# Simulating Heat Transfer

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 November 16, 2012, 11:40 Simulating Heat Transfer #1 New Member   Felix Join Date: Nov 2012 Posts: 15 Rep Power: 11 Hi everybody, I'm new to this forum and I hope you can help me. Concerning my thesis i have got to simulate convective heat transfer inside an autoclave as well as the heat transfer on a Tool placed inside the autoclave. I am still learning to use ANSYS CFX at the moment. So I want to do some simple test simulations about convective heat transfer from fluid to a solid square inside a duct. I want to simulate the heating of the square from an initial temperature of 300 K, while to sourrounding fluid has a temperature of 1000 K. The fluid domain is meshed with tetrahedrons and inflation layers near the square. The Solid is meshed with a structured mesh. The fluid is air at 25°C, Reference pressure 1 atm, Turbulence Model SST, while the square is of aluminium. I have set both, fluid and solid Heat Transfer to Thermal energy. I have set a domain Interface between solid and fluid and have set heat transfer to conservative interface flux with a contact resistance of 120 W/mēK. The condition at the inlet of the duct are 3 m/s Normal Speed and a temperature of 1000 K. Outlet is Relative Pressure 0 Pa. First i did a steady state simulation with the above parameters and afterwards a transient simulation with initial values from the steady state simulation. The timesteps are 2 min and total time is 30 min. My first results do not seem to be correct. The steady state simulation shows a uniform temperature profile in the square AND the fluid of 1000K. The transient simulation doenst change over time and has the same temperature profile in every timestep. Acutally I am expecting a time dependent heating of the square. 1. Do i have to set the temperature at the inlet to 1000 or 300 K in the steady state simulation? 2. Do I have to choose smaller timesteps? 3. What am I doing wrong? Hope you get my problem. If not, please ask me. Thanks for help. Greetings.

 November 17, 2012, 03:09 #2 New Member   Join Date: Oct 2011 Posts: 20 Rep Power: 12 I suppose you have to use coupled boundary condition between for Fluid sloid interface. It is taken by default even if u dont define any interface and flux conservation is satisfied there. Inlet at 1000k in Fluid domain is fine, Initialize solid domain with 300 K.

November 17, 2012, 20:09
#3
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Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
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Why do you have a contact resistance between solid and fluid domains? What does this represent?

Quote:
 The steady state simulation shows a uniform temperature profile in the square AND the fluid of 1000K.
Of course - the solid has no heat generation so its steady state temperature is the same as the surroundings. Basic stuff.

Quote:
 Acutally I am expecting a time dependent heating of the square.
Then do a transient simulation with an initial condition as you specified (300K object, 1000K autoclave).

Here is the general FAQ on accuracy: http://www.cfd-online.com/Wiki/Ansys..._inaccurate.3F

 November 19, 2012, 12:08 #4 New Member   Felix Join Date: Nov 2012 Posts: 15 Rep Power: 11 Thank you for answering. The problem was that there has been a uniform temperature profile in transient simulation. I did the setup without contact resistance and just a transient simulation now. This gives better results now. Do I have to adjust the solid and the fluid mesh for a better result(Contact Sizing)?

November 19, 2012, 17:52
#5
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Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 16,923
Rep Power: 132
Quote:
 I did the setup without contact resistance and just a transient simulation now. This gives better results now.
Fluid in contact with a solid has no contact resistance. The interface boundary takes care of the conduction/convection. But note unless you have radiation activated it ignores the radiation.

Quote:
 Do I have to adjust the solid and the fluid mesh for a better result(Contact Sizing)?
You always have to check your mesh for accuracy. If you don't then how do you know how accurate your simulation is?

 Tags heat transfer, temperature profile, transient