Falin Chen

Effects of porosity change of gas diffuser on performance of proton exchange membrane fuel cell

Abstract An investigation is made of the effects of the change of the porosity of the gas diffuser layer (GDL) on the performance of a proton exchange membrane (PEM) fuel cell. The analysis of fuel cell performance with non-uniform porosity of GDL is a necessity because the presence of liquid water in the GDL leads to a non-uniformly distributed porosity in the GDL. To implement this performance analysis, a half-cell model which considers the oxygen mass fraction distribution in the gas channel, the GDL and the catalyst layer, and the current density and the membrane phase potential in the catalyst layer and the membrane is investigated. Four continuous functions of position are employed to describe the porosity, and differential equations are derived based on oxygen transportation and Ohm’s law for proton migration and solved numerically. Results show that a fuel cell embedded with a GDL with a larger averaged porosity will consume a greater amount of oxygen, so that a higher current density is generated and a better fuel cell performance is obtained. This explains partly why fuel cell performance deteriorates significantly as the cathode is flooded with water (i.e. to give a lower effective porosity in the GDL). In terms of the system performance, a change in GDL porosity has virtually no influence on the level of polarization when the current density is medium or lower, but exerts a significant influence when the current density is high. This finding supports the scenario proposed by previous studies that the polarization at high current density corresponds mainly to mass transfer through (or the concentration activation of) the membrane assembly.

Convective instability in ammonium chloride solution directionally solidified from below

The stabilities og salt-finger and plume convection, two major flows characterizing the fluid dynamics og NH 4 Cl solutions cooling from below, are investigated by theoretical and experimental approaches. A linear stability analysis is implemented to study theoretically the onset of salt-finger convection. Special emphasis is placed on the competition between different instability modes. It is found that in most of the cases considered, the neutral curve consists of two separated monotonic branches with a Hopf bifurcation branch in between; the right-hand monotonic branch corresponding to the boundary-layer-mode convection is more unstable than the left-hand monotonic branch corresponding to the mushy-layer mode

Convection in superposed fluid and porous layers

A nonlinear computational investigation of thermal convection due to heating from below in a porous layer underlying a fluid layer has been carried out. The motion of the fluid in the porous layer is governed by Darcys equation with the Brinkman terms for viscous effects and the Forchheimer term for inertial effects included. The motion in the fluid layer is governed by the Navier-Stokes equation. The flow is assumed to be two-dimensional and periodic in the horizontal direction, with a wavelength equal to the critical value at onset as predicted by the linear stability theory. The numerical scheme used is a combined Galerkin and finite-difference method, and appropriate boundary conditions are applied at the interface. Results have been obtained for depth ratios

Double-diffusive fingering convection in a porous medium

\hat{d}=0, 0.1, 0.2, 0.5

Onset of plume convection in mushy layers

and 1.0, where

Instability of Poiseuille flow in a fluid overlying a porous layer

\hat{d}

Throughflow effects on convective instability in superposed fluid and porous layers

is the ratio of the thickness of the fluid layer to that of the porous layer. For

Stability of Taylor–Dean flow in a small gap between rotating cylinders

\hat{d}=0.1

On the axisymmetry of annular jet instabilities

, up to R m (Rayleigh number of the porous layer) equal to 20 times the critical

Convective instability in superposed fluid and anisotropic porous layers

R_{{\rm m}_{\rm e}}