Effects of catalytic walls on hydrogen/air combustion inside a micro-tube

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1 Applied Catalysis A: General 332 (2007) Effects of catalytic walls on hydrogen/air combustion inside a micro-tube Guan-Bang Chen a, Chih-Peng Chen b, Chih-Yung Wu b, Yei-Chin Chao 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 Received 19 May 2007; received in revised form 2 August 2007; accepted 2 August 2007 Available online 10 August 2007 Abstract Numerical simulations with detailed heterogeneous and homogeneous chemistries of hydrogen/air mixture reactions inside a catalytic microtube were performed. The characteristics of heterogeneous and homogeneous interaction are delineated in terms of flow velocity, tube diameter and wall thermal conductivity. With the catalytic wall, the homogeneous combustion is obviously weakened. The heterogeneous reactions will consume part of the fuel near the entrance and will make the homogeneous combustion region shift downstream. The micro-tube can be divided into two regions. The upstream region is dominated by the heterogeneous reaction and the downstream region is dominated by the homogeneous combustion. With increasing inlet velocity, the region dominated by heterogeneous reactions expanded downstream and finally occupied the whole tube. Decreasing the tube diameter enhanced the heterogeneous reactions. However, the increased heat released at the wall will be beneficial for homogeneous combustion. The maximum allowed inlet velocity for homogeneous combustion first increased and then decreased to almost zero at 0.2 mm tube. This means that homogeneous combustion can not be sustained inside such a small catalytic micro-tube. Finally, three different wall materials are simulated. Higher wall temperature gradient for lower wall thermal conductivity will promote the homogeneous combustion shift upstream and will have a wider temperature distribution. Since the homogeneous combustion is ignited and sustained by the heat from the heterogeneous reaction, the effect of wall thermal conductivity is not as obvious as it is in the micro-tube without catalyst walls. # 2007 Elsevier B.V. All rights reserved. Keywords: Numerical simulation; Hydrogen; Catalyst; Micro-tube 1. Introduction Micro-scale combustors for micro-power generation and micro-propulsion systems are receiving intensive attention and interest in the combustion research community recently [1,2]. When compared with the traditional large-scale combustors, a micro-combustor can be defined as a reactor with the characteristic length scale located between sub-millimeter and micrometer. For power generation, micro-scale combustion of conventional fuels can usually produce much higher power density than the most advanced lithium batteries. In addition, it can provide longer operational cycles and can reduce the mass and volume fractions of the power system. Micro-reactors can also have lower manufacturing cost than their larger counterparts. Since it is likely that the micro-scale combustion will operate at a lower temperature, it is expected to produce less NO x * Corresponding author. Tel.: x63690; fax: address: ycchao@mail.ncku.edu.tw (Y.-C. Chao). emissions. Micro-combustors can also serve as a micro-reformer to produce hydrogen for fuel cells or many useful chemicals for other purposes. For micro- and pico-satellite missions, microscale combustion can be used in the micro-propulsion systems, which are designed and manufactured using micro-electromechanical system (MEMS) technologies. When the size of a conventional combustor is reduced to submillimeter scale, thermal and radical quenching effects become the outstanding issues in maintaining intensive reactions in the combustor. The feature of high surface-to-volume ratio of the micro-scale devices poses specific challenges to researchers and designers in the design of micro-combustors. With suitable thermal management and fine balance between flow residence and chemical times, stable combustion in a micro-reactor has been demonstrated [2 4]. Many useful strategies were proposed, such as the Swiss roll and heat-recirculating combustor 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 X/$ see front matter # 2007 Elsevier B.V. All rights reserved. doi: /j.apcata

2 90 G.-B. Chen et al. / Applied Catalysis A: General 332 (2007) intensive reaction and can reduce heat loss through the reactor and radical depletion on the wall. Being limited by the size, it is very difficult to obtain information inside a micro-reactor by systematic experimental studies, and in fact, most past works acquired only outlet information. Unlike the difficulties encountered in experimental measurements, numerical simulation provides a convenient and cost-effective approach to study the micro-combustion phenomena and mechanism. Maruta et al. [8] computationally 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 nonadiabatic conditions, the extinction curve exhibited U-shaped dual-limit behavior due to heat loss and insufficient residence time compared to chemical time. Raimondeau et al. [9] performed numerical simulations of methane/air flame propagation in a straight