan (cstc2013kjrc-qnrc90002) and National Natural Science Foundation of China (Project No. 50906103). References [1] Loy CC, Bo F. The development of a micropower (micro-thermo photovoltaic) device. J Power Sources 2007;165:455–80. [2] Bagheri G, Hosseini SE, Wahid MA. Effects of bluff body shape on the flame stability in premixed micro-combustion of hydrogen-air mixture. Appl Therm Eng 2014;67:266–72. [3] Yang WM, Chou SK, Chu C, et al. Microscale combustion research for application to micro thermophotovoltaic systems. Energy Convers Manage 2003;44:2625–34. [4] Yang WM, Chua KJ, Pan JF, et al. Development of micro-thermophotovoltaic power generator with heat recuperation. Energy Convers Manage 2014;78:81–7. [5] Kang YH, Wang QH, Lu XF, et al. Experimental and theoretical study on the flow, mixing, and combustion characteristics of dimethyl ether, methane, and LPG jet diffusion flames. Fuel Process Technol 2015;129:98–112. [6] Cao HL, Zhang K, Xu JL, Wei XL. Diffusion combustion in a micro heatrecirculating combustor using porous media. J Combust Sci Tech 2011;17:394–400. [7] Chen CH, Ronney PD. Three-dimensional effects in counter flow heatrecirculating combustors. Proc Combust Inst 2011;33:3285–91. [8] Zhong BJ, Hong ZK. Numerical simulation of catalytic combustion of CH4 in micro-scale. J Eng Therm 2003;24:173–6. [9] Zhang L, Yan YF, Li LX, Ran JY. Numerical investigation of premixed catalytic combustion of methane in micro-combustor. CIESC J 2009;60:627–33. [10] Fan AW, Wan JL, Maruta K, Yao H, Liu W. Interactions between heat transfer, flow field and flame stabilization in a micro-combustor with a bluff body. Int J Heat Mass Transf 2013;66:72–9. [11] Fan AW, Wan JL, Liu Y, Pi BM, Yao H, Liu W. Effect of bluff body shape on the blow-off limit of hydrogen/air flame in a planar micro-combustor. Appl Therm Eng 2014;62:13–9. [12] Yan YF, Pan WL, Zhang L, Tang WM, Yang ZQ, Tang Q, et al. Numerical study on combustion characteristics of hydrogen addition into methane air mixture. Int J Hydrogen Energy 2013;38:13463–70. [13] Yan YF, Tang WM, Zhang L, Pan WL, Yang ZQ, Chen YR, et al. Numerical simulation of the effect of hydrogen addition fraction on catalytic microcombustion characteristics of methane-air. Int J Hydrogen Energy 2014;39:1864–73. [14] Kang YH, Lu XF, Wang QH, et al. Effect of H2 addition on combustion characteristics of dimethyl ether jet diffusion flame. Energy Convers Manage 2015;89:735–48. [15] Kang YH, Lu XF, Wang QH, et al. Experimental and modeling study on the flame structure and reaction zone size of dimethyl ether/air premixed flame in an industrial boiler furnace. Energy Fuels 2013;27:7054–66. [16] Zhang L, Zhu JC, Yan YF, Pan WL, Yang ZQ, Chen YR, et al. Numerical investigation on the transient characteristics of hydrogen production from catalytic autothermal reforming of methane in a micro combustor with multiple cylinders. J Nat Gas Sci Eng 2014;19:251–7. [17] Yan YF, Tang WM, Zhang L, Pan WL, Li LY. Thermal and chemical effects of hydrogen addition on catalytic micro-combustion of methane/air. Int J Hydrogen Energy 2014;39:19204–11. [18] Deutschmann O, Schwiedernoch R, Maier LI, Chatterjee D. Natural gas conversion in monolithic catalysts: interaction of chemical reactions and transport phenomena. Stud Surf Sci Catal 2001;136:251–8. [19] Hosseini SE, Wahid MA. Investigation of bluff-body micro-flameless combustion. Energy Convers Manage 2014;88:120–8. [20] Wan JL, Fan AW, Maruta K, Yao H, Liu W. Experimental and numerical investigation on combustion characteristics of premixed hydrogen/air flame in a micro-combustor with a bluff body. Int J Hydrogen Energy 2012;37:19190–7. [21] Zhang L, Zheng SW, Yang ZQ. Experimental study on the combustion characteristic of low concentration methane in the inert particles. J Eng Therm Energy Power 2013;28:78–81.