Fundamental performance and the transient characteristics of a small tubular SOFC were numerically investigated. The model simultaneously treats momentum, heat and mass transfer, electrochemical phenomena and an electric circuit. Transient characteristics of a cell upon a sudden change of the cell terminal voltage or the air flow rate were examined. During its process, a steady simulation was first conducted and its result was used as the initial condition for the transient calculation. The transient calculation continued until a steady state under the new boundary condition was obtained. Discussions on the time response of the cell performance and thermal field were made in order to find the key factors mainly affecting the transient characteristics. To confirm what factors affect on the transient behavior, calculations were conducted for two different cell geometries, which have different cell diameters, but have same ratios of cell heat capacity to the heat generating area. As a result, it was found that the transient characteristics of a cell are primarily governed by the balance between the heat capacity of a cell and the heat generation by electrochemical reactions. The large heat capacity of the cell is one of the key problems affecting the transient behavior, limiting its quick response. A numerical demonstration is shown in which a rapid cell status change was achieved by controlling its operation conditions.

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