tubular micro-channel with detailed gasphase chemistry. It was found that radial gradients and temperature discontinuity at 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 [10] 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 microcombustion dimensions, of conductivity and thickness of the wall, of external heat losses and of operating conditions on the combustion characteristics and flame stability. Norton and Vlachos [11] also reported CFD results on the microcombustion stability of propane/air mixtures. Hua et al. [12] performed numerical simulations to study the combustion of premixed hydrogen air mixtures in a series of chambers with various dimensions from millimeter to micrometer level. While combustion in micro-combustors has been studied, the effects of catalytic walls on combustion characteristics inside micro-channels are still not fully understood nor well documented. Most previous computational studies dealt with heterogeneous or homogeneous reactions separately and concentrated on the extinction limit or flame stability. Although a catalyst is used to maintain reaction and reduce heat loss, the interaction between heterogeneous and homogeneous reactions in a micro-scale reactor is still not well understood. In this work, an axisymmetric CFD model of a catalytic micro-tube with detailed multi-step gas phase and surface phase reactions for premixed H 2 /air mixtures is used to investigate the effects of catalytic wall on homogeneous combustion inside a microtube. The simplicity in micro-tube geometry facilitates simple practical flow systems for fundamental research on reaction interaction. Hydrogen is employed since it has a fast reaction rate and reliable detailed chemical mechanisms. 2. Numerical model and chemical mechanism In this work, a commercial code, CFD-ACE [13], is modified to incorporate with detailed gas-phase and surface Fig. 1. Schematic of computational domain. reaction mechanisms in Chemkin formats to simulate the flow and reaction characteristics inside the catalyst tube. The tube geometry is simple for fundamental studies and is easy to fabricate. The channel is modeled as a cylindrical tube with 1 cm in length. The numerical model consists of axisymmetric Navier Stokes equations, mass and energy conservation equations and one species equation for each chemical species. The geometry of the catalytic micro-tube is shown in Fig. 1 and simulations of one radius of the micro-tube are performed due to the axial symmetry. In the simulation, the channel wall is assumed thermally thin so that the axial heat conduction along the wall is neglected. Axial heat conduction within the wall may have a significant effect and will be considered in some later study cases. 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 is specified at the inlet and the flow 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 the air and radiation. It is described as: q 00 ¼ hðt w 300ÞþesðT 4 w 3004 Þ (1) where the heat transfer coefficient h is 20 W/m 2 /K in the study and T w is the wall temperature. The wall emissivity e is 0.5 and s is the Stefan-Boltzmann constant. At the exit, pressure is specified at a constant pressure of 101 kpa and an extrapolation scheme is used for species and temperature. The far-field boundary condition is not adopted at the exit. Because once the reaction zone shifts past approximately half the length of the channel, it may no longer describe the system properly [11]. Initially, uniform meshes are used in the preliminary tests and subsequently non-uniform meshes are used with more grids distributed in the reaction region to provide sufficient grid resolution in the computational domain. The number of grids varied depending on the tube diameter. Grid independence check has been performed and a non-uniform mesh with the distribution of grid points in the axial and radial directions is used for the largest tube. The simulation convergence is decided when the residuals of 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 by equations of state. The mixture viscosity, specific heat and thermal conductivity are calculated from a mass average of species properties and thermal diffusivity is also involved. Chemical reaction mechanisms are used in the gas phase and on the wall. The tube is modeled with the inner wall coated with

3 G.-B. Chen et al. / Applied Catalysis A: General 332 (2007) Table 1 Surface reactions of hydrogen on Platinum Reaction A b E a (KJ/mol) H 2 + 2PT(S)! 2H(S) STICK FORD/PT(S) 1/ 2H(S)! H2 + 2PT(S) 3.7E H(*) O 2 + 2PT(S)! 2O(S) STICK 2O(S)! O 2 + 2PT(S) 3.7E O(*) H + PT(*)! H(S) STICK O + PT(*)! O(S) STICK OH + PT(*)! OH(*) STICK H 2 O + PT(*)! H 2 O(S) STICK H(*) + O(*) = OH(*) + PT(S) 3.7E H(*) + OH(*) = H2O(*) + PT(S) 3.7E OH(*) = H 2 O(*) + O(*) 3.7E OH(*)! OH + PT(S) 1.0E H 2 O(*)! H 2 O + PT(S) 1.0E A: pre-exponential factor in the Arrhenius expression; b: temperature exponent; E a : activation energy. platinum. The homogeneous reaction mechanism of hydrogen air combustion composes of 9 species and 19 reaction steps; these are adopted from the mechanism proposed by Miller and Bowman [14]. The surface reaction mechanism shown in Table 1 is compiled primarily from that proposed by Deutschmann et al. [15]. These reaction mechanisms have been used in the previous study and the comparisons with experimental results are satisfactory [16,17]. 