# 3D convection and conduction

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• Last Post 10 July 2019
dnelo13 posted this 28 June 2019

Hi there, it's Daniel. I want to make a 3d simulation of the heating of a cool product inside a box which is closed. The main idea is tha the box is under air convection so it is heated by the ambient by a heat conduction coeficient. As the walls of the box get hot, the air inside the box get hot too and heat the product inside the box. What would be the boundary conditions? How could I relate the air inside the box with the walls of the box? It would be a transient analysis.

peteroznewman posted this 30 June 2019

Hi Daniel,

A simple additional boundary condition is the bottom of the box. Assume the box is sitting on an insulated surface.

For simplicity, let the cool product on the inside be a cube sitting on the bottom of the box in the center with air all around it.

In DesignModeler, create the cube solid and the box solid, using Add Frozen, so you have two separate solids. Then use Boolean to Subtract the cube from the box, using Keep Tool so you have two solids, the cube and the box with the cube shaped hole in the bottom.  Now select these two solids and Form New Part to share topology.

This is about the simplest geometry you can make to do this simulation.  The box solid represents the air inside the box. Five faces of the box can have a convective boundary condition with the warm outside air temperature. On those five faces, you can also define a wall thickness and conductivity for the box material. The cube solid represents the product, and you will need to define that as a solid with a specific heat capacity and give it an initial temperature. The air inside the box can have the same initial air temperature.  You will want Gravity included to allow the natural convection to occur inside the box.

You will also need to imprint a small square face into the side of the box solid to represent a pressure equalization hole. As the air inside the box warms up, it will expand and a small volume will need to exit the box. The face on the side of the box is defined as a Pressure Outlet boundary condition.

Here are a few relevant discussions.

One

Two

Video

dnelo13 posted this 01 July 2019

Hi Peteroznewman,

thank you so much for your advices. I've just create the geometry, separating the two bodies, and I have set up the convective boundary condition at the five external faces of the box.

However, I don't know where can I define the conductivity box material. I've attached the boundary condition I defined.

Besides, how could I define the cool product temperature if it is define as an interior?

peteroznewman posted this 01 July 2019

I'm also learning Fluent, so I don't know everything, but I am good at guessing. I see a Material Name on the lower left side of that dialog box with an Edit... button next to it.  Does that let you define a conductivity?

I believe you can set the initial temperature on the cold solid and the cold air in the box in the Solution Initialization panel.

On one of the five faces, did you make a very small imprinted face to have a pressure outlet?

dnelo13 posted this 06 July 2019

Hi there,

I have set up a 2D simulation for an easily calculation. I have defined the material box conductivity and I have applied the air convection to the exterior walls, however, when I define the interior temperature of the cube, it keeps like a constant, and it has no variation with the ambient inside the box.

I've attached the boundary condition that I've defined.

Keep on trying.

peteroznewman posted this 07 July 2019

Do not apply a Boundary Condition of Temperature to the Product wall. That forces the solution to keep it at that temperature through all time, which is what you got. Delete that Boundary Condition.

The product and the air should both get an Initial Condition of 3 C to start the simulation, then the convection from the warm air outside the box will heat the air inside the box and the airflow will heat the product over time.

Where is your small edge on the box defined as the Pressure Outlet to accommodate the increase in air volume as it warms up?

dnelo13 posted this 08 July 2019

I though I defined wrong the boundary conditions because it was a very good insulation box, but everything is okay.

The pressure outlet is defined at the bottom of the box, I think it's okay.

peteroznewman posted this 08 July 2019

If the entire bottom face is an outlet, then cool air that descends near the surface of the product will leave the domain. That is not correct. It should hit a floor and flow out toward the warm box wall, be heated up and rise. You only need a very small outlet on the edge, maybe somewhere in the center of the box wall.

rwoolhou posted this 10 July 2019

You also need ideal gas (assuming it's a gas) to model a sealed domain: this allows the temperature & pressure to float to maintain the volume.