标题: | 平板式微型甲醇蒸汽重组器热质传特性与流道设计之研究 Study on Heat and Mass Transfer Characteristics and Flow Channel Design in a Plate Methanol Steam Micro-Reformer |
作者: | 薛清益 Hsueh, Ching-Yi 陈俊勋 曲新生 Chen, Chiun-Hsun Chu, Hsin-Sen 机械工程学系 |
关键字: | 微型重组器;甲醇;热质传;数值分析;micro-reformer;methanol;heat and mass transfer;numerical analysis |
公开日期: | 2010 |
摘要: | 本论文系以數值分析探讨平板式微型甲醇蒸汽重组器(包含甲醇蒸汽重组器与甲醇触媒燃烧器)之热质传现象,本研究首先针对微型甲醇蒸汽重组器,探讨几何效应与热流效应对甲醇转化率及气体浓度分布之影响,以俾获得较佳的流道设计与操作条件,接着并加入甲醇触媒燃烧器,其结果可以提供平板式微型甲醇蒸汽重组器一个完整的设计资讯。 本研究探讨的议题主要分为二个部份:第一部份是以微型甲醇蒸汽重组器为主,并不考虑甲醇触媒燃烧器。首先建立一甲醇蒸汽重组器之二维流道數学模型,并探讨几何参数与热流参数对重组器性能与流道内热质传现象之影响。研究结果发现当壁面温度由200度升高至260度时,甲醇转换效率约提升49%,结果也显示当入口甲醇与水之燃料比由1.0变为1.6时,流道出口之CO浓度会从1.72%降低至0.95%。而选用较长的流道长度、较低的流道高度、较大的触媒高度、较大的触媒孔隙度、较高的壁面温度与较低的雷诺数等参数可以有效提升微型重组器之性能。接着建立甲醇蒸气重组器之三维流道数学模型,并探讨不同流道高宽比与流道几何尺寸对气体传输现象与微型甲醇蒸汽重组器性能之影响。结果显示,壁面传导效应对于模型之温度分布会有显着的影响,因此在分析模型中,必须考虑壁面传导效应之影响。结果亦显示,较低的流道高宽比会有较好的微型甲醇蒸汽重组器性能,主要是由于较低的流道高宽比会有较大的化学反应面积,而流道尺寸较小时,则会有较佳的甲醇转化率,此乃肇因于较小的流道尺寸会有较均匀的温度分布,因此能有效提升燃料使用率。最后将已经建立之三维流道模组进一步扩展至具蛇型流道之微型甲醇蒸汽重组器,并利用数值方法探讨壁面温度、入口燃料比与雷诺数对具蛇型流道之微型甲醇蒸汽重组器性能与传输现象之影响。结果显示,藉由降低雷诺数与提高入口燃料比可以有效提升甲醇转换效率。而加热壁面在蛇型流道之顶端(Y=1)或底部(Y=0)时,吾人发现加热壁面在流道顶端时,会有较佳的甲醇转换率,此乃肇因于加热壁面在流道顶端时,会有较大的化学反应。 而本论文第二部份主要是利用数值方法针对微型甲醇蒸气重组器并搭配触媒燃烧器之热质传特性与性能进行研究,首先建立甲醇蒸气重组器搭配触媒燃烧器之三维流道數学模型,来探讨不同流动形式与几何参数对微型甲醇重组器性能之影响,结果显示采用逆向流比起平行流可以有效改善重组器10%的效能,主要是由于逆向流有较佳的热管理能力,因此能有效改善重组器之转换效率,结果也显示,适当的几何参数会有较佳的热管理能力与甲醇转换率,而当燃烧器有较大的雷诺数时,会有较大的壁面温度,因此能有效提升甲醇转化率。接着建立具不同流道形状(蛇型流道与直通流道)之三维甲醇蒸汽重组器搭配甲醇触媒燃烧器模型,并探讨不同流道对甲醇转化率与传输现象之影响。结果显示,具蛇型流道之微型甲醇蒸汽重组器与甲醇触媒燃烧器会有最佳的甲醇转换率,此乃肇因于采用蛇型流道作为微型甲醇蒸汽重组器与甲醇触媒燃烧器之流道时,会有较佳的热管理能力。本论文之数值模型可以有效的分析微型重组器内传输现象,其结果将有助于今后平板式微型重组器之设计。 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. |
URI: | http://140.113.39.130/cdrfb3/record/nctu/#GT079414815 http://hdl.handle.net/11536/40769 |
显示于类别: | Thesis |
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