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 gas-phase 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 [18,19]. 3. Results and discussion 3.1. Combustion characteristics for different reaction models In this study, the interaction between heterogeneous and homogeneous reactions of hydrogen air inside a catalytic micro-scale reactor is the main issue. First, to clearly identify and to highlight the effects of heterogeneous reactions, we apply three different reaction modes by different combinations of the homogeneous and heterogeneous reaction mechanisms. They are both homogeneous and heterogeneous, homogeneousalone and heterogeneous-alone mechanisms, respectively. Fig. 2 shows the computed temperature contours and OH concentration contours. Fig. 3 shows the fuel and H 2 O concentration contours. For this case, a stoichiometric fuel/ air mixture flows into a 1 mm tube and the inlet velocity is 2 m/ s, which is close to the flame speed of hydrogen. There are obvious differences in these figures. In the homogeneous-alone case, the highest temperature and highest OH concentration are in the fluid region and the flame structure displays a cone shape. The heat is dissipated by the wall and the temperature is decreased along the radial direction. On the assumption of an inert wall, the OH concentration near the wall reaches a certain value. OH concentration contours can be used to identify whether combustion occurs inside the micro-tube. High OH concentration can be used to mark the reaction zone and high temperature regions. For the heterogeneous-alone case, reactions can only occur on the wall. Therefore, the highest temperature is found on the wall near the inlet and heat transports downstream by convection. The highest temperature is also lower than these for the homogeneous-alone case or the complete case. The highest OH concentration also appears on the wall and its strength is apparently weak since OH species has high absorption ability. The difference of OH concentration also reveals the variation of heterogeneous reaction and homogeneous reaction. For the complete case, the high temperature regions exist both on the wall near the entrance and in the fluid region. The high temperature zone is not simply heat convection from the wall and both homogeneous and heterogeneous reactions exist inside the catalyst tube. The homogeneous reaction occurs in the fluid region but it shifts slightly downstream when compared with the homogeneousalone case (see the OH concentration). To some extent, the homogeneous reaction is obviously affected by the presence of heterogeneous reactions. In Fig. 3, the fuel concentration also exhibits this phenomenon. Once homogeneous combustion occurs, the fuel concentration displays a sharp decrease in the axial direction. In the complete case, the fuel along the centerline can move farther downstream than it can in the heterogeneous case. It seems that homogeneous combustion is delayed by the heterogeneous reactions. However, in the heterogeneous-alone case, since fuel is only consumed on the wall, fuel must diffuse to the wall before consumption. The axial fuel concentration distribution depicting the fuel consumption along the axial direction is smoother when compared to that in the fuel concentration across the reaction zone for the other two cases. Several authors have identified that the homogeneous reaction will be inhibited by catalyst, especially in fuel lean conditions. From the results of Figs. 2 and 3, even in the high fuel concentration condition (F = 1 in our cases), the effect of catalytic inhibition still exists. The product concentration contour also exhibits some differences among these reaction

4 92 G.-B. Chen et al. / Applied Catalysis A: General 332 (2007) Fig. 2. The computed temperature contour and OH concentration contour for three different reactions: both homogeneous and heterogeneous, homogeneous-alone and heterogeneous-alone. models. In particular, our simulations show that the homogeneous reaction can not be ignored inside a micro-catalyst reactor for most conditions Effect of flow velocity In a catalytic micro-combustor, the inlet velocity is an important operation parameter and it can be operated over a larger range than is possible in a traditional combustor. Figs. 4 6 show the temperature, OH mass fraction and fuel mass fraction along the central axis for different inlet velocities. The inlet velocities are from 2 to 20 m/s and some representative cases are shown here. The tube diameter is 1 mm. In Fig. 4, the temperature increases along the central axis due to the heat released by reactions. For smaller inlet velocities, the temperature sharply increases and the fuel abruptly decreases close to the entrance due to the occurrence of the homogeneous combustion. When the inlet velocity Fig. 3. The computed fuel and H 2 O concentration contours for three different reactions: both homogeneous and heterogeneous, homogeneous-alone and heterogeneous-alone.

