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Dry Wall with Air Cavity

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Old   October 8, 2014, 00:09
Question Dry Wall with Air Cavity
  #1
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Rusthi
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Hi,

I need to produce a CFD model of a dry wall (3m tall, 0.6m wide) with a cavity of 90mm thick and 16mm plaster boards on either sides of the cavity (interior and exterior).

The steps in the conjugate heat transfer model should include the followings.

1) I need to model for variable exterior temperature on the exterior plasterboard wall surface (Use profile option to define variable temperature boundary condition)
2) Then the heat should conduct through the solid plasterboard (I need to define variable Specific heat and Conductivity --> User-defined materials)
3) Then the inside surface of the outer wall cavity will heat up and Radiate and Convect Heat to the air cavity inside and to the opposite face of the internal plaster board)
4) Due to the previous step the the opposite face inside the cavity will heat up and the internal plasterboard conduct heat
5) Then the internal surface should radiate and convect heat to ambient temperature.

Please help me to sort out and define all these steps in Ansys-Fluent

Thank you in advance
Rusthi

Last edited by rusthimim; October 19, 2014 at 21:59. Reason: Added Model Information
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Old   October 19, 2014, 22:01
Default Need help
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Someone please help me on this. Thanks
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Old   October 20, 2014, 06:36
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Tarjei
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Hi, rusthimim!

You need to specify your question further - as of now, it reads like you're asking us to do your homework. Please provide information about what you've tried so far, as well as a description of the errors or problems you've encountered.
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Old   October 21, 2014, 22:08
Default Heat transfer modeling
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Hi there,

I have tried the following.

1) I modeled 3 solid bodies using Solidworks and imported the assembly to Ansys design modeler. This includes outside solid plasterboard and inside solid plasterboard with Air cavity Fluid sandwiched between outside and inside plasterboard.
2) I defined the named selection for outside surface wall, inside surface wall and sides of solids and fluid.
3) In Fluent, I included a profile data for temperature variation on the outer wall. I used boundary condition to have the variable temperature profile on the outside wall surface.
4) I prepared a udf for conductivity and specific heat for solid plasterboards and interpreted that into new material properties. I used incompressible ideal gas for inside cavity air.

#include "udf.h"
DEFINE_SPECIFIC_HEAT(my_user_cp, T, Tref, h, yi)
{
real cp;
if (T < 373.15)
{
cp = 950;
}
else if (T >= 373.15 && T <= 473.15)
{
cp = 17500;
}
else
{
cp = 950;
}
*h = cp*(T-Tref);
return cp;
}
#include "udf.h"
DEFINE_PROPERTY(my_user_Conductivity,c,t)
{
real k;
real T = C_T(c,t);
if (T < 413.15)
{
k = 0.25;
}
else if (T >= 413.15 && T <= 1073.15)
{
k = 0.15;
}
else if (T > 1073.15 && T <= 1273.15)
{
k = 0.4;
}
else
{
k = 0.8;
}
return k;
}

5) I defined surface to surface radiation applied to all walls.
6) I defined mixed boundary condition on the inside wall surface with heat transfer coefficient of 10 and emissivity of 0.9. Same applied to the sides.
7) I turned on Pressure based, Transient, gravity, and energy equations at the start of the analysis.
8) In Mesh Interface I created coupled walls on outer and inside wall surface connected with air cavity.

Now when I run the model, I am not getting the temperature variation on the inside wall surface (facet maximum). This is a conjugate heat transfer with natural convection and radiation inside the cavity and outside.

Please give me you thought on this. Is there any mistakes in the method or in the UDF?
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Old   October 22, 2014, 03:50
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When you say that you do not get the temperature variation on the inside wall surface, what do you actually get? Do you get a constant, seemingly realistic, temperature or something different?

To me, your procedure looks fine. I have never done any radiation calculation using Fluent, though, so I do not know how that is supposed to be set up.

Good luck!
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Old   October 22, 2014, 18:04
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Rusthi
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Thanks for you reply.

I tried to get Facet maximum temperature on the outside wall using monitors. The outside wall graph matches well with my boundary condition input profile. However my inside wall area weighted average temperature value is showing a smooth graph but not matching well with the experimental results.

Can anyone else please check my UDF and tell me whether it is correct or not?

As this is a natural convection and radiation, what reference value and zone I should define?

And my temperature profile is defined for every 60 sec. Can I run the simulation for every second? in this way will the software interpolate values?

Please help me on this. Thanks
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Old   October 23, 2014, 02:07
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There does not seem to be any glaring mistakes in your UDF -- but why have the twenty-fold jump in cp at 100 deg C? I usually set "Print to console" for the applicable values in the UDF, just to get some control of how it behaves.

Reference values and zones aren't of that much importance here, I think, as my understanding is that these are used for returning dimensionless and other scaled values. All the time you're only looking at the physical quantities as they are used in the calculations, the reference values won't change your results.

How large is the temperature difference across the wall? It shouldn't be any problem using a temperature profile defined for every 60 seconds, as you said, Fluent interpolates (linearly, I think) between these values.

Can I assume that you getting a smooth value for the inner wall temperature, implies that the solution procedure itself behaves nicely (e.g., your residuals converge steadily)?

How do the other, cavity-faced, walls behave? If there's an error in your physical description of the problem, best case scenario it solely applies to the inner plaster wall, the conductivity through this. Try to locate where you start to see unexpected behavior.
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Old   October 23, 2014, 02:40
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Quote:
Originally Posted by tarjeiba View Post
There does not seem to be any glaring mistakes in your UDF -- but why have the twenty-fold jump in cp at 100 deg C? I usually set "Print to console" for the applicable values in the UDF, just to get some control of how it behaves.

The jump comes in because of water vaporization in the plaster where chemical reactions occur. When I monitored the Cp and K, the Cp value is not working as it stays at a constant value. However the conductivity value working well. I think there is a problem in my Cp UDF?

Reference values and zones aren't of that much importance here, I think, as my understanding is that these are used for returning dimensionless and other scaled values. All the time you're only looking at the physical quantities as they are used in the calculations, the reference values won't change your results.



How large is the temperature difference across the wall? It shouldn't be any problem using a temperature profile defined for every 60 seconds, as you said, Fluent interpolates (linearly, I think) between these values.

Can I assume that you getting a smooth value for the inner wall temperature, implies that the solution procedure itself behaves nicely (e.g., your residuals converge steadily)?

All my residuals are staying around a constant value (continuity residual around 1e+00)

How do the other, cavity-faced, walls behave? If there's an error in your physical description of the problem, best case scenario it solely applies to the inner plaster wall, the conductivity through this. Try to locate where you start to see unexpected behavior.
I tried to run the analysis using constant Cp and k values. This way the graphs are looking very smooth. And my expected values at the end of the time step is matching well. However, The important thing I am trying to locate is the peaks in temperature variation due to the change in Cp and k value of plasterboard. That is why I tried to use UDF. I firmly believe the problem in the UDF.

Please help me on this. Thanks for you time. I really appreciate it.
Attached Images
File Type: jpg 2.jpg (19.7 KB, 4 views)
File Type: jpg Residuals.jpg (33.2 KB, 3 views)
File Type: jpg 3.jpg (36.2 KB, 3 views)

Last edited by rusthimim; October 23, 2014 at 02:47. Reason: MOre information
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conjugate heat transfer, natural convection, transient bc

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