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air combustion in a micro planar combustor with parallel separating plates
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 0 ( 2 0 1 5 ) 2 3 9 6 e2 4 0 3
Available online at www.sciencedirect.com
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Numerical study of premixed hydrogen/air combustion in a micro planar combustor with parallel separating plates Aikun Tang a,b, Jianfeng Pan a,*, Wenming Yang b, Yiming Xu a, Zhiyong Hou a a b
School of Energy and Power Engineering, Jiangsu University, Zhenjiang 212013, China Department of Mechanical Engineering, National University of Singapore, Singapore 117576, Singapore
article info
abstract
Article history:
Recently, micro power generation devices based on the micro combustion process have
Received 10 October 2014
been extensively stimulated by the persistent breakthrough of MEMS techniques. As a
Received in revised form
major component, the design of a micro combustor is very critical when determining the
4 December 2014
power output of these devices. In this paper, a new type of micro planar combustor for
Accepted 9 December 2014
micro thermophotovoltaic system is proposed, which has multi-mode heat transfer pas-
Available online 7 January 2015
sages in the combustion channel. A three-dimensional CFD model with a skeletal reaction mechanism embedded is established for premixed hydrogen-air combustion in the micro
Keywords:
combustor. Simulation studies are conducted to investigate the basic combustion char-
Micro combustion
acteristics and the effect of structure parameters on the combustion processes. It is found
Numerical simulation
that compared to single-passage combustor, the new combustor can achieve a higher
Separating plates
mean temperature of the radiation wall due to the enhancement of heat transfer, and it
Heat transfer enhancement
becomes more obvious with the increase of plate number. The length of heat transfer
Temperature distribution
passage also significantly affects the average value and uniformity of the external wall temperature distribution. Some useful designing parameters have been obtained so as to give a reference for the similar combustor. Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction With the great impact of micro machining technology, persistent breakthroughs have been obtained on the exploitation of micro machinery and electromechanical products. However, the MEMS development has reached a bottleneck period due to the limitations of its power supply system. In recent years, due to the advantages of high energy density,
long working time as well as small volume, several combustion-based micro power generators have attracted widespread attention around the world and are expected to well solve this problem [1e3]. As one of the typical power generation devices, micro thermophotovoltaic (MTPV) system utilizes thermal energy to heat up the micro combustor's wall through the combustion of hydrocarbon fuels, and then electricity can be generated when the high-energy photon released from the high temperature
* Corresponding author. Tel.: þ86 511 88780210; fax: þ86 511 88780216. E-mail address: [email protected] (J. Pan). http://dx.doi.org/10.1016/j.ijhydene.2014.12.018 0360-3199/Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 0 ( 2 0 1 5 ) 2 3 9 6 e2 4 0 3
radiation wall impinges on the PV cells. Compared to other types of power generators, the greatest superiorities of MTPV system are as follows: (1) there is no moving components in the device; (2) it is easy to manufacture and assemble [2]. In a typical MTPV system, the micro combustor is the most important part, as its design will determine the stability of the combustion flame and the external wall temperature distribution, which has a direct impact on the output power density and the energy conversion efficiency of the system. Various micro combustors for the MTPV system have been proposed by scholars in the past decade. Yang et al. [4] presented a micro cylindrical combustor with backward facing step so as to control the position of the flame. Pan et al. [5] investigated the micro combustion characteristics of premixed hydrogeneoxygen in a micro planar combustor through both experimental and computational methods, and believed that a planar micro combustor is more appropriate for modular assembly. Then, Yang et al. [6] made a detailed comparison study between cylindrical and planar micro combustors, and pointed out that planar combustor has a higher radiation efficiency. Fan et al. [7,8] developed a micro combustor with a bluff body which can bring about extension in the blow-off limit by 3e5 times. Park et al. [9,10] designed a micro heat-recirculating combustor with a cylindrical configuration, and an overall system efficiency of 2.12% was obtained by experimental test. Federici et al. [11] compared the flame stability of a heat recirculation reactor with a singlechannel micro planar combustor through a 2D computational model. Jiang et al. [12,13] further analyzed the positive effect of heat recuperation through entropy generation theory. Also, Porous media were used to strengthen the micro combustion process in the MTPV system. Chou et al. [14,15] studied the porous media combustion characteristics both in cylindrical and planar combustor, which indicated that porous media had a significant effect of enhancing the heat transfer between combustion products and emitter wall. Furthermore, a combustor with catalytic wall was presented by Yang el al. [16], and the influence on the system power generation was also investigated. When designing a combustor for the MTPV system, the heat transfer enhancement between burned gas and the inner wall should also be given careful consideration. However, little research on this aspect has been reported. A new type of micro planar combustor with multi-mode heat transfer passages is studied in this paper by a simulation method. A comparative calculation of the combustors with and without parallel separating plates is conducted to analyze the active effects brought by the new design concept. Combustion characteristics of premixed hydrogen-air are obtained by changing some important structure parameters, so as to provide some valuable reference for further optimization.
