combustion of hydrogen air in micro combustors with catalytic pt layer wang

7/28/2019 Combustion of Hydrogen Air in Micro Combustors With Catalytic Pt Layer Wang

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Combustion of hydrogen-air in micro combustors with catalytic Pt layer

Yang Wang, Zhijun Zhou, Weijuan Yang *, Junhu Zhou, Jianzhong Liu, Zhihua Wang, Kefa Cen

State Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou 310027, Zhejiang, China

a r t i c l e i n f o

Article history:

Received 2 April 2009

Received in revised form 30 November 2009Accepted 13 December 2009

Available online 12 January 2010

Keywords:

Catalytic combustion

Micro combustor

Stability limits

Hydrogen

a b s t r a c t

Micro power generators have high power density. However, their key components micro combustors

have low stability. In this experiment, catalyst is applied to improve the stability. The catalytic micro

combustor is made from an alumina ceramic tube. It has inner diameter of 1 mm, outer diameter of2.02 mm and length of 24.5 mm. It is prepared through impregnation of aqueous solution of H 2PtCl6.

The flammability limits and surface temperatures under different operation conditions are measured.

The flow rates range from 0.08 to 0.4 L/min. According to the experimental results, catalyst is effective

to inhibit extinction. For example, At 0.8 L/min, the stability limit is 0.19314.9 in the non-catalytic com-

bustor. After applying catalyst, the lean limit is near 0, and the rich limit is 29.3. But catalyst is less effec-

tive to inhibit blow out. Increasing flow rates also inhibits extinction. In the non-catalytic combustor,

while the flow rates increase from 0.08 to 0.2 L/min, the lean stability limit decreases from 0.193 to

0.125. The experimental results indicate that catalyst induces shift downstream in the stoichiometric

and rich cases. The numeric simulation verifies that the heterogeneous reaction weakens the homoge-

neous reaction through consuming fuels. Thus, the insufficient heat recirculation makes the reaction

region shift downstream. However, lean mixture has intense reaction in the catalytic combustor. It is

attributed to the high mass diffusion and low thermal diffusion of lean mixture.

2009 Elsevier Ltd. All rights reserved.

1. Introduction

Micro power generators provide electricity for portable elec-

tronic devices. They convert chemical energy into electricity di-

rectly, thus have longer operation period [1] and higher energy

density than the conventional batteries [24].

The first micro power generator is suggested in 1996 in Massa-

chusetts Institute of Technology [5]. The prototype is a gas turbine

generator less than 1 cm3. To overcome the high friction of micro

turbine, energy converters based on thermoelectric or thermo-

photovoltaic material are lately proposed [1,6].

Micro combustors are the key components of micro power gen-

erators. They also produce hydrogen for fuel cell [7,8]. But their

stability is low [9]. There are many problems different from themacro-scale combustors. For example, high quenching distance,

radical quenching, and uniform temperature distribution [10].

The high surface to volume ratio increases the heat loss. Blow

out also occurs easily [11].

Micro combustors require improvement. Swiss-roll combustors

could increase the enthalpy of the reactants, thus stabilize flame

[12,13]. Increasing the heat recirculation along the combustor wall

helps inhibit blow out [14]. Catalytic reaction helps stabilize micro

combustion [15,16]. Processing interior wall of micro combustors

chemically inhibits radical quenching [17].

Computational fluid dynamics (CFD) simulation is effective to

investigate the micro combustion [1820]. The thermal behavior

of combustor wall affects micro combustion, thus simulation of

fluid should be combined with solid wall [2123].

There have been literatures about the catalytic micro combus-

tors. Their ignition [24,25] and reaction processes [26,27] are stud-

ied. Catalytic combustors with different structures are also

compared [28]. Numeric simulation predicts that catalytic com-

bustors have weak reaction intensity [29], which affects their per-

formances accordingly [30]. It is attributed to the competition

between homogeneous and heterogeneous reactions [31]. Solution

has been proposed to intensify the homogeneous reaction [32].However, there are few experiments about the comparison of reac-

tion intensity between catalytic and non-catalytic combustors. In

this paper, this comparison is made. The combustors are investi-

gated under different operation conditions. The catalytic combus-

tor shows high stability, but also weak reaction intensity.

Combining experimental data and numeric simulation, the details

of weakening process are analyzed.