5 G.-B. Chen et al. / Applied Catalysis A: General 332 (2007) Fig. 4. Temperature profile along the central axis for different inlet velocities. increases, the temperature increase and the fuel consumption show two different slopes. These two slopes stand for two different reaction modes. The first slope is smoother and the region is dominated by heterogeneous reaction. Since fuel consumption is dominated by diffusion, the heat is transported from the wall to tube center by convection and the temperature increase is smooth. However, the second slope is due to homogeneous combustion. Since homogeneous combustion occurs in the tube center and the chemical time scale is much smaller than the flow time scale, the temperature increase and the fuel consumption are intense. From these results, it is obvious that the reaction inside the catalyst tube can be divided into two sections. The forward region is dominated by heterogeneous reaction and the rearward one is dominated by homogeneous reaction. At the lower inlet velocity, heterogeneous reaction is weak and homogeneous combustion Fig. 5. Fuel mass fraction profile along the central axis for different inlet velocities. Fig. 6. OH mass fraction profile along the central axis for different inlet velocities. occurs near the flow entrance. The temperature first sharply increases and then decreases along the axial direction. The heat loss effect is relatively obvious in low flow-rate cases. When the inlet velocity increases, the residence time decreases and the heat transport to the downstream increases. In Fig. 5, when the inlet velocity increases to 10 m/s, homogeneous combustion still maintains near the tube exit. This inlet velocity much exceeds the hydrogen flame speed (2 m/s). To attain complete combustion inside a micro-reactor depends on the ratio between the flow residence time and the chemical reaction time (the Damköhler number). Increasing flow rate represents smaller residence time, which is detrimental for a complete reaction. However, the existence of a catalyst wall will extend the homogeneous combustion region and this will help to reach complete reaction in the micro-tube. When the inlet velocity increases further to exceed 10 m/s, the temperature along the central axis increases smoothly in general. The fuel consumption does not have a sharp decrease. At this condition, homogeneous combustion can not occur inside the catalyst reactor and the whole reactor is dominated by heterogeneous reaction. In Fig. 6, the peak OH concentration decreases and shifts downstream with increase of the inlet velocity. When the inlet velocity exceeds 10 m/s, OH concentration will substantially decrease. The existence of a heterogeneous reaction has two effects on homogeneous combustion. On the one hand, the heterogeneous reactions will consume part of the fuels in the upstream region. The reduced fuel concentration and the reaction product will weaken the homogeneous reaction. On the other hand, for the micro-scale combustor, high temperature and radicals produced on the wall by heterogeneous reactions will help to enhance gas phase reaction and to overcome heat losses from the gas phase. Therefore, the homogeneous combustion can be sustained inside the micro-reactor with the size smaller than the quench distance. These two competing effects will determine whether gas phase combustion can occur inside the micro-catalytic combustor. To help us further examine the effects of inlet velocity on the micro-reactor performance, Fig. 7 shows the

6 94 G.-B. Chen et al. / Applied Catalysis A: General 332 (2007) Fig. 7. Average temperature and fuel conversion ratio at the outlet for different inlet velocities. average temperature and fuel conversion at the outlet. When homogeneous combustion occurs near the entrance for smaller inlet velocities, the outlet temperature is low due to the heat loss. As the inlet velocity increases, homogeneous combustion moves downstream and the outlet temperature increases. As shown in the figure, if homogeneous combustion occurs inside the catalyst tube, almost complete combustion can be attained. When the inlet velocity exceeds 10 m/s and homogeneous combustion can not be maintained inside the micro-tube, the outlet temperature and