FractalLike FlowFields with Minimum Entropy Production for Polymer Electrolyte Membrane Fuel Cells The fractalxe2x80x90type flowxe2x80x90fields for fuel cell (FC) applications are promising, due to their ability to deliver uniformly, with a Peclet number Pe1, the reactant gases to the catalytic layer. We review fractal designs that have been developed and studied in experimental prototypes and with CFD computations on 1D and 3D flow models for planar, circular, cylindrical and conical FCs. It is shown, that the FC efficiency could be increased by design optimization of the fractal system. The total entropy production (TEP) due to viscous flow was the objective function, and a constant total volume (TV) of the channels was used as constraint in the design optimization. Analytical solutions were used for the TEP, for rectangular channels and a simplified 1D circular tube. Case studies were done varying the equivalent hydraulic diameter (Dh), crossxe2x80x90sectional area (Dxcexa3) and hydraulic resistance (DZ). The analytical expressions allowed us to obtain exact solutions to the optimization problem (TEPmin, TVconst). It was shown that the optimal design corresponds to a nonxe2x80x90uniform width and length scaling of consecutive channels that classifies the flow field as a quasixe2x80x90fractal. The depths of the channels were set equal for manufacturing reasons. Recursive formulae for optimal nonxe2x80x90uniform width scaling were obtained for 1D circular Dh xe2x80x90, Dxcexa3 xe2x80x90, and DZ xe2x80x90based tubes (Cases 1xe2x80x903). Appropriate scaling of the fractal system providing uniform entropy production along all the channels have also been computed for Dh xe2x80x90, Dxcexa3 xe2x80x90, and DZ xe2x80x90based 1D models (Cases 4xe2x80x906). As a reference case, Murrayxe2x80x99s law was used for circular (Case 7) and rectangular (Case 8) channels. It was shown, that Dhxe2x80x90based models always resulted in smaller crossxe2x80x90sectional areas and, thus, overestimated the hydraulic resistance and TEP. The Dxcexa3 xe2x80x90based models gave smaller resistances compared to the original rectangular channels and, therefore, underestimated the TEP. The DZ xe2x80x90based models fitted best to the 3D CFD data. All optimal geometries exhibited larger TEP, but smaller TV than those from Murrayxe2x80x99s scaling (reference Cases 7,8). Higher TV with Murrayxe2x80x99s scaling leads to lower contact area between the flowxe2x80x90field plate with other FC layers and, therefore, to larger electric resistivity or ohmic losses. We conclude that the most appropriate design can be found from multixe2x80x90criteria optimization, resulting in a Paretoxe2x80x90frontier on the dependencies of TEP vs TV computed for all studied geometries. The proposed approach helps us to determine a restricted number of geometries for more detailed 3D computations and further experimental validations on prototypes.

Soft PolymerElectrolyteFuelCell Tube Realizing AirHoseFree Thin McKibben Muscles Among the various pneumatic actuators, thin McKibben muscles are particularly attractive owing to their high performance in terms of high contraction ratio, high flexibility, and productivity. One of the challenges for thin McKibben muscles and other pneumatic actuators is the fact that they generally need air hoses and air sources like compressors. This necessarily leads to bulky systems. To solve this problem, we propose and study a new approach to driving pneumatic actuators that exploits electrolysissynthesis of water by a polymer electrolyte fuel cell (PEFC). This method could inprinciple have high applicability to thin McKibben muscles. However, one challenge must still be addressed for the realization of an electrically driven thin McKibben muscle: the development of a tubeshaped soft PEFC. This paper proposes an electrically driven thin McKibben muscle with a tubeshaped soft PEFC realized through a flowing nonelectrolytic plating method. First, the proposed thin McKibben muscle is briefly described. Then, a novel method for plating a tubeshaped soft PEFC using reflux is introduced, and its fundamental operation is tested. After the evaluation of the PEFC, we report a prototype realization of the proposed thin McKibben muscle. The developed prototype has a length of over 170 mm, a diameter of 4 mm, and high flexibility for bending. Finally, we elaborate on a driving experiment through which fundamental characteristics were experimentally evaluated. The prototype thin McKibben muscle succeeded in electrically controlling contraction and expansion motion with a flexible and bendable structure.