Structure of micro combustor In this work, a new type of micro combustor with parallel separating plates is designed for the MTPV system application as shown in Fig. 1. The micro combustor is also a planar structure in appearance, except that there are some
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Fig. 1 e Schematic diagram of the micro planar combustor with parallel separating plates.
separating plates arranged inside the combustor. The upper wall (radiation wall) is drawn as a transparent plate so as to visualize the internal structure. The overall dimensions are 18 mm long (x-direction), 9 mm wide (y-direction) and 3 mm tall (z-direction). The wall thickness is 0.5 mm, so the rectangular combustion channel height is 2 mm. Several parallel plates are set up in the combustion channel which are some distance away from the inlet and divide the channel to several parts. The plate thickness is 0.5 mm, and all the shaped passages share the same width of 2 mm. In the simulation process, plate length and number of plates are chosen as variable parameters. Premixed hydrogen and air will flow into the channel from the inlet, and then begin to react at the front region (before the plate zone), leading to the burned gas being expelled through the outlet via each micro passage. Both combustor and plates are made of the 316 stainless steel which can withstand a high temperature of 2000 K.
Computational model Considering the structural characteristic of the new type combustor, a 3D computational model is developed. The mesh independence has been checked as shown in Fig. 2. It can be seen that the temperature obtained by the mesh with medium size (243,000 cells) is very close to that of fine mesh (486,000 cells), as such the medium size mesh is used in the following simulation. The basic governing equations of gas phase include the mass conservation equation, the momentum conservation equation, the chemical components transport equation and the energy conservation equation. The model is solved by the finite volume method and an implicit solver using the underrelaxation method, and Fluent is chosen as the calculation software. All of the governing equations are discretized by a first-order upwind scheme, and SIMPLE algorithm is used to deal with the pressureevelocity coupling. A skeletal mechanism for hydrogen-air gas phase reaction which has 9 species and 19 reversible elementary reactions is applied in this simulation [17]. Ronney et al. [18] pointed out that the modeling of turbulent flow is recommended when the Reynolds number is higher than 500, so as to obtain a more
Centerline temperature (K)
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Results and discussions
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Comparison of single-passage and multi-passage combustors
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97200 cells (0.25,0.10,0.10) 243000 cells (0.10,0.10,0.10) 486000 cells (0.10,0.10,0.05)
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Fig. 2 e Centerline temperature profiles of combustor at different mesh densities.
accurate simulation result. And increasing number of scholars employed the standard k-ε model during the simulation process [19]. Considering the effect of improving mixture turbulence by separating plates, the standard k-ε model and the finite-rate model are chosen as flow and the combustion chemistry models. The mixture gas density and specific heat are calculated using incompressible-ideal-gas law and mixing-law, respectively. The gas thermal conductivity and viscosity are calculated as a mass fraction-weighted average of all species [20,21], and kinetic-theory is applied when calculating the gas mass diffusivity [22]. For each species, specific heat is calculated using a piecewise polynomial fit of temperature, and the thermal conductivity and viscosity are calculated by kinetic-theory. In the process of computation, Dufour effects, gas radiation and viscous dissipation are neglected [5]. For boundary conditions, velocity-inlet is chosen at the inlet, the velocity and mass fraction of each component are set. Pressure-outlet boundary condition is specified at the outlet zone. Considering both convection and radiation existing between the radiation wall and the surrounding area, mixed thermal conditions are chosen for all of the external walls. The convective heat transfer coefficient and wall emissivity are set to be 15 W/(m2$K) and 0.65, respectively. In order to verify the applicability of computational model, a single channel micro-combustor with the same dimensions and material as Fig. 1 was fabricated and tested. The basic experimental set-up was given in Ref. [5]. The flow rates of each gas and equivalence ratio can be accurately controlled by two flow controllers. An infrared thermographer (Camera model: ThermovisionTM A40) is adopted to measure the external wall temperature distribution of the micro combustor. The maximum measurable temperature of this apparatus can reach 2000 C, its measuring accuracy is ±2% of reading, and a 200 mm close-up lens is equipped on the camera so as to obtain more clear images. From Fig. 3, it can be seen that not only the temperature distribution patterns, but also the centerline temperature distributions of the radiation wall have shown a good agreement, and the maximum deviation between simulation and experiment results is about 3.5%.
A comparison study on micro combustors with and without parallel separating plates has been performed so as to understand basic working characteristic for the new type of combustor. To better distinguish the two types of combustors, they will be defined as single-passage combustor and multipassage combustor respectively in the following sections. Here, number of plates is 4 and length is 14 mm, as such there are five passages and each passage is 1.2 mm in width. Firstly, temperature distributions on the center section of the two combustors are shown in Fig. 4. Due to the rapid flame speed, the mixture of hydrogen and air can be ignited easily. The reactions inside the two combustor designs can be completed in a very small region, which is only about 3 mm away from the inlet. Compared with the single-passage combustor, the setting up of parallel separating plates will decrease the equivalent diameter of each passage and increase the flow velocity of exhaust gas, so the mean heat transfer coefficient reaches 208 W/(m2$K) which is only 67 W/
Fig. 3 e Experimental validation of computational model. (a) Comparison of temperature distribution patterns on the combustor external wall, (b) Comparison of centerline temperature distributions along the combustor external wall.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 0 ( 2 0 1 5 ) 2 3 9 6 e2 4 0 3
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Fig. 4 e Comparison of temperature distributions on the combustor center section (inlet velocity: 6 m/s). (a) single-passage combustor, (b) multi-passage combustor.
