*3.6. Effect of Bottom Non-Uniform Temperature Distribution*

During the continuous operation of a PEMFC, the heat transmitted at the bottom of the cooling plate is not always uniform and constant. The temperature downstream of the coolant is always higher than the temperature upstream of the coolant due to heat exchange. After a lengthy period of operation, the temperature downstream of the cooling plate is higher than that upstream, and the temperature falls from high to low from downstream to upstream for cooling plates with a serpentine flow field, model 2, model 4 and model 5 cooling flow field distribution. To investigate the impact of non-uniform temperature on the heat transfer of the cooling plate, the uniform heat flow at the bottom is altered into a temperature gradient distribution from 324 K to 310 K. Figure 15 shows the temperature distribution results of the middle surface of the flow field.

As can be seen from Figure 15, the four models show similar temperature distributions in the case of non-uniform temperature distribution. The highest temperature of the four models is 323.99 K of model 5, and the lowest temperature is 323.9 K; the difference is not obvious. The lowest average temperature is 316.828 K of model 2 and the highest is 316.99 K of model 5. Due to the low temperature in the upstream, the heat exchange capacity of the coolant is small, and the heat exchange is mainly concentrated in the downstream region, which causes the serpentine flow field to avoid the heat accumulation generated by the longer flow channel, thus showing temperature performance similar to that of other flow channels.

**Figure 15.** Effect of bottom non-uniform temperature distribution.
