Enhancement of hydrogen reaction in a micro-channel by catalyst segmentation

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1 INTERNATIONAL JOURNAL OF HYDROGEN ENERGY 33 (2008) Available at journal homepage: Enhancement of hydrogen reaction in a micro-channel by catalyst segmentation Guan-Bang Chen a,, Yei-Chin Chao b,1, Chih-Peng Chen b a Department of Computer Science and Information Engineering, Diwan University, 87-1, Nansh Li, Madou Jen, Tainan 721, Taiwan, ROC b Institute of Aeronautics and Astronautics, National Cheng Kung University, Tainan 701, Taiwan, ROC article info Article history: Received 31 October 2007 Received in revised form 24 February 2008 Accepted 28 February 2008 Available online 16 April 2008 Keywords: Numerical simulation Multi-segment catalyst Micro-reactor abstract The paper addresses the combustion characteristics of multi-segment catalysts in a microreactor by numerical simulation with detailed heterogeneous and homogeneous chemistries. The effect of multi-segment catalyst is delineated in terms of different catalyst dispositions, different flow conditions and different reactor properties. With a fixed total catalyst length (1 cm), multi-segment catalyst reveals better performance than single catalyst. The space between catalyst segments reduces the inhibition of homogeneous reactions by catalyst and promotes homogeneous reactions in this region since the neighboring catalysts help to maintain a high wall temperature. Therefore, homogeneous combustion can shift upstream with the multi-segment catalyst. The results of different catalyst dispositions show that more catalyst segments has better performance but the catalyst space distance has no obvious effects due to the fast reaction rate of hydrogen. For different flow conditions, the results indicate multi-segment catalyst disposition has better conversion ratio even though there is no homogeneous combustion in the fluid region for fuel-lean condition. The results for different inlet velocities show that multi-segment catalyst has no obvious benefit on lower inlet velocity. However, it can extend the blowout velocity. Finally different reactor dimension and wall material are simulated. Although heterogeneous reactions strengthen in small channel, multi-segment catalyst still has obvious benefit. The results of different wall thermal conductivity do not have obvious difference for multi-segment catalyst. These results can be used in the design of a catalytic micro-reactor for hydrogen/air reactions. & 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. 1. Introduction With increasing demands of micro-devices using MEMS technology, micro-power generation systems and micro-scale combustors with characteristic length scales ranging from few millimeters to sub-millimeters are receiving intensive research interest recently [1,2]. Combustion converts chemical energy of hydrocarbon fuels into thermal energy, which can then be transformed into electricity via mechanical or thermal-electric devices. Even with a relatively inefficient conversion of hydrocarbon fuels, the energy density is still much higher than the most advanced lithium battery. In addition, it can provide longer operational cycles and reduce mass and volume fractions of the power system. Microcombustors can also be used in many specific applications. Examples include a micro-reformer to produce hydrogen for Corresponding author. Tel.: x862; fax: addresses: gbchen@dwu.edu.tw (G.-B. Chen), ycchao@mail.ncku.edu.tw (Y.-C. Chao). 1 Tel.: X63690; fax: /$ - see front matter & 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi: /j.ijhydene

2 INTERNATIONAL JOURNAL OF HYDROGEN ENERGY 33 (2008) fuel cells and other useful chemical species for other purposes, and a micro-thruster for micro- and pico-satellite missions, etc. In practice, there are challenges to maintain a stable combustion inside a micro-reactor. High surface-to-volume ratio of a micro-scale device leads to enhanced heat loss to the surroundings. In addition, thermal and radical quenching on the wall are two primary mechanisms of flame extinction in a micro-combustor. Thermal quenching occurs when too much heat is removed from the flame to sustain combustion. Radical quenching occurs when excessive adsorption of radicals on the walls results in extinction of homogeneous reactions. With suitable thermal management and fine balance between flow residence and chemical times, stable combustion in a micro-reactor has been demonstrated [2 4]. There have been several studies on how to solve these problems and many useful strategies were proposed, such