## Cfd Approach

The computational grid definition is based on a compromise between accuracy and efficiency. One type of grid for a single cell is illustrated in Fig. 1, in which x, y and z respectively represent: the fuel gas channel direction, the stack direction and oxidant gas direction. The height of a cell (in the y direction) has been increased by, a ratio of 200:1. The x and z grid have 16 and 8 uniform elements respectively. The x grid is more dense than the z grid because the change of a fuel gas in the cell direction is often more significant than that of an oxidant gas (the utilization of fuel gas is normally higher than that of oxidant gas). In the y direction, there are five zones: two for half-separators and three for fuel gas, cell unit and oxidant gas. There are three grids for each of the two half-separators, nine for cell unit and 15 for fuel and oxidant gas respectively. ■

Fig. 1 Dimensions and computational grids for a single cell

Under transient conditions, a time-step grid must also be specified. To ensure convergence of the numerical solution, for a step or ramp change, a fine grid is often required. For example, a +10% step change in voltage can be approximately simulated by 100 grids in a very short interval with a +0.1% step change in voltage taking place in each grid.

Fig. 1 Dimensions and computational grids for a single cell

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