(m2$K) in the single-passage micro combustor. In addition, the arrangement of plates provides better contacts between the radiation wall and the high temperature exhaust gas, so more heat is absorbed by the solid walls, which could account for the gas temperature inside the new combustor design decreasing gradually as seen in Fig. 4b. Fig. 5 gives the external wall temperature distributions of the two combustors. It can be seen that the high temperature zone of the single-passage combustor is situated at the front part, which is about 6 mm long. However, when it comes to multi-passage combustor, the high temperature zone becomes bigger and has a larger distance away from the inlet. A comparison of the external wall mean temperature at different inlet velocities is shown in Fig. 6. In both cases, wall temperature gradually rises with the increasing inlet velocity, and the growing rate is relatively larger at low flow rates. When increasing the inlet velocity from 2 m/s to 4 m/s, the mean wall temperature increases by 130 K and 187 K respectively. But the corresponding increments are only 36 K and 37 K from 6 m/s to 8 m/s. Thusly one can conclude that merely increasing the mixture flow rate is not an effective way to improve the working efficiency of the MTPV system. Meanwhile, it should be noted that the mean temperature of the multi-passage combustor at all investigated velocities are higher than that of the single-passage combustor, and the relative temperature rise are 33 K, 90 K, 102 K and 103 K respectively. It is significant that the mean temperature of the multi-passage case can reach 1365 K at 4 m/s case, which is almost the same value of single-passage combustor at velocity of 8 m/s. The above data indicates that the effect of improving
heat transfer intensity by separating plates will become more apparent at high flow rates. For micro thermophotovoltaic system, the increasing of wall temperature is the most direct and effective way to promote power output and energy conversion efficiency [5,6]. From this point of view, the quantity and quality of radiation energy can get a boost by simply adding plates, which can fully demonstrate the significant superiority of the new design concept over other designs.
Effect of number of passages To further study the combustion characteristic and working performance of the multi-passage combustor, the effects of number of passages are investigated. In this section, the number of passages varies from two to eight, and the plate length is kept the same of 14 mm for each case. As the number of plates increases, both the number of passages and width of each passage will change. The effects of number of passages on the mean wall temperature and the heat transfer coefficient are shown in Fig. 7. It can be seen that the increasing of number of plates brings a steady rise to the mean wall temperature. The average growth rate at two inlet velocities are about 22 K (at 4 m/s case) and 26 K (at 6 m/s case) respectively when adding a new plate into the channel. Compared with the single-passage combustor, the mean temperature of the external wall can be increased by only 13 K (at 4 m/s case) and 19 K (at 6 m/s case) if just one plate is set up. However, with regards to the eight-passage combustor, the increment reaches 145 K and 172 K respectively, so the positive effect is very
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 0 ( 2 0 1 5 ) 2 3 9 6 e2 4 0 3
Fig. 5 e Comparison of temperature distributions on the external wall (inlet velocity: 6 m/s). (a) single-passage combustor, (b) multi-passage combustor.
phenomenon that the eight-passage combustor has a wall temperature rise of 83 K when the inlet velocity increases from 4 m/s to 6 m/s, which is 22 K larger than that of twopassage combustor. It is certain that the growth of the mean wall temperature will bring an increase in the radiation energy. In Fig. 8, a comparison of total radiant energy and usable radiation energy are given so as to further analyze the effect of number of passages. Here, usable radiation energy refers to short-wave photons whose wavelength is shorter than the cut-off wavelength of the PV cells (e.g. for GaSb cell, 1.78 mm). Both curves
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single-passage combustor multi-passage combustor
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Fig. 6 e Comparison of the external wall mean temperature at different inlet velocities.
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Mean temperature of external wall (K)
Mean temperature of external wall (K)
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Heat transfer coefficient (W·m ·K )
significant. The root cause for this phenomenon is a variation of heat transfer intensity which can be seen from Fig. 7. The heat transfer coefficient will increase with the amount of passages, and the tendency become more obvious when the passage number exceeds five. At the 4 m/s case, heat transfer coefficient of the eight-passage combustor will reach 320 W/ (m2$K) which is five times as much as that of two-passage combustor. In addition, the heat transfer coefficient growth rate versus velocity is relatively lower when number of passages is less, but it will increase sharply when more plates are arranged into the combustion channel. This accounts for the
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Fig. 7 e Comparison of the external wall mean temperature and the heat transfer coefficient at different number of passages (upper line: 6 m/s; lower line: 4 m/s).