as the Swiss roll and heat-recirculating combustors to reduce heat losses [4,5], and the catalytic combustor to enhance reaction and to suppress radical depletion on the wall [3,6,7]. Catalytic reactions often have lower activation energies than homogeneous reactions. Using the catalyst wall can ensure intensive reaction and can reduce heat loss through the reactor and radical depletion on the wall. Boyarko et al. [8] investigated catalytic combustion of hydrogen oxygen mixture in platinum tubes and Volchko et al. [9] studied catalytic combustion of rich methane/oxygen mixtures in platinum tubes. Although their conditions are based on micro-propulsion applications, their experimental results indicated that catalytic reaction of methane can be selfsustained in a platinum tube with 0.4 mm inner diameter. Being limited by the size, it is very difficult to obtain information inside a micro-reactor by systematic experimental studies, and in fact, most studies acquired only outlet information. On the other hand, numerical simulation provides a convenient and cost-effective approach to study the microcombustion phenomena and mechanism. Maruta et al. [10] numerically studied the extinction limits in a micro-catalytic channel with multi-step reactions. They showed that, for adiabatic walls, the equivalence ratio at the extinction limit monotonically decreased with increasing Reynolds number, while for non-adiabatic conditions, the extinction curve exhibited the U-shaped dual-limit behavior due to heat loss and insufficient residence time compared to chemical time. Raimondeau et al. [11] performed numerical simulations of methane/air flame propagation in a straight tubular microchannel with detailed gas-phase chemistry. It was found that radial gradients and temperature discontinuity on the wall were negligible in very small reactors. The near-entrance heat loss and radical quench on the wall were key factors that control flame propagation inside the micro-channels. Norton and Vlachos [12] conducted two-dimensional CFD simulation to analyze the premixed methane/air flames stability in a micro-combustor, consisting of two parallel, infinitely wide plates of 1 cm length and small separation distance. They studied the effects of micro-combustion dimensions, conductivity and thickness of the wall, external heat losses and operation conditions on the combustion characteristics and flame stability. Norton and Vlachos [13] also reported CFD results on micro-combustion stabilities of propane/air mixtures. Hua et al. [14] performed numerical simulations to study premixed hydrogen air combustion in a series of chambers with various dimensions from millimeter to micrometer levels. Kaisare et al. [15] discussed the appropriate reactor length, wall thickness, and reactor opening size for self-sustained homogeneous combustion in parallel plate channels. An optimum gap width in the range of mm provides the largest operation range of self-sustained combustion. They also found that size effects are stronger for methane than for propane combustion. Karagiannidis et al. [16] numerically investigated the hetero-/homogeneous steady combustion and the stability limits of fuel-lean methane catalytic microreactors at pressure of 1 and 5 bar. They found that gas-phase combustion could be sustained in catalytic micro-reactors with gaps as low as 0.3 mm. While combustion in micro-combustors has been studied, most previous computational studies dealt with heterogeneous or homogeneous reactions separately and concentrated on the extinction limits and flame stabilities. The effects of catalytic walls on combustion characteristics inside micro-channels are still not fully understood nor well documented. Our previous study has systematically discussed the effect of catalyst walls on hydrogen/air combustion in a micro-tube [17]. The aim of this paper is to extend the previous work by studying the effect of different catalyst dispositions. Although a catalyst can be used to maintain reaction and reduce radical depletion, the study of the effect of different catalyst dispositions in a micro-scale reactor is scarce. In this work, an axisymmetric CFD model with detailed multi-step gas-phase and surface phase reaction mechanisms for premixed H 2 /air mixtures is used to investigate the effects of catalyst segmentation on combustion performance in a catalytic micro-reactor. Hydrogen is employed since it has a fast reaction rate, reliable detailed chemical mechanisms and is also an important and clean fuel in the future. 