Flow field design manifold

Note that the flow field design of the bipolar plate is a critical factor for reactant supply and water management of stack and individual cells. As will be discussed later, a suboptimal flow field and manifold design can lead to an uneven distribution of reactants, and by this to fuel and air starvation. 

The design and assembly of PEM fuel cell components, such as flow fields and manifolds, can have a significant influence on water management and feed flows, which will in mrn affect the durability of fuel ceU components. For example, an improper design of the flow fields can result in water blockage, and improper manifold design can induce poor cell-to-cell flow distribution, both of which may cause localized fuel starvation. This localized fuel starvation can then induce an increased local anode potential to levels at which carbon oxidation or even water electrolysis may occur to provide the required protons and electrons for the oxygen reduction reaction (ORR) at the cathode. These reactions will induce corrosion of the carbon support and will result in a permanent loss of electrochemically active area at the anode. 

There have been many internal configurations of the PAFC stack plates, the most recent involving the use of carbon paper substrate as an electrolyte reservoir and a flow distributor, as discussed. The flow field design in a PAFC is similar to a PEFC, but there is no special provision for flooding, since this is not an issue with the medium-temperature PAFC. Typically, the flow fields are aligned in plane and perpendicular to one another (cross-flow), and external manifolding is used, as shown in Figures 7.21 and 7.22. By... 

Also a simulation of the flow field in the methanol-reforming reactor of Figure 2.21 by means of the finite-volume method shows that recirculation zones are formed in the flow distribution chamber (see Figure 2.22). One of the goals of the work focused on the development of a micro reformer was to design the flow manifold in such a way that the volume flows in the different reaction channels are approximately the same [113]. In spite of the recirculation zones found, for the chosen design a flow variation of about 2% between different channels was predicted from the CFD simulations. In the application under study a washcoat cata-... 

The die design equation proposed by Pearson (60) utilizes a constant die lip opening, but an approach-channel-taper that varies with the die width. Thus, in the region between the manifold and the die lip opening both the pressure and flow fields are two-dimensional. This may affect the flow in the die lip region, since the fluid is viscoelastic with memory of this recent upstream flow experience. 

Chen et al recently reported the fabrication and characterization of a high power self-breathing PEMFC with optimally designed wet KOH etched flow-fields and electrodes [49]. Ni/Cu/Au layers are used for current collecting, the 1.5 /im-thick in-between layer instead of a thick Au layer allowing to reduce the fabrication cost. The MEA is composed of a classical Nafion -1135 membrane with Pt-alloy sprayed carbon paper on both sides. The two silicon electrodes are sandwiched and pressed at RT with the MEA, and then sealed with epoxy. A base-chip formed by drilled Pyrex glass anodically bonded with KOH eched silicon acted both as H2 inlet/outlet manifolds and as a support for a... 

One of the most important aspects of fuel cell design is the reactant flow through the manifolds and flow field, because parasitic losses from driving the flow through the cell can... 

Straight channels with small manifolds—This design has the same shortcomings and, in addition, has inherent maldistribution of reactant gases, because the channels immediately below or above the manifold receive most of the flow. The early fuel cells built with such a flow field exhibited low and unstable cell voltages. 

The injection process introduces the prepared sample or reagent into the flowing carrier stream within the manifold. Ideally, the injector system should be designed so as to provide a high sample flow rate. Injection systems typically employ electrokinetic mobility or hydrodynamic pressure techniques. In the former systems, the sample flow into the microchannel is controlled by the application of an external electric field to the reservoir, while in the latter systems, a pressure difference is created in the reservoir using either a positive pressure (pistmi-type) technique or a suction pressure (vacuum) technique. 

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