Serpentine channels

Figure 21. Comparison of local current density distributions in a two-channel serpentine PEFC at Eceii — 0.4...

Siegel et al. recently presented a CFD cell model with a six-channel serpentine flow field [34]. The model is isothermal and steady state. The description of the catalyst layer follows an agglomerate approach, which takes diffusivity and solubility of gases in phosphoric acid into account. The submodel for the temperature dependence of the conductivity of the phosphoric acid is critically discussed. In the range 150-160 °C, good agreement with experimental results was obtained. 

Figure 31.10 Calculated polarization and power curves of two-channel serpentine PEFC at a cell temperature of 80°C, a pressure of 1.5 atm, fully humidified inlets, and an-ode cathode stoichiometry 2 at lAcm [33].

FIGURE 3.18 Photograph of a PCB segmented electrode with a seven-channel serpentine flow field current collector used on the anode side of the cell (a) and a close-up of the PCB (b), showing the flow channels [72],... 

This is also (see [R 6]) a commercial chip ( Radiator ), provided by MCS, Micro Chemical Systems Ltd., The Deep Business Center [20]. A bottom plate contains an extensively wound serpentine channel. A top plate covers this microstructure. The two reactant solutions enter via capillary tubing through holes in the top plate. The first reactant is fed at the start of the serpentine path and the second enters this path in a short distance. Shortly before the end of the serpentine, a third stream can enter which may serve, e.g., for dilution and thus quenching of the reaction. After a very short passage, the diluted streams enter via a fourth port analytics. Commercially available capillary connectors were employed. 

Reactor type Chip micro reactor with extended serpentine path and ports for two-step processing Serpentine micro channel width depth 100 pm 25 pm... 

This device is based on multiple parallel bi-lamination using bifurcation cascade for generating multiple thin fluid olamellae [25]. The first feed stream is split into multiple sub-streams via a bifurcation cascade in a similar way this is done for the second feed stream in another level. The corresponding sub-streams enter via nozzles into the first level. Here, the end of the channels of the bifurcation cascade and the nozzles lie next to each other. Thereby, bi-laminated sub-streams are formed and enter many parallel channels of an inverse-bifurcation cascade. These are recombined to multilayered stream in one main channel which has a serpentine shape, i.e. comprises extended length. 

A coolant channel is guided through the metal block in a serpentine fashion [15], Hence reactant and coolant flows are orthogonal. A thermocouple measures the temperature at the product outlet of the single-channel thin-film micro reactor. 

This micro-chip reactor comprises a liquid inlet port which splits into two channels of equal passage [21]. These split channels merge with a third channel, which is connected to a second port for gas feed, in such way that the two liquid streams encompass the gas stream. This triple-stream feed section is followed by a long serpentine channel passage which ends in a third outlet port (Figure 5.11). 

Figure 5.11 Schematic of the single-channel chip micro reactor. Divergent inlet channels (A) secondary inlet channel (B) serpentine irradiation channel (center) outlet channel (Q [21. ...

An example of a transparent PEMFC was presented by Spemjak, Prasad, and Advani [87], who used a 10 cm transparent fuel cell to investigate different cathode DL materials (with and without MPLs) influence on water management. The FF channels had a single-path serpentine design with rectangular channel cross sections 1 mm deep and 0.8 mm wide. In these researchers study, the analyzed images corresponded to those in the lower section of the cathode s active area (closest to the outlet) because most of the water droplets were observed in this area away from the inlet. To observe how different DLs affected the water transport in the anode, this side was also visualized (see Section 4.3.3.2). 

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. 

Park and Li [267] also performed an experimental and numerical study in which a serpentine FF with and without diffusion layers was analyzed (see Figure 4.30). For the case in which a DL was not used, an impermeable plate was placed between the anode and cathode plates in order to perform the pressure drop tests. It was observed that the pressure difference between the two cases was as large as 80% of the pressure drop in the case without the DL. It was also explained that the reason for the large pressure difference between both cases was due to the cross-flow phenomena between adjacent channels through the porous diffusion layer. These researchers also performed the pressure drop tests when the fuel cell was running it was observed that the pressure drops were higher in this case than when the cell was inactive. It was believed that this pressure difference was a result of liquid water blocking reactant flow in either the channels or the DLs. 

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. 

J. Park and X. Li. An experimental and numerical investigation on the cross flow through gas diffusion layer in a PEM fuel cell with a serpentine flow channel. Journal of Power Sources 163 (2007) 853-863. 

The steam reformer is a serpentine channel with a channel width of 1000 fim and depth of 230 fim (Figure 15). Four reformers were fabricated per single 100 mm silicon wafer polished on both sides. In the procedure employed to fabricate the reactors, plasma enhanced chemical vapor deposition (PECVD) was used to deposit silicon nitride, an etch stop for a silicon wet etch later in the process, on both sides of the wafer. Next, the desired pattern was transferred to the back of the wafer using photolithography, and the silicon nitride was plasma etched. Potassium hydroxide was then used to etch the exposed silicon to the desired depth. Copper, approximately 33 nm thick, which was used as the reforming catalyst, was then deposited by sputter deposition. The reactor inlet was made by etching a 1 mm hole into the end... 

The reactor consisted of two inlets with a serpentine delay section and an additional inlet to perform the addition of the reagent in the second step. Channels were of 150 pm width and 50 pm depth. The amine and sodium nitrite solutions were injected separately at a rate of 3.5 pl/min, and (3-naphtol was added via the third inlet at a flow rate of 7 pl/min (Scheme 31). 

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