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Contributor
Posts: 23
Registered: ‎02-04-2013

transient heat analysis - convection coefficient

196 Views, 4 Replies
11-21-2013 09:54 AM

Hi all,

I am working on a transient heat analysis. It is a three-layer masonry wall model, where I know the ambient temperatures on both sides. I am trying to correlate the in-wall temperature values to the values obtained on-site by means of monitoring.

The problem is that I can obtain relatively comparable temperature values only when I use temperature independent convection coefficient in the range of 50 J/(s.C.m2), which is non-sense. Even then, I cannot obtain the desired level of correlation.

What do you think the problem is?

Thanks,

y

Distinguished Contributor
Posts: 141
Registered: ‎06-13-2012

Re: transient heat analysis - convection coefficient

11-25-2013 07:32 PM in reply to: yasmind

Hi yasmind,

Lets name the surfaces on your wall inside and outside, where do you measure the temperature? inside or outside or in between?

What are all your boundary conditions? Are there any other besides the convection on the inside and outside surfaces?

I suppose 'on-site' is an uncontrolled environment. Do you consider the heat escaping/entering via ground?

I tend to think of walls as long exposed to the BCs, why not run as steady state?

Other than that, if the temperature gets too high, perhaps you should consider heat released by radiation.

Regards

Ilyas

Contributor
Posts: 23
Registered: ‎02-04-2013

Re: transient heat analysis - convection coefficient

12-08-2013 10:27 AM in reply to: ilyas

Hi Ilyas,

Thank you very much for your message. Sorry for my late reply, I didnt get an e-mail notification for some reason.

We measured the temperature inside, outside and at twopoints in-wall. I am trying to apply the indoors and outdoors temperature values by using convection loading (because we measured the ambient temperature, and not the surface temperature) and trying to reproduce in-wall the temperature values. It is a time-dependant analysis, that's why I do not use the steady state option.

Well, my model represents only a part of a wall, obviously. I am not sure what my boundary conditions are. I am completely new to heat transfer analyses, sorry.

I am quite convinced that I need to find a way to include the efefct of solar radiation. I don't know how to do that. I was not able to find any videos, tutorials etc. regarding radiation loading in Autodesk. Please help!!!!

Thanks,

y

Contributor
Posts: 23
Registered: ‎02-04-2013

Re: transient heat analysis - convection coefficient

12-08-2013 10:29 AM in reply to: ilyas

Well, probably I should also mention that we placed the in-wall sensors inside the holes drilled in the wall, so we're actually measuring the temperature of the air inside the hole, rather than the temperature of the material itself. What do you think the effect of this is?

Thanks,

y

Distinguished Contributor
Posts: 141
Registered: ‎06-13-2012

Re: transient heat analysis - convection coefficient

12-08-2013 07:02 PM in reply to: yasmind

1. I don't think there would be a huge issue as to the way you measure the temperature, as long as the hole is not too big and the sensor is touching the surface.
2. If you are running as transient, make sure you apply initial temperature, otherwise, it would start off from the default nodal temperature value and would throw off your results. IMO, if ambient convection and solar radiation are all that you consider, it should be run as steady state.
3. You don't need to apply BC on surfaces that you assume as symmetry or extend for a long distance as the program will run as if it is adiabatic. Adiabatic is equivalent to symmetry, would this make sense? Can anyone verify this? Otherwise, if you have heat coming from/escaping to the ground, you should apply a BC to represent it.
4. I have limited knowledge in radiation, but as far as I know, there is no shortcut to solar radiation in Sim Mech. You may need to calculate for yourself how much heat is contributed by the sun and apply that as Heat Source. I hope I don't post anything misleading.

Regards
Ilyas