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CFD热分析软件准确度讨论

2015-06-03

    Tony 硕士毕业于斯坦福大学,拥有底特律大学本科学历,专业从事电源热设计工作长达20年.针对业界流行的热仿真准确度问题,他提出了他的观点. 

 

以下为全文,供大家参考.

Dear Tony,

There is disagreement here in my cross-functional work team about how to determine the percent? error when comparing measured temperatures with CFD predictions.? Sometimes when component 
temperature predictions seem close to measured numbers we feel good but I think the actual percent error may be quite high.? I realize this is not a simple issue - but let's assume for the sake of simplicity that the measured values are accurate and the CFD model is as good as it can get.? How would you calculate the error in temperature predictions in the following examples? 

                                        Scenario 1     Scenario 2  
Ambient temperature                         65C            23C  
Measured component temperature              92C            86C  
CFD predicted component temperature         95C            89C 

In which scenario would you say the CFD tool did a better job?

 Rhett from Atlanta

 

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Dear Rhett,

You ran a CFD analysis and came out within 3 degrees C of a measured result?? Congratulations!? You are better at CFD modeling than I am.? If I could routinely predict electronic component temperatures to within plus or minus 3 degrees C, I would not bother with calculating the percent error.? Who cares what the relative accuracy is?? Temperature is what you really want to predict, and it seems to me from your examples that you are doing a great job.

But I can read between the lines of your e-mail, (using a special feature of Microsoft Outlook), and I detect a more general question.? I am assuming from the way you frame your question that your disagreement is between two different ways of computing the percent error.? One way would be to compare the "error" between the prediction and the measurement to the measured component temperature.? The second way would be to compare the error to the temperature rise above ambient of the measurement.? Let's try it both ways: 
?

                 Error    % Error based on Tcomponent      % Error based on (Tcomponent - Tambient)  
Scenario 1         3C          3/92 = 3%                           3 / (92 - 65) = 11%  
Scenario 2         3C          3/86 = 3%                           3 / (86 - 23) = 5%
 

If the basis for your error calculation is just the component temperature measurement, the percent error for both scenarios is about the same, and quite low, at about 3%.

But if you base the percent error on the component temperature rise above ambient, you get a somewhat different picture.? The percent error for the two scenarios is quite different.? The percent error for Scenario 1 is more than double that for Scenario 2.?? (In my opinion, the error, and the percent error, are still quite low, but that all depends on what you expect your CFD tool to accomplish.)

You gave away your own view on the matter when you supplied the ambient temperature for each scenario.? Of course the only correct way is to base the percent error on the temperature rise!? The Celsius temperature scale is a relative one.? You can't make ratios of temperatures.?? Is 100C twice as hot as 50C?? What is twice as cold as 0 degrees C?

If you can stand to hear some more explaining on this topic, I'd like to give you a more general reason to do it my way.? More general than just the fact that Celsius degrees are based only on the boiling and freezing points of water.? It has to do with basic error analysis, something that more people should be doing when they report CFD or measurement results.

Your question is:? How accurately can CFD predict temperature???

But to be more specific, you should be asking : How accurately can CFD do what it is supposed to do??

Because CFD does not predict temperatures.

Whoa!? Don't panic, FLOTHERM and IcePak folks!? You'll be agreeing with me in a second.? Of course your CFD (and any other software like it) does not calculate temperatures.? They can't.

Let's look at the simplest problem you could solve with CFD:? a single heated object surrounded by an ambient fluid.? You wouldn't use CFD to solve it because you could do it by hand.? You would use an equation like this:

Tobject = Tfluid + Power / (h x Area)

Your CFD tool doesn't calculate Tobject .? The most you can claim that it is responsible for is the temperature rise above the fluid, (Tobject - Tfluid).? The fluid temperature is a boundary condition!? You supply it as an input number, and the CFD tool just spits it back out at you as a piece of the object temperature result.? Does it make any sense to include the magnitude of input temperature when you are trying to estimate how well the CFD is working?

That's what I mean when I claim that CFD doesn't predict temperature.? It predicts the temperature RISE of the objects in the system above the ambient fluid.? (In a real problem, CFD does a lot more than that -- calculating the temperature rise of the fluid as it passes through the system.? My point is that CFD does not calculate its own boundary conditions, especially the ambient temperature.)

So for judging the accuracy of CFD predictions, we should ignore the ambient, and look only at the temperature rise, given by:

Tobject - Tfluid = Power / (h x Area)

Now we can talk about some error analysis. Check out the right side of this equation and you might notice that the temperature rise calculated by CFD depends on a couple of other inputs -- Power and Area.? The temperature rise is directly proportional to the Power value supplied by you, the user.? A 10% overestimate of component power will lead to a 10% overprediction of the temperature rise.? The same thing applies to the Area.? If you specify the geometry in your CFD problem incorrectly, that will cause errors in the temperature rise.

This concept is actually useful when you are trying to decide how accurate your CFD prediction is.? Before you start comparing temperatures, compare the power values you put into your CFD tool to the actual component power in your measurement.? If your power estimates were 30% off, how could you expect the temperature rise predictions to be any closer than that?

Now you can see why I was stunned by your scenarios.? If you had a real circuit board for which CFD had consistently predicted component temperatures to within 3 degrees C, and even if the percent error was as high as 11%, that would mean you had known the component power to within 11% of the value it was when you measured it.? Rhett, can you really get your power estimates that close before you build your circuits?? If so, please share the secret with me.? Together, we will rule the electronics cooling world.

    关于CFD热分析软件作用,笔者认为关键是帮助我们分析问题的风险,准备应对方案,预测调整后的趋势(杜绝盲目打样测试),考虑项目成功率这方面价值评估上.在项目初始阶段,我们可以依据(已往经验,小型模拟试验,软件CFD模拟)三条路径来评测项目方案,准备好风险应对策略(无论这应对策略是由热流本身,结构,还是电气上,都是需要提前沟通好确认OK的。)若在项目后期引入CFD软件热分析可以杜绝我们盲目打样,“碰运气”作业方式来处理研发的问题。

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