the fuel conversion ratio at the tube exit obviously decrease. Nevertheless, the exit residual fuel and the high temperature from the catalytic micro-tube will help to stabilize the gas-phase combustion at the tube exit [3] Effect of tube diameter Fig. 8. OH concentration contour along the tube center for different tube diameters (V = 5 m/s). cannot be sustained inside the micro-reactor. When the tube diameter decreases, the maximum allowed inlet velocity will first increase. When the diameter decreases to 0.6 mm, homogeneous combustion can even be sustained at the inlet velocity of 18 m/s. This velocity value much exceeds the hydrogen flame speed and the catalyst wall exhibits a good stabilization for homogeneous combustion. However, when the tube diameter decreases to 0.4 mm, the maximum allowed velocity also decreases to 4 m/s. This result clearly indicates that the inhibition effect of heterogeneous reaction surpasses the enhancement effects of heat and radical generation on the homogeneous combustion. When the tube diameter further decreases to 0.2 mm, homogeneous combustion can not occur inside the catalyst tube for any cases of inlet velocity in our simulation. To further examine the effect of tube size, we studied five different tube diameters in this work. They are from 1 to 0.2 mm. Fig. 8 shows the OH concentration contour along the tube center for different tube diameters. The inlet velocity is 5 m/s. In the figure, the OH concentration shifts upstream and becomes weaker with the decrease of tube diameter. This is due to the enhancement of heterogeneous reaction. The heterogeneous reaction is affected by the mass diffusion time. The mass diffusion time can be estimated by the following equation t D ¼ R2 D AB (2) where D AB, and R are the mass diffusivity, and the diffusive length. For smaller diameter tubes, the fuel in the bulk gas needs less diffusive time to reach the catalytic surface. The heat and radicals produced also help to maintain homogeneous combustion with higher inlet velocities. Fig. 9 shows the maximum allowed inlet velocity for homogeneous combustion inside the micro-reactor with different tube sizes. Once the inlet flow rate exceeds these values, homogeneous combustion Fig. 9. The maximum allowed inlet velocity for homogeneous combustion inside the micro-reactor with different tube sizes.

7 G.-B. Chen et al. / Applied Catalysis A: General 332 (2007) Fig. 11. The computed OH mass fraction contours for both homogeneous and heterogeneous (top) and heterogeneous-alone reaction (bottom): D = 0.4 mm, V = 5 m/s. This can be also identified by the substantial decrease of OH concentration in the centerline. In a micro-catalytic reactor, the interaction between heterogeneous and homogeneous reactions can be divided into three types. In the first type, the homogeneous combustion is weakened by the presence of the catalyst but it can be sustained over a large inlet velocity range, as mentioned in the previous section. Temperature, major species and radicals have obvious differences depending on whether homogeneous reactions are included or not. The second type of interaction is shown in Figs. 10 and 11. Fig. 10 shows the temperature, H 2 mass fraction and H 2 O mass fraction contours. Fig. 11 includes the OH mass fraction contours. In this case, the tube diameter is 0.4 mm and the inlet velocity is 5 m/s. In these figures, the results of computation using both heterogeneous and homogeneous reaction mechanisms are shown on the top and the results using only heterogeneous reaction mechanism are shown at the bottom. The results are almost identical for temperature and major species contours whether homogeneous reactions are included or not. The homogeneous reactions are obviously inhibited by catalyst wall and they have little effect on the spatial distribution of temperature and major species. However, homogeneous reactions still have significant effects on OH radical concentration distribution, as shown in Fig. 11. Finally, the third type of interaction is that homogeneous reactions can be completely neglected. The whole tube is dominated by heterogeneous reactions. In summary, these three characteristic reaction types can be schematically depicted in terms of tube diameter and