2. Numerical model and chemical mechanism In this work, a commercial code, CFD-ACE [18] is modified to incorporate with detailed gas-phase and surface reaction mechanisms in CHEMKIN formats to simulate the flow and reaction characteristics inside the micro-reactor. For simplicity, the micro-reactor is modeled as a two-dimensional system with a gap width L between the two parallel plates in the numerical simulation. In practical applications, a micro-reactor with large aspect ratio can be fabricated by using MEMS technology. The numerical model consists of two-dimensional Navier Stokes equations, mass and energy conservation equations and species equation for each chemical species. Fig. 1 shows the schematic of the catalytic micro-reactor modeled in this work. The computational domain contains both the gas-phase and the surrounding channel walls. Due to the symmetry, simulations are performed in one half of the micro-channel. The reactor is 3 cm in length and the wall thickness is 0.2 mm. The twodimensional heat conduction equation for the wall is also solved simultaneously in order to have a better description of

3 2588 INTERNATIONAL JOURNAL OF HYDROGEN ENERGY 33 (2008) the wall temperature. For the catalyst bed disposition, most previous researches used single section of uniform catalyst. In this work, effects of catalyst segmentation on fuel reaction enhancement are studied. The inner wall is coated with platinum catalyst and the total catalyst length is 1 cm for cases both with and without catalyst segmentation for comparison. The 1 cm catalyst length is divided into multiple segments and the length of one segment and the space distance (d) between two segments are variable. The boundary conditions are as follows. The concentration of the hydrogen air mixture is specified at the inlet. The inlet temperature for the fuel/air mixture is 300 K. A uniform velocity profile is specified at the inlet and the flow field is laminar for all cases studied. The thermal boundary condition at the wall is the heat loss to the ambient air. The exterior heat loss includes the heat convection by air and thermal radiation. It is described as q 00 ¼ hðt w 300ÞþesðT 4 w 3004 Þ, (1) where the heat transfer coefficient h equals to 20 W=m 2 =Kin the study. T w is the wall temperature and the value of 300 K is specified for ambient air temperature. The wall emissivity e is 0.5 and s is the Stefan Boltzmann constant. At the exit, pressure is specified with a constant ambient pressure of 101 kpa and an extrapolation scheme is used for species and temperature. In the simulations, uniform meshes are used in the preliminary tests and subsequently non-uniform meshes are used with more grids distributed in the reaction region Unifom Inlet 4 q'' = h( T 300) + ( w 4 w εσ T 300 ) 0.2mm Platinum L/2 Wall Fluid d Centerline Outlet near the wall to provide sufficient grid resolution in the computational domain. The number of grids varied depending on the channel dimensions. Grid independence test has been performed and the final grid density is determined when the centerline profiles of temperature and species concentration do not show obvious difference. Under these criteria, a non-uniform mesh with the distribution of grid points in the axial and transverse directions is used for the largest channel. The simulation convergence is decided when the residuals of all governing equations approaches steady states. With the convergent criteria, the results reported in this work are achieved with the residuals smaller than Regarding the gas properties and transport coefficients, the mixture density is calculated using the ideal gas law. The mixture viscosity and thermal conductivity are calculated from a mixture average of species properties and thermal diffusivity is also involved. Chemical reaction mechanisms are used in the gas phase as well as on the catalyst surface of the inner wall. The reaction rate is represented by the modified Arrhenius expression; for the heterogeneous reactions, all temperature exponents are set to zero. The homogeneous reaction mechanism of hydrogen air combustion composed of nine species and 19 reaction steps; these are adopted from the mechanism proposed by Miller and Bowman [19]. The surface reaction mechanism shown in Table 1 is compiled primarily from that proposed by Deutschmann et al. [20]. These reaction mechanisms have been used in previous studies and the comparisons with experimental results are satisfactory [21 23]. For hydrogen fuel, five surface species (H(S), O(S), OH(S), H 2 O(S), PT(S)) describe the coverage of the surface with adsorbed species. PT(S) denotes free surface sites which are available for adsorption. The