This study considers four inlet flow patterns, including uniform anode and cathode gas (Pattern I), uniform anode and non-uniform cathode gas (Pattern II),
non-uniform anode and uniform cathode gas (Pattern III), and both non-uniform anode and cathode gas (Pattern IV), shown in Figure 4-2. Non-uniform deviation is 0.5 for all cases, and all non-uniform profiles in the stacking direction progressively increase as calculated by Equations X(4.9)X andX(4.10)X. Figure 4-3 depicts cell temperature distribution on the bottom, middle, and top layer in different patterns.
Each layer in Figure 4-3(a) has a similar temperature contour outline, which increases along the y direction cathode gas flow. Meanwhile, temperature on the top layer is slightly higher than on the bottom layer, because the anode gas flows next to the top end plate, and the cathode gas flows next to the bottom end plate. The end plate is adiabatic with the surrounding as well as the molar flow rate of anode gas is smaller than that of cathode gas. Therefore, the top cell has less cooling due to the anode gas flow between the top cell and the top end plate. The bottom cell has more cooling due to the cathode gas flow between the bottom cell and the bottom end plate. The boundary effect on cell temperature can be neglected because temperature difference between the top and bottom layer is below 10°C . Figure 4-3(b) shows cell temperature distribution on the top, middle, and bottom layer in a ten-layer MCFC with a Pattern II inlet flow configuration. Temperature difference in the same position of different layers in this figure is clearly varied, over 30°C. A cathode gas not only supports oxygen reaction, but also cools the cell because of its flow rate larger than the anode gas. Cathode gas molar flow rate in Pattern II increases with an increase in the stacking direction, so top layer cell temperature is lowest because
most cathode gas flow rate is in this stack. Oppositely, the bottom layer cell has highest temperature due to lacking cathode gas in this layer. Figure 4-3(c) shows cell temperature distribution on the top, middle, and bottom layer in a ten-layer MCFC when the inlet flow configuration is Pattern III, or a non-uniform inlet anode gas. Anode gas on the top layer has more molar flow rate than the other layers, but cell temperature is highest in the ten-layer MCFC stack. Anode gas in the fuel cell provides reactant hydrogen and its flow rate is always restricted for promoting higher fuel utilization. Therefore, anode gas slightly affects the temperature field. Highest temperature on the top layer results from more reaction heat generation, induced by larger molar flow rate of anode gas. The cooling role in this situation is weaker than the anode gas-heating role when the molar flow rate of anode gas increases. Figure 4-3(d) depicts Pattern IV cell temperature in different layers, with both non-uniform inlet anode and cathode gas. Cell temperature distributions in this figure are similar to those in Figure 4-3(b), with only non-uniform inlet cathode gas. Increasing heat generation and cooling ability effect of anode gas with more molar flow rate are both nearly equivalent, therefore non-uniform anode gas slightly induces higher cell temperature on the top layer and lower cell temperature on the bottom layer, compared to those in Figure 4-3(b). Non-uniform molar flow rate of cathode gas consequently dominates the temperature field change in an MCFC stack. Highest cell temperature occurs at the cathode gas exit on the bottom layer, which has the lowest molar flow rate.
Figure 4-4 shows the cell temperature isotherm on the top layer and exit face of anode and cathode gas. Cell temperature in Figure 4-4(a) increases in the cathode gas flow direction, attaining highest temperature at the top middle part of the exit face.
Temperature difference between the top middle and bottom middle exit face of cathode gas is close to 10 °C because of end plate boundary effect mentioned in Figure 4-3(a). Figure 4-4(b) depicts the cell temperature isotherm on the top layer and exit face of anode and cathode gas. Cathode gas molar flow rate in the stacking direction progressively increases in Pattern II. Considerable change results in the cell temperature isotherm on exit faces compared to those in Figure 4-4(a) due to higher molar flow rate near the top layers. The non-uniform cathode gas changes the hot spot from the top middle to the bottom middle on the cathode gas exit face. Figure 4-3 illustrates a smaller non-uniform anode gas effect on temperature distribution than that of non-uniform cathode gas, so the isotherm in Figure 4-4(a) and the isotherm in Figure 4-4(c) are similar. The non-uniform anode gas moves the isotherm near the top half layers toward the exit corner because of more anode gas molar flow rate in these layers. Pattern IV isotherms in Figure 4-4(d) are similar to those in Figure 4-4(b) because of the same non-uniform cathode gas inlet flow in the stacking direction. Non-uniform anode gas effect on temperature distribution is observable as indicated by Figure 4-4(d) to 4-4(b) comparison.
Figure 4-5 depicts average cell temperature in different layers with I, II, III, and IV flow- patterns. Temperature profile along the cell number in Pattern I is the most
uniform as expected, because Pattern I has uniform inlet anode and cathode gas.
Temperature profile in Pattern II is the most non-uniform due to non-uniform inlet cathode gas, and variation of average cell temperature is close to 2%. Pattern IV has more uniform average cell temperature along the stacking direction than Pattern II, although Pattern IV has both non-uniform anode and cathode gas. Non-uniform inlet anode gas profile induces less molar flow rate in the bottom layer, dropping current density, while simultaneously decreasing chemical reaction heat. Average cell temperature in the Pattern IV bottom layer is therefore lower than that in Pattern II.
Pattern III slightly changes average cell temperature in each layer compared to Pattern I, showing non-uniform anode gas effect on cell temperature, as illustrated in Figure 4-3(c).
Figure 4-6 depicts cell voltage of each layer in Pattern I, II, III, and IV flow configuration. Meanwhile, the continuous, dashed, dash-dotted, and dash-double-dotted lines represent the cell voltage distribution in Pattern I, II, III, and IV, respectively. In this figure, continuous and dashed lines are close to straight lines, and their cell voltages in each layer are near 0.8V. The Pattern II is different to the Pattern I in the inlet flow of cathode gas, but their cell voltage distributions in the stacking direction are similar. Therefore, the effect of non-uniform inlet flow of cathode gas on cell voltage distribution can be neglected. Oppositely, the cell voltage distributions in the Pattern III and Pattern IV show severe changes in Fig. 4-6.
The dash-dotted and dash-double-dotted lines increase from 0.6V on the bottom layer
to over 0.9V on the top layer. Pattern III and Pattern IV have same non-uniform inlet flow of anode gas in the stacking direction, and their cell voltage distributions are similar to each other. Consequently, the non-uniform inlet flow of anode gas dominates the cell voltage of each layer. When the molar flow rate of anode gas increase, the fuel concentration will slowly drops along its flowing direction, and keep a higher electrical performance. Therefore, the cell voltage will increase in an increase of the molar flow rate. In Figure 4-6, the variation of the cell voltage in Pattern III and IV are over 40%, hence the non-uniform inlet flow effect of anode gas in the stacking direction on electrical performance is very apparent.