flow velocity in Fig. 12. In Fig. 12, reaction regions are divided by lines A and B into three characteristic regions. In region I when the velocity is low and the diameter is relatively large, homogeneous combustion is affected by heterogeneous reactions and a homogeneous reaction can still be sustained inside the tube. The homogeneous reaction rate obviously surpasses the catalytic reaction rate. When the velocity is increased or the diameter is reduced, the characteristic reaction in the tube moves into region II where the surface catalytic reaction rate is enhanced and exceeds homogeneous reaction, and thus the catalytic reaction becomes the dominant reaction in the tube. Homogeneous combustion can not be sustained inside the tube but gas phase reactions still affect the expression of intermediate species. In this region, the catalytic reaction rate exceeds the homogeneous reaction rate. When operation conditions are in region III, with very high inlet velocity and relatively large tube diameter, the homogeneous reaction is inhibited to have almost no effects. Fig. 10. The computed temperature, H 2 mass fraction and H 2 O mass fraction contours for complete (top) and heterogeneous-alone reaction (bottom): D = 0.4 mm, V = 5 m/s. Fig. 12. Three reaction regions for the effect of heterogeneous reaction on homogeneous reaction.

8 96 G.-B. Chen et al. / Applied Catalysis A: General 332 (2007) Effect of wall material The effect of heat conduction through the tube wall for a catalytic micro-reactor was also investigated. In this case, the tube diameter is 1 mm and the wall thickness is 0.1 mm. The inner surface of the wall is coated with platinum. The inlet velocity is 2 m/s. Three kinds of characteristic wall materials are selected from practical considerations. They are platinum, silicon and alumina. These materials are often used in the catalyst reactors and their heat conductivities are close to 78, 23and 3.3 W/m/K, respectively. In the micro-reactor with non-catalytic walls, it has been shown in the literature that the wall material is critical to ignition and flame stability inside the channel. The wall thermal conductivity has two competing effects. On the one hand, the heat conduction in the wall will provide a convenient route for heat transfer from the post-combustion region to preheat the unburned mixture in the upstream region. On the other hand, the radial heat conduction to the environment will delay flame Fig. 14. OH mass fraction profile along the central axis for different wall thermal conductivity values. ignition and even cause extinction. The reported simulation results indicated that moderate wall thermal conductivity is essential for flame ignition and stabilization near the microreactor entrance [10,11]. As for the micro-catalytic reactor, Fig. 13 shows the temperature distribution along the interior wall and along the central axis for different wall materials. For low wall thermal conductivity, a high temperature gradient exists on the wall and the hot spot can cause the material to melt or degrade the catalyst. The wall temperature distribution tends to become uniform as the wall thermal conductivity increases. In Fig. 13(b), the temperature distribution along the central axis shows a higher temperature in the low wall thermal conductivity. Fig. 14 shows the OH distribution along the central axis. A lower OH concentration is found and the flame location shifts slightly downstream for high thermal wall conductivity. A high temperature gradient on the wall will make the homogeneous combustion shift upstream and the system will have a higher peak temperature. However, this behavior is different from that in the micro-reactor without catalyst walls. In that case, the flame core will shift downstream when the wall thermal conductivity decreases [12]. Our simulations indicate that the effect of wall thermal conductivity is not as evident as that in the micro-reactor without catalyst walls. Since the heat to ignite the homogeneous reaction is primarily from the catalytic reaction on the wall, not from the upstream heat conduction, the effect of wall thermal conductivity is then not so obvious. 4. Conclusions Fig. 13. Temperature profile along the wall (a) and the central axis (b) for different wall thermal conductivity values. Numerical simulations of hydrogen/air reaction in a catalytic micro tube are performed to identify the role of the catalytic wall in the reaction characteristics and to verify its sustaining and competing effects on homogeneous combustion in a micro tube. The parameters investigated include inlet flow

