MIT10_626S11_lec17

MIT10_626S11_lec17 - III. Transport Phenomena Lecture 17:...

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Unformatted text preview: III. Transport Phenomena Lecture 17: Forced Convection in Fuel Cells (I) MIT Student Last lecture we examined how concentration polarisation limits the cur rent that can be drawn from a fuel cell. Reducing the thickness of the porous electrode will increase this limiting current, but we want to increase the limiting current without changing the geometry of the cell. We can do this by imposing a uid ow, as convection can work faster than diffusion alone. We are interested in the case of high power, i.e. P max I lim V O . 1 Membrane-Electrode Assemblies Figures 1 and 2 show side and top views of a membrane-electrode assembly used to produce a convective ow. The reactant gas is forced into inlet channels and drawn out of separate outlet channels, and can only pass between the two by travelling through the porous electrode. This moves gas past the catalyst layer, and some of the gas will reach the catalyst and react. 2 General Analysis Flow velocity is related to the pressure gradient by Darcys law: u =- K p. (1) K is the permeability of the uid, just a constant. For incompressible ow, we also have . u = 0 . (2) 1 Lecture 17: Forced convection in fuel cells (I) 10.626 (2011) Bazant Porous electrode Catalyst layer Fuel in Fuel out Membrane Figure 1: Membrane-electrode assembly Inlet channels Outlet channels In Out Pump Figure 2: Interdigitated ow channels 2 Lecture 17: Forced convection in fuel cells (I) 10.626 (2011) Bazant Even if the uid is actually compressible, this equation must hold in the steady state where the amount of uid at a particular location is not chang ing in time. Combining these two equations, we have 2 p = 0 . (3) To maintain a steady state, the total rate at which concentration is changing due to convection and diffusion must be zero, i.e. u . c = D 2 c (4) - K p. c = D 2 c. (5) There are different boundary conditions at the inlet and outlet, on the walls and on the membrane. At the inlet and outlet, pressure and concentration are both fixed....
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MIT10_626S11_lec17 - III. Transport Phenomena Lecture 17:...

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