gasphase chemical kinetics with CHEMKIN format and heterogeneous chemical kinetics with surface CHEMKIN are imported into the code. Details of the chemical reaction rate formulation and CHEMKIN format can be found in the user s manual [24,25]. Fig. 1 Schematic of computational domain. Table 1 Surface reactions of hydrogen/air on platinum Reaction A b E a (KJ/mol) H 2 þ 2PTðSÞ ¼42HðSÞ STICK FORD/PT(S) 1/ 2HðSÞ ¼4H 2 þ 2PTðSÞ 3:7E þ :4 6:0HðSÞ O 2 þ 2PTðSÞ ¼42OðSÞ STICK 2OðSÞ ¼4O 2 þ 2PTðSÞ 3:7E þ :2 60OðSÞ H þ PTðSÞ ¼4HðSÞ STICK O þ PTðSÞ ¼4OðSÞ STICK OH þ PTðSÞ ¼4OHðSÞ STICK H 2 O þ PTðSÞ ¼4H 2 OðSÞ STICK HðSÞ þoðsþ ¼OHðSÞþPTðSÞ 3:7E þ HðSÞþOHðSÞ ¼H 2 OðSÞþPTðSÞ 3:7E þ OHðSÞ ¼H 2 OðSÞþOðSÞ 3:7E þ OHðSÞ ¼4OH þ PTðSÞ 1:0E þ H 2 OðSÞ ¼4H 2 O þ PTðSÞ 1:0E þ A; pre-exponential factor in the Arrhenius expression; b; temperature exponent; E a ; activation energy.

4 INTERNATIONAL JOURNAL OF HYDROGEN ENERGY 33 (2008) experiments simulations Temperature (K) Fuel concentration (%) Fig. 2 Comparisons of predicted and measured outlet temperature (K) for a 1000 lm platinum tube (F ¼ 1, V ¼ 15 m=s). Fig. 3 Computed contours of temperature (K) and fuel mass fraction for different catalyst dispositions (F ¼ 1, V ¼ 20 m=s). 3. Results and discussion 3.1. Model validation First, in order to validate the numerical model and chemical mechanisms used in the study, the simulations for a single platinum micro-tube were performed and the results are compared with experimental results of our previous study (for details of the experiment, see [3]). Fig. 2 shows the predicted outlet temperature of a typical case of velocity 15 m/s in a 1000 mm micro-tube and they agree well with the experimental data for the range of equivalence ratios test. Especially, the simulations also successfully predicted the gas-phase combustions near the exit at higher equivalence ratio cases. Therefore, the agreement suggests the present numerical simulations can be used to investigate the reaction characteristics inside a micro-reactor where experimental measurement is very difficult Combustion characteristics for catalysts of multiple segments In this study, the combustion characteristics of multisegment catalyst dispositions for hydrogen air reactions inside a micro-reactor under different catalyst dispositions, flow conditions and reactor properties are investigated. First, to clearly identify the outstanding combustion characteristics, the results of 1 cm single catalyst and the results of 1 mm catalyst with 10 segments are compared for illustration. The separation distance between segments is 1 mm in the multisegment case. Fig. 3 shows the computed temperature contours and fuel mass fraction contours in the microreactors for the two cases. Fig. 4 shows OH and H mass fraction contours. In these figures, the results of the single catalyst case are shown in part (a) and the results of the multi-segment catalyst case are shown in part (b). For these Fig. 4 Computed contours of OH mass fraction and H mass fraction for different catalyst dispositions (F ¼ 1, V ¼ 20 m=s). cases, stoichiometric fuel/air mixture flows into the reactor of 1 mm channel height and the inlet velocity is 20 m/s. The wall thermal conductivity is 23 W/m/K and it is the same for other figures unless the effect of wall thermal conductivity is investigated. High velocity case is shown here to emphasize the significant differences between these two catalyst dispositions. This velocity far exceeds the flame speed of hydrogen and homogeneous combustion cannot occur in a micro-reactor with inert walls in this condition. However, it is shown that the existence of catalyst walls can effectively extend the blowout velocity of hydrogen in a micro-tube [17]. As shown in the figures, homogeneous combustion occurs in the centerline for both cases but the flame location has large variance, as can be found from high OH concentration regions in Fig. 4. In the single catalyst case, some of the fuel is consumed by heterogeneous reactions inside the catalyst section and there is unburned fuel at the outlet of the catalyst section. The high temperature fuel and products leaving the catalyst section stabilize homogeneous combustion downstream of the catalyst section. The highest temperature and

