This section compares a simple parallel flow field with a tapered parallel flow field. Simulation parameters are 323 K, 4 atm, GDL porosity 0.5, and both sides stoichiometries 3. The parameters were resulted from taking both RSM levels and reference . The difference between the two flow fields is the inclination of the main channels. Ratio equals zero and 4 for simple and tapered flow fields, respectively. The inlet and outlet are adjusted. Figure shows tapering enhanced the uniformity of velocity distribution between 21 channels resulting in a more uniform water saturation distribution at the reaction site. The uniformity increases limiting current density by 41 % from 0.88 A cm -2 to 1.24 A cm -2 . The tapered parallel flow field is selected for the optimization.