Artículo Traducción (1)

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Combustion characteristics and performance evaluation of premixed methane/air with hydrogen addition in a micro-planar combustor Aikun Tang a,b , Yiming Xu a , Jianfeng Pan a,n , Wenming Yang b , Dongyue Jiang b , Qingbo Lu a a School of Energy and Power Engineering, Jiangsu University, Zhenjiang 212013, China b Department of Mechanical Engineering, National University of Singapore, Singapore 117576, Singapore HIGHLIGHTS Hydrogen addition has a dramatic function on ame stabilizing of methane combustion. The reaction rate of methane will be apparently raised by burned hydrogen. The effects on the growth of wall temperature and radiant energy are investigated. Hydrogen addition ratio should exceed 10% so as to ensure a good operation effect. article info Article history: Received 27 November 2014 Received in revised form 18 February 2015 Accepted 14 March 2015 Available online 28 March 2015 Keywords: Micro-combustion Hydrogen addition Numerical simulation Flame position Radiant energy abstract Combustion performance of premixed methane/air with hydrogen addition in a micro-planar combustor is numerically investigated, aiming to extend the application of various fuels in a micro- thermophotovoltaic system. The simulation results show that hydrogen addition has signicant effect on raising methane reaction rate and ame stability. With the increase of hydrogen mass fraction, the location of the ame shifts toward the combustor inlet gradually, and the temperature gets a steady growth. For the micro-combustor with a channel height of 3 mm, it is found that the ame of pure methane is far away from inlet, which does not meet the basic working requirement of the micro- thermophotovoltaic system. However, the combustor wall temperature will be signicantly improved when a small amount of hydrogen is added into the mixture, which is mainly due to the extension of ammability range. Along with the increase of H 2 addition ratio, both the total radiant energy and the proportion of usable radiant energy to total input chemical energy increase dramatically, which brings a signicant improving of electricity output and system efciency. & 2015 Elsevier Ltd. All rights reserved. 1. Introduction With the development of micro-electro-mechanical systems (MEMS), great attention has been paid to the design of its energy supply system. Recently, several combustion-based micro-power generators have aroused the research interest of scholars around the world, which have the advantages of high energy density, small volume as well as long working time (Ju and Maruta, 2011; Chia and Feng, 2007; Chou et al., 2011). Apart from serving as electric source for MEMS such as micro-pumps and micro-robots, the micro-power generators can also provide adequate electricity or power for portable electronics, wireless communication equip- ments, vehicles, and military devices (Chia and Feng, 2007). However, due to the very short history, the technology is still far away from mature. So, it is essential to further improve the performance of these micro-power devices. The micro-thermophotovoltaic (MTPV) system is one of the typical micro-power generators, which consists of three major components: micro-combustor, PV cells and optical lter. Due to its high energy density, no moving parts and convenient to fabricate, it is superior to other portable power generation systems (Chou et al., 2011). During operation, rstly, the chemical energy from hydrocarbon fuel is released during the combustion process. The outer wall of combustor can reach a high-temperature state and emits photons. Then, the short-wave radiation will transmit the lter, and arrive at the surface of PV cells. Finally, free electron can be generated due to photoelectric effect. Therefore, the output power density and energy conversion efciency of the MTPV system are directly inuenced by the micro-combustion process and the outer wall temperature. For this reason, various research works on the structure optimization of micro-combustor have been carried out in the Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/ces Chemical Engineering Science http://dx.doi.org/10.1016/j.ces.2015.03.030 0009-2509/& 2015 Elsevier Ltd. All rights reserved. n Corresponding author. Tel.: þ86 511 88780210; fax: þ86 511 88780216. E-mail address: [email protected] (J. Pan). Chemical Engineering Science 131 (2015) 235242

Transcript of Artículo Traducción (1)

Page 1: Artículo Traducción (1)

Combustion characteristics and performance evaluation of premixedmethane/air with hydrogen addition in a micro-planar combustor

Aikun Tang a,b, Yiming Xu a, Jianfeng Pan a,n, Wenming Yang b, Dongyue Jiang b, Qingbo Lu a

a School of Energy and Power Engineering, Jiangsu University, Zhenjiang 212013, Chinab Department of Mechanical Engineering, National University of Singapore, Singapore 117576, Singapore

H I G H L I G H T S

� Hydrogen addition has a dramatic function on flame stabilizing of methane combustion.� The reaction rate of methane will be apparently raised by burned hydrogen.� The effects on the growth of wall temperature and radiant energy are investigated.� Hydrogen addition ratio should exceed 10% so as to ensure a good operation effect.

a r t i c l e i n f o

Article history:Received 27 November 2014Received in revised form18 February 2015Accepted 14 March 2015Available online 28 March 2015

