Gas Flow-Field

One of the major losses in PEM fuel cells is the mass transfer loss, which is caused by the lack of reactant gas concentration distribution at the electrode-catalyst reaction surface. In order to reduce this resistance, a high-performance gas flow channel design has to be developed. 

The contact surface area of the reactant gas on the bipolar plates is increased by decreasing spacing between the channels and increasing the number of channels, which contributes to the local distribution of gas, overall reaction of the gas, and the local current density. 

Removal of water from the electrode surface in the form of water vapor or liquid water droplets may lead to a complex mulhphase flow depending on the size and type of gas flow charmels used. The mulhphase flow is generally characterized by a number of different flow regimes such as bubbly flow, slug flow, plug flow, and armular flow. 

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The steady states of such systems result from nonlinear hydrodynamic interactions with the gas flow field. For the convex flame, the flame surface area F can be determined from the relation fSl = b zv, where Sl is the laminar burning velocity, the cross-section area of the channel, and w is the propagation velocity at the leading point. 

At the most detailed level of description, the gas flow field is modeled at scales... 

Hoekstra, A. J., Gas flow field and collection efficiency of cyclone separators , Ph.D. Thesis, Delft University of Technology, Delft, Netherlands (2000). 

Velocity seals are more recent developments in air seal design. They use conical baffles to redirect and focus the purge gas flow field just below the flare tip to sweep air from the flare stack. Some velocity seal designs can reduce the purge gas flow rate requirement to about 1/10 of the rate needed without the seal. Also, some velocity seal designs reportedly require only about 25 to 33 percent of the purge gas used in diffusion seals (AICliE-CCPS, 1998). More details about air (purge reduction) seals may be found in API RP 521 (2007). 

Figure 5.2. Calculated gas flow fields in the near-nozzle region of free-fall atomizers. Primary gas pressure 0.140 MPa secondary gas pressure 0.189 MPa angle of secondary gas nozzle relative to the spray centerline 10° angle of primary gas nozzle relative to the spray centerline (a) 0°, (b) 22.5°, and (c) 30° designed for minimizing recirculation gas flow. (Reprinted from Ref. 612.)...

In diffusion combustion of unmixed gases the combustion intensity is limited by the supply of fuel and oxidizer to the reaction zone. The basic task of a theory of diffusion combustion is the determination of the location of the reaction zone and of the flow of fuel and oxidizer into it for a given gas flow field. Following V. A. Schvab, Ya.B. considered (22) the diffusion equation for an appropriately selected linear combination of fuel and oxidizer concentrations such that the chemical reaction rate is excluded from the equation, so that it may be solved throughout the desired region. The location of the reaction zone and the combustion intensity are determined using simple algebraic relations. This convenient method, which is universally used for calculations of diffusion flames, has been named the Schvab-Zeldovich method. 

We will consider the cold-gas-convex surface of the flame front as a curved cell of the flame which had been formed after the plane flame lost its stability. The steady state of the convex flame is a result of the nonlinear hydrodynamic interaction with the gas flow field (see Zeldovich, 1966, 1979). In the linear approximation the flame perturbation amplitude grows in time in accordance with Landau theory, but this growth is restricted by nonlinear effects. 

Fig. 1.2 Principles, functions and schematic of a flat planar SOFC where the PEN is stacked between two bipolar plates (interconnect with gas flow fields).

In computation using the stochastic trajectory model, the Monte Carlo approach is commonly employed. It is necessary to calculate several thousands, or even tens of thousands, of trajectories to simulate the particle flow field. The central issue in developing the stochastic trajectory model is how to model the instantaneous turbulent gas flow field. The method... 

Previously, we had shown from elemental mapping that Cr diffuses from the gas flow field into the bond layer attached to the oxygen electrode. This Cr was seen to substitute for Co in the (La,Sr)Co03 bond material, reducing its conductivity, and the displaced Co reacted with more Cr to form CoCr204 crystals on the open surfaces of the bond layer situated under the gas flow fields (Mawdsley, 2009). 

The final element on the outer side of the unit cell (Figs. 1 and 2) is the current collector plate, which typically contains the machined gas flow field. These two functions of current collector and gas flow field may be fulfilled, in principle, by two separate components but, in most of the cells and and stacks tested so far, the flow field is machined in the current collector plate using a range of geometries, e.g., a single serpentine channel, parallel channel flow and series-parallel combinations. The specific flow-field geometry may be critical in fulfilling the requirements of effective water supply and effective liquid water removal from the cathode. 

FIGURE 7.18 Results of Eulerian-Eulerian simulations, (a) Liquid flow field (maximum velocity = 6.5 m/s), (b) Gas flow field (maximum velocity = 8 ms ), (c) Contours of gas volume fraction (8 uniform contours between 0 [A] to 0.07 [G]). 

The L79 is easy to build and uses a few common materials in a unique way (see illustration, next page). It consists of a six layered sandwich composed of one PVC end plate, two PC (printed circuit) board elec-trode/gas flow field plates, one 12 PEM MEA (membrane electrode assembly) layer, and 2 rubber gaskets, one of which also acts as a gas supply line. 

In the L79 design, both the gas flow fields and electrodes are made from single sided copper clad circuit board. Copper clad circuit board comes in a variety of sizes and is either clad with copper on one side only, or both sides. Boards also differ in the type of base material they are made of. This fuel cell requires the FR-4 glass epoxy resin base, clad with copper on one side only. We used a 1 ounce, which is coated with copper to a thickness of. 0014". 

These two plates are the positive and negative electrodes and gas flow fields for the membrane electrode assembly. 

The L79 is a planar variation of some of the construction ideas involved in the L78 stack, as presented in Build Your Own Fuel Cells, which also has printed circuit board for the electrode plates. While working with this material, we realized that flow fields could be routed in the circuit board, so that the material could function for a dual purpose as both a collector plate, and a gas flow field. 

An example of a stainless steel serpentine-based gas flow field is reported in Fig. 3.4 for a simple small size PEM stack (64 cm as active area). 

Fig. 3.4 Serpentine gas flow field in a bipolar plate of a simple PEM stack...

A. N. Tifford, The Thermodynamics of the Laminar Boundary Layer of a Heated Body in a High-Speed Gas Flow Field, J. Aeronaut. Sci. (12) 241-251,1945. 

The carrier gas flow field can be visualized by numerically solving the complete Navier-Stokes equations for the deposition geometry. The momentum balance is given by... 

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