The degradation or failure caused by thermal stress is a serious problem for solid oxide fuel cell (SOFC), especially in preheating process. The common working temperature for SOFC is more than 700 ℃, so it should be preheated to startup temperature (e.g. 600 ℃). The thermal stress induced by temperature gradient in SOFC is a crucial factor that results in the degradation or failure of SOFC, therefore there are many studies on the optimization of preheating process.
Numerical model is an important tool in the study of SOFC preheating process, however there exists a serious discrepancy between the model results and experimental results. The numerical model always gives a very high temperature gradient in the SOFC which can result in SOFC crack according to the material permissible stress, and this result disagrees with the practical experimental result. In this paper, a hot gas preheating model of SOFC is developed and the model is verified by comparing with model results from the literature. Then, the location of maximum temperature gradient and distribution of temperature gradient in the SOFC are studied by this model, and the extremely high temperature gradient at entrance is analyzed. Some conclusions are given below.
1) The maximum temperature gradient is always located at the edge of SOFC nearby the gas entrance. The variation of temperature rise rate and velocity of hot gas show negligible effect on the position of maximum temperature gradient in the gas flow direction. For single channel preheating method, the maximum temperature gradient is at the gas entrance. For the dual channel preheating method, the maximum temperature gradient is always at the cathode gas entrance whatever gas feeding way is co-flow or counter-flow, because the thermal conductivity of cathode is lowest.
2) There is an extremely high temperature gradient at the gas entrance, and the temperature gradient sharply decreases along the gas flowing direction at the small entrance section. The extremely high temperature gradient may result from the uniform inlet temperature and velocity set in the model, and the entrance effect can greatly enhance the heat transfer between gas and SOFC component due to the large difference in velocity and temperature at the entrance section.
3) The entrance extension of gas channel can give rise to a fully developed velocity distribution and reduce the temperature gradient at SOFC entrance, however, there is always a high temperature gradient at the entrance section of SOFC due to the uniform inlet gas temperature. Therefore, the maximum temperature gradient given by numerical model as a criterion of SOFC safety can overestimate the thermal stress, and the distribution of temperature gradient in SOFC should be analyzed together to optimize the preheating process.
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