行政院國家科學委員會專題研究計畫 期中進度報告
子計畫三：微型質子交換膜燃料電池元件薄膜電極體與流道
板界面封裝暨結構可靠度之研究(1/3)
計畫類別： 整合型計畫 計畫編號： NSC93-2218-E-110-008- 執行期間： 93 年 08 月 01 日至 94 年 07 月 31 日 執行單位： 國立中山大學機械與機電工程學系(所) 計畫主持人： 錢志回 共同主持人： 陳太平 計畫參與人員： 吳以德、王中鼎、蔡耀葳、李翊鋒、石益三 報告類型： 精簡報告 報告附件： 出席國際會議研究心得報告及發表論文 處理方式： 本計畫可公開查詢中 華 民 國 94 年 5 月 11 日
Abstract
This is the first year mid-term report of subproject-3 of a three-year main project. The main aim of this subproject is to analysis the interface reliability of MEA and flow field plate during package process, by using the experimental and numerical method, as well as evaluates bonding strength of the interface. In the first year, in order to study the reliability of flow field plate, the study is concentrated on predicting the stress distribution between the sputtered Ag layer and the SU8 substrate of a micro-channel in polymer electrolyte membrane fuel cell (PEMFC) through simulation of stress state and flow field in the channel by commercial package software ANSYS® 8.0. In the beginning, a single straight micro-channel flow field design was used for studying the effect of flow-induced wall shear stress and pressure on the channel wall. Then, the results are used to predict the stress distribution between the sputtered Ag layer and the SU8 substrate of the micro-channel. In the first half of this year, the efforts were concentrated on set up the numerical model and the analysis process. In the second half of this year, the efforts will be concentrated on studying the effects of the variation of micro-channel construction and the gas inlet velocity and pressure on the interface stress distribution in a flow field plate.
Keywords: Fuel cells; Flow field plate; Micro-channels; Interface Reliability
摘要
此為三年期整合計畫的子計畫三之第一年期中報告。本子計畫的主要目的為分析薄膜 電極體（MEA）與流道板界面於實際運作時界面封裝及結構可靠度評估,並建立實驗與數 值模擬等分析模式。在第一年為了研究流道的可靠度，將研究重點置於藉由商用套裝軟體
ANSYS®8.0 來分析在 PEMFC 微流道中濺鍍層 Ag 和基材 SU8 間的應力分佈情況。在初期
階段，先分析單一長直微管流場內流體作用於管壁之剪應力以及壓力的分佈；再將此結果 運用來分析微流道中濺鍍層 Ag 和基材 SU8 的間應力分佈情況。本年度前半年，計畫成果 著重於數值模型以及解析過程之建立；後半年將會著重在微流道結構變化和入口氣體速度 及壓力對於界面應力分佈的影響。 關鍵詞: 燃料電池; 流道板; 微流道; 界面可靠度 1. Introduction
Fuel cell is an electrochemical device that converts chemical energy (oxidation potential) into electrical energy directly. It operates like a battery and has similar characteristics with a battery. It produces electricity from supplying to the cell with the oxidant (typically air) and hydrogen. A fuel cell is not limited by its internal energy storage capacity as a battery. The electrochemical reactions are [1] ：
Anode ： H22H++2e -Cathode： 2 1 O2+2H++ e- H2O Net reaction：H2+ 2 1 O2 H2O
Figure 1 [2] illustrates the basic construction of a fuel cell ： a positively charged anode, a negatively charged cathode, and an electrolyte. Hydrogen and oxidant are
2 fed to the anode and cathode, respectively. When hydrogen and oxidant pass through the electrolyte, the electrochemical reaction occurs and the cell continues to produce electrical energy and heat. In order to guiding the electrons, one thin Ag film was sputtered on the SU8 to form the flow field plate. Figure 2 shows the structure of the flow field plates.
Figure1. The construction of a Polymer Electrolyte Membrane Fuel Cell
(PEMFC)
Figure2. The construction of Flow field plate
2. Literature Review
Kumar and Reddy [1] optimized the
dimension value for channel width, land width and channel depth to 1.5, 0.5 and 1.5mm, respectively. Studies on the effect of channel shapes showed that triangular and hemispherical shaped cross-section resulted in increase in hydrogen consumption by around 9%. Consequently, their conclusion would lead to improve fuel cell efficiency.