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 0 ( 2 0 1 5 ) 2 3 9 6 e2 4 0 3
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Fig. 8 e Comparison of total radiant energy and usable radiant energy at different number of passages (inlet velocity: 6 m/s).
presents a linear growth of the radiation energy with the increasing number of passages, indicating that an extra 5.4 W total radiation energy and 1.6 W useful radiation energy can be obtained with the addition of one plate. At the inlet velocity of 6 m/s, the useful radiation energy of the eight-passage combustor is reaches 17.6 W, which is about 2.2 times as much as that of the two-passage combustor. If all other system working conditions remain unchanged, the same growth rate of the system conversion efficiency can be obtained.
Effect of plates arrangement distance When designing the multi-passage combustor, the length of parallel separating plates is shorter than that of the combustor so as to form a combustion region near the inlet. To study the effect of the distance of plates from the inlet of
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the combustor to the leading edge of the passage, eight different beginning distances are chosen, which are 1 mme8 mm away from the inlet, and for the purposes of this section the number of passages is maintained at six. To simplify, the arrangement distance is defined as the distance of plates from the inlet of the combustor to the leading edge of the passage. Fig. 9 displays the external wall temperature distributions at four selected arrangement distances, from which we can see that the size and location of the high temperature zone are strongly influenced by the beginning position of plates. It is found that the high temperature zone will move downstream with the increase of the arrangement distance. Meanwhile, due to the decrease of maximum temperature (1551 K at 2 mm case and 1513 K at 8 mm case), the shape and size of high temperature zone have shown some changes. The area of high temperature zone for the 8 mm case is only about half as much as the 2 mm case. But when it comes to the uniformity of the temperature distribution, the maximum temperature gradient of the 8 mm case is only 152 K, which is 47 K lower than that of the 2 mm case. In fact, the high temperature gradient will produce a great thermal stress to the surface, which can lead to a material fracture when exceeding a certain degree. The mean wall temperature comparison of the external wall is shown in Fig. 10. The variation pattern at two inlet velocities are very similar, the mean temperature reduces slowly when the plate arrangement position moves downstream. There is little difference when the plate position is very close to the inlet, so the mean temperature of the 1 mm case and the 2 mm case are nearly the same, and about only 1 K higher than the 3 mm case. Besides, the largest temperature differences at the two velocities are only 19 K and 18 K, respectively. As a result, it can be concluded that effect of plate arrangement distance on mean wall temperature is relatively less than that of number of passages. In order to explain this phenomenon clearly, the comparison of heat transfer coefficient and maximum flame
Fig. 9 e Comparison of temperature distributions on the external wall at different plates arrangement distances (inlet velocity: 6 m/s). (a) 2 mm, (b) 4 mm, (c) 6 mm, (d) 8 mm.
Mean temperature of external wall (K)
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Conclusions
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In this paper, a new type of micro planar combustor with parallel separating plates is designed for the MTPV system, and an extensive investigation about working performance of this multi-passage combustor has been conducted through a 3D simulation method. Some important conclusions are summarized as follows:
inlet velocity 4m/s inlet velocity 6m/s
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Fig. 10 e Comparison of the external wall mean temperature at different plates arrangement distances.
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temperature are shown in Fig. 11. The extension of separating plates and passage length will result in a continuous increase of heat transfer intensity, so the average heat transfer coefficient of 1 mm case can reach 408 W/(m2$K) which is about 2.2 times as much as that of the 8 mm case. Meanwhile, it should be noted that the plate arrangement distance also has a relationship with the temperature distribution of the flame in the micro combustion channel. A prior arrangement of obstacles will lead to a change of flame shape and a decline of flame temperature. The maximum flame temperature values are only 1958 K and 1962 K at the 1 mm and the 2 mm cases, however it reaches 2016 K when arrangement distance is 4 mm, and has exceeded 2100 K at the 8 mm case. According to the principle of heat transfer, flame or mixture temperature is the inherent driving force of heat exchange, which is bound to affect the temperature distribution of the external wall at a certain degree. Based on the simulation results, it can be concluded that at higher flow rates (e.g. inlet velocity 8 m/s), the arrangement distances of 2 mm or 3 mm will be an appropriate structural parameter.
1940
Fig. 11 e Comparison of the heat transfer coefficient and the maximum flame temperature at different plates arrangement distances (inlet velocity: 6 m/s).
(1) The setting up of parallel separating plates can help to enhance heat transfer between mixture and combustor inner walls, and the mean wall temperature can be increased by more than 100 K when compared to the single-passage combustor. (2) The more plates that are arranged, the higher the mean wall temperature will be, and the system conversion efficiency will get a continuous growth due to the increase of usable radiation energy. (3) The arrangement distance of the separating plates has a dual effect on both internal heat transfer intensity and flame temperature, and a prior arrangement of plates is favorable for MTPV application.
