Three-Dimensional Model of a Plate Methanol Steam Micro-Reformer with

The research above has shown that micro-reformer performance can be enhanced by suitable thermo-fluid parameters. However, there has been a limited amount of work investigating the effects of the different flow field designs on thermo-fluid parameters, especially for the serpentine flow field. Therefore, the objective of this section is to establish a three-dimensional serpentine flow field model of the plate methanol steam micro-reformer to investigate its transport phenomena and methanol conversion efficiency.

Figure 2-3 presents schematics of the three-dimensional plate methanol steam micro-reformer considered in this work. The inlet cross-section of the channel is 1 mm x 1 mm. The thicknesses of the catalyst layers are set at 50 μm. The base conditions of the properties are as follows; the operating pressure is 1 atm and the inlet temperature is 120 . ℃ The inlet flow velocity is 1 m/s and the molar ratio of H2O/CH3OH is 1.3 in the first example.

The physical properties of the channels are listed in Table 4-6.

In order to study the effect of grid number on the numerical results, the grid independence was examined in preliminary test runs. For simplification of the analysis, three grid configurations were evaluated for the single channel of the plate methanol steam micro-reformer at a wall temperature of 230 . The single channel length is 33 mm,℃ and the

cross-section of the channel is 1 mm x 1 mm. The thicknesses of the catalyst layer are set at 50 um. The inlet flow velocity is 1 m/s and the molar ratio of H2O/CH3OH is 1.3. The physical properties of the channel are listed in Table 4-6. The numbers of grid lines in the x, y and z directions were 265x36x9, 133x26x5, and 67x16x3. The influence of grid lines on the local temperatures is shown in Table 4-7. The deviations of local temperatures are 0.04-1.1%

for grids 67x16x3 and 133x26x5, and 0.4-2.8% for grids 133x26x5 and 265x36x9. Therefore, Grid 132x25x4 was chosen for the simulation in the present study as a tradeoff between accuracy and CPU computation time.

In order to compare the numerical results and experimental data, a micro-reformer with parallel flow field is tested. Fig. 4-15 shows a comparison of the present prediction with previous experimental data. The solid symbols denote the experimental results of Park et al.

[16] and the curve is the present prediction. Only small discrepancies between the numerical results and the experimental data have been found. The numerical model accurately predicted the methanol conversion and the gas distributions. Hence, the proposed three-dimensional numerical model is adequate for analyzing the heat and mass transfer in a micro-reformer.

4.3.1 Effects of Thermo-Fluid Parameters on the Plate Methanol Steam Micro-Reformer with Serpentine Flow Field Performance

The wall temperature, Tw, on the heated wall is important. To this end, the effects of wall temperatures on the dimensionless temperature distributions along the centerline of the serpentine flow field at a Reynolds number of 41 and a H2O/CH3OH molar ratio of 1.3 are

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variation when X>0.3. This is due to thermal equilibrium for X>0.3, in which the temperature has a more uniform distribution. In addition, it is found in Fig. 4-25(a) that the dimensionless temperature distributions are almost the same for various values of wall temperature. Figure 4-25(b) shows the effects of the wall temperature Tw on the distribution of CH3OH and H2

mole fraction along the centerline of the serpentine flow field. The CH3OH mole fraction gradually decreases along the flow directions due to the chemical reaction, whereas the H2

mole fractions increase when the flow moves downstream. It is also clearly seen that the CH3OH consumption and the H2 mole fractions increase with an increase in the wall temperature. A lower CH3OH mole fraction and a higher H2 mole fraction along the serpentine flow field represent a higher methanol conversion.

The velocity distributions along the centerline of the serpentine flow field for various values of wall temperature are presented in Fig. 4-26(a). Significant variations are seen clearly at the turning points of the flow channel due to the change of velocity direction. The fuel flow velocity increases from the inlet to outlet, due to increases in the temperature caused by significant density variation. The rise in fuel velocity becomes relatively insignificant since it is fully developed thermally at X>0.3. The thermo-fluid parameters affect not only the methanol conversion efficiency, but also pressure loss (the difference between local and inlet pressures) in the serpentine flow channel. Large pressure drops in the channel mean that more pumping work is needed to pump the reactants. Thus, pressure loss is a significant issue. An exploration of pressure loss for various wall temperatures along the centerline of the serpentine flow field is presented in Fig. 4-26(b). Local pressure loss increases along the serpentine flow field. A larger pressure loss occurs at higher wall temperatures. In addition, clearly observed variations in the pressure loss appear at the turning points, due to the velocity change. The pressure losses are higher for the serpentine flow field than for the parallel flow channel, so reducing them is a priority for future use with the plate methanol micro-reformer.

