how to set the boundary conditions correctly? (cube with 10,000 W losses)

LisaKTL · January 25, 2021, 01:14

Hello,

I’m trying to solve this problem: Cube (steel) 1m^3 with 10 000W loss. I want to see how this cube heat up.
I don’t understand how to do boundary conditions correctly. I set cube as subdomain with source 10 000 [W m^3]. If I set air around this cube with “opening” temperature is too high. If I set “inlet” and “outlet” with Static Pressure 1 [Pa] and 0 [Pa] then temperature is closer to real temp., but still high.

I don’t understand what should i do to Ansys to understand me.

Thank you

ghorrocks · January 25, 2021, 04:54

This is not a CFD problem. It is just a heat transfer problem, so you don't need any fluid domains. Just set the cube as a solid domain and apply 10kW heat loss on the outer boundary.

But I don't see why you want to model this at all. There are analytical solutions for the temperature versus time for this case, or if you just want the average temperature it simplifies to a single equation. So no need to model anything.

LisaKTL · January 25, 2021, 07:13

When I’m doing just cube and just loss there is like 17 000K temperature. That’s unreal.
I’m modelling that just to understand how it works in Ansys.

ghorrocks · January 26, 2021, 17:10

The high temperature could be either:

Numerical Error - in other words you are not solving the equations accurately. Usually this means you just need to converge tighter. So run the simulation for more time steps. This is very common in conjugate heat transfer simulations (like what you propose) as the solid time scales are just much slower than the heat transfer time scales, so getting the heat transfer to converge using default settings is tricky. Usually we add imbalances as a convergence criteria for CHT simulations to mitigate this.

Physical Error - The physical setup of your simulation is wrong. Are you sure a 10kW heat load is correct? Are you sure the material properties are correct?

January 25, 2021, 01:14	how to set the boundary conditions correctly? (cube with 10,000 W losses)	#1
LisaKTL New Member Liza Join Date: Jan 2021 Posts: 5 Rep Power: 5	Hello, I’m trying to solve this problem: Cube (steel) 1m^3 with 10 000W loss. I want to see how this cube heat up. I don’t understand how to do boundary conditions correctly. I set cube as subdomain with source 10 000 [W m^3]. If I set air around this cube with “opening” temperature is too high. If I set “inlet” and “outlet” with Static Pressure 1 [Pa] and 0 [Pa] then temperature is closer to real temp., but still high. I don’t understand what should i do to Ansys to understand me. Thank you

January 25, 2021, 04:54		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,703 Rep Power: 143	This is not a CFD problem. It is just a heat transfer problem, so you don't need any fluid domains. Just set the cube as a solid domain and apply 10kW heat loss on the outer boundary. But I don't see why you want to model this at all. There are analytical solutions for the temperature versus time for this case, or if you just want the average temperature it simplifies to a single equation. So no need to model anything. __________________ Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.

January 26, 2021, 17:10		#4
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,703 Rep Power: 143	The high temperature could be either: Numerical Error - in other words you are not solving the equations accurately. Usually this means you just need to converge tighter. So run the simulation for more time steps. This is very common in conjugate heat transfer simulations (like what you propose) as the solid time scales are just much slower than the heat transfer time scales, so getting the heat transfer to converge using default settings is tricky. Usually we add imbalances as a convergence criteria for CHT simulations to mitigate this. Physical Error - The physical setup of your simulation is wrong. Are you sure a 10kW heat load is correct? Are you sure the material properties are correct? aero_head likes this. __________________ Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.

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January 25, 2021, 07:13		#3
LisaKTL New Member Liza Join Date: Jan 2021 Posts: 5 Rep Power: 5	When I’m doing just cube and just loss there is like 17 000K temperature. That’s unreal. I’m modelling that just to understand how it works in Ansys.