5 2590 INTERNATIONAL JOURNAL OF HYDROGEN ENERGY 33 (2008) highest fuel consumption gradient are near the catalyst section outlet. Complete combustion is achieved at the axial distance of approximate 1.38 cm from the inlet. This is a traditional case of catalytically stabilized homogeneous combustion. However, in the case of multi-segment catalysts, the homogeneous combustion obviously shifts upstream and complete combustion occurs near the inlet of the channel. The radical distribution shown in Fig. 4 can further identify the difference between these two cases. H radical is the first product in the hydrogen decomposition reaction and it is often used to identify the start of reaction. OH concentration contours can be used to identify whether combustion occurs. High OH concentration can be used to mark the reaction zone and high temperature regions. For the single catalyst case, the OH concentration inside the catalyst section is very sparse since OH species is produced near the wall and has high absorption ability. The highest OH and H concentrations are found behind the catalyst section near the homogeneous reaction region. High H concentration layer can be found to extend from the wall near the catalyst section outlet to the centerline to connect the high OH concentration region in the center. For the multi-segment catalyst case, the high H concentration layer is seen to extend from the first catalyst segment gap and similarly it extend to the centerline to connect the high OH concentration region. This radical behavior clearly shows the phenomenon of catalytically stabilized homogeneous combustion in the micro-channel. From Fig. 4 it obviously shows that different wall catalyst dispositions make the homogeneous combustion shift upstream. Furthermore, the multi-segment catalyst disposition can help to maintain the wall temperature and reduce heat loss to the surroundings. Therefore, the wall will have a higher and uniform temperature distribution. This characteristic is advantageous when used in the design of a reactor for micro-thermophotovoltaic system. To more clearly identify the reaction characteristics of multi-segment catalyst disposition, Fig. 5 shows the fuel and OH mass fraction distributions along the axial direction close to the inner wall. For the single catalyst case, the fuel consumption shows two distinct characteristics. The first smooth decrease region is caused by catalytic combustion and it is obvious under the kinetic controlled region. The reaction in the catalyst section is slow, the amount of fuel consumed is small and the wall temperature is low (see Fig. 3). Owing to the homogeneous combustion in the second characteristic region, the fuel concentration near the wall then abruptly decreases. For the multi-segment catalyst case, the fuel is significantly decreased at the first catalyst gap and then smoothly decreases due to the occurrence of the homogeneous reaction between the catalyst gaps. At these gaps, high OH concentration peaks are found. The radicals produced here will convect downstream and induce homogeneous combustion in the centerline of the channel, see Fig. 4. The existence of heterogeneous reaction has two effects on homogeneous combustion. On the one hand, heterogeneous reaction will consume part of the fuels in the upstream region. The reduced fuel concentration and the reaction product will degenerate the homogeneous reaction. On the other hand, for the micro-scale combustor, high temperature OH mass fraction 4 1cm PT 1mm PTx cm PT 1mm PTx10 and radicals produced on the wall by heterogeneous reactions will enhance gas-phase reaction in the volume and help to overcome heat loss from the gas phase. These two competing effects will determine whether gas phase combustion can occur inside the micro-catalytic combustor. From the results of these figures, even in the high velocity condition (20 m/s in our cases), homogeneous combustion can exist near the entrance of the micro-channel by changing the catalyst disposition. In addition, the performance of the multisegment catalyst is superior to that of the single catalyst Combustion characteristics under different catalyst disposition types 0.03 Fig. 5 Fuel mass fraction profile and OH mass fraction profile along the wall for different catalyst dispositions. In this study, variations of multi-segment dispositions are also tested and three of them are shown here as representative cases. These three types are 1 mm catalyst with 10 segments, 2 mm catalyst with five segments and 5 mm catalyst with two segments. The channel separation distance (L) is 1 mm and the catalyst gap (d) is 1 mm. The effect of gap distance (d) will be discussed in the next paragraph. The single catalyst result is also shown here for comparison. Fig. 6 shows the fuel mass fraction and OH mass fraction

