MIT10_626S11_lec17

# MIT10_626S11_lec17 - III Transport Phenomena Lecture 17...

This preview shows pages 1–4. Sign up to view the full content.

This preview has intentionally blurred sections. Sign up to view the full version.

View Full Document

This preview has intentionally blurred sections. Sign up to view the full version.

View Full Document
This is the end of the preview. Sign up to access the rest of the document.

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 ﬂuid ﬂow, 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 ﬂow. 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 Darcy’s law: u =- K ∇ p. (1) K is the permeability of the ﬂuid, just a constant. For incompressible ﬂow, 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 ﬂow channels 2 Lecture 17: Forced convection in fuel cells (I) 10.626 (2011) Bazant Even if the ﬂuid is actually compressible, this equation must hold in the steady state where the amount of ﬂuid 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....
View Full Document

{[ snackBarMessage ]}

### Page1 / 9

MIT10_626S11_lec17 - III Transport Phenomena Lecture 17...

This preview shows document pages 1 - 4. Sign up to view the full document.

View Full Document
Ask a homework question - tutors are online