Acknowledgments This work is supported by National Natural Science Foundation of China (No. 51206066, No. 51376082), China Postdoctoral Science Foundation (No. 2014M551514), Natural Science Foundation of Jiangsu Province (No. BK20131253), Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), and Scientific Research Starting Foundation for Advanced Talents of Jiangsu University (No. 11JDG139).
references
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[15] Chou SK, Yang WM, Li J, Li ZW. Porous media combustion for micro thermophotovoltaic system applications. Appl Energy 2010;87:2862e7. [16] Yang WM, Chou SK, Shu C, Li ZW, Xue H. Study of catalytic combustion and its effect on microthermophotovoltaic power generators. J Phys D Appl Phys 2005;38:4252e5. [17] Hua JS, Wu M, Kumar K. Numerical simulation of the combustion of hydrogen-air mixture in micro-scaled chambers (part I: fundamental study). Chem Eng Sci 2005;60:3497e506. [18] Kuo CH, Ronney PD. Numerical modeling of non-adiabatic heat-recirculating combustors. Proc Combust Inst 2007;31:3277e84. [19] Jiang DY, Yang WM, Chua KJ, Ouyang JY. Thermal performance of micro-combustors with baffles for thermophotovoltaic system. Appl Therm Eng 2013;61:670e7. [20] Norton DG, Vlachos DG. Combustion characteristics and flame stability at the microscale: a CFD study of premixed methane/air mixtures. Chem Eng Sci 2003;58:4871e82. [21] Norton DG, Vlachos DG. A CFD study for propane/air microflame stability. Combust Flame 2004;138:97e107. [22] Zarvandi J, Tabejamaat S, Baigmohammadi M. Numerical study of the effects of heat transfer methods on CH4/ (CH4 þ H2)-air pre-mixed flames in a micro-stepped tube. Energy 2012;44:396e409.
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Numerical study of premixed hydrogen/air combustion in a micro planar combustor with parallel separating plates Aikun Tang a,b, Jianfeng Pan a,*, Wenming Yang b, Yiming Xu a, Zhiyong Hou a a b
School of Energy and Power Engineering, Jiangsu University, Zhenjiang 212013, China Department of Mechanical Engineering, National University of Singapore, Singapore 117576, Singapore
article info
abstract
Article history:
Recently, micro power generation devices based on the micro combustion process have
Received 10 October 2014
been extensively stimulated by the persistent breakthrough of MEMS techniques. As a
Received in revised form
major component, the design of a micro combustor is very critical when determining the
4 December 2014
power output of these devices. In this paper, a new type of micro planar combustor for
Accepted 9 December 2014
micro thermophotovoltaic system is proposed, which has multi-mode heat transfer pas-
Available online 7 January 2015
sages in the combustion channel. A three-dimensional CFD model with a skeletal reaction mechanism embedded is established for premixed hydrogen-air combustion in the micro
Keywords:
combustor. Simulation studies are conducted to investigate the basic combustion char-
Micro combustion
acteristics and the effect of structure parameters on the combustion processes. It is found
Numerical simulation
that compared to single-passage combustor, the new combustor can achieve a higher
Separating plates
mean temperature of the radiation wall due to the enhancement of heat transfer, and it
Heat transfer enhancement
becomes more obvious with the increase of plate number. The length of heat transfer
Temperature distribution
passage also significantly affects the average value and uniformity of the external wall temperature distribution. Some useful designing parameters have been obtained so as to give a reference for the similar combustor. Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction With the great impact of micro machining technology, persistent breakthroughs have been obtained on the exploitation of micro machinery and electromechanical products. However, the MEMS development has reached a bottleneck period due to the limitations of its power supply system. In recent years, due to the advantages of high energy density,
long working time as well as small volume, several combustion-based micro power generators have attracted widespread attention around the world and are expected to well solve this problem [1e3]. As one of the typical power generation devices, micro thermophotovoltaic (MTPV) system utilizes thermal energy to heat up the micro combustor's wall through the combustion of hydrocarbon fuels, and then electricity can be generated when the high-energy photon released from the high temperature
* Corresponding author. Tel.: þ86 511 88780210; fax: þ86 511 88780216. E-mail address: [email protected] (J. Pan). http://dx.doi.org/10.1016/j.ijhydene.2014.12.018 0360-3199/Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 0 ( 2 0 1 5 ) 2 3 9 6 e2 4 0 3
radiation wall impinges on the PV cells. Compared to other types of power generators, the greatest superiorities of MTPV system are as follows: (1) there is no moving components in the device; (2) it is easy to manufacture and assemble [2]. In a typical MTPV system, the micro combustor is the most important part, as its design will determine the stability of the combustion flame and the external wall temperature distribution, which has a direct impact on the output power density and the energy conversion efficiency of the system. Various micro combustors for the MTPV system have been proposed by scholars in the past decade. Yang et al. [4] presented a micro cylindrical combustor with backward facing step so as to control the position of the flame. Pan et al. [5] investigated the micro combustion characteristics of premixed hydrogeneoxygen in a micro planar combustor through both experimental and computational methods, and believed that a planar micro combustor is more appropriate for modular assembly. Then, Yang et al. [6] made a detailed comparison study between cylindrical and planar micro combustors, and pointed out that planar combustor has a higher radiation efficiency. Fan et al. [7,8] developed a micro combustor with a bluff body which can bring about extension in the blow-off limit by 3e5 times. Park et al. [9,10] designed a micro heat-recirculating combustor with a cylindrical configuration, and an overall system efficiency of 2.12% was obtained by experimental test. Federici et al. [11] compared the flame stability of a heat recirculation reactor with a singlechannel micro planar combustor through a 2D computational model. Jiang et al. [12,13] further analyzed the positive effect of heat recuperation through entropy generation theory. Also, Porous media were used to strengthen the micro combustion process in the MTPV system. Chou et al. [14,15] studied the porous media combustion characteristics both in cylindrical and planar combustor, which indicated that porous media had a significant effect of enhancing the heat transfer between combustion products and emitter wall. Furthermore, a combustor with catalytic wall was presented by Yang el al. [16], and the influence on the system power generation was also investigated. When designing a combustor for the MTPV system, the heat transfer enhancement between burned gas and the inner wall should also be given careful consideration. However, little research on this aspect has been reported. A new type of micro planar combustor with multi-mode heat transfer passages is studied in this paper by a simulation method. A comparative calculation of the combustors with and without parallel separating plates is conducted to analyze the active effects brought by the new design concept. Combustion characteristics of premixed hydrogen-air are obtained by changing some important structure parameters, so as to provide some valuable reference for further optimization.