Guo and Li [3] focused on the size effect induced by the variation of dominant factors and phenomena in the flow and heat transfer as the device scale decreases. For example, surface friction induced flow compressibility makes the fluid velocity profiles flatter and leads to higher friction factors and Nusselt numbers; surface roughness is likely responsible for the early transition from laminar to turbulent flow and the increased friction factor and Nusselt number and, other effects, could lead to different flow and heat transfer behaviors from that at conventional scales. Tang, Yang, and Ku[4] simulated a three-dimensional (3-D) thin-wall model with flow-structure interactions was introduced and solved using ADINA to investigate the wall deformation and flow properties of blood flow in carotid arteries with symmetric and asymmetric stenoses. The Navier-Stokes equations were used as the governing equations for the fluid. The tube wall was assumed to be hyperelastic, homogeneous, isotropic and incompressible. The nonlinear large strain Ogden material model was used for the wall with the elastic properties determined experimentally for a silicone tube with a 78% stenosis by diameter. Ausiello, Apicella, and Davidson [5] found that for adhesive and composite of different rigidities, FEM analysis allows the determination of the optimal adhesive layer thickness leading to maximum stress release while preserving the interface integrity. Application of a thin layer of a more flexible adhesive (low elastic modulus) leads to the same stress relief as thick layers of less flexible adhesive (high elastic modulus).
Carmai, Baik, Dunne, Grant, and Cantor [6] developed models from an existing diffusion bonding theory, and is implemented into finite element software. The finite element simulations, and results of experiments, showed that diffusion bonding can lead to localized deformation, the inhibition of
consolidation, and a resulting
inhomogeneous distribution of consolidated
and unconsolidated regions during
component manufacture. A further effect of the diffusion bonding is to increase the level of component distortion which results from the constraint imposed on the consolidating composite. The interface model presented enables the simulation of practical forming processes so that process variables such as temperature and pressure can be chosen to ensure appropriate finished component properties.
3. Research Objective
The efficiency of the fuel cell depends on both the kinetics of the electrochemical process and performance of the components. In the first year, the study is concentrated on predicting the flow-induced wall shear stress and pressure on a micro-channel wall and the stress distribution between the sputtered Ag layer and the SU8 substrate of the micro-channel in PEMFC. This would help one to have a better understanding of the reliability of the micro-channel and the life of the designed fuel cell.
4. Numerical Model 4.1 Model Used
A single straight micro-channel with
rectangular cross-section which was isolated from a bipolar plate was adopted as the model. The cross-sectional area is 200×10-6 m(height) by 200×10-6 m (width), and the length of the channel is 2×10-2 m. Figure 3 shows the meshed model of the chosen channel. The ANSYS®8.0 element type FLUID142 was chosen to analyze the flow field in the micro-channel. The Definition of the proportion we took in ANSYS®8.0 was that one line meshed in different sizes. If the proportion value was less than -1, it means that the mesh size of the line edge was smaller then the size of line center. The flow field was chosen as a single-path design.
Figure 4 shows the meshed
composite layers of a portion (length = 5× 10-5 m) of the micro-channel wall. The wall layers were composed of sputtered Ag layer, adhesion layer, and SU8 substrate. The thickness of Ag and SU8 are 400×10-9m and 200 × 10-6 m, respectively. Three different ANSYS®8.0 element types were chosen in this model. They were SOLID45(Ag),
INTER195(Adhesion Layer), and
SOLID45(SU8) from top to bottom,
respectively. This model was used for simulating the distribution of stresses in the interface of the tube wall. Table 1 shows the material properties of Ag and SU8 [7] [8].
Table.1 Material properties Modulus of
Elasticity Poisson’s Ratio
Ag 76GPa 0.37
SU8 5.63Gpa 0.32
Property
4 Figure.3 The meshed rectangular
cross-sectional channel model
Figure.4 The meshed composite layers of a portion of the micro-channel wall
The following assumptions were used： (1) Steady state and laminar flow. (2) The effect of gravity is neglected. (3) Isothermal conditions exist in the
model.
(4) The flow field in the tube wall is assumed no-slip.
(5) The materials are all assumed isotropic and homogeneous 4.2 Boundary Conditions
The mass-flow-inlet of the hydrogen
reactant gas was kept constant as 8 c.c./s. The operating temperature and pressure were set as 298 K and 2 atm, respectively. According to the mass-flow equation, the inlet velocity can be found from：
Q = A×V (1)
where Q is the mass-flow-inlet of the reactant gas, and A is the cross-sectional area. The inlet velocity was calculated as 200 m/s and it was normal to the cross-section of the channel. For the flow field boundary conditions, the velocity of the each channel wall was set as zero and the inlet velocity was 200m/s. There was no pressure in the end of the channel. For the composite layer model, all DOF of the substrate (SU8) were set as zero in every direction and all DOF of the Ag layer were free. The applied loadings which acted on the top Ag surface were transferred from the datum of the flow-induced wall shear stress and flow field pressure.
