平板式微型甲醇蒸汽重組器熱質傳特性與流道設計之研究

Abstract

本論文係以數值分析探討平板式微型甲醇蒸汽重組器（包含甲醇蒸汽重組器與甲醇觸媒燃燒器）之熱質傳現象，本研究首先針對微型甲醇蒸汽重組器，探討幾何效應與熱流效應對甲醇轉化率及氣體濃度分佈之影響，以俾獲得較佳的流道設計與操作條件，接著並加入甲醇觸媒燃燒器，其結果可以提供平板式微型甲醇蒸汽重組器一個完整的設計資訊。 本研究探討的議題主要分為二個部份：第一部份是以微型甲醇蒸汽重組器為主，並不考慮甲醇觸媒燃燒器。首先建立一甲醇蒸汽重組器之二維流道數學模型，並探討幾何參數與熱流參數對重組器性能與流道內熱質傳現象之影響。研究結果發現當壁面溫度由200度升高至260度時，甲醇轉換效率約提升49％，結果也顯示當入口甲醇與水之燃料比由1.0變為1.6時，流道出口之CO濃度會從1.72％降低至0.95％。而選用較長的流道長度、較低的流道高度、較大的觸媒高度、較大的觸媒孔隙度、較高的壁面溫度與較低的雷諾數等參數可以有效提升微型重組器之性能。接著建立甲醇蒸氣重組器之三維流道數學模型，並探討不同流道高寬比與流道幾何尺寸對氣體傳輸現象與微型甲醇蒸汽重組器性能之影響。結果顯示，壁面傳導效應對於模型之溫度分佈會有顯著的影響，因此在分析模型中，必須考慮壁面傳導效應之影響。結果亦顯示，較低的流道高寬比會有較好的微型甲醇蒸汽重組器性能，主要是由於較低的流道高寬比會有較大的化學反應面積，而流道尺寸較小時，則會有較佳的甲醇轉化率，此乃肇因於較小的流道尺寸會有較均勻的溫度分佈，因此能有效提升燃料使用率。最後將已經建立之三維流道模組進一步擴展至具蛇型流道之微型甲醇蒸汽重組器，並利用數值方法探討壁面溫度、入口燃料比與雷諾數對具蛇型流道之微型甲醇蒸汽重組器性能與傳輸現象之影響。結果顯示，藉由降低雷諾數與提高入口燃料比可以有效提升甲醇轉換效率。而加熱壁面在蛇型流道之頂端(Y=1)或底部(Y=0)時，吾人發現加熱壁面在流道頂端時，會有較佳的甲醇轉換率，此乃肇因於加熱壁面在流道頂端時，會有較大的化學反應。 而本論文第二部份主要是利用數值方法針對微型甲醇蒸氣重組器並搭配觸媒燃燒器之熱質傳特性與性能進行研究，首先建立甲醇蒸氣重組器搭配觸媒燃燒器之三維流道數學模型，來探討不同流動形式與幾何參數對微型甲醇重組器性能之影響，結果顯示採用逆向流比起平行流可以有效改善重組器10％的效能，主要是由於逆向流有較佳的熱管理能力，因此能有效改善重組器之轉換效率，結果也顯示，適當的幾何參數會有較佳的熱管理能力與甲醇轉換率，而當燃燒器有較大的雷諾數時，會有較大的壁面溫度，因此能有效提升甲醇轉化率。接著建立具不同流道形狀(蛇型流道與直通流道)之三維甲醇蒸汽重組器搭配甲醇觸媒燃燒器模型，並探討不同流道對甲醇轉化率與傳輸現象之影響。結果顯示，具蛇型流道之微型甲醇蒸汽重組器與甲醇觸媒燃燒器會有最佳的甲醇轉換率，此乃肇因於採用蛇型流道作為微型甲醇蒸汽重組器與甲醇觸媒燃燒器之流道時，會有較佳的熱管理能力。本論文之數值模型可以有效的分析微型重組器內傳輸現象，其結果將有助於今後平板式微型重組器之設計。