6 INTERNATIONAL JOURNAL OF HYDROGEN ENERGY 33 (2008) cm PT 1mm PTx10 2mm PTx5 5mm PTx mm PTx10 2mm PTx5 5mm PTx2 OH mass fraction cm PT 1mm PTx10 2mm PTx5 5mm PTx Fig. 6 Fuel mass fraction profile and OH mass fraction profile along the centerline for different catalyst dispositions Fig. 7 Fuel mass fraction along the wall for different catalyst dispositions. 5 5 d=1mm d=2mm d=3mm distributions along the centerline for different catalyst dispositions. In Fig. 6, 1 mm catalyst type has the best performance and the homogeneous combustion shifts downstream with increase of catalyst segment length. When the catalyst segment length is increased, the inhibition effect on homogeneous reaction increases. Moreover, the increase of the catalyst segment length also shifts the catalyst gap and the intensive homogeneous reaction in the first gap downstream. Fig. 7 shows the fuel mass fraction close to the inner wall. The location of the highest fuel decrease always occurs in the first catalyst gap for every catalyst disposition type. It implies that there is strict homogeneous reaction in the gap region. In the catalyst gaps, the produced high temperature radicals will move downstream and it is the homogeneous reaction in the catalyst gap that promotes global homogeneous combustion in the central region of the channel. Next, the catalyst space distance (d) is also investigated in this study. Fig. 8 shows fuel mass fraction and OH mass fraction along the centerline for different separation distances. Here, 10 segments of 1 mm catalyst are used and the space distances are 1, 2 and 3 mm, respectively. In Fig. 8, the fuel mass fraction distributions for three cases almost OH mass fraction d=1mm d=2mm d=3mm Fig. 8 Fuel mass fraction profile and OH mass fraction profile along the centerline for different catalyst space distance.

7 2592 INTERNATIONAL JOURNAL OF HYDROGEN ENERGY 33 (2008) overlap. As to the OH mass fractions, there is only minor deviation between cases. These results indicate that the gap distance does not obviously affect the reaction. The catalyst gap can maintain high temperature due to the heterogeneous reactions in the neighboring catalytic surface. In addition, hydrogen has a fast reaction rate, therefore, light-off can occur in a short residence time. Short catalyst gaps also indicate that the reactor length can be reduced and still has a better performance than the traditional single segment catalyst Combustion characteristics under different flow conditions To further examine the operational characteristics of multisegment catalyst disposition, different fuel concentrations and inlet velocities are studied in this work. For the fuel concentration, fuel-lean conditions are simulated and an equivalence ratio of 0.3 is shown as a representative case. Fig. 9 shows the computed temperature, fuel and OH mass fraction contours. Part (a) is for the single catalyst and part (b) is for the multi-segment catalyst ð1mm 10Þ. Although the equivalence ratio is fuel-lean, the fuel still cannot be completely consumed in both cases. This is due to the low surface reaction rate and low wall temperature for lean catalytic reaction to sustain the global homogeneous reaction in the volume. The temperature is lower and more uniform for multi-segment catalyst since the fuel is consumed in a longer distance. It is a benefit to avoid high wall temperature and to extend operation conditions. For single catalyst, fuel consumption is primarily by heterogeneous reactions and there is no central homogeneous reaction to further consume the residual fuel leaving the catalyst section. The overall fuel conversion ratio of the reactor is about 71%. For multisegment catalyst, the catalyst was dispersed in a longer distance. The fuel is not only consumed by heterogeneous reactions but also consumed by homogeneous reaction in the catalyst gaps. There is an obvious decrease of fuel mass fraction in the near wall region. Homogeneous reactions occur in the catalyst gaps and generate high OH concentration. However, unlike the case of stoichiometric fuel, the residual fuel is too lean to result in global homogeneous combustion in the flow field. The near wall reactions also consumed much fuel and the overall fuel conversion ratio is increased to about 92%. Besides, the wall has a more uniform temperature distribution. From these results, it is obvious that multi-segment catalyst dispositions still have better performance even in fuel-lean conditions. Without changing the catalyst length and the total reactor length, the multisegment catalyst exhibits better reaction characteristics even though homogeneous combustion cannot occur inside the micro-reactor. In a catalytic micro-reactor, the inlet velocity is another important operation