4.3 Analysis Process
5. Results and Discussions
In order to study the convergence of the channel mesh proportions, the flow-induced wall shear stress distribution corresponding to different mesh proportions with the same reactant gas inlet velocity and pressure were computed. The inlet velocity was chosen as
200 m/s and the inlet pressure was 2 atm. Figures 5 to 8 show the results. Figure 9
shows the mesh proportions versus
maximum wall shear stress. From the convergence of the curve shown in the Figure 9, one can see that the proportion -20 is fine enough. Therefore, the results obtained corresponding to the proportion -20 will be used to compute the stress distribution between the sputtered Ag layer and the SU8 substrate of the micro-channel. Figure 10 shows the flow-induced pressure corresponding to proportion -20. The primitive results of the Von Mises stress distribution at the channel wall composite layer is shown in Figure 11. It can be seen that the maximum stress occurs at the inlet of the wall. Figure 12 shows the stress concentration zone.
.
Figure.5 Wall shear stress(MPa) corresponding to mesh proportion -5 Computational Fluid
Mechanics
Element Type Chosen
Channel Modeling
Proper Mesh Proportion Determined
Compute Flow-Induced Wall Shear Stresses and
Normal Pressure
Solid Mechanics Analysis
Element Type Chosen
Compute Interface Stress Distribution
6 Figure.6 Wall shear stress(MPa) corresponding to mesh proportion -10
Figure.7 Wall shear stress(MPa) corresponding to mesh proportion -15
Figure.8 Wall shear stress(MPa) corresponding to mesh proportion -20
Solution Converge with Mesh Proportion Variation
0 1000 2000 3000 4000 5000 -25 -20 -15 -10 -5 0 Mesh Proportion W al l S he ar S tr es s (M P a)
Figure.9 Mesh proportions v.s. maximum wall shear stress
Figure.10 The flow-induced pressure (MPa) of channel
Figure.11 The Von Mises stress (Mpa) of the composite layer
Figure.12 The stress concentration zone of the composite layer
6. Conclusion and Self Commentary A computational there-dimensional straight channel model for predicting the flow-induced wall shear stress and pressure distribution of a Micro-channel was developed. Simulations were performed by using the commercial package software Ansys®8.0. Proper channel mesh proportion was determined. Then the results were used to study the stress distribution between the sputtered Ag layer and the SU8 substrate of the micro-channel. Some primitive results have obtained.
The purpose of this study is to study the peeling phenomenon between Ag and SU8 of a micro-channel. In the first half of this year, the efforts were concentrated on set up the numerical model and the analysis process. In the second half of this year, the efforts will be concentrated on studying the effects of the variation of micro-channel construction and the gas inlet velocity and pressure on the interface stress distribution in a flow field plate.
8. References
[1] A. Kumar and R. G. Reddy, “Effect of channel dimensions and shape in the flow-field distributor on the performance of polymer electrolyte membrane fuel cells”, Journal of Power Sources, 113, (2003), pp.11–18.
[2] S. Haasl, “Assembly of micro-systems for optical and fluidic applications”, Ph. D. Dissertation, The Royal Institute of Technology, Stockholm, Sweden, 2005. [3] Z. Y. Guo and Z. X. Li, “Size effect on
microscale single-phase flow and heat transfer”, International Journal of Heat and Mass Transfer, 46, (2003), pp. 149–159. [4] D. Tang, C. Yang, D. N. Ku, “A 3-D
thin-wall model with fluid-structure interactions for blood flow in carotid arteries with symmetric and asymmetric stenosis”, Computers and Structures, 72, (1999), pp. 357-377.
[5] P. Ausiello, A. Apicella, and C. L. Davidson, “Effect of adhesive layer properties in stress distribution in composite restorations-a 3D finite element analysis”, Dental Materials, 18, (2002), pp. 295-303.
[6] J. Carmai, K.H. Baik, F.P.E. Dunne, P.S. Grant, and B. Cantor, “Interface effects during consolidation in titanium alloy components locally reinforced with matrix-coated fibre composite”, Acta Materialia, 50, (2002), pp. 4981–4993. [7] http://www.matweb.com
