*3.2. Pressure Distribution*

Figure 9 shows the pressure distribution of six different cooling channels, and the inlet mass flow is 0.002 kg/s. As can be seen from the figure, compared with other situations, the pressure distribution of model 4 and model 5 is more uniform. Model 0 and model 2 show the largest pressure difference in the reaction area, and the maximum pressure can reach 49,265.57 Pa and 79,226.62 Pa, respectively. The reason for the large pressure drop of model 0–model 3 is the long coolant transportation distance, while the coolant flow area of model 4 and model 5 is wide, the flow channels cross and connect with each other, and the pressure drop is reduced. The pressure loss produced by the long channel length is avoided due to the large number and small length of model 4 and model 5 channels. Model 1 has four inlets and four outlets in comparison to the serpentine flow field. It can be seen that the multi-inlet and multi-channel design helps to lessen the flow field's pressure loss. It can be summarized that the pressure drop can be effectively reduced by using a uniform plate flow field and honeycomb structure flow field.

#### *3.3. Effect of Heat Flux*

Figure 10 shows the effect of heat flux at the bottom of the cooling plate on the average temperature, maximum temperature difference, maximum temperature and temperature uniformity index of the cooling plate. It can be seen from Figure 10a–c that with the increase in bottom heat flux, the average temperature, maximum temperature difference and maximum temperature of the cooling plate increase significantly, among which the traditional single-channel serpentine flow field cooling plate increases the most. In Figure 10c, when the bottom heat flux is 4000 W/m3, the serpentine flow field cooling plate represented by model 0 maintains a good temperature since the heat flux remains within model 0's heat exchange capacity. When the heat flux is increased to 5000 W/m3, the temperature of model 0 rises significantly due to heat accumulation produced by the serpentine flow field's lengthy channel. For the maximum temperature difference, the effect of heat flux on the maximum temperature difference of model 2 and model 3 is slighter than that of other types of flow fields. In addition to the traditional serpentine cooling channel, the increase in heat flux has the same effect on the average temperature and maximum temperature of different types of cooling flow fields. For the new designed flow field structure, model 3 and model 4 show the highest temperature uniformity index, which shows that the deviation between the quantitatively measured surface temperature and the average temperature of the heat transfer surface of the flow channel structure is small, the temperature uniformity is high, and it has better heat dissipation performance. This is due to the uniform distribution of cooling channels and weaker blockage of the multi-helix flow field and honeycomb structure flow field.

**Figure 10.** Effect of heat flux on heat transfer characteristics. (**a**) Average temperature, (**b**) temperature difference, (**c**) maximum temperature, (**d**) temperature uniformity index.

### *3.4. Effect of Fluid Reynolds Number*

Figure 11a shows the maximum temperature of the cooling plate under different Reynolds numbers of the coolant. The boundary conditions of the numerical simulation are shown in Table 2, where the mass flow rate of the inlet is adjusted to achieve different Reynolds numbers. The results show that the maximum temperature of each type of cooling plate decreases with the increase in Reynolds number, because the larger mass flow at the inlet accelerates the heat dissipation. Figure 11b shows that the increase in Reynolds number will also increase the pressure drop in the channel due to the addition of more fluid flow. The rising trend of the Reynolds number of model 0 and model 2 is faster, because the fluid congestion in these two channels is more likely to occur.

Figure 12 shows the variation in the difference between the maximum temperature and the minimum temperature of each cooling flow field at different Reynolds numbers. The increase in the Reynolds number brings more flow of coolant, which alleviates the polarization of the working temperature of all types of cooling plates and improves the heat transfer capacity of the fuel cell cooling plates. Due to the multi-helical flow field structure with good heat dissipation performance, the temperature difference of model 3 always remains at a low value with the increase in Reynolds number.

The temperature uniformity index can present the temperature uniformity numerically. The closer the temperature uniformity index is to 1, the more uniform the temperature of the cooling flow field is. As can be seen from Figure 13, with the increase in Reynolds number, the temperature of all flow channels becomes more and more uniform. The traditional single-channel serpentine flow field maintains the lowest temperature uniformity, and the temperature uniformity of the multi-spiral flow field of model 3 is always the strongest. On the whole, increasing the Reynolds number can improve the heat transfer effect of the cooling plate.

**Figure 11.** Effect of Reynolds number on heat transfer characteristics. (**a**) Maximum surface temperature, (**b**) pressure drop.

**Figure 12.** Effect of Reynolds number on temperature difference.

**Figure 13.** Effect of Reynolds number on temperature uniformity index.

### *3.5. Flow Distribution Improvement*

It can be found from Figure 14a that the center temperature of the multi-serpentine cooling plate is high due to the transfer of heat from the inlet to the outlet and the winding of the cooling channel in the middle of the cooling plate. Therefore, the mass flow of the four inlets is redistributed with the total flow unchanged, as shown in Figure 14b, where half of the flow of the external cooling channel is distributed to the internal winding channel. The temperature uniformity at the bottom of the distributed cooling plate is improved, the uniformity index is increased from 0.9978508 to 0.9980883, the maximum temperature is reduced from 321.1978 K to 319.3245 K, the temperature difference is also reduced, and the average temperature is also reduced by 1.2096 K.

**Figure 14.** Effect of changing inlet flow on temperature distribution.