2. Experiment apparatus

Fig. 1 gives the schematic diagram of the experimental appara-

tus. It is made up of gas feed system, measurement instruments,

0196-8904/$ - see front matter 2009 Elsevier Ltd. All rights reserved.doi:10.1016/j.enconman.2009.12.021

* Corresponding author.

E-mail address: [email protected](W. Yang).

Energy Conversion and Management 51 (2010) 11271133

Contents lists available at ScienceDirect

Energy Conversion and Management

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / e n c o n m a n
http://dx.doi.org/10.1016/j.enconman.2009.12.021mailto:[email protected]://www.sciencedirect.com/science/journal/01968904http://www.elsevier.com/locate/enconmanhttp://www.elsevier.com/locate/enconmanhttp://www.sciencedirect.com/science/journal/01968904mailto:[email protected]://dx.doi.org/10.1016/j.enconman.2009.12.021


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and the micro combustor. Hydrogen and air come from the high-

pressure tanks and flow through the pressure reducing valves

and flow meters. After premixed, they are injected into the micro

combustor. Two thermal mass flow controllers (D07-7B) are used

to control the flow rates of mixture and hydrogen, respectively. A

digital data acquisition system (WSP-D806) read the flow-rate sig-

nal and transfers it to the computer. An infrared camera (Therma-

Cam S65) measures the temperature distribution on the combustor

surface, which is placed with a distance of 30 cm from the combs-

utor. The micro combustor is made of an alumina ceramic tube. Its

has length of 24.5 mm, inner diameter of 1 mm and outer diameterof 2.02 mm.

The catalytic layer is prepared on the surface of the micro com-

bustor by impregnation. The combustor is impregnated in the

aqueous solution of hydrochloroplatinic acid (H2PtCl6) [24]

(47.4 g/L). After 12 h, H2PtCl6 is dispersed onto the surface. Then,

the micro combustor is heated at 500 C for 2 h. Finally, its surface

turns black, indicating Pt attached. Fig. 2 shows the transverse sec-

tion of the micro combustor under micro scope. The weight of the

combustor increased by 0.0579 g after impregnation. The weight

difference is multiplied by the solution concentration and divided

by combustor surface area, then the Pt site density on the combus-

tor surface is about 3.98 ug/mm2.

The X-ray powder diffraction patterns (XRD) of the Pt layer are

shown in Fig. 3. The peaks around 40

, 48

, 68

indicate the exis-tence of Pt. The peak around 26 indicates alumina ceramic.

Both C (catalytic) and N (non-catalytic) combustors are tested

under the similar operation conditions. The stability limits at the

total flow rates from 0.08 to 0.4 L/min are measured. The upper

and lower stability limits in different cases are found following

the process below. Start the micro combustor at the equivalence

ratio of 1 to achieve sable flame and wait until it reaches stability.

Adjust the flow rate of hydrogen until extinction or blow out oc-

curs, with the total flow rate fixed. Adjust the total flow rate and

repeat the steps above until the flammable region over the flow-

rate range from 0.08 to 0.4 L/min is obtained.

Heat loss rate (Qloss) and average heat conduction rate (Qcon) of

the micro combustor are calculated with Eqs. (1) and (2). The for-

mer one is integrated from the location of peak temperature (xp) to

the nozzle outlet (xn), which is the reaction area. The latter one is

calculated from the location of peak temperature to the upstream

end of combustor (xend), which is the preheating area.

Qloss Qcov Qrad

Zxpxn

DT h db e T4

w T4

1 dSw 1

Qcon

Rxpxend

Sc k dTwdx

dx

xp xend2

Nomenclature

Qa available chemical energy [W]k thermal conductivity [W/(m K)]T temperature [K]x axial location [mm]Qsurf heat loss rate on the combustor surface [W]

At transverse section area of the combustor [mm2]

ER equivalence ratio, mH2/mAir/(mH2/mAir)stoixp axial location of the wall peak temperatureTp peak temperature on the combustor wall [C]Qcon average heat conduction rate on the combustor wall [W]Qex exhaust-gas heat loss [W]

Fig. 2. Part of the transverse section of the micro combustor. The black rim

indicates the catalytic lay of Pt. The overall picture is on the top left.

Fig. 3. X-ray powder diffraction spectra of alumina ceramic after impregnation ofH2PtCl6. The tiny pictures on the top left and right show the spectra detail.

Fig. 1. Schematic of the experimental apparatus. The solid lines are the gas routes,the long-dashed lines are the signal routes.

1128 Y. Wang et al. / Energy Conversion and Management 51 (2010) 11271133


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Qloss is the heat loss rate; Qcov is the natural convective heat loss

rate; Qrad is the radiative heat loss rate;DTis the temperature dif-

ference between wall and ambient; Sw is the external-wall area; h

is the convective heat transfer coefficient;db is the black body radi-

ation constant; e is the radiative emissivity; Tw is the wall temper-

ature; T1

is the ambient temperature; Qcon is the heat conduction

rate; Sc is the cross-section area; k is the thermal conductivity of

wall; x is the axial location on the combustor wall, the subscriptsp, n and end are peak temperature, nozzle outlet and combustor

upstream end, respectively.

The errors in calculating heat loss and heat conduction are ana-

lyzed. The independent parameters measured in the experiments

are: wall temperature, ambient temperature, combustor dimen-

sion. A constant value is taken as the emissivity of the micro com-

bustor, its error is lower than 4.6%. Uncertainty of empirical

correlation of h is less than 5% [33]. Error of Tw is 2%, according

to the instruction of infrared camera. Errors of x and combustor

diameter are both less than 1%. Error of k is 8.4%. Conclusively,

the maximum possible uncertainty of Qloss and Qcon are 12.7%

and 8.7%, respectively. Qex has the similar uncertainty as Qloss.

Numeric simulation is applied to investigate the combustion

process. The conservation equations of mass, chemical species,

momentum and energy are solved for two-dimensional steady

low Re number laminar flow, heat transfer and chemical reaction

using the FLUENT 6.0 [34] CFD package. Grid with cells in the

gas/solid phase is employed. The first-order upwind scheme is

used for discretization and the Simple algorithm is employed for

the pressurevelocity coupling [35]. The micro combustor is mod-

eled as an axisymmetric 2D system (shown in Fig. 4), and only half

of it is simulated for simplicity. Non-uniform mesh is applied, and

its density is about 2 103 node/mm2 in the reaction region. Fixed

velocity and pressure are specified at the inlet and the outlet,

respectively. The fluid and solid phases are linked with coupled

interface. Measured surface temperature is specified on the outer

edge of combustor. It is more accurate than simply specifying it

as convective boundary [36].Detailed chemistry kinetics are applied in both homogenous

and heterogeneous reactions. The chemical mechanism provide

by Chemkin 4.0 [37] database is imported into Fluent 6.0 for sim-

ulation. The homogeneous mechanism involves eight gaseous spe-

cies (H2, O2, H2O, O, OH, H, HO2, and H2O2) and 20 reaction steps.

The heterogeneous mechanism involves five surface species

(Pt(s), H(s), H2O(s), OH(s) and O(s)) and 12 reaction steps.

3. Results and discussion

3.1. Flammability limits

Stability limits are shown in Fig. 5. Extinction occurs mainly in

the lean cases. On the contrast, blow out occurs mainly in the rich

cases. Catalyst is more effective to inhibit extinction. For example,

at 0.8 L/min, extinction occurs. In the N combustor, the stability

limit is 0.19314.9. In the C combustor, the lean limit decreases

to almost 0 (beyond the measurement range of flowmeter), and

the rich limit increases to 29.3. At 0.4 L/min, blow out occurs.

The stability limits are 0.2495.23 and 0.2495.17 in the N and C

combustors, respectively.

Increasing flow rates inhibits extinction at lower flow rates. For

example, in the N combustor, extinction occurs in the lean cases.

While the flow rates increase from 0.08 to 0.2 L/min, the lean limits

decrease from 0.193 to 0.125.

To compare the stability between lean and rich cases, Qa (avail-

able chemical energy at the critical ER) at different flow rates are

analyzed (given in Fig. 6). The lean cases require less Qa to sustain

Fig. 4. The schematic of the combustor model. Only half of the combustor is

simulated. The solid phase has heat exchange with the fluid. Heterogeneous

reaction occurs on the catalytic surface. The temperature distribution measured on

the combustor surface is imported as the boundary condition.

Fig. 5. Stability limits of the micro combustors at flow rates from 0.08 to 0.4 L/min.(E: extinction, B: blow out).

Fig. 6. Qa vs. flow rate from 0.08 to 0.4 L/min.

Fig. 7. T contour in the N combustor at 0.08 L/min [K].

Y. Wang et al. / Energy Conversion and Management 51 (2010) 11271133 1129


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the combustion, thus they are more stable. For example, in the N

combustor, at 0.2 L/min, Qa is 2.11 and 4.70 W in the lean and rich

cases, respectively.

3.2. Numeric simulation

Fig. 7 gives the T (temperature) contour at 0.08 L/min in the N

combustor. The preheating process exists at the inlet, where T in-creases along the axis until it reaches the ignition point (847 K).

The reactants are preheated by the heat diffusion upstream and

heat recirculation. The peak temperature is 1774 K and locates

around 1.5 mm length, indicating the reaction center. After reac-

tion, T decreases continuously due to the heat loss. H2 mass frac-

tion contour is given in Fig. 8. It decreases abruptly along the

axis. At 1 mm length, it reaches 0.006. The sharp increase of T

and abrupt decrease of H2 indicate intensive homogeneous reac-

tion. It is verified by the OH mass fraction in Fig. 9, which is 0.0035.

Figs. 1012 give the simulation results at 0.2 L/min in the N

combustor. The phenomenon indicates that larger amount of fuel

intensifies the reaction, thus increases the reaction temperature.

Because the fuels require more residence time for preheating and

reaction, the reaction region extends and shifts downstream. Com-

pared with 0.08 L/min, the peak values of T and OH increase to

1806 K and 0.00968. Reaction center shifts downstream to

1.7 mm length. H2 reaches 0.004 at 1 mm length.

Fig. 1315 give the simulation results at 0.2 L/min in the C com-

bustor. Compared with the N combustor at 0.2 L/min, the results

show that under the similar operation condition, catalytic combus-

tor has lower reaction intensity. The peak temperature in the reac-

tion region decreases to 1749 K, and the peak OH decreases to

0.00933. According to the Arrhenius Equation, (Eq. (3), k is the rate

constant of chemical reactions; T is the temperature, A is the pre-

factor and R is the gas constant and Ea is the activation energy),the lower reaction temperature induces slower reaction rate. The

lower OH concentration indicates that the homogeneous reaction

intensity decreases [32]. Due to the slow reaction rate, the reaction

center shifts downstream further to 2 mm length. The H2 contour

shows that the H2 reaches 0.004 at 1 mm.

k AeEaRT 3

Fig. 1618 give the simulation results at 0.4 L/min in the C com-

bustor. The simulation at higher flow rate shows that the catalytic

combustor has low reaction intensity because the heterogeneous

reaction weakens the homogeneous reaction through consuming

fuel in the gaseous phase [31]. According to Fig. 17, there is H2 con-

centrating at the catalytic surface near the inlet, where the mass

fraction is 0.032. Accordingly, the H2 concentration near the axisdecreases. At the inlet, the premixture has initial mass fraction of

0.28 before reaction. Through preheating, the catalytic surface ad-

Fig. 8. H2 mass fraction contour of the fluid region in the N combustorat 0.08 L/min

[kg/kg].

Fig. 9. OH mass fraction contour of the fluid region in the N combustor at 0.08 L/

min [kg/kg].

Fig. 10. T contour in the N combustor at 0.2 L/min [K].

Fig. 11. H2 massfraction contour of thefluid region in the N combustor at 0.2 L/min

[kg/kg].

Fig. 12. HO mass fraction contour of the fluid region in the N combustor at 0.2 L/

min [kg/kg].

Fig. 13. T contour in the C combustor at 0.2 L/min [K].



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sorbs gaseous species and heterogeneous reaction develops [38].

Then more H2 diffuses to the catalytic surface. Therefore, the

absorption and diffusion of H2 decreases its concentration in the

gaseous phase [39]. The homogeneous reaction does not occur un-

til it reaches the ignition temperature. According to the Arrhenius

Equation, its reaction rate decreases with the fuel concentration.

Therefore, the homogeneous reaction weakens due to lack of fuels.

In Fig. 16, the heat released by the catalytic surface at the preheat-ing region also verifies the exist of heterogeneous reaction region,

which coincides with the H2-concentrating region. The OH contour

in Fig. 18 shows the extension and shift downstream of reaction re-

gion due to the slow reaction rate and high feed amount of fuel.

Compare our simulation results with the ones by Guan-Bang

Chen [31], whose combustor has the similar diameter of 1 mm as

ours and operated with lower flow rates ranging from 1.3 105

to 1.3 104 L/min. Both results show the similar phenomenon,

such as the shift downstream of flame and the extension of reac-

tion region at higher flow rates. Moreover, the reaction tempera-

tures and the mass fractions of OH have the similar quantity

level, which are around 1800 K and 0.009 kg/kg, respectively. How-

ever, the simulation result by Chen shows more obvious shift

downstream of flame in the combustor, which is around 10 mm

from the inlet. But the flame in our combustor locates within

5 mm from the inlet, which should be farther than Chens, due to

its higher flow rates. In the micro combustor, the temperature dis-

tribution on its surface affects the flame location importantly by

heat recirculation [14]. Because our boundary condition of surface

temperature is obtained from the experiment, the induced simula-

tion result should be more accurate.

3.3. Heat loss

Heat loss of the upstream half of combustor affects the combus-

tion directly, and is analyzed. It is from the axial location of 0 to

10 mm. The heat released from combustion is lost by Qsurf (heat

loss of the combustor surface) and Qex

(heat loss of the exhaust

gas).

Fig. 19 gives Qsurf and Qex at different flow rates. N combustor

has slightly higher Qsurf than C combustor, because catalyst weak-

ens the reaction. For example, at 0.2 L/min, Qsurf are 10.0 and

9.57 W in the N and C combustor, respectively. Therefore, catalyst

does not decreases Qsurf effectively. It inhibits extinction mainly

because heterogeneous reaction occurs at low temperature.

Increasing flow rates also inhibits extinction. Because at high

flowrates, there is lower percentages of heat lost from the combus-

Fig. 14. H2 mass fraction contour of the fluid region in the C combustor at 0.2 L/min

[kg/kg].

Fig. 15. HO mass fraction contour of the fluid region in the C combustor at 0.2 L/

min [kg/kg].

Fig. 16. T contour in the C combustor at 0.4 L/min [K].

Fig. 17. H2 mass fraction contour of the fluid region in the C combustor at 0.4 L/min[kg/kg].

Fig. 18. HO mass fraction contour of the fluid region in the C combustor at 0.4 L/min [kg/kg]. Fig. 19. Qsurf vs. flow rate at ER of 1 (stoichiometric).



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tor wall [10]. At 0.08 L/min, Qsurf occupies 84.5% of the total energy

in the N combustor. At 0.2 L/min, it decreases to 78.9%.

3.4. Shift downstream in catalytic combustor

Fig. 20 shows the profiles ofxp (location of surface peak temper-

ature) at different cases. Data from 0.28 to 0.4 L/min in the rich

cases is not given, because the combustor does work under the

conditions. The profiles show that catalyst induces shift down-

stream except in the lean cases. At ER of 1, xp stays around

0.5 mm in the N combustor, and shifts to 3 mm in the C combustor.

At ER of 5.55,xp keeps increasing with the flow rates. Butxp in the C

combustor is 4 mm higher than in the N combustor. At ER of 0.34,

xp increases with flow rates in the N combustor. But in the C com-

bustor, the reaction region is fixed around 2 mm.

Fig. 21 gives the profiles of Tp (peak surface temperature) vs.

flow rate. Low Tp indicates weak reaction. Catalyst decreases Tpin the rich and stoichiometric cases, but increases Tp in the lean

case. For example, at 0.2 L/min and ER of 1, Tp are 982 and783 C in the N and C combustors, respectively. At ER of 5.55, Tp de-

creases from 750 to 561 C after applying catalyst. But at ER of 0.34,

Tp increases from 561 to 686 C after applying catalyst. Comparing

Figs. 20 and 21, the profiles ofTp and xp have the similar trends. It

indicates that weak reaction induces shift downstream.

Qcon (average heat conduction on the combustor wall) in the

preheating region is analyzed. It makes an important route for heat

recirulation [40], thus affects the shift downstream [1]. The pre-

heating region is from the location of peak surface temperature

to the upstream end of the combustor.

Fig. 22 gives the profiles ofQcon at different flow rates. Catalyst

decreases Qcon in the rich and stoichiometric cases, but increases

Qcon in the lean cases. Take 0.2 L/min as an example, at ER of 1, Qcondecreases from 13.3 to 7.45 W after applying catalyst. At ER of 5.55,

Qcon decreases from 9.64 to 2.61 W. But at ER of 0.34, Qcon increases

from 6.73 to 8.21 W. Comparing Figs. 20 and 22, the profiles ofQconand xp have the similar trends. It indicates that shift downstream

occurs due to insufficient heat recirculation.

Conclusive, catalyst weakens the homogeneous reaction in the

stoichiometric and rich cases. The lower and uniform temperaturedistribution [14] decreases the heat recirculation, thus causes shift

downstream. But in the lean cases, the combustor has intense reac-

tion after applying catalyst. It is attributed to the high mass diffu-

sion and low thermal diffusion of reactants. The heterogeneous

reaction rate depends on mass diffusion [14]. In the lean cases,

the diffusion of hydrogen determines the heterogeneous reaction

rates, which is higher than oxygen in the rich cases

(5.83 104 m2/s vs. 1.53 104 m2/s). The low thermal diffusion

of lean mixture also helps preserve heat in the reaction region

(2.2 107 of air vs. 1.3 104 m2/s of hydrogen). Therefore, the

heterogeneous reaction is more intense in the lean cases than in

the other cases.

4. Conclusion

According to the comparison between C and N combustors, cat-

alyst inhibits extinction. But it is less effective to inhibit blow out.

At 0.8 L/min, the stability limit is 0.19314.9 in the N combustor.

With catalyst, the lean limit is close to 0, and the rich limit is

29.3. Heterogeneous reaction occurs at much lower temperature

than homogeneous reaction, thus catalyst inhibits extinction. The

comparison ofQsurf also shows that catalyst decreasesQsurf slightly,

but it is less important to inhibit extinction. At 0.2 L/min with ER of

1, Qsurf are 10.0 and 9.57 W in the N and C combustor, respectively.

Increasing flow rates inhibits extinction. Qsurf occupies less per-

centages of the total energy at higher flow rates, thus the intense

reaction inhibits extinction. In the N combustor, from 0.08 to0.2 L/min, the percentage of Qsurf decreases from 84.8% to 78.9%.

Numeric simulation also verifies the intense reaction. The peak

temperature increases from 1774 to 1806 K, and the peak OH mass

fraction increases from 0.0035 to 0.0096.

Catalyst induces shift downstream in the rich and stoichiome-

tric cases. C combustor has weak reaction and exhibits uniform

temperature distribution, thus the heat recirculation is insufficient.

At 0.2 L/min with ER of 1, xp stays around 0.5 mm in the N combus-

tor, and shifts to 3 mm in the C combustor. Accordingly, Tp de-

creases from 982 to 783 C, and Qcon decreases from 13.3 to

7.45 W. According to numeric simulation, heterogeneous reaction

consumes part of the fuel, thus weakens the homogeneous reac-

tion. However, C combustor shows intense reaction in the lean

cases, where shift downstream is inhibited. It is attributed to thehigh mass diffusion and low thermal diffusion of lean mixture.

Fig. 20. xp vs. flow rate at ER of 0.34 (lean), 1 (stoichiometric) and 5.55 (rich).

Fig. 21. Peak temperature vs. flow rate at ER of 0.34 (lean), 1 (stoichiometric) and5.55 (rich).

Fig. 22. Qcon vs. flow rate at ER of 0.34 (lean), 1 (stoichiometric) and 5.55 (rich).



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Considering that extinction mainly occurs in the lean cases, it also

explains why catalyst inhibits extinction more effectively.

Acknowledgements

This work is supported by the National Nature Science Founda-

tion of China (No. 50606030), Research Fund for the Doctoral Pro-

gram of Higher Education of China (No. 20060335124), Program ofIntroducing Talents of Discipline to University (No. B08026) and

National Science Foundation for Distinguished Young Scholars

(No. 50525620).

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combustion of hydrogen air in micro combustors with catalytic pt layer wang

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