parameter and it can be operated in a larger range than that in a traditional combustor. Fig. 10 shows the fuel mass fractions along the centerline for different inlet velocities. The inlet velocities are varied from 5 to 40 m/s and some representative cases are shown here. In Fig. 10, for smaller inlet velocity (5 m/s), heterogeneous reaction is weak and homogeneous combustion occurs near the flow entrance. The fuel mass fraction abruptly decreases close to the entrance due to the occurrence of the homogeneous combustion. Therefore, the effect of catalyst gap on enhancing reaction in the micro-reactor is not obvious and there is no obvious difference between single and multisegment catalysts. The fuel mass fraction distributions under 5 m/s inlet velocity for different catalyst dispositions almost overlap (not shown). For higher velocities, multi-segment 5 5 1cm PT,5m/s 1cm PT,20m/s 1cm PT,40m/s 1mm PTx10,5m/s 1mm PTx10,20m/s 1mm PTx10,40m/s Fig. 9 Computed contours of temperature, fuel mass fraction and OH mass fraction with equivalence ratio 0.3 and inlet velocity 20 m/s: (a) 1 cm PT; (b) 1 mm PT Fig. 10 Fuel mass fraction profile along the centerline for different inlet velocities.

8 INTERNATIONAL JOURNAL OF HYDROGEN ENERGY 33 (2008) catalyst has specific effects on enhancing and sustaining homogeneous combustion inside the catalytic micro-reactor. When the inlet velocity increases, heterogeneous reactions are strengthened and the advantage of multi-segment catalyst appears. The fuel consumption profile shows two different slopes. The first slope is smooth and the region is dominated by heterogeneous reaction. The second slope of sudden decrease of fuel mass fraction is due to homogeneous combustion. For single catalyst under higher inlet velocities, the homogeneous combustion shifts downstream and always occurs downstream after the catalyst exit. However, for multisegment catalyst, increasing flow rate represents smaller residence time, which is detrimental for complete reaction but the existence of the catalyst gaps will enhance the reaction and shift the homogeneous combustion upstream even though the inlet velocity increases to as high as 40 m/s. From these results, the catalyst wall can extend the blowout velocity of hydrogen/air reaction inside a micro-reactor and the multi-segment catalyst is an even preferable choice. To further examine the effect of multi-segment catalyst under different reactor properties, different channel separation distance and different wall materials are studied in this work. The channel separation distances are reduced from 1 to 0.2 mm. Fig. 11 shows the fuel mass fraction distributions along the centerline for different separation distances. The results of single catalyst are shown here for comparison. The inlet velocity is 20 m/s. When the separation distance decreases, heterogeneous reactions are enhanced since fuel in the bulk gas needs less diffusive time to reach the catalytic surface. The homogeneous combustion shifts upstream and becomes weaker with decrease of channel separation distance. In general, the multi-segment catalyst has superior performance than the single catalyst. When the channel separation distance decreases to 0.2 mm, the multi-segment catalyst still exhibits better reaction performance than single catalyst. Finally, different wall materials are modeled to investigate the effect of wall thermal conductivity on multi-segment catalyst performance. In this case, the channel separation distance is 1 mm and the wall thickness is 0.2 mm. The inlet velocity is 20 m/s. Three kinds of wall thermal conductivity are selected from low to high value and they are 78, 23 and 3.3 W/m/K, respectively. This range covers many practical materials for catalyst micro-reactors. In the micro-reactor without catalytic walls, it has been shown that the wall material is critical to ignition and flame stability inside the channel. However, in the micro- reactor with catalytic walls, the wall material is not as critical to ignition and flame stability since the heat to ignite the homogeneous reaction is 3.5. Combustion characteristics under different reactor properties 5 k=3.3 W/m/K k=23 W/m/K k=78 W/m/K Fig. 12 Fuel mass fraction profile along the centerline for different wall thermal conductivity values cm PT,L=0.2mm 1cm PT,L=0.5mm 1cm PT,L=1mm 1mm PTx10,L=0.2mm 1mm PTx10,L=0.5mm 1mm PTx10,L=1mm 5 5 k=3.3 W/m/K k=23 W/m/K k=78 W/m/K Fig. 11 Fuel mass fraction profile along the centerline for different channel gap widths. Fig. 13 Fuel mass fraction profile along the wall for different wall thermal conductivity values.