This dissertation aims to examine numerically heat and mass transport phenomena in the plate methanol steam micro-reformer (including methanol steam micro-reformer and methanol catalytic combustor). The first focus is to investigate the effects of geometric and thermo-fluid parameters on the methanol conversion and gas concentration distributions of the methanol steam micro-reformer in order to obtain better channel designs and operating conditions. Furthermore, a methanol steam micro-reformer with a methanol catalytic combustor is considered in the present work. The results can provide comprehensive information for designing the plate methanol steam micro-reformer. This study can be divided into two parts. In the first part, the research only considered the plate methanol steam micro-reformer, namely the methanol catalytic combustor is not included in it. Firstly, a 2-dimensional channel model of the methanol steam micro-reformer is established to investigate effects of geometric and thermo-fluid parameters on performance and heat and mass transfer phenomena in micro-reformer channels. The results of the modeling suggest that the methanol conversion could be improved by 49 %-points by increasing the wall temperature from 200 ℃ to 260 ℃. The results also show that the CO concentration would be reduced from 1.72% to 0.95% with the H2O/CH3OH molar ratio values ranging from 1.0 to 1.6. The values of parameters that enhance the performance of micro-reformer were identified, such as longer channel length, smaller channel height, thicker catalyst layer, larger catalyst porosity, lower Reynolds number and higher wall temperature. Secondly, a 3-dimensional channel model of the methanol steam micro-reformer is developed to investigate the effects of various height and width ratios and channel geometric size on the reactant gas transport characteristics and micro-reformer performance. The predictions show that conduction through the wall plays a significant effect on the temperature distribution and must be considered in the modeling. The predicted results also demonstrated that better performance is noted for a micro-reformer with lower aspect-ratio channel. This is due to the larger the chemical reaction surface area for a lower aspect-ratio channel. The results indicate that the smaller channel size experiences a better methanol conversion. This is due to the fact that a smaller channel has a much more uniform temperature distribution, which in turn, fuel utilization efficiency is improved for a smaller channel reformer. Finally, the established 3-dimensional channel model of a plate methanol steam micro-reformer extends to be a plate methanol steam micro-reformer with serpentine flow field. A numerical investigation of the transport phenomena and performance of a plate methanol steam micro-reformer with serpentine flow field as a function of wall temperature, fuel ratio and Reynolds number are presented. The methanol conversion is improved by decreasing the Reynolds number or increasing the S/C molar ratio. When the serpentine flow field of the channel is heated either through top plate (Y=1) or the bottom plate (Y=0), we observe a higher degree of methanol conversion for the case with top plate heating. This is due to the stronger chemical reaction for the case with top plate heating. In the second part, a numerical study is performed to examine the characteristics of heat and mass transfer and the performance of a plate methanol steam micro-reformer with a methanol catalytic combustor. Firstly, a three-dimensional channel numerical model of a micro-reformer with combustor is developed to examine the effects of various flow configurations and geometric parameters on micro-reformer performance. Comparing the co- and counter-current flows via numerical simulation, the results show that the methanol conversion for counter-current flow could be improved by 10%. This is due to the fact that counter-current flow leads to a better thermal management, which in turn improves fuel conversion efficiency. The results also reveal that the appropriate geometric parameters exist for a micro-reformer with a combustor to obtain better thermal management and methanol conversion. With a higher Reynolds number on the combustor side, the wall temperature is increased and the methanol conversion can thus be enhanced. In addition, the three-dimensional models of a plate methanol steam micro-reformer and a methanol catalytic combustor with the parallel flow field and the serpentine flow field have been established to investigate the performance and transport phenomena in the micro-reformer. The methanol conversion of the micro-reformer with the serpentine flow field and the combustor with the serpentine flow field is the best due to a better thermal management in the micro-reformer. The numerical model provides an efficient way to characterize the transport phenomena within the micro-reformer, and the results will benefit the future design for the plate methanol steam micro-reformer.