[mjkey]

Hