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针对固体氧化物燃料电池热循环失效问题，建立了固体氧化物燃料电池热气体预热动态模型，研究了电池内最大温度梯度分布规律和入口异常高温度梯度形成的原因，结果表明：在热气体参数和预热方式变化时，电池内最大温度梯度始终处于电池入口边缘处的电极表面；电池入口处存在异常高的温度梯度，且在入口一小段区域内，温度梯度沿流动方向迅速下降；其原因是模型中入口采用均一的平均速度和温度，“入口效应”强化气体与电池换热；采用入口段延长的方式可使入口速度充分发展，降低电池内最大温度梯度，但由于均一温度入口并未优化，入口处仍然存在很大的温度梯度和温度梯度变化；因此采用数值模拟研究电池预热升温安全性时，仅采用最大温度梯度作为安全性判据会高估电池内热应力。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 700oC, so it should be preheated to startup temperature (e.g. 600oC) and 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 is a serious contradiction between the model results and experiment. 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 experiment. In this paper, a hot gas preheating model of SOFC is developed and the model is verified by comparing with model results in 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 as following:
1) The maximum temperature gradient is always located in the edge of SOFC nearby the gas entrance. The variation of temperature rise rate and velocity of hot gas shows 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 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 the lowest.
2) There is extremely high temperature gradient at the gas entrance, and the temperature gradient sharply decrease 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 components due to the large velocity and temperature difference at the entrance section.
3) The entrance extension of gas channel can give 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.-
Keywords:
- Solid oxide fuel cell /
- Temperature gradient /
- Numerical model /
- Entrance effect
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