Structure of micro combustor In this work, a new type of micro combustor with parallel separating plates is designed for the MTPV system application as shown in Fig. 1. The micro combustor is also a planar structure in appearance, except that there are some
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Fig. 1 e Schematic diagram of the micro planar combustor with parallel separating plates.
separating plates arranged inside the combustor. The upper wall (radiation wall) is drawn as a transparent plate so as to visualize the internal structure. The overall dimensions are 18 mm long (x-direction), 9 mm wide (y-direction) and 3 mm tall (z-direction). The wall thickness is 0.5 mm, so the rectangular combustion channel height is 2 mm. Several parallel plates are set up in the combustion channel which are some distance away from the inlet and divide the channel to several parts. The plate thickness is 0.5 mm, and all the shaped passages share the same width of 2 mm. In the simulation process, plate length and number of plates are chosen as variable parameters. Premixed hydrogen and air will flow into the channel from the inlet, and then begin to react at the front region (before the plate zone), leading to the burned gas being expelled through the outlet via each micro passage. Both combustor and plates are made of the 316 stainless steel which can withstand a high temperature of 2000 K.
Computational model Considering the structural characteristic of the new type combustor, a 3D computational model is developed. The mesh independence has been checked as shown in Fig. 2. It can be seen that the temperature obtained by the mesh with medium size (243,000 cells) is very close to that of fine mesh (486,000 cells), as such the medium size mesh is used in the following simulation. The basic governing equations of gas phase include the mass conservation equation, the momentum conservation equation, the chemical components transport equation and the energy conservation equation. The model is solved by the finite volume method and an implicit solver using the underrelaxation method, and Fluent is chosen as the calculation software. All of the governing equations are discretized by a first-order upwind scheme, and SIMPLE algorithm is used to deal with the pressureevelocity coupling. A skeletal mechanism for hydrogen-air gas phase reaction which has 9 species and 19 reversible elementary reactions is applied in this simulation [17]. Ronney et al. [18] pointed out that the modeling of turbulent flow is recommended when the Reynolds number is higher than 500, so as to obtain a more
Centerline temperature (K)
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accurate simulation result. And increasing number of scholars employed the standard k-ε model during the simulation process [19]. Considering the effect of improving mixture turbulence by separating plates, the standard k-ε model and the finite-rate model are chosen as flow and the combustion chemistry models. The mixture gas density and specific heat are calculated using incompressible-ideal-gas law and mixing-law, respectively. The gas thermal conductivity and viscosity are calculated as a mass fraction-weighted average of all species [20,21], and kinetic-theory is applied when calculating the gas mass diffusivity [22]. For each species, specific heat is calculated using a piecewise polynomial fit of temperature, and the thermal conductivity and viscosity are calculated by kinetic-theory. In the process of computation, Dufour effects, gas radiation and viscous dissipation are neglected [5]. For boundary conditions, velocity-inlet is chosen at the inlet, the velocity and mass fraction of each component are set. Pressure-outlet boundary condition is specified at the outlet zone. Considering both convection and radiation existing between the radiation wall and the surrounding area, mixed thermal conditions are chosen for all of the external walls. The convective heat transfer coefficient and wall emissivity are set to be 15 W/(m2$K) and 0.65, respectively. In order to verify the applicability of computational model, a single channel micro-combustor with the same dimensions and material as Fig. 1 was fabricated and tested. The basic experimental set-up was given in Ref. [5]. The flow rates of each gas and equivalence ratio can be accurately controlled by two flow controllers. An infrared thermographer (Camera model: ThermovisionTM A40) is adopted to measure the external wall temperature distribution of the micro combustor. The maximum measurable temperature of this apparatus can reach 2000 C, its measuring accuracy is ±2% of reading, and a 200 mm close-up lens is equipped on the camera so as to obtain more clear images. From Fig. 3, it can be seen that not only the temperature distribution patterns, but also the centerline temperature distributions of the radiation wall have shown a good agreement, and the maximum deviation between simulation and experiment results is about 3.5%.