9 2594 INTERNATIONAL JOURNAL OF HYDROGEN ENERGY 33 (2008) primarily from the catalytic reaction on the wall, not from the upstream heat conduction [17]. As for the micro-reactor with multi-segment catalysts, Figs. 12 and 13 show the fuel mass fraction distributions along the centerline and along the axial direction close to the wall for different wall thermal conductivity values. For high wall thermal conductivity, homogeneous combustion shifts upstream slightly but the difference is insignificant. From Fig. 13, the three fuel distributions are almost the same except for the slight differences in the initial catalyst gaps. Since heterogeneous reactions in the catalyst help to maintain high temperature in the catalyst gap, the effect of different wall thermal conductivities on the micro-reactor performance with multi-segment catalyst is not as pronounced. 4. Conclusions In this paper, the benefits of multi-segment catalyst for a micro-reactor are investigated by numerical simulation with detailed heterogeneous and homogeneous chemistries of hydrogen/air reactions. Specially, the combustion characteristics of multi-segment catalyst are studied in terms of different catalyst dispositions, different flow conditions and different reactor properties. The following findings are obtained from this study. 1. With a fixed total catalyst length (1 cm), the multi-segment catalyst reveals better performance than the single catalyst. Heterogeneous reactions help to maintain a high wall temperature and the gap space between catalyst segments reduces the inhibition effect of the catalyst and promotes homogeneous reactions in this region. Therefore, homogeneous combustion shifts upstream. In addition, smaller catalyst length has better performance and the catalyst space distance has no obvious effects due to the fast reaction rate of hydrogen. 2. Even though there is no global homogeneous combustion in the volume for fuel-lean conditions, the multi-segment catalyst disposition still has better performance due to the fact that much of the fuel is consumed by homogeneous reactions in the gap regions between catalyst segments. Different inlet velocities show that multi-segment catalyst disposition has no obvious effect on lower inlet velocities but it can extend the blowout velocity when compared to single catalyst case. 3. Although heterogeneous reaction is enhanced in a smaller channel, multi-segment catalyst still exhibits better performance. The results for different wall thermal conductivity values show that wall materials do not have an obvious effect on multi-segment catalyst. In general, no matter whether homogeneous combustion occurs or not, the reactor with multi-segment catalyst reveals better reaction performances than the one with traditional single catalyst. Using multi-segment catalyst disposition for hydrogen/air reactions, the reactor dimension can be significantly reduced and its characteristics of uniform and high wall temperature distribution are advantageous for the reactor design of a micro-thermophotovoltaic system. Acknowledgments The support of the computer time and the numerical packages provided by the National Center for High-Performance Computing, Taiwan (NCHC-Taiwan) throughout the research phase are sincerely acknowledged. R E F E R E N C E S [1] Sirignano WA, Pham TK, Dunn-Rankin D. Miniature scale liquid-fuel film combustor. In: Proceedings of the 29th symposium (international) on combustion. Pittsburgh: The Combustion Institute; p [2] Fernandez-Pello AC. Micro-power generation using combustion: issues and approaches. In: Proceedings of the 29th symposium (international) on combustion. Pittsburgh: The Combustion Institute; p [3] Chao YC, Chen GB, Hsu CJ, Leu TS, Wu CY, Cheng TS. 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