Keywords:Micro-combustionHydrogen additionNumerical simulationFlame positionRadiant energy

a b s t r a c t

Combustion performance of premixed methane/air with hydrogen addition in a micro-planar combustoris numerically investigated, aiming to extend the application of various fuels in a micro-thermophotovoltaic system. The simulation results show that hydrogen addition has significant effecton raising methane reaction rate and flame stability. With the increase of hydrogen mass fraction, thelocation of the flame shifts toward the combustor inlet gradually, and the temperature gets a steadygrowth. For the micro-combustor with a channel height of 3 mm, it is found that the flame of puremethane is far away from inlet, which does not meet the basic working requirement of the micro-thermophotovoltaic system. However, the combustor wall temperature will be significantly improvedwhen a small amount of hydrogen is added into the mixture, which is mainly due to the extension offlammability range. Along with the increase of H2 addition ratio, both the total radiant energy and theproportion of usable radiant energy to total input chemical energy increase dramatically, which brings asignificant improving of electricity output and system efficiency.

& 2015 Elsevier Ltd. All rights reserved.

1. Introduction

With the development of micro-electro-mechanical systems(MEMS), great attention has been paid to the design of its energysupply system. Recently, several combustion-based micro-powergenerators have aroused the research interest of scholars aroundthe world, which have the advantages of high energy density,small volume as well as long working time (Ju and Maruta, 2011;Chia and Feng, 2007; Chou et al., 2011). Apart from serving aselectric source for MEMS such as micro-pumps and micro-robots,the micro-power generators can also provide adequate electricityor power for portable electronics, wireless communication equip-ments, vehicles, and military devices (Chia and Feng, 2007).However, due to the very short history, the technology is still far

away from mature. So, it is essential to further improve theperformance of these micro-power devices.

The micro-thermophotovoltaic (MTPV) system is one of thetypical micro-power generators, which consists of three majorcomponents: micro-combustor, PV cells and optical filter. Due toits high energy density, no moving parts and convenient tofabricate, it is superior to other portable power generation systems(Chou et al., 2011). During operation, firstly, the chemical energyfrom hydrocarbon fuel is released during the combustion process.The outer wall of combustor can reach a high-temperature stateand emits photons. Then, the short-wave radiation will transmitthe filter, and arrive at the surface of PV cells. Finally, free electroncan be generated due to photoelectric effect. Therefore, the outputpower density and energy conversion efficiency of the MTPVsystem are directly influenced by the micro-combustion processand the outer wall temperature.

For this reason, various research works on the structureoptimization of micro-combustor have been carried out in the

Contents lists available at ScienceDirect

journal homepage: www.elsevier.com/locate/ces

Chemical Engineering Science

http://dx.doi.org/10.1016/j.ces.2015.03.0300009-2509/& 2015 Elsevier Ltd. All rights reserved.

n Corresponding author. Tel.: þ86 511 88780210; fax: þ86 511 88780216.E-mail address: [email protected] (J. Pan).

Chemical Engineering Science 131 (2015) 235–242

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past decade. Yang et al. (2007) tested a micro-cylindrical combus-tor with backward facing step, and found that the step is veryuseful in controlling flame position and widening the operationalrange of mixture flow rate. Pan et al. (2010) presented a designconcept of modular TPV power generator based on the sub-millimeter planar combustor. The combustion characteristics ofpremixed hydrogen–oxygen were investigated through bothexperimental and computational methods. Yang et al. (2011,2012) also conducted similar studies about premixed hydrogen–air in the micro-planar micro-combustor. The SiC porous mediafoam was employed in the combustion channel to enhance heattransfer between the hot gas and the wall (Yang et al., 2011). Amicro-combustor with a percolated platinum tube was proposedby Li et al. (2013), which served as catalyst, emitter, and flamestabilizer to overcome the critical heat loss and improve the flameinstability. Wan et al. (2012) developed a micro-combustor with abluff body which can extend the blow-off limit by 3–5 times.Besides, several kinds of micro-heat-recirculating combustors forthe MTPV system were reported by Park et al. (2011, 2012), Yanget al. (2014) and Jiang et al. (2013a, 2013b), which all have positiveeffects on improving the temperature of emitters. In addition,studies of flames propagating in micro-channels were alsoreported (e.g. Veeraragavan and Cadou (2011), Kurdyumov andJimenez (2014)), which would provide some promotional effectson the improving of working performance of micro-combustor.

For the MTPV system, the scholars prefer to choose hydrogen asthe fuel of the combustion process. This is typically attributed tothe higher heating value and ignition propensity of hydrogen. Liet al. (2009) pointed out that the blowout phenomenon occurswhen the velocity of premixed methane and air exceed 1 m/s,while the blowout limit of hydrogen case can reach 8 m/s in thesame combustion channel. Similar problem of propane combus-tion in a single channel micro-combustor was also found byFederici and Vlachos (2008), and they employed a heat recircula-tion combustor to increase the blowout limit of propane airmixture. In the study of conventional scale combustion, it wasfound that the combustion of hydrocarbon fuel could be greatlyimproved by adding a small amount of hydrogen into the mixture,so a great deal of research on this subject have been conducted(Khalil and Gupta, 2013; Wang et al., 2009; Sabia et al., 2007;Titova et al., 2014; Cuoci et al., 2013). When it comes to micro-scale combustion, Norton and Vlachos (2005) realized the self-ignition of propane/air mixtures with the assistance of hydrogenaddition in a catalytic micro-burner. Yan et al. (2014) studied thehydrogen assisted catalytic combustion of methane on platinum,and concluded that hydrogen addition has a great influence onlowering the methane ignition temperature and shortening theignition time. Seshadri and Kaisare (2010) also investigated theself-ignition property of hydrogen, in which two different modesof hydrogen-assisted propane ignition were compared. A novelcatalytic micro-reactor which combined the concept of catalystsegmentation and cavities was designed by Li et al. (2012), and theenhancement effects on H2/CO/CH4 multi-fuel combustion wereanalyzed through numerical simulation. Zarvandi et al. (2012)conducted a study of CH4/air combustion with hydrogen additionin a micro-stepped tube, and found that adding hydrogen couldintensify the production of some crucial species which are veryvital for establishing a stable combustion. Tacchina et al. (2010)investigated the combustion performance of CH4/H2/air mixturesusing different catalysts, and concluded that the presence of H2 inthe fuel will improve the conversion of CH4 due to an increase inthe production of OH radicals.

At the condition of micro-scale, the blending fuel combustionprocess may present very different characteristic compared withconventional scale combustion, which is subjected to the problemsof short residence time and large heat loss. In contrast, the

research on blending fuel combustion in micro-combustor is verylimited, and most of the studies mainly focus on the aspect ofcatalytic combustion. However, very few related research for themicro-thermophotovoltaic system using blending fuel has beenreported. In this paper, the combustion process of premixedmethane/air with hydrogen addition in a micro-planar combustorfor the MTPV system is studied by a simulation method. Thepositive effects of hydrogen addition on the growth of walltemperature distribution and radiant energy output are alsodiscussed, so as to give a reference for further improvement ofthe system working performance.

2. Construction and verification of a computational model

2.1. Geometric model and grid generation

In this work, a micro-planar combustor is designed, as shownin Fig. 1. The dimensions of the micro-combustor are 18 mm inlength, 9 mm in width and 4 mm in height. The wall thickness is0.5 mm, so the height of straight combustion channel is 3 mm.During the operation process, premixed fuel (methane and hydro-gen) and air flow into the straight parallel channel through arectangle inlet which is located at one end of the combustor. Theburned gas will be expelled out from an outlet at the other end ofcombustor. The material of 316 stainless steel is chosen ascombustor wall, which can withstand a temperature of 2000 1Cand has a large emissivity at high-temperature state.

A 3D uniform grid is developed to predict the combustion and theheat transport in the micro-combustor. Half of the combustor space isemployed as computational domain to save the calculation time.Centerline gas temperature profiles at three different mesh densitieshave been checked so as to determine the grid size, as seen in Fig. 2.After comparison, the meshwith 0.1 mm density in all three directionsand 324,000 total cells number is finally chosen.

Combustor wall

Fuel/air inlet

18 mm

Product outlet

0.5 mm

3 mmUniform velocity profile

Heat loss : convection & radiation

Fig. 1. Schematic diagram of micro-planar combustor.

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162000 cells (0.20,0.10,0.10)324000 cells (0.10,0.10,0.10)

648000 cells (0.10,0.10,0.05)

Fig. 2. Centerline temperature profiles of combustor at different mesh densities(10% hydrogen addition, inlet velocity: 2 m/s).

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2.2. Methods of model building

Important parameters of the computational model are shownin Table 1. A skeletal mechanism for methane–air combustion isemployed in the computational model, which has 16 species and25 reversible reactions (Li et al., 2009). The mechanism has beenproved to be applicable in the simulation of micro-combustion.Due to the low inlet flow velocity, laminar model and laminarfinite-rate model are chosen as flow and chemistry reactionmodels, respectively. The basic governing equations of gas phaseinclude the mass conservation equation, the momentum conser-vation equation, the chemical components transport equation andthe energy conservation equation, which are expressed as follows.

Continuity:

∂∂xi

ρui� �¼ 0 ð1Þ

where ρ is the density, ui is the velocity components of xi direction(i¼1, 2, 3).

Momentum:

∂∂xj

ρuiuj�τij� �¼ � ∂p

∂xið2Þ

where p is the pressure, τij is the stress tensor.Species:

∂∂xj

ðρujmlþ Jl;jÞ ¼ Rl ð3Þ

where ml is the mass fraction of species l, Jl;j is the diffusion flux ofspecies l in the xj direction, Rl is the production rate of species l bychemical reaction.

Energy:

∂∂xj

ðρujhþFh;jÞ ¼ uj∂pxjþτij

∂ui

∂xjð4Þ

where h is the total enthalpy, Fh;j is the energy flux of the xjdirection.

All of the governing equations are discretized by a first-orderupwind scheme, and SIMPLE algorithm is used to deal with thepressure–velocity coupling. Fluent is chosen to solve the governingequations implicitly and the convergence criterions of all residualsare set to be 10�6. Compared to case of one-step reaction mechan-ism, the calculation is more difficult to get converged, so stiff

chemistry solver and under-relaxation method are used to overcomethis question.

In our simulation, the specific heat of each component iscalculated using a piecewise polynomial fitting of temperature,while both of thermal conductivity and viscosity are calculated bykinetic-theory. The density and specific heat of the fuel and airmixture are calculated by incompressible-ideal-gas law and mix-ing-law, respectively. The mixture gas thermal conductivity andviscosity are calculated as a mass fraction-weighted average of allspecies (Norton and Vlachos, 2003), and the kinetic-theory isselected for mass diffusivity calculation of mixture (Zarvandiet al., 2012).

The boundary conditions in the computational model are set asfollows. At the inlet, velocity-inlet boundary condition is chosen. Auniform flat velocity profile is assumed, and the mass fraction ofeach component is specified. Pressure-outlet boundary conditionis employed at the exit face, the constant pressure and tempera-ture of mixture are given, and the mass fraction of each product isestimated. No slip and zero diffusive flux species boundaryconditions are employed at the gas–solid interfaces. For eachouter wall, both convection and radiation heat transfer withenvironment should be considered, so mixed thermal conditionsare set for them, the convective heat transfer coefficient and wallemissivity are taken to be 15 W/(m2 K) and 0.65, respectively.

2.3. Validation of the computational model

The experimental validation has been conducted on the micro-planar combustor with the fuel of pure methane. The experimen-tal setup is given in the literature (Pan et al., 2010), and an infraredthermographer (Camera model: ThermovisionTM A40) is adoptedto measure the outer wall temperature distribution of combustor.It is found that the premixed methane and air cannot be ignitedwhen the channel height is less than 3 mm. This is the reason whythe 3 mm inner height combustor is tested and simulated in thispaper. Fig. 3(a) shows the comparison on combustor wall tem-perature distribution patterns of experiment and simulation atinlet velocity of 0.6 m/s. Fig. 3(b) illustrates the centerline tem-perature profiles of experimental and simulation result at twodifferent velocity conditions. The comparisons demonstrate thatan excellent agreement between simulation results and experi-mental tests is achieved. The maximum deviations of temperaturein Fig. 3(b) is 2.3% (inlet velocity 0.4 m/s) and 3.7% ( inlet velocity0.6 m/s), respectively.

Table 1Important parameters of the computational model.

Parameters Methods

Flow LaminarReaction Finite-rate (mechanism with16 species and 25 reversible reactions ) (Li et al., 2009; Zarvandi et al., 2012)Discretization First-order upwindPressure–velocity coupling SIMPLE algorithmSolver Segregate/implicit (under-relaxation method) (Li et al., 2009)

Software FluentMixture physical properties Density: incompressible-ideal-gas law (Norton and Vlachos, 2003)

Specific heat: mixing-law (Zarvandi et al., 2012)Thermal conductivity: mass-weighted-mixing law (Jiang et al., 2013a; Li et al., 2009; Norton and Vlachos, 2003)Viscosity: mass-weighted-mixing law (Jiang et al., 2013a; Li et al., 2009; Norton and Vlachos, 2003)Mass diffusivity: kinetic-theory

Species physical properties Specific heat: a piecewise polynomial fitting of temperature (Jiang et al., 2013a; Li et al., 2009)Thermal conductivity: kinetic-theory (Zarvandi et al., 2012)Viscosity: kinetic-theory (Zarvandi et al., 2012)

Boundary conditions Inlet: velocity-inletExit face: pressure-outletExternal wall: mixed thermal conditions

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3. Results and discussions

3.1. Combustion characteristics

To obtain the effect of hydrogen addition on methane combus-tion in the micro-channel, the cross section temperature distribu-tions of the case with pure methane and the case with 10%hydrogen addition are compared. Here, the addition ratio means10% of methane is replaced by hydrogen with the same massfraction. Thus, the total fuel mass will remain constant. Comparingto the methane combustion, more oxygen will be consumed inhydrogen combustion of the same mass fraction. Therefore, theoriginal methane–air equivalence ratio is chosen to be 0.8 so as toensure full combustion of mixed fuel. As shown in Fig. 4(a), themethane/air flame location is greatly influenced by inlet velocity,which is very similar to the 2D simulation results by Li et al. (2009).At each case, the high temperature flame zone is attached to theinner wall of combustor, and will be a little backward at the centralregion of channel. The location of flame apparently moves towardthe outlet while mixture flow rate is increasing. The position ofmaximum temperature flame is about 6 mm far from the inletwhen the inlet velocity is 0.4 m/s. But the high temperature flamezone is very close to the outlet when the inlet velocity is 0.6 m/s. Ifthe inlet velocity is further increased, not only incomplete combus-tion but even total blow-off would happen. The result implies theshort flammability range of methane/air micro-combustion.

When 10% of hydrogen is added to the mixture, a great changewill take place in the micro-combustion process. As shown in

Fig. 4(b), the position of high temperature zone is obviously shiftedto the inlet when velocity is 0.6 m/s, and actually there is littledifference while the inlet velocity is less than 0.6 m/s. When itcomes to the case of 1 m/s, the high temperature flame zonebegins at the inner wall attachment point and is still close to theinlet. And the position of flame front at the centerline is only 2 mmaway from the inlet. This phenomenon is very similar to the purehydrogen combustion (Pan et al., 2010; Yang et al., 2012). There-fore, it is concluded that the mixture will be ignited very rapidlydue to the highly combustible characteristic of hydrogen. And themixed fuel exhibits more combustion characteristics of hydrogenwhen inlet velocity is relatively low.

As a result, the inlet velocity is further increased so as to examinethe flammability range of methane with hydrogen addition. It can beseen from Fig. 4(b) that the shortening of residence time byincreasing inlet velocity has arose some difference in flame shape.Although the reaction positions near the inner wall do not changemuch with an increase in flow rate, there is an obvious phenomenonthat the flame of the central zone moves backward, and the flamefront becomes more and more sharp. This is due to the flame stretchwhich is caused by longer warm-up time. The chemical reactiontakes place 4 mm away from inlet along the centerline for the 2 m/scase, and 7 mm away from inlet for the 3 m/s case. It is very close tothe exit when the velocity reaches 4 m/s. These calculation dataillustrate that the flammability range has been expanded from0.6 m/s to 4 m/s, which indicates a positive effect of stabilizingflame location by 10% hydrogen addition.

A comparison on different addition ratios is conducted so as tofurther analyze the effect of hydrogen addition. The five addition

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experimental data (0.4 m/s)simulation result (0.4 m/s)experimental data (0.6 m/s)simulation result (0.6 m/s)

Fig. 3. Experimental validation of computational model (equivalence ratio¼1.0).(a) Comparison of temperature distribution patterns on the combustor outer wall,(left: experiment; right: simulation; inlet velocity: 0.6 m/s), (b) Comparison ofcenterline temperature profiles along the combustor outer wall.

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T/K 0.3 m/s

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2.0 m/s

3.0 m/s

Fig. 4. Comparison of temperature distributions on combustor cross section atdifferent inlet velocities. (a) Pure methane (b) 10% hydrogen addition.

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ratios are 5%, 10%, 15%, 20% and 25%, respectively. It should bepointed out that the mass fraction of mixed fuel and oxygen arevery close to the stoichiometric balance when hydrogen additionratio reaches 25%, so incomplete combustion would be caused ifmore methane is replaced by hydrogen.

First, we compared the centreline chemical reaction rate ofdifferent addition ratios (as shown in Fig. 5). Here we adopt thereaction-19 (CH4þOH-CH3þH2) for comparison which can pro-vide some useful reference for methane reaction rate. When theaddition ratio is 5% and inlet velocity is 1.5 m/s, the mixture cannotbe ignited until arriving at the position of 10 mm away from inletalong the centerline, and the reaction rate will reach the highestvalue at 11 mm. However, the reaction will be closer to the inletzone obviously while the hydrogen addition ratio exceeds 10%, andthe locations of maximum reaction rate are only 4 mm (10%),3 mm (15%), 1 mm (20%) and 0.5 mm (25%) away from the inlet,respectively. In fact, the maximum velocity for the occurrence ofcombustion is 2 m/s when the hydrogen addition ratio is 5%,which is about only half as much as the 10% case. As a result, morehydrogen should be added into the mixture so as to achieve aneffective improvement of flammability. Furthermore, the max-imum rate of reaction-19 will be raised gradually along with theincrease of hydrogen fraction. The case with 25% hydrogen addi-tion can reach 2.1 times as much as that of 5% case. The resultdemonstrates that the addition of hydrogen also has an effectiveimpact on improving reaction rate of methane.

Fig. 6 shows the centerline temperature profiles at differentaddition ratios. It can be seen that the high temperature zone is alittle behind with the reaction position in the Fig. 5. Apart from the5% case, all of the mixture can climb from normal temperature tothe highest temperature within a 5 mm distance, which is due tothe rapid chemical reaction. Then, the mixture temperature willdecline slowly alone the flow direction as a result of heatabsorption by the combustor wall. Meanwhile, it should be notedthat the maximum value of flame temperature has get a steadygrowth with the increase of addition ratio. The adiabatic flametemperature of methane and hydrogen with air are 2226 K and2400 K, respectively (Turns, 2011), so this is an inevitable trend. Inaddition, the growth of methane reaction rate also has animportant promoting effect on the flame temperature increasing.At the five hydrogen addition cases, the highest flame temperaturecan be increased by about 94 K when 5% of the methane isreplaced by hydrogen. Thus, the maximum value of flame tem-perature can reach 2276 K at 25% case, which has already exceededthe adiabatic flame temperature of methane.

3.2. Working performance of micro-combustor

For the micro-combustor of the MTPV system, besides theacquisition of a steady combustion process, much more concernshould be paid to the outer wall temperature distribution andradiant energy output. A comparison of the outer wall meantemperature at different inlet velocities and addition ratios isshown in Fig. 7. First, it should be noted that the outer walltemperature of pure methane combustion case is only 868 K atinlet velocity of 0.6 m/s. Obviously, the further thermophotovoltaicconversion could hardly carry out due to such low temperature.When 5% and 10% of methane are replaced by hydrogen, the meanwall temperature can reach 980 K and 1039 K, which have shownan apparent promoting effect. However, the growth trend willbecome very slowly while more hydrogen is added into themixture, and the increment form 10% to 25% case is only 22 K.As mentioned before, hydrogen will occupy a predominant posi-tion in the mixed fuel when inlet velocity is relatively low andhydrogen addition ratio is larger than 10%. The fuel is ignitedimmediately once entering the combustion channel, which lead tolittle variation in the mean temperature of mixture. As a result, theassistance of wall temperature increasing will become quitelimited. However, the characteristic of methane will be revealedapparently when the mixture flow rate is further increased. The

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91-noitcaerfoetar

citeniK

(kgm

ol/m

3 .s-1)

Distance from inlet (mm)

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Fig. 5. Kinetic rate of reaction-19 at different addition ratios (inlet velocity: 1.5 m/s).

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Fig. 6. Centerline temperature profiles at different addition ratios (inlet velocity:1.5 m/s).

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Fig. 7. Outer wall mean temperature at different addition ratios and inletvelocities.

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reaction position gradually shifts forward according to the increas-ing addition ratio, and the mixture temperature also increasescontinuously (as seen from Fig. 6). As a result, a linear growthtendency of wall temperature can be found at each high velocitycase. The outer wall temperature of 10% addition case is higherthan 1200 K at inlet velocity of 2 m/s, which has reached the basicworking requirement of the MTPV system. When increasing thehydrogen mass fraction from 10% to 25%, another 87 K (at 2 m/scase) and 102 K (at 3 m/s case) temperature increments will begained. From this point of view, the positive effect of hydrogenaddition is very remarkable.

Fig. 8 shows the outer wall temperature distributions at differentaddition ratios. The high temperature zone (T41200 K) of the fourcases are all situated at the front part of outer wall except for 5% case,which keep in step with the high flame temperature locations inFig. 6. It can be predicted that the location of high temperature zoneat 10% case will also move backward when inlet velocity is furtherincreased. Meanwhile, as the hydrogen mass fraction increased, thearea of high temperature zone has presented an apparent growingtrend. At the 25% hydrogen addition case, 62% part of the outer wallis higher than 1200 K, and the region above 1300 K occupies 1/3 ofthe total wall area, which brings an average temperature of 1243 K tothe outer wall. However, the maximum temperature of 5% case isonly 1186 K, which also indicates that the hydrogen addition ratiohigher than 10% is favorable. Besides, the largest temperaturedifferences at the five addition ratios are 339 K, 211 K, 251 K, 273 Kand 290 K respectively, so the minimum value is at the 10% case.Thus, we can conclude that there is a close relationship between theuniformity of wall temperature distribution and the mixture ignitionlocation, and the uniformity can be controlled by proper setting ofhydrogen addition ratio and inlet velocity. In fact, the promotion oftemperature distribution uniformity also has an important effect onreducing thermal stress of surface, which is absolutely critical toprolong the service life of micro-combustor.