The effects of the Reynolds number (Re) on the local dimensionless temperature, CH3OH and H2 mole fraction distributions at Tw=230 are analyzed in ℃ Fig. 4-27. A careful examination of Fig. 4-27(a) reveals that the dimensionless temperature increases as the fluid moves downstream due to the heated wall. In addition, the dimensionless temperature rise is lower for a higher Re. This is due to the fact that a higher Reynolds number significantly increases the heat leaving the flow channel, which decreases the temperature rise. In Fig.

4-27(b), the CH3OH mole fraction clearly decreases along the serpentine flow field. This is due to the temperature rise caused by the strong chemical reaction, which in turn increases the methanol consumption. Also, the methanol mole fraction distributions decrease with a lower Reynolds number. The results also show that a higher H2 mole fraction is noted for the lower Reynolds number. This is due to the longer gas resident time which results in a better methanol conversion and a higher H2 production.

Figure 4-28 presents the effects of the H2O/CH3OH (S/C) molar ratio on the CH3OH, H2

and CO mole fraction distributions at Tw=230℃. Figure 4-28(a) shows that more efficient methanol conversion is noted at a higher H2O/CH3OH molar ratio. It is also found that a higher molar ratio of H2O/CH3OH causes the H2 mole fraction to fall. The CO mole fraction distributions are also shown in Fig. 4-28(b). The CO concentration decreases with an increase in H2O/CH3OH molar ratio. This is because the higher H2O concentration would enhance the water-gas-shift reaction, which in turn reduces the CO concentration. However, the higher H2O/CH3OH molar ratio also reduces the H2 concentration at the channel outlet.

Figures 4-29 and 4-30 present the CH3OH, H2 and CO mole fraction distributions for T =230 along the middle cross℃ -section of the serpentine flow field (Y=0.5) and the

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catalyst layer. A comparison between Figs. 4-29 and 4-30 shows that the methanol consumption is higher at the interface compared with the middle cross-section. This is because the interface has a higher temperature and chemical reaction rate. It is evident that the mass fraction of H2 and CO increases along the middle cross-section of the serpentine flow field and along the interface between the flow channel and catalyst layer. This confirms that a methanol micro-reformer with a higher CH3OH consumption indicates a higher H2 and CO concentration, as would be expected.

Figure 4-31 presents the effects of the Reynolds number Re on the methanol conversion η and H2 production rate QH2 for various values of wall temperature Tw. In Fig. 4-31, the value of QH2 increases and η decreases with an increase in Re. This can be attributed to the fact that a lower Re implies an increase in the fuel residence time and temperature distributions in the micro-reformer, which in turn improves the methanol conversion. As for the QH2, a higher QH2 is found for the case with a higher Re due to the higher inlet flow rate.

In addition, η and QH2 increase with an increase in wall temperature Tw. Besides, it is important to note that a higher Reynolds number will not necessarily provide a better H2

production rate. When the methanol conversion is too small, a higher Reynolds number may provide a lower H2 production rate.

4.3.2 Effects of Various Heated Plates on Micro-Reformer Performance

The effects of various heated plates of the channel on the temperature distributions and CH3OH mole fraction distributions are examined in Fig. 4-32. In Fig. 4-32, two cases of heated plates were tested, top heated plate (Y=1) or bottom heated plate (Y=0). The effects of top versus bottom heated plates of the channel on temperature distributions along the center line of the serpentine flow field are shown in Fig. 4-32. A top heated plate has a higher

temperature distribution than a bottom heated plate. This is because a bottom heated plate has a smaller temperature rise due to fuel convection effects, while a top heated plate with the heated position near the catalyst layer has a more significant temperature rise. For a bottom heated plate, it is also apparent that marked temperature variations occur at the turning points of the flow channel due to the change of the heat flow direction. The top heated plate clearly has a larger methanol consumption than the bottom heated plate. These phenomena make it obvious that the stronger chemical reaction for the case with the top heated plate is due to a higher temperature distribution.

4.4 Three-Dimensional Channel Model of a Plate Methanol Steam