9 G.-B. Chen et al. / Applied Catalysis A: General 332 (2007) velocity (residence time), tube diameter and wall thermal conductivity. The following conclusions are obtained from this study. The existence of a heterogeneous reaction has two effects on homogeneous combustion. On the one hand, heterogeneous reactions will consume part of the fuels in the upstream region. The reduced fuel concentration and the reaction product will weaken the homogeneous reaction. On the other hand, for the micro-scale combustor, high temperature and radicals produced on the wall by heterogeneous reactions will help to enhance gas phase reaction and to overcome heat loss from the gas phase. Decreasing the tube diameter will enhance the heterogeneous reactions. However, the increased heat released at the wall will be of benefit for homogeneous combustion. The maximum allowed inlet velocity for homogeneous combustion first increased and then decreased to almost zero at a 0.2 mm tube. The effect of heterogeneous reaction on homogeneous reaction can be divided to three types. In the first type, the homogeneous combustion is weakened by the catalyst but it can be sustained in a large inlet velocity region. In the second type, the homogeneous reactions only have significant effects on the amounts of intermediate species. In the third type, homogeneous reactions can be completely neglected. Decreasing fuel concentration, increasing inlet velocity or decreasing tube size will make the interaction shift from the first type to the third type. The higher wall temperature gradient for low wall thermal conductivity will promote the homogeneous combustion shift upstream and will result in a higher temperature distribution. Since the homogeneous combustion is ignited and sustained by the heat from heterogeneous reactions, the effect of wall thermal conductivity is not as obvious as that in the micro tube without catalyst walls. Acknowledgements The support of the computer time and the numerical packages provided by the National Center for High-Performance Computing, Taiwan throughout the research phase are sincerely acknowledged. References [1] W.A. Sirignano, T.K. Pham, D. Dunn-Rankin, in: Proceedings of the 29th Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 2002, pp [2] A.C. Fernandez-Pello, in: Proceedings of the 29th Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 2002, pp [3] Y.C. Chao, G.B. Chen, C.J. Hsu, T.S. Leu, C.Y. Wu, T.S. Cheng, Combust. Sci. Tech. 176 (2004) [4] L. Sitzki, K. Borer, S. Wussow, E. Schuster, K. Maruta, P. Ronney, A. Cohen, AIAA , 38th AIAA Space Sciences & Exhibit, Reno, NV. [5] P. Ronney, Combust. Flame 135 (2003) 421. [6] G. Veser, Chem. Eng. Sci. 56 (2001) [7] D.G. Norton, E.D. Wetzel, D.G. Vlachos, Ind. Eng. Chem. Res. 43 (2004) [8] K. Maruta, K. Takeda, J. Ahn, K. Borer, L. Sitzki, P.D. Ronney, O. Deutschmann, in: Proceedings of the 29th Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 2002, pp [9] S. Raimondeau, D.G. Norton, D.G. Vlachos, R.I. Masel, in: Proceedings of the 29th Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 2002, pp [10] D.G. Norton, D.G. Vlachos, Chem. Eng. Sci. 58 (2003) [11] D.G. Norton, D.G. Vlachos, Combust. Flame 138 (2004) 97. [12] J. Hua, M. Wu, K. Kumar, Chem. Eng. Sci. 60 (2005) [13] CFDRC, CFD-ACE, Huntsville, Alabama, [14] J.A. Miller, C.T. Bowan, Prog. Energy Combust. Sci. 15 (1989) 287. [15] O. Deutschmann, R. Schmidt, F. Behrendt, J. Warnatz, in: Proceedings of the 26th Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1996, pp [16] C.P. Chen, Y.C. Chao, C.Y. Wu, J.C. Lee, G.B. Chen, Combust. Sci. Tech. 178 (2006) [17] O. Deutschmann, L.I. Maier, U. Riedel, A.H. Stroemman, R.W. Dibble, Catal. Today 59 (2000) [18] R.J. Kee, F.M. Rupley, E. Meeks, J.A. Miller, Sandia National Laboratories Report SAND , [19] M.E. Coltrin, R.J. Kee, F.M. Rupley, E. Meeks, Sandia National Laboratories Report SAND , 1996.