A comparison study on micro combustors with and without parallel separating plates has been performed so as to understand basic working characteristic for the new type of combustor. To better distinguish the two types of combustors, they will be defined as single-passage combustor and multipassage combustor respectively in the following sections. Here, number of plates is 4 and length is 14 mm, as such there are five passages and each passage is 1.2 mm in width. Firstly, temperature distributions on the center section of the two combustors are shown in Fig. 4. Due to the rapid flame speed, the mixture of hydrogen and air can be ignited easily. The reactions inside the two combustor designs can be completed in a very small region, which is only about 3 mm away from the inlet. Compared with the single-passage combustor, the setting up of parallel separating plates will decrease the equivalent diameter of each passage and increase the flow velocity of exhaust gas, so the mean heat transfer coefficient reaches 208 W/(m2$K) which is only 67 W/
Fig. 3 e Experimental validation of computational model. (a) Comparison of temperature distribution patterns on the combustor external wall, (b) Comparison of centerline temperature distributions along the combustor external wall.
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Fig. 4 e Comparison of temperature distributions on the combustor center section (inlet velocity: 6 m/s). (a) single-passage combustor, (b) multi-passage combustor.
(m2$K) in the single-passage micro combustor. In addition, the arrangement of plates provides better contacts between the radiation wall and the high temperature exhaust gas, so more heat is absorbed by the solid walls, which could account for the gas temperature inside the new combustor design decreasing gradually as seen in Fig. 4b. Fig. 5 gives the external wall temperature distributions of the two combustors. It can be seen that the high temperature zone of the single-passage combustor is situated at the front part, which is about 6 mm long. However, when it comes to multi-passage combustor, the high temperature zone becomes bigger and has a larger distance away from the inlet. A comparison of the external wall mean temperature at different inlet velocities is shown in Fig. 6. In both cases, wall temperature gradually rises with the increasing inlet velocity, and the growing rate is relatively larger at low flow rates. When increasing the inlet velocity from 2 m/s to 4 m/s, the mean wall temperature increases by 130 K and 187 K respectively. But the corresponding increments are only 36 K and 37 K from 6 m/s to 8 m/s. Thusly one can conclude that merely increasing the mixture flow rate is not an effective way to improve the working efficiency of the MTPV system. Meanwhile, it should be noted that the mean temperature of the multi-passage combustor at all investigated velocities are higher than that of the single-passage combustor, and the relative temperature rise are 33 K, 90 K, 102 K and 103 K respectively. It is significant that the mean temperature of the multi-passage case can reach 1365 K at 4 m/s case, which is almost the same value of single-passage combustor at velocity of 8 m/s. The above data indicates that the effect of improving
heat transfer intensity by separating plates will become more apparent at high flow rates. For micro thermophotovoltaic system, the increasing of wall temperature is the most direct and effective way to promote power output and energy conversion efficiency [5,6]. From this point of view, the quantity and quality of radiation energy can get a boost by simply adding plates, which can fully demonstrate the significant superiority of the new design concept over other designs.
Effect of number of passages To further study the combustion characteristic and working performance of the multi-passage combustor, the effects of number of passages are investigated. In this section, the number of passages varies from two to eight, and the plate length is kept the same of 14 mm for each case. As the number of plates increases, both the number of passages and width of each passage will change. The effects of number of passages on the mean wall temperature and the heat transfer coefficient are shown in Fig. 7. It can be seen that the increasing of number of plates brings a steady rise to the mean wall temperature. The average growth rate at two inlet velocities are about 22 K (at 4 m/s case) and 26 K (at 6 m/s case) respectively when adding a new plate into the channel. Compared with the single-passage combustor, the mean temperature of the external wall can be increased by only 13 K (at 4 m/s case) and 19 K (at 6 m/s case) if just one plate is set up. However, with regards to the eight-passage combustor, the increment reaches 145 K and 172 K respectively, so the positive effect is very
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Fig. 5 e Comparison of temperature distributions on the external wall (inlet velocity: 6 m/s). (a) single-passage combustor, (b) multi-passage combustor.
phenomenon that the eight-passage combustor has a wall temperature rise of 83 K when the inlet velocity increases from 4 m/s to 6 m/s, which is 22 K larger than that of twopassage combustor. It is certain that the growth of the mean wall temperature will bring an increase in the radiation energy. In Fig. 8, a comparison of total radiant energy and usable radiation energy are given so as to further analyze the effect of number of passages. Here, usable radiation energy refers to short-wave photons whose wavelength is shorter than the cut-off wavelength of the PV cells (e.g. for GaSb cell, 1.78 mm). Both curves
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Fig. 6 e Comparison of the external wall mean temperature at different inlet velocities.
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significant. The root cause for this phenomenon is a variation of heat transfer intensity which can be seen from Fig. 7. The heat transfer coefficient will increase with the amount of passages, and the tendency become more obvious when the passage number exceeds five. At the 4 m/s case, heat transfer coefficient of the eight-passage combustor will reach 320 W/ (m2$K) which is five times as much as that of two-passage combustor. In addition, the heat transfer coefficient growth rate versus velocity is relatively lower when number of passages is less, but it will increase sharply when more plates are arranged into the combustion channel. This accounts for the
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Fig. 7 e Comparison of the external wall mean temperature and the heat transfer coefficient at different number of passages (upper line: 6 m/s; lower line: 4 m/s).
