Thermal analysis - heat capacity

Roland R · March 4, 2015, 12:48

Hello,

I have a transient thermal simulation including fluid and solid domains. If I reduce the specific heat capacity value in the solid domain than what will be the difference? I expect that the reduced heat capacity in solid will result the same final temperature distribution and values in solid domain but the time of temp. changing will reduce. Please confirme me if this is true.

Thanks
Roland

evcelica · March 4, 2015, 14:16

Yes, in steady state specific heat in solid domain doesn't matter. But In transient you will get the wrong solution on both domains. I'm not sure why you would want to do this?

Roland R · March 5, 2015, 12:59

Quote:

Originally Posted by evcelica

Yes, in steady state specific heat in solid domain doesn't matter. But In transient you will get the wrong solution on both domains. I'm not sure why you would want to do this?

Thanks your quick answer

I have a model which inlcudes a transient temperature/pressure boundary conditions. The T and p change periodically at the inlet as a result I have to calculate it as transient case. The averaged T and p (instead of the sinus signal) are wrong solution because the periodic mass flow (and velocity) is also important boundary condition...

My main problem is that the time step of fluid domain has to be very small because the frequency of sinus signal is high, about 50Hz but the temperature will increase in the solid very very slowly... I would like to reach a special solution where the temp. of solid can increase very fast...

Do you understand my problem?

regards
Roland

ghorrocks · March 5, 2015, 17:26

Your problem is a typical one for CFD - different parts of the simulation have vastly different time scales, so you need a fine time scale to resolve the small time scale, but run it forever to resolve the large time scale.

If you are running this steady state you can use acceleration factors to even things out and achieve convergence quicker, but in transient you have to be clever.

In a transient simulation your options include:
* Simulate the fast time scale and get its output to the long time scale (in your case heat to the solid). Then do a simulation of the long time scale thing using the heat input. So you split the simulation into a fast and slow one.
* Replace the transient input (temp/pressure) and replace it with a weighted average. Then do a steady state simulation on the long time scale.
* Simulate the fast time scale to get the heat. Then use a simple ODE solver to get the temperature of the solid over time.

Which approach is best will depend on your exact requirements. But the basic idea is you split the simulation into a fast and a slow time scale one, and link them with the physical parameter which links the time scales.

March 4, 2015, 12:48	Thermal analysis - heat capacity	#1
Roland R Senior Member Roland Rakos Join Date: Mar 2009 Posts: 131 Rep Power: 17	Hello, I have a transient thermal simulation including fluid and solid domains. If I reduce the specific heat capacity value in the solid domain than what will be the difference? I expect that the reduced heat capacity in solid will result the same final temperature distribution and values in solid domain but the time of temp. changing will reduce. Please confirme me if this is true. Thanks Roland

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March 4, 2015, 14:16		#2
evcelica Senior Member Erik Join Date: Feb 2011 Location: Earth (Land portion) Posts: 1,188 Rep Power: 23	Yes, in steady state specific heat in solid domain doesn't matter. But In transient you will get the wrong solution on both domains. I'm not sure why you would want to do this?

March 5, 2015, 17:26		#4
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,872 Rep Power: 144	Your problem is a typical one for CFD - different parts of the simulation have vastly different time scales, so you need a fine time scale to resolve the small time scale, but run it forever to resolve the large time scale. If you are running this steady state you can use acceleration factors to even things out and achieve convergence quicker, but in transient you have to be clever. In a transient simulation your options include: * Simulate the fast time scale and get its output to the long time scale (in your case heat to the solid). Then do a simulation of the long time scale thing using the heat input. So you split the simulation into a fast and slow one. * Replace the transient input (temp/pressure) and replace it with a weighted average. Then do a steady state simulation on the long time scale. * Simulate the fast time scale to get the heat. Then use a simple ODE solver to get the temperature of the solid over time. Which approach is best will depend on your exact requirements. But the basic idea is you split the simulation into a fast and a slow time scale one, and link them with the physical parameter which links the time scales.