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bighead April 14, 2009 21:25

Transient Modeling in Flotherm
Hey guys, I have some queries regarding transient modeling in Flotherm. I have two objects that are placed together in the domain. However, both are at different initial conditions,ie Block 1 is at 100C and 2 is at 25C. The time scales for both model are of same order of magnitude, hence I cannot ignore one or the other.

1) Is it even possible to model the transient heat transfer interaction ? If so, how can I do it ?

RobinB April 16, 2009 09:00

Certainly doable in FloTHERM.

First of all make sure that you have attached material attributes to both blocks with the correct thermal conductivity, density and specific heat values. The latter two are particularly important for a transient simulation.

Next use Initial Subdomain objects collocated with the two cuboids to define the initial temperatures.

Set up the other aspects of a transient simulation in the usual way (e.g. end time, transient grid etc.)

Then press 'Go' !

bighead April 16, 2009 10:02

Thxs RobinB, I will give it a try. It didnt cross my mind to use the subdomain. Just a question. How will the solver treat the interface when the two domain meet ??

RobinB April 16, 2009 11:09

The subdomains simply define initial conditions for the transient simulation. At the end of the first time step the temperature field in the two cuboids will have changed based on their thermal conductivity, thermal mass and surrounding air side HTCs.

bighead April 16, 2009 22:56

I had tried using the initial subdomain and the results are definitely much better. Also, after I initialise the solution and plot the temperature contours for both bodies, I noticed a temp gradient across a very thin layer at the interface location. Probably guess that is to ensure no temperature jump at interfaces. But thxs anyway RobinB for the help.

RobinB April 17, 2009 02:43

The temp gradient of the initial fields at the interface will be due to how the cell temperature scalar values are interpolated (smoothed). To see the true real values graphically select the plane plot and change the 'Fill Type' from 'Interpolated' to 'Cell Fill'

bighead April 23, 2009 05:33

Thxs RobinB, everything works fine now.

bighead May 3, 2009 23:27

Hey Robin, just a short question for you. The initial conditions for sub domain works fine. The main domain is at 90C while the sub is at 25C. However, at the beginning of the first time step. depending on the model (TIM thickness), I am getting large temperature changes (90 to 83C in 0.2s). I had tried to use smaller timesteps but still the same time happen. Do you have any idea how to resolve this ?

RobinB May 5, 2009 06:15

The initial conditions are just that, the conditions at t=0s for a transient simulation. Once the transient simulation starts heat will start to flow from hot to cool blocks, very rapidly at first where the dT is high. As t->infinity both blocks will assume the same temperature.

In addition to this model I think you will need to set either a fixed T or fixed Q (heat source) on the side where the package sits.

I'm more than happy fielding this support issue here but really you would get as good (if not better!!) service from your local support office.

naveen kannan May 21, 2013 12:31

Transient analysis

Is there any tech note or instructions manual on how to perform transient analysis in Flotherm/Flovent?

RobinB May 21, 2013 12:37

Yep, the Tech Note MG245418 on SupportNet outlines a typical transient model definition process.

naveen kannan May 21, 2013 12:47

Thanks Robin. I'll take a look at that. I guess my keywords didn't work on support net. Will get back if I have any questions.

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