According to the hydrogen addition plan, the total mass flowrate will remain constant when inlet velocity is fixed. However,the total input chemical energy of the mixed fuel is bound tochange because of the heating value difference between methaneand hydrogen. Fig. 9 gives a comparison of input chemical energyand the output radiant energy at four selected hydrogen additionratios. The high heating values of methane and hydrogen are55.5 MJ/kg and 141.9 MJ/kg, respectively (Turns, 2011). So the totalchemical energy increases gradually with the increase of hydrogenmass fraction. The total radiant energy has also got a steadygrowth due to the increasing of wall mean temperature.

Fig. 10 shows the specific proportions of these two energies atdifferent inlet velocities and addition ratios. First, the proportionpresents a steady growth with the increase of hydrogen massfraction at the 2 m/s case. This indicates that the growth rateof outer wall radiant energy is larger than that of input chemi-cal energy. Therefore, it is suggested that the mass fraction of

hydrogen should be improved as much as possible if the oxidant isenough. Good agreements have been shown at both 1.5 m/s and3 m/s cases in Fig. 10, however, the variation trend is ratherdifferent at a low inlet velocity case (e.g. for 0.6 m/s case). It canbe seen that the proportion of radiant energy to total chemicalenergy will get a big increment from 0% to 10% hydrogen additionratio, which are 20.8% and 36.7% respectively. When the hydrogenmass fraction exceeds 10%, the proportion goes into a slow decline

13501300125012001150110010501000950900850

T/K

Fig. 8. Comparison of temperature distributions on outer wall at different addition ratios (inlet velocity: 1.5 m/s). (a) 5% (b) 10% (c) 15% (d) 20% (e) 25%.

10 15 20 250

25

50

75

100

125

150

175

200

Ene

rgy

(W)

Hydrogen addition ratio (%)

Total chemical energy Radiant energy

Fig. 9. Total chemical energy and radiant energy at different addition ratios (inletvelocity: 2 m/s).

0 5 10 15 20 255

10

15

20

25

30

35

40

Rygrenelaci

mehc/ygrenetnaida

(%)

Hydrogen addition ratio (%)

inlet velocity 0.6 m/sinlet velocity 1.5 m/sinlet velocity 2.0 m/sinlet velocity 3.0 m/s

Fig. 10. Proportion of radiant energy to total chemical energy at different additionratios and inlet velocities.

A. Tang et al. / Chemical Engineering Science 131 (2015) 235–242240

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rather than a further increasing. This phenomenon is mainlycaused by the low growth rate of wall temperature, which isclearly shown in Fig. 7. Thus, like hydrogen addition ratio, themixture flow rate also should not be too low so as to ensure a goodoperation effect. Besides, it should be noted that the proportion ofradiant energy to total chemical energy decreases evidently withthe increase of inlet velocity, while hydrogen ratio is at the samecondition. It follows that although the values of mean temperatureand output radiant energy will get a continuous increment byrising mixture flow rate. However, the growing trend is signifi-cantly lower than that of input chemical energy which presents alinear relationship with the inlet velocity.

As shown in Fig. 9, the total radiant energy from the outer wallreaches 35 W when the addition ratio is 10%. Actually only a smallportion of photons could be converted to electricity, whosewavelength is shorter than the cut-off wavelength of the PV cells(e.g. for GaSb cell, 1.78 μm). To further study the improving effecton working performance by hydrogen addition, detailed propor-tions of usable radiant energy to total radiant energy and chemicalenergy are displayed in Fig. 11. First, these two proportions are alsogoing up with the increase of hydrogen addition ratio, whichshows similar trend as mean wall temperature, total radiantenergy as well as its proportion in chemical energy. Take the caseof 3 m/s for example, the values of usable radiant energy havereached 5.0 W and 9.4 W at the 10% and 25% addition ratios, andthe proportion of usable radiant energy to total chemical energywill rise from 1.95% to 3.05%. As a result, it is concluded that theoverall system efficiency can be improved by 50% if other systemworking conditions remain unchanged. Therefore, from the per-spectives of both power output and system efficiency, the positiveeffects of hydrogen addition are very significant. Meanwhile, it isfound that the two proportions present an opposite tendency withthe increase of inlet velocity. The proportion of usable radiantenergy to total radiant energy at each addition ratio can be raisedby about 2% from 2 m/s to 3 m/s, and the specific value hasreached 16% at 25% addition case. However, this growth can onlymake a limited compensation to the adverse effect brought by theslow increasing rate of total radiant energy. Thus, proportions ofusable radiant energy to total chemical energy at inlet velocity of2 m/s are 2.2%, 2.7%, 3.0% and 3.3% respectively, which are 0.2%higher than that of each 3 m/s calculation result on average.Consequently, when burning mixed fuel such as methane andhydrogen, although the merely increasing of mixture flow rate cancontribute to the boost of output power, it will not contribute tothe improvement of the system efficiency.

