Effects of Channel with Various Height and Width Ratios on Micro-Reformer

Using previously stated the numerical model, the effects of aspect ratios of channel on methanol conversion and transport phenomena were emphasized. The various cases for different aspect ratios are shown in Table 4-3. The aspect ratios, γ, are defined as follows:

1 2

WR

δ + δ

γ = (4-1)

Where (δ1+δ2) and WR are the channel height and width, respectively. The corresponding hydraulic diameters in Table 4-3 are fixed to be 0.286 mm.

In the present study, the mass fractions of the inlet reactant gas including methanol vapor and water vapor of 0.38 and 0.62, respectively, were tested. Thus, the molar ratio of H2O/CH3OH was kept constant at 1.1. The inlet flow velocity of 0.266 m/s used and therefore, the corresponding Reynolds number Re was 3.14 for each test. The geometric dimensions and

physical properties of the channel are listed in Table 4-4.

The grid independence was examined in preliminary test runs. Three grid configurations were evaluated for the channel of the plate methanol steam micro-reformer at a wall temperature of 200 . The numbers of grid lines in the x, y and z directions were: 41x1℃ 6x18, 51x21x23, and 71x31x33. The influence of grid lines on the local methanol mole fraction is shown in Table 4-5. The deviations of methanol mole friction are 0.4% for grids 41x16x18 and 51x21x23, and 0.4% for grids 51x21x23 and 71x31x33. Grid 51x21x23 was, therefore, chosen for the simulation in the present study as a tradeoff between accuracy and CPU computation time.

For aspect ratio of γ=0.5, Fig. 4-16 presents the local distributions of the different species at wall temperatures of 200 and 260℃ along the center of the channel. Overall inspection ℃ of Fig. 4-16 disclosed that the CH3OH and H2O mole fractions decrease along the channel, while the H2, CO2 and CO mole fractions increase along the center of the channel. The results demonstrate that as the mole fractions of the products increase as the wall temperature increases. The results also show that the methanol conversion is about 49% for a wall temperature of 200 , wit℃ h a gas composition of 24％ CH3OH, 28% H2O, 36% H2, and 12%

CO2 at the channel outlet. However, the CO concentration is only 244 ppm. For a wall temperature of 260 , the results℃ indicates that the methanol conversion is greater than 96%, with a product gas composition of 74.05% H2, 24.28% CO2, and 1.67% CO at the channel outlet. For the PEMFC, the CO concentration must be less than 10 ppm which can be achieved using a CO oxidation reactor. The utilization of the preferential oxidizer (PrOx) reactor or water-gas-shift reaction can reduce the CO concentration in the gas from the

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and CO concentration (ppm). In this work, the catalyst thicknesses are fixed to be 30μm. A careful examination of Fig. 4-17 reveals that the methanol conversion increases with an increase in wall temperature. Thus, the methanol conversion can be improved by increasing wall temperature, which in turn, increases the chemical reaction rate. It is also found form Fig.

4-17 that the methanol conversion increases with a decrease in aspect ratio due to the large chemical reaction area for a low aspect-ratio channel. This implies that better performance is noted for a lower aspect-ratio channel of micro-reformer. The CO concentration (ppm) leaving the channel was also studied. In Fig. 4-17, the results show that the CO concentration increases with increasing wall temperature. This can be made plausible by noting the fact that the reaction rate of the endothermic reverse water-gas-shift reaction increases as the wall temperature increases. Additionally, it is also found that the CO concentration (ppm) increases with decreasing aspect ratios. A height/width ratio of γ=0.25 and a wall temperature of 260 ℃ yielded the better methanol conversion (greater than 98%), but the CO concentration was also higher, and was greater than 16000 ppm. Hence, the CO concentration in the outlet gases must be reduced for further use in a PEMFC.

The effects of aspect ratios on local methanol conversion distributions along the channel center line at T w=200 °C and T w=260 °C are presented in Fig. 4-18. An overall inspection of Fig. 4-18 reveals that the aspect ratios of channel have a considerable impact on the local methanol conversion distributions along the center line. It is found that the predicted methanol conversion increases along the channel. In addition, methanol conversion efficiency increases with decreasing aspect ratios of channel. As for the effects of wall temperature, better methanol conversion is noted for a case with a higher wall temperature owing to a stronger chemical reaction rate.

