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Student Number 93343037 Author Chia Ming(黃家明) Author's Email Address 93343037@cc.ncu.edu.tw Statistics This thesis had been viewed 1511 times. Download 789 times. Department Mechanical Engineering Year 2009 Semester 1 Degree Ph.D. Type of Document Doctoral Dissertation Language English Title Optimizations of Flow Distributors and Anodic Microstructures for Planar SOFC Date of Defense 2010-01-26 Page Count 95 Keyword anodic microstructures cell tests flow uniformity numerical simulations solid oxide fuel cell Abstract This thesis aims to study two facets of the polarization problems in planar solid oxide fuel cells (SOFC) concerning optimization of both flow distributors and anodic microstructures with an emphasis on the degradation mechanisms due to effects of flow uniformity in flow channels and flow resistances in porous electrodes. The first objective is to investigate the effect of flow uniformity in various flow distributors to the cell performance of planar SOFC using both numerical simulations and experimental measurements. The former involves several 3-D numerical models implemented by CFD-RC packages which have been used to simulate various hydraulic rib-channel experiments previously performed by Yen (2004) in our laboratory. Numerical flow data were found in good agreement with the experimental results obtained by Yen (2004). Then the validated numerical models were used to evaluate the effect of flow uniformity modulating by various different designs of flow distributors to the cell performance of a single-cell stack. It was found that a new design, using simple small guide vanes equally-spaced around the feed header of the double-inlet/single-outlet module, can effectively improve the degree of flow uniformity in flow distributors resulting in 11% increase of the peak power density (PPD). For experimental measurements, a test rig was established, so that the power-generating characteristics as well as the ac impedance spectra of the single-cell stack using different designs of flow distributors can be measured. The goal is to show how exactly the cell performance would vary with a change in the degree of flow uniformity in these aforementioned flow distributors. We found that by improving the degree of flow uniformity in flow distributors, values of PPD of the single-cell stack can be indeed increased up to 14%. Furthermore, we also found that the ohmic resistance and the low-frequency arc of the single-cell stack can be reduced respectively 32% and 40% when using the optimal flow distributors and the operating voltage is set at 0.6 V.

The second objective is to investigate effects of porosity (e), permeability (k) and tortuosity (t) of anodic microstructures to the cell performance of a single-unit planar anode-supported SOFC using 3-D electrochemical flow models with measured porous transport properties. In particular, we measured an effective viscosity (mu_e) in the Brinkman equation commonly used to predict flow properties in porous electrodes of SOFC. It is found that, contrary to the popular scenario, mu_e is not equal to the fluid viscosity (mu_f), but it is several orders in magnitude smaller than mu_f. This difference can result in more than 10% difference on values of PPD. Our numerical analyses reveal three points. (1) While keeping k and ? fixed with e varying from 0.2 to 0.6, the highest PPD occurs at e = 0.3 where the corresponding triple-phase-boundary length is a maximum. (2) The value of PPD increases slightly with k when k ≤ 10-11 m2 because of the diffusion limitation in anode. (3) The value of PPD decreases with t when t > 1.5 due to the accumulation of non-depleted products. Hence, a combination of e = 0.3, k = 10-11 m2, and t = 1.5 is suggested for achieving higher cell performance of planar SOFC. These results should be useful for further improving cell performance and longevity of planar SOFCs.Table of Content 中文摘要i

各章中文一頁總結ii

Abstractx

致謝xii

List of Tablesxiv

List of Figuresxv

Chapter 1 Introduction1

1.1 Solid Oxide Fuel Cells1

1.2 Why and What to Study?3

1.2.1 Effect of Flow Uniformity3

1.2.2 Effect of Anodic Microstructures5

1.3 Thesis Outline7

Chapter 2 Literature Review12

2.1 Researches on Optimization of Flow Distributors12

2.2 Researches on Flow Dynamics at Porous Interface15

2.3 Researches on Optimization of Anodic Microstructures20

2.4 Researches on AC Impedance Measurements of SOFCs22

Chapter 3 Experimental Methods33

3.1 Test Rig33

3.2 Testing Platform34

Chapter 4 Numerical Methods43

Chapter 5 Numerical Results for Optimization of Flow Distributors51

5.1Flow Uniformity in Various Flow Distributors51

5.2 Effect of Flow Uniformity on Cell Performance52

5.3Effect of Reynolds Number on Cell Performance54

Chapter 6 Numerical Results for Optimization of Anodic Microstructures62

6.1 Effect of Viscosity Ratio on Velocity Profile in Porous Channel62

6.2 Effect of Viscosity Ratio on Cell Performance63

6.3 Effect of Anodic Microstructures on Cell Performance65

Chapter 7 Performance Measurements of Single-Cell Stacks73

7.1 Effect of Flow Uniformity on Cell Performance73

7.2 Variations of Impedance Spectra with Flow Uniformity75

Chapter 8 Conclusions and Future Work85

8.1 Conclusions85

8.2 Recommendations for Future Work86

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