Finally, based on the temperature predictions, the performanceof the MTPV system is estimated. It is assumed that two GaSb PVcells are arranged parallel to the external wall of micro-combustor,which has the same surface area. The distance between the PV celland external wall of micro-combustor is 1 mm. Meanwhile, it issupposed that there is a good cooling system behind the PV cell.That is, the temperature of the PV cell is kept 300 K and itsefficiency is not changed. During the calculation process, viewfactor between each elementary face of external wall and PV cell isconsidered, and the detailed computational model can be found inour previous work (Pan et al., 2010). Electricity output andefficiency of the MTPV system at four addition ratios are calcu-lated, as shown in Fig. 12. It can be seen that both the electricityoutput and system efficiency improve significantly with theincrease of hydrogen addition ratio. Under the same condition ofinlet velocity 3 m/s, the electricity output exceeds 1.48 W whenhydrogen addition ratio is 25%, which is nearly twice as much asthe 10% case. And due to the increase of wall temperature andusable radiant energy, the system efficiency reaches 0.48% at25% case.

Furthermore, it should be noted that a great amount of radiantenergy from the outer wall cannot be used to photo-electricconversion due to the cell bandgap. It not only creates greatwastage of energy, but also increases the cooling load of cells.Therefore, besides intensification of micro-combustion process,the successful development of lower bangap cell is also veryessential to the working performance improving of the MTPVsystem.

4. Conclusions

In this paper, the effect of hydrogen addition on methane/aircombustion in a micro-planar combustor is investigated through3D simulation method. Also, the radiation performance of themicro-combustor at different addition ratios and flow rates areanalyzed and compared, which are absolutely crucial to the MTPVsystem. Main conclusions are summarized as follows:

(1) Hydrogen addition has a significant function on flame stabiliz-ing, and the flammability range can be extended from 0.6 m/sto 4 m/s when 10% of methane is replaced by hydrogen.

(2) The reaction rate of methane will be increased significantly bythe burned hydrogen. Along with the increase of additionratio, the flame location apparently shifts toward the inlet andthe flame temperature gets a steady growth.

10 15 20 2510

11

12

13

14

15

16

17

18

19

20

Hydrogen addition ratio (%)

ygrenetnaidar/ygrenetnaidar

elbasU

(%)

Usa

ble

radi

ant e

nerg

y / c

hem

ical

ene

rgy

(%)

1.001.251.501.752.002.252.502.753.003.253.503.75

3 m/s

3 m/s

2 m/s

2 m/s

Fig. 11. Proportions of usable radiant energy to total radiant energy and chemicalenergy at different addition ratios.

10 15 20 250.75

1.00

1.25

1.50

1.75

Hydrogen addition ratio (%)

Ele

ctri

city

out

put(

W)

Syst

em e

ffic

ienc

y(%

)

0.25

0.30

0.35

0.40

0.45

0.50

Fig. 12. Electricity output and system efficiency at different addition ratios (inletvelocity: 3 m/s).

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(3) The mean wall temperature is raised with the increase ofhydrogen mass fraction, resulting in an increment of radiantenergy and the proportion of usable radiant energy to totalchemical energy, as well as the electricity output and systemefficiency.

(4) It is found that the hydrogen addition ratio should be higherthan 10% so as to ensure a good operation effect.

Acknowledgments

This work is supported by National Natural Science Foundationof China (Nos. 51206066 and 51376082), China PostdoctoralScience Foundation (No. 2014M551514), Natural Science Founda-tion of Jiangsu Province (No. BK20131253), Priority AcademicProgram Development of Jiangsu Higher Education Institutions(PAPD), and Scientific Research Starting Foundation for AdvancedTalents of Jiangsu University (No. 11JDG139).

Appendix A. Supporting information

Supplementary data associated with this article can be found inthe online version at http://dx.doi.org/10.1016/j.ces.2015.03.030.

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