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Fig. 8 e Comparison of total radiant energy and usable radiant energy at different number of passages (inlet velocity: 6 m/s).
presents a linear growth of the radiation energy with the increasing number of passages, indicating that an extra 5.4 W total radiation energy and 1.6 W useful radiation energy can be obtained with the addition of one plate. At the inlet velocity of 6 m/s, the useful radiation energy of the eight-passage combustor is reaches 17.6 W, which is about 2.2 times as much as that of the two-passage combustor. If all other system working conditions remain unchanged, the same growth rate of the system conversion efficiency can be obtained.
Effect of plates arrangement distance When designing the multi-passage combustor, the length of parallel separating plates is shorter than that of the combustor so as to form a combustion region near the inlet. To study the effect of the distance of plates from the inlet of
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the combustor to the leading edge of the passage, eight different beginning distances are chosen, which are 1 mme8 mm away from the inlet, and for the purposes of this section the number of passages is maintained at six. To simplify, the arrangement distance is defined as the distance of plates from the inlet of the combustor to the leading edge of the passage. Fig. 9 displays the external wall temperature distributions at four selected arrangement distances, from which we can see that the size and location of the high temperature zone are strongly influenced by the beginning position of plates. It is found that the high temperature zone will move downstream with the increase of the arrangement distance. Meanwhile, due to the decrease of maximum temperature (1551 K at 2 mm case and 1513 K at 8 mm case), the shape and size of high temperature zone have shown some changes. The area of high temperature zone for the 8 mm case is only about half as much as the 2 mm case. But when it comes to the uniformity of the temperature distribution, the maximum temperature gradient of the 8 mm case is only 152 K, which is 47 K lower than that of the 2 mm case. In fact, the high temperature gradient will produce a great thermal stress to the surface, which can lead to a material fracture when exceeding a certain degree. The mean wall temperature comparison of the external wall is shown in Fig. 10. The variation pattern at two inlet velocities are very similar, the mean temperature reduces slowly when the plate arrangement position moves downstream. There is little difference when the plate position is very close to the inlet, so the mean temperature of the 1 mm case and the 2 mm case are nearly the same, and about only 1 K higher than the 3 mm case. Besides, the largest temperature differences at the two velocities are only 19 K and 18 K, respectively. As a result, it can be concluded that effect of plate arrangement distance on mean wall temperature is relatively less than that of number of passages. In order to explain this phenomenon clearly, the comparison of heat transfer coefficient and maximum flame
Fig. 9 e Comparison of temperature distributions on the external wall at different plates arrangement distances (inlet velocity: 6 m/s). (a) 2 mm, (b) 4 mm, (c) 6 mm, (d) 8 mm.
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In this paper, a new type of micro planar combustor with parallel separating plates is designed for the MTPV system, and an extensive investigation about working performance of this multi-passage combustor has been conducted through a 3D simulation method. Some important conclusions are summarized as follows:
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temperature are shown in Fig. 11. The extension of separating plates and passage length will result in a continuous increase of heat transfer intensity, so the average heat transfer coefficient of 1 mm case can reach 408 W/(m2$K) which is about 2.2 times as much as that of the 8 mm case. Meanwhile, it should be noted that the plate arrangement distance also has a relationship with the temperature distribution of the flame in the micro combustion channel. A prior arrangement of obstacles will lead to a change of flame shape and a decline of flame temperature. The maximum flame temperature values are only 1958 K and 1962 K at the 1 mm and the 2 mm cases, however it reaches 2016 K when arrangement distance is 4 mm, and has exceeded 2100 K at the 8 mm case. According to the principle of heat transfer, flame or mixture temperature is the inherent driving force of heat exchange, which is bound to affect the temperature distribution of the external wall at a certain degree. Based on the simulation results, it can be concluded that at higher flow rates (e.g. inlet velocity 8 m/s), the arrangement distances of 2 mm or 3 mm will be an appropriate structural parameter.
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Fig. 11 e Comparison of the heat transfer coefficient and the maximum flame temperature at different plates arrangement distances (inlet velocity: 6 m/s).
(1) The setting up of parallel separating plates can help to enhance heat transfer between mixture and combustor inner walls, and the mean wall temperature can be increased by more than 100 K when compared to the single-passage combustor. (2) The more plates that are arranged, the higher the mean wall temperature will be, and the system conversion efficiency will get a continuous growth due to the increase of usable radiation energy. (3) The arrangement distance of the separating plates has a dual effect on both internal heat transfer intensity and flame temperature, and a prior arrangement of plates is favorable for MTPV application.
Acknowledgments This work is supported by National Natural Science Foundation of China (No. 51206066, No. 51376082), China Postdoctoral Science Foundation (No. 2014M551514), Natural Science Foundation of Jiangsu Province (No. BK20131253), Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), and Scientific Research Starting Foundation for Advanced Talents of Jiangsu University (No. 11JDG139).
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