Effects of Various Flow Configurations and Geometric Parameters on

In the present section, a three-dimensional channel model of a micro-reformer with combustor is developed to examine the effects of various flow configurations and geometric parameters on micro-reformer performance. A three dimensional model is analyzed to understand heat and mass transfer in the channels of a methanol steam micro-reformer with methanol catalytic combustor. The influences of wall conduction effects on the transport phenomena of heat and mass transfer in a micro-reformer with combustor are important.

Therefore, the local temperature distributions along the centerline of the top reforming channel (Y=0.333) and the CH3OH mole fraction distributions along the centerline of the reforming channel (Y=0.167) are presented in Fig. 4-34. In this work, X denotes the dimensionless distance from the flow channel inlet to the outlet. It is clearly seen in Fig.

4-34(a) that the temperature distributions with the wall thermal conduction effect show a lower and more uniform distribution than that without a wall conduction effect. This implies that the effects of wall conduction on the thermal development in a micro-reformer with combustor are important. It is also found in Fig. 4-34 that the effects of wall conduction lead to a higher methanol distribution than without a wall conduction effect due to a smaller value of the temperature distribution. However, the wall conduction effects on the heat and mass transport phenomena are remarkable and cannot be neglected in the modeling. Therefore, their effect should be considered in this work.

The influences of the flow configurations on the transport phenomena and the

performance of micro-reformers are important. To this end, the effects of the flow configurations for co- and counter-current flow on the temperature distributions along different axial location lines and on the local distributions of the mole fractions of various species along the centerline of the reforming channel are presented in Fig. 4-35. Fig. 4-35(a) discloses that the temperature distributions are much more uniform due to the shorter thermal entrance length. It is also obvious that a higher temperature distribution is noted for the counter-current flow. This is due to the fact that counter-current flow leads to better thermal management. Figure 4-35(b) shows the local distributions of the different species for co- and counter-current flow along the centerline of the reforming channel (Y=0.167). An overall inspection of Fig. 4-35(b) reveals that both the mole fractions of CH3OH and H2O decrease as the fluid moves downstream, while the H2, CO2 and CO mole fractions increase with axial location. In addition, a lower CH3OH mole fraction along the centerline of the channel represents a higher methanol conversion rate. The methanol conversion rate is greater than 91% for the counter-current flow, with a product gas composition of 73.2% H2, 25.1 % CO2

and 1.7% CO at the outlet of the reforming channel. Comparing co- and counter-current flow via numerical simulation, the results show that the methanol conversion efficiency for counter-current flow could be improved by 10% due to a higher temperature distribution.

In order to explain the effectiveness of the geometric parameters for a micro-reformer with combustor in the thermal management, the temperature and CH3OH mole fraction distributions for various geometric parameters were investigated. For a fixed Reynolds number, Figure 4-36 demonstrates the effects of the combustion flow channel heights on the temperature distributions along the centerline of the top reforming channel and on the

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lower CH3OH mole fraction is noted for a system with a greater combustion flow channel height due to a stronger chemical reaction for a higher temperature distribution. This means that a higher efficiency methanol conversion is enhanced via a greater combustion flow channel height.

An exploration of the temperature distributions for various reforming flow channel heights along the centerline of the top reforming channel is presented in Fig. 4-37(a). For fixed Re, the results show that a higher temperature distribution is found for a micro-reformer channel with a lower reforming flow channel height. This is because a higher channel height has a greater hydraulic diameter. As for the effects of reforming flow channel heights on the CH3OH mole fraction distributions along the centerline of the reforming channel, an overall inspection of Fig. 4-37(b) reveals that better micro-reformer performance is noted for a lower reforming channel height. This implies that the chemical reaction rate is slower for a system with a greater reforming channel height. This seems plausible as a stronger chemical reaction is experienced for a micro-reformer channel with a higher wall temperature.

The effects of the channel widths on the temperature distributions and CH3OH mole fraction distributions for a fixed Reynolds number were also investigated. Figure 4-38(a) presents the effects of channel widths on the temperature distributions along the centerline of the top reforming channel (Y=0.333). It is shown in Fig. 4-38(a) that the local temperature distribution increases with a decrease in the channel width. This may be because a rather narrow channel decreases the heat leaving the flow channel. In Fig. 4-38(b), the methanol conversion of the micro-reformer is slightly enhanced with a wider channel width. It is important to note that a higher temperature distribution will not necessarily provide better methanol conversion, because the channel width increases with increasing catalyst reaction area, which in turn increases the chemical reaction rate.

Figure 4-39 shows the effects of the steel widths on the temperature distributions and

CH3OH mole fraction distributions. Fig. 4-39 (a) presents the effects of the steel widths on the temperature distributions along the centerline of the top reforming channel (Y=0.333).

Comparison of the corresponding curves of the steel widths WL=0.25 mm, 0.5 mm, and 1.0 mm indicates that a higher temperature distribution is found with wider steel. As for the effects of the steel width on the CH3OH mole fraction distributions along the centerline of the reforming channel (Y=0.167), the results reveal that they have similar CH3OH mole fraction distributions. This is due to having similar temperature distributions. Therefore, the steel width does not have a significant impact on the methanol conversion.

4.4.2 Effects of the Reynolds Number (Re) on Heat and Mass Transfer