Serpentine Flow Channel Design

Single serpentine channels, (a) Serpentine flow channels, (b) Single channel curvilinear bends, (c) Single channel square bends. 

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Most fuel cell designs employ flow channels to distribute the reactants across the MEA area and to assist pushing liquid water out of the fuel cell the serpentine flow charmel represented in Figure 3.1 is the most popular flow channel design. Because reactants are consumed and water is produced in the fuel cell reaction, there is a decrease in the reactant concentrations between the flow channel inlet and outlet. There is also an increase in the water... 

Different flow channel design configurations, (a) Straight parallel channels, (b) Single serpentine curvilinear bends, (c) Single serpentine channel with square bends, (d) Dual 2-mm serpentine parallel channels, (e) Dual 1.2-mm serpentine parallel channels, (f) Dual 1-mm serpentine parallel channels, (g) Four 0.5-mm serpentine parallel channels. 

Kumar and Reddy (2003) studied the optimization of the channel dimensions and shape in the flow-field of bipolar/end plates. Single-path serpentine flow-field design was used for studying the effect of channel dimensions on the hydrogen consumption at the anode. They carried out simulations from 0.5 to 4 mm for... 

An optimum relationship between the DL and the flow field channels is a key factor in the overall improvement of fhe fuel cell s performance at both high and low current densities. Currently, flow field designs are typically serpentine, interdigitated, or parallel [207,264]. The FF plate performs several functions If is a current collector, provides mechanical support for the electrodes, provides access channels for the reactants to their respective electrode surfaces and for the removal of producf water, and it prevents mixing of oxidant, fuel, and coolant fluids. 

Kramer et al. [272] used this same technique to compare two different flow field designs— inferdigifafed and serpentine— and their interactions with the cathode diffusion layer. If was shown thaf the bottom of the interdigitated channels got plugged with liquid water that was not removed properly. On the other hand, the serpentine FF could transport the water inside the channels more effectively, but inside the cathode DL accumulation of wafer was still evident. 

Figure 3.36. Different design options for the gas flow channels (A) straight, (B) inter-digitated, (C) serpentine and (D) spiral. For the interdigitated design, the incoming flow must proceed through the gas diffusion layer to reach the outlet channel.

Another unique silica-based approach to microscale DNA extraction currently underdevelopment utilizes a serpentine channel design, combined with an immobilized silica-bead solid phase and fluidic oscillation. This method, developed by Chung et al., relies on silica beads immobilized on the plasma-oxidized surface of the polymethylmethacrylate (PMMA) channels, instead of a packed-silica solid phase, as depicted in Figure 43.1c. Following bead immobilization, the solutions required for DNA binding, purification, and release are flowed back-and-forth through the device. This fluidic oscillation over the immobilized phase results in marked improvement of recovery and extraction efficiency over the same extraction methods with free beads. This method represents yet another variation of silica-based purifications that has been accomplished in microfluidic systems, exploiting previously optimized chemistries. In summary, the development of macroscale, commercial, silica SPE protocols has enabled the facile translation of DNA, and now RNA, extraction into microfluidic systems for a variety of applications. 

Reliable forms of water management have been developed based on continuous flow field design and appropriate operating adjustments. For this reason, flow field designs often feature serpentine channels or unstructured flow passages. The flow-plates (which also serve as bipolar plates) are typically made of graphite, an injection-molded and cured carbon material, or a metal. 

FIGURE 11.12 Parallel serpentine channel design on graphite plate. (Reprinted from Marvin, R.H. and C.M. Carlstrom. 2002. Fuel cell fluid flow plate for promoting fluid service. U.S. Patent 6500580. With permission.)... 

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