Effects of the Geometric Parameters on the Heat and Mass Transfer and

The influences of the geometric parameters and thermo-fluid parameters on the performance of micro-reformer are considered of great importance. To this end, the effects of the channel length (L=22 mm, 33 mm, and 44 mm), channel height (H=0.1 mm, 0.2 mm, and 1.0 mm), catalyst thickness (δ2=10 µm, 30 µm and 50 µm) and catalyst porosity (ε=0.28, 0.38, and 0.48) on the methanol conversion and CO concentration in the micro-reformer were investigated.

For fixed Reynolds number (Re=2.2), the effects of geometric parameters on methanol conversion of micro-reformer channel are presented in Fig. 4-2(a). The results show that the methanol conversion increases with an increase in the wall temperature Tw for all geometric conditions, implying that a better micro-reformer performance can be archived at higher wall temperature. By comparing the results of Tw = 200 ℃ and Tw = 260 with ℃ L=33 mm, H=0.2 mm, δ2=30 µm and ε=0.38, it shows that the methanol conversion for Tw = 260 ℃ could be improved by 49% relative to that of Tw = 200 ℃. Comparison of the corresponding curves of the channel lengths of L=22 mm, 33 mm, and 44 mm indicates that better methanol

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methanol conversion. Additionally, it is seen in Fig. 4-2(a) that the methanol conversion in the micro-reformer is enhanced by the increased catalyst thickness. The channel with thicker catalyst layer has a larger chemical reaction area, which in turn causes a better methanol conversion. The results show that methanol conversion improves from 80% to 99% at Tw = 260 with the catalyst thickness ranging from 10μm to 50μm. The effects of the porosity of ℃ catalyst layer (ε=0.28, 0.38, and 0.48) on methanol conversion of micro-reformer channel are also shown in Fig. 4-2(a). It is found that the methanol conversion increases with an increase in the porosity of catalyst layer. This means that reaction surface is enlarged via an increase in the catalyst porosity. In addition, the best methanol conversion is noted for the case with L=33 mm, H=0.2 mm, δ2=50 µm and ε=0.38 at Tw = 260 . This implies that the appropriate ℃ channel geometry and catalyst thickness are very critical for improving methanol conversion.

The CO concentration must be reduced for further use in a PEM fuel cell. Therefore, the CO concentration distributions for various geometric parameters are presented in Fig. 4-2(b).

It is clearly observed that the CO concentration increases with increasing wall temperature.

This is because the endothermic reverse water-gas-shift reaction increases as the wall temperature increases. It is clear in Fig. 4-2(b) that lower CO concentration is found for a micro-reformer with a shorter channel length. A detailed comparison of the corresponding curves shows that lower CO concentration is noted for a micro-reformer with a thinner catalyst thickness or a lower porosity. It is clearly seen that the CO concentration is about 16000ppm for the case with L=33 mm, H=0.2 mm, δ2=30 μm, ε=0.38 and a wall temperature of 260 . ℃

The effects of inlet fuel ratio on the CO concentration (ppm) at the outlet were also investigated. The effect of the molar ratio of H2O/CH3OH on the CO concentration for various geometric parameters and wall temperatures are shown in Fig 4-3. A careful inspection of Fig. 4-3 discloses that the CO concentration decreases with an increase in the

inlet molar ratio of H2O/CH3OH. This is due to the fact that the higher H2O concentration enhances the water-gas-shift reaction which, in turn, reduces the CO concentration. The results also show that the CO concentration would be reduced from 1.72% to 0.95% at Tw = 260 ℃with the H2O/CH3OH molar ratio values ranging from 1.0 to 1.6. However, the higher molar ratio of H2O/CH3OH also reduces the H2 concentration at the channel outlet. It is also found that the effects of the H2O/CH3OH molar ratio on the CO concentration are more significant for a case with a higher wall temperature.

The impact of channel height on temperature distribution along the centerline of the channel, at a fixed Reynolds number, was examined for the heights 0.1 mm, 0.2 mm and 1cm.

The hydraulic diameters of channel vary depending on channel heights. A higher channel height has a greater hydraulic diameter. Fig. 4-4 illustrates that the centerline temperature increases along the channel as a consequence of the heated wall. For a smaller channel height, the temperature distribution is much more uniform due to the shorter thermal entrance length.

This kind of uniform temperature distribution improves the chemical reaction rate. Therefore, as shown in Fig. 4-2 (a), the methanol conversion of the micro-reformer is slightly enhanced with the smaller channel height at higher wall temperature. A comparison of the temperature distributions for wall temperatures of 200 and ℃ 260 indicates that the centerline ℃ temperature increases with an increase in the wall temperature.

Figure 4-5 shows the effects of wall temperature on the cross-sectioned temperature at different axial locations for H=1.0mm at the wall temperatures of 200 and ℃ 260 . It is ℃ clearly found that near the entrance (X=0.076), the cross-sectioned temperature shows a significant variation. As the flow move downstream, the cross-sectioned temperature becomes

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expected, there is a large difference on the velocity scale between the catalyst layer and flow channel. The velocity distributions in the catalyst layer are down to two orders of magnitude smaller than that in the flow channel, indicating that gas diffusion is the dominant transport mechanism in the porous media. The results also indicate the wall temperature of 260 ℃ generates higher gas velocity distributions than that of 200 .℃

Figure 4-7 shows the local distributions of the different species at wall temperatures of 200 ℃and 260 along the centerline of the channel for the same operating conditions. Fig.℃ 4-7 discloses that both the mole fractions of the CH3OH and H2O decrease as the fluid moves downstream, while the H2, CO2 and CO mole fractions increase with axial location. Fig. 4-7 clearly demonstrates that the mole fractions of the products increase with an increase in the wall temperature. In addition, Fig. 4-7 (b) shows that the methanol conversion is greater than 99% at a wall temperature of 260 , with ℃ a product gas composition of 74.7% H2, 23.6%

CO2 and 1.7% CO at the outlet of the channel. The results agree reasonably with experimental data [16]. For the PEM fuel cell, the CO concentration should be less than 10 ppm, so cleanup step is required after reforming. The utilization of the PrOx or water-gas-shift reaction equipment can reduce the CO concentration in the gas from the micro-reformer.

Studies of the reactant gas transport in micro-reformer channels have shown that a detailed understanding of the local distribution of the CH3OH mole fraction along the channel is important for designing the micro-reformer. Therefore, the effects of geometric parameters on the local distributions of the CH3OH mole fraction along the channel center line are presented in Fig. 4-8. The results reveal that geometric parameters have a considerable impact on the local CH3OH distributions. It is found that the CH3OH mole fractions decrease as the fluid moves downstream due to the chemical reaction. For various channel heights, there appears to be little variation in the CH3OH mole fraction distributions. The higher methanol concentration is noted for a system with a longer channel length or with a lower catalyst layer

thickness and porosity. This implies that the chemical reaction rate is weaker for a system with a shorter channel length or with a lower catalyst layer thickness and porosity. The effect of wall temperature on the local CH3OH mole fraction can be found by comparing the corresponding curves in Figs. 4-8 (a) and (b). It is clear that smaller methanol concentration is noted for a case with a higher wall temperature. This can be explained by the fact that a stronger chemical reaction is experienced for a micro-reformer channel with a higher wall temperature.

The distributions of the H2 mole fraction along the channel are shown in Fig. 4-9 for various geometric parameters and wall temperatures. A higher H2 mole fraction along the channel represents a higher methanol conversion. Thus, the variation of the H2 fraction is opposite to that of the CH3OH mole fraction in Fig. 4-8. In Fig. 4-9, a higher H2 mole fraction is found for a micro-reformer channel with a longer channel length or with a higher catalyst thickness, porosity and wall temperature. Figure 4-10 presents the effects of the geometric parameters on the local CO mole fraction distribution along the center line at wall temperatures of 200 and 260℃ . ℃ The trends of the variations in Fig. 4-10 can be interpreted in a similar way as for the results in Fig. 4-8 since a higher methanol conversion results in a higher CO production.

4.1.2 Effects of Thermo-Fluid Parameters on the Heat and Mass transfer and