[clementbrun]

Dear Fluent Community,

I am currently simulating air at rest confined in two concentric tubes with a constant heat condition on the inner radius and a convection condition characterised by a convective exchange coefficient.

I have defined the thermal properties of air that change with temperature (density, heat capacity, conductivity, viscosity).

The delta T is very large, almost 900°C in the real world. I've opted for a laminar model for viscosity. Is this correct? Do I need to change any other parameters?

[CFDKareem]

Originally Posted by clementbrun

Dear Fluent Community,

I am currently simulating air at rest confined in two concentric tubes with a constant heat condition on the inner radius and a convection condition characterised by a convective exchange coefficient.

I have defined the thermal properties of air that change with temperature (density, heat capacity, conductivity, viscosity).

The delta T is very large, almost 900°C in the real world. I've opted for a laminar model for viscosity. Is this correct? Do I need to change any other parameters?

Check out the Grashof number which is a dimensionless number which gives the ratio of buoyancy to vicious forces. Between 10^8 < Gr < 10^9 the natural convection flow will transition to turblent flow. Try calculating this value for your number to estimate if you should use a turbulent or laminar solver.

[clementbrun]

Thank you for your reply.

In my case, I opted for the Laminar solver. I have one last question to ask you. I think I'm getting good results, but one result worries me. When I increased the inside diameter to 22mm instead of 21mm, I got a temperature difference of more than 100°C.

In reality, I've tested and we don't have a big difference as the simulation results tell us. I have no idea where the problem lies in my conditions/simulations. Do you have an idea ?

Thank you in advance

[CFDKareem]

It's hard to say without knowing the exact experimental setup. On the outer radius I assume there is a solid wall. Since you are applying a convective heat transfer coefficient directly to the fluid domain it would likely overestimate the heat flux through this boundary. I would try either adding a wall thickness to the wall boundary or modeling the solid, assuming there is a solid wall on the outside.

[clementbrun]

Thank you very much !

[LoGaL]

I do not get how air at 1000K sits at rest near air at 300K and there's no circulating flow (like in a pot). Unless gravity is neglected

[clementbrun]

Hi, I probably misspoke. There is no air movement in the current system, the air is trapped. But in reality, yes, there are natural air movements. I observed this by making a contour of the speed and I could see the circulation of the fluid.