Flotherm瞬态分析

如何设定瞬时问题??
电子散热分析中，通常以稳态(Steady State)结果作为分析依据，在此状况下可以观察到整个模型热场流场稳定后的结果。

如需观察温度随时间变化的曲线图，则必须设定随时间变化的热源(Transient Attribute)、瞬时时间网格(Transient Grid)，以便观察各时间点整体计算区域的热量传递速度及流体扰动状态。

以下以一实际案例解释FLOTHERM中瞬时设定的流程。

瞬时问题建立流程：
一、模型建立与几何尺寸设定：
模型建立请参照本公司之”FLOTHERM基础课程训练教材”一书。

以下为几何尺寸设定
二、PCB板热源设定：
三、瞬时热源设定：
Enclosure的六个面上均提供如上图之发热量随时间变化的Transient Source。

四、数值结果显示及说明：
由上图结果可以得知，在350秒的时候NC1芯片已超过最大容许温度摄氏125度，所以在350秒的时候此装置已无法正常运作。

五、图形化后处理：
问答集
Question 1：使用FLOTHERM解决瞬时问题需注意的事项。

Answer：
(1) 必须设定材质的密度(Density)及比热(Specific Heat)。

一般求解稳态的案例中，因密度及比热不随时间的变化，所以可以忽略不设定。

(2) 时间网格( Transient Grid )。

FLOTHERM为一自动网格系动热流仿真软件，当设定瞬时热源以后，软件本身会自动设定其时间网格。

所以当求解瞬时问题时，可先设定瞬时热源再设定时间网格。

Question 2：如何设定瞬时热源?!
Answer：瞬时热源顾名思义为发热量随时间而改变，FLOTHERM中提供以下几种热源设定。

Question 3：瞬时收敛性的问题。

Answer：瞬时问题收敛性有关于模型本身物理性的网格与时间性网格的设定。

当无法收敛的时候两者都必须作网格收敛性的测试。

详情请参阅Flomerics 网站( /)中的技术文件”Convergence and Transient Simulation”
何谓局部加
密?? <TOP>
局部网格加密理论
Validation - 1D Conduction
Without validation of the most basic of cases there is no way trust can be gained in the localized grid solution of more complex models. This model shows that a perfect linear variation in T in this conduction only analysis is predicted both with and without the use of localized grid. It is my belief that if this case was set up and solved in competitive codes that offered such a gridding capability there would be ‘kinks’ in the T profile, kinks that are highly sensitive to the quality of the grid.
Validation - 2D Conduction,Getting slightly more complex...
Validation - 2D Convection,
The first graph indicates that there is a difference. The second graph is for a much finer grid both localized and non-localized. This therefore demonstrates that the difference observed in the first graph was because BOTH localized and non-localized were not grid independent.
" Don't assume that if localized does not equal non-localized that the localized grid is wrong some how"。