1 0。5 973 0。8 3 873 Counter-flow, Co-flow

0。5

2 1。0 973 0。8 3 873 Co-flow

1。5

0。8

3 0。5 973 0。9 3 873 Co-flow

1

3

4 0。5 973 0。8 2 873 Co-flow

1

Fig。 3。 Hydrogen mass fraction distributions on the interface between anode/electrolyte in the co-flow case (a) and counter-flow case (b) under first working conditions。

and 987 K in MOLB-type SOFC in co-flow case (see Fig。 2(a))。 But, in counter-flow case, average temperature is 996 K with maximum and minimum temperatures of 1088 and 923 K (see Fig。 2(b))。 So the co-flow case has the more uniform tempera- ture distribution and smaller temperature difference (6T) from air inlet to outlet。 A further difference is that the temperature of PEN increases uniformly along the direction of fuel flow, and is highest near the fuel outlet in co-flow case。 However, the temperature of PEN rises rapidly, reaching a maximum near the fuel inlet, and then gradually drops for counter-flow case。 This is due to the offsetting effects of air near the inlet, at its coolest,

being aligned with the fuel inlet。 As a consequence, the temper- ature gradient is smaller in co-flow case, which must result in the smaller thermal stress in the PEN although the thermal stress distributions were not researched in the paper。

Fig。 3(a and b) illustrates the hydrogen mass fraction distribu- tions in the co-flow and counter-flow cases, respectively。 Along the direction of fuel flow, the mass fractions of hydrogen on the interface between the electrolyte and anode decrease due to the electrochemical reaction。 In particular, of the two flow case, the hydrogen mass fraction near the fuel outlet is less in co-flow case, so the more fuel is consumed and fuel utilization is higher accordingly。

On the basis of above analysis about the temperature distri- bution, it was found that the co-flow case is advantageous to improve the fuel utilization and mitigate the steep temperature gradient, and hence to reduce the internal stresses。 For the co- flow case, other working conditions influenced the performances of SOFC were also studied。 Firstly, we characterized the effects of the fuel gas on current density and temperature distributions。 Fig。 4 shows the current density distributions of PEN。 Changing

the delivery rate of fuel with 0。5, 1。0 and 1。5 m s−1, respectively,

the average current density gradually increases (see Fig。 4(a–c)), but the current density is less uniform。 This indicates that it is effective for the improvement of the electrical performance to increase the delivery rate of fuel。 Fig。 5(a) shows the temperature distributions of the mid-plane in the X-direction。 With the incre- ment of the delivery rate of fuel, the average temperature and temperature difference (6T) from fuel inlet to outlet of PEN rise, so the temperature gradient also rises, which may cause thermal stress。