[aicha]

Hi all
Itry to use equilibrium thermal model for modeling heat transfert in 2D porous media (packed bed of rocks traversed by air). I have followed all of these steps:

1- I enable energy equation;
2- I have defined the properties of air at 337 ° C;
3- I have defined the porous zone ( porosity, viscous resistance);
4- Boundary conditions: mass flow inlet (in inlet pipe) and pressure outlet (outlet pipe);
The air flows into the porous media at 520 °C and sort at 25°C. As a result, I have obtained a uniform temperature throughout the bed which is false.
please advice me and help me to find the right temperature distribution.
Thank you very mutch for all replys

[pakk]

Originally Posted by aicha

The air flows into the porous media at 520 °C and sort at 25°C. As a result, I have obtained a uniform temperature throughout the bed

I don't understand this. With "sort at 25°C", I assume you mean "exits at 25°C", as one of your other posts says something about the French language, but be aware that "sort" in English is not the same as "sortir" in French.

But if it is 520 °C at the inlet, and 25°C at the exit, then it is clearly not uniform. So why do you say it is uniform?

And what is the thermal boundary conditions at the walls of the porous media? Not adiabatic, I hope?

[aicha]

Thank you very mutch Pakk,
Yes I mean that the air exits at 25°C.
I mean by uniform that i get the same temperature in all positions of my porous media. I must get a high temperature near to the inlet and a low temperature near to the air outlet.
I assume that the boundary conditions at the walls is adiabatic.

[pakk]

If the temperature is uniform, it is the same at the inlet as at the outlet.
But you say that the temperature at the outlet is lower than at the inlet.

Which one is it? Is it uniform and the outlet temperature is 520C, or is it not uniform and is the outlet temperature 25C?


And if your boundaries are adiabatic, how would your porous media cool down? There is no place where the heat can go to, so the (total?) temperature of your gas would be the same everywhere, and I expect your outlet to be at 520 C.

In reality, heat will be lost at the outside of your porous media. Apply a non-adiabatic condition there, then you'll see a temperature gradient. Which boundary condition you need there, depends on the problem you are modeling.

[aicha]

Thank you very mutch Pakk
In reality, i should get the temperature at the outlet is lower than at the inlet; but, i obtain that the temperature is uniform (T=520°C) in all position, and the outlet temperature also is equal to T=520°C.
The system is a packed bed of rocks used for storage thermal energy derived from solar energy.The heated air flows from the solar collectors into a rocks bed, where thermal energy is transferred to the rocks. The recovery of this stored enery is usually obtained by reversing the flow in the bed, the cool air enters at the bottom.
I choise adiabatic boundary conditions at the walls to simplify the calculation.

[pakk]

Originally Posted by aicha

Thank you very mutch Pakk
In reality, i should get the temperature at the outlet is lower than at the inlet; but, i obtain that the temperature is uniform (T=520°C) in all position, and the outlet temperature also is equal to T=520°C.
The system is a packed bed of rocks used for storage thermal energy derived from solar energy.The heated air flows from the solar collectors into a rocks bed, where thermal energy is transferred to the rocks. The recovery of this stored enery is usually obtained by reversing the flow in the bed, the cool air enters at the bottom.
I choise adiabatic boundary conditions at the walls to simplify the calculation.

You chose adiabatic boundary conditions at the walls to simplify the calculation, but by doing that, you remove physics!

Energy is put into the system by the hot air, and escapes the system through the boundaries. The goal of your simulation is to find the equilibrium situation between those two, when the amount of energy going into the system is the same as the amount of energy leaving the system.
By putting adiabatic boundary conditions, you isolate the system. Heat can not leave the system. You will never find the balance that you are looking for in that way.

[aicha]

Thank you very mutch Pakk
My system has two pipes: inlet pipe and outlet pipe.
The hot air enters from the inlet pipe and after heat exchange with the rocks, it comes out with a lower temperature. So, the air escapes the system through the outlet pipe but no trough the boundary.
Yes, the goal of my simulation is to use the equilibrium thermal model
described in this website "https://www.sharcnet.ca/Software/Fluent14/help/flu_ug/flu_ug_sec_bc_porous_media.html".
The equilibrium situation is between the fluid and the solid (air and rocks).

[pakk]

Originally Posted by aicha

Thank you very mutch Pakk
My system has two pipes: inlet pipe and outlet pipe.
The hot air enters from the inlet pipe and after heat exchange with the rocks, it comes out with a lower temperature. So, the air escapes the system through the outlet pipe but no trough the boundary.
Yes, the goal of my simulation is to use the equilibrium thermal model
described in this website "https://www.sharcnet.ca/Software/Fluent14/help/flu_ug/flu_ug_sec_bc_porous_media.html".
The equilibrium situation is between the fluid and the solid (air and rocks).

I understand that. Of course the air does not escape through the boundary. I am not sure if you think that I meant that, but to be sure: I said that the in reality heat escapes through the boundary, so you need to set up your boundary conditions in the simulation such that heat can escape through the boundary. Otherwise, nothing will ever cool down.

[aicha]

You are absolutely right Pakk
But my rocks bed is insulated to minimize heat loss that's why I have assumed that adiabatic boundary conditions.

[pakk]

Then, if the insulation is 100% effective, your hot air will never cool down, and everything in your system will get the same temperature as the gas at the inlet.

If that is really what you want, then it is ok. But then you would not have needed to do a simulation for that.

If you want to see the effects of the heat losses through the walls in your solution, you need to include it in your simulation.


Or are you interested in the transient behavior? Like, the rocks initially are cold (25 C?), and you want to know how warm they are after for example one hour of blowing hot air in? Then you need to do a transient simulation, and then the adiabatic boundaries might be reasonable. But what you have now, steady state and adiabatic, will never give interesting results.

[aicha]

Thank you very mutch Pakk,
I want to study the transient behavior, but i use steady state, may be this is the problem. therefore, I must use the unsteady state. Is it true?

[LuckyTran]

Absolutely. You must be running unsteady if you want to see transient behavior.