Figure 4-19 shows the effects of the aspect ratios of channel on the distribution of the H2

and CO mole fractions, respectively, along the channel center line. It is clear in Fig. 4-19(a)

that the H2 mole fraction along the channel center line represents the methanol conversion, with a higher H2 fraction indicating higher methanol conversion rates. Thus, the variation of the H2 fraction is opposite to that of the CH3OH mole fraction. It is clearly observed from Fig.

4-19(a) the H2 mole fraction increases as the aspect ratio decreases and wall temperature increases. The trends of the variations in Fig. 4-19(b) can be interpreted in a similar way to the data in Fig. 4-19(a). This is because that a higher methanol conversion results in a higher CO production. These phenomena are more obvious at a higher wall temperature (260 ), ℃ because the methanol micro-reformer produces more CO for a higher wall temperature.

Figure 4-20(a) presents the reactant gas velocity distributions in the center of the channel along the direction of flow from the inlet to the outlet for wall temperatures of 200 and 260℃

for the various aspect ratios

℃ . It is clearly seen in Fig. 4-20(a) that the velocity slowly increases from the inlet to the outlet. The wall temperature of 260 ℃ generates higher gas velocity distributions than that of 200 .℃ The results show that the methanol conversion of the micro-reformer is independent of the velocity distributions in the center of the channel due to the chemical reaction dominating methanol conversion. Then, to explore the pressure loss caused by the various aspect ratios, the local pressure losses (the difference between local and inlet pressures) along the channel center line for wall temperatures of 200 and 260℃ ℃ under various aspect ratios are presented in Fig. 4-20(b). In Fig. 4-20(b), it is found that the local pressure loss increases along the flow channel. A careful examination of Fig. 4-20(b) discloses that larger pressure loss is noticed for a case with a higher wall temperature (260 ). ℃ Additionally, the results show that larger pressure loss is found for a smaller aspect ratio (γ=0.25). The higher pressure losses mean that excess work has to be done in pushing the

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chemical reaction. When X>0.03, the fluid temperature increase insignificantly along channel because there is a thermal equilibrium beyond this point. It is also shown that the temperature rises with the decrease in aspect ratios, γ. As the γ decreases, the temperature distributions become much more uniform. Therefore, a smaller aspect ratio leads to a better methanol conversion. Afterwards, to explore the change in steam reforming reaction rate caused by the various aspect ratios of the channels, the local steam reforming reaction rates along the interface between the catalyst layer and the flow channel are presented in Fig. 4-21(b). For a wall temperature of 260℃, larger aspect ratios can improve the steam reforming reaction rate, as a result of the methanol being consumed in the forward region of channel inlet. For a lower wall temperature of 200℃, the change in steam reforming reaction rate follows a similar trend to that of the wall temperature 260 . In addition, it is found that℃ a higher steam reforming reaction rate is found for a case with a higher wall temperature due to a stronger steam reforming reaction.

Figure 4-22 demonstrates the effects of the Reynolds number on the methanol conversion and the H2 production rate of the channel in the plate methanol steam micro-reformer for wall temperatures of 260 . ℃ An overall inspection of Fig.4-22 discloses that the methanol conversion increases with decreasing Reynolds number Re. This can be made plausible by noting the fact that the fuel can spend more time in the channel for a lower Reynolds number Re. As for the H2 production rate, the results show that the H2 production rate increases as the Re increases. Additionally, the effects of the aspect ratios on the methanol conversion and H2 production rate are shown in Fig. 4-22. It is clear that the methanol conversion increases with a decrease in aspect ratio. When γ=0.5 with a Re of 3.14, the methanol conversion was greater than 86%, and the H2 production rate was 184 cm³/min.

The plate methanol steam micro-reformer produced hydrogen to supply fuel cell, Park et al.

[16] presented that the H2 production rate was only 186cm³/min, which could supply a 15W

fuel cell.

4.2.2 Effects of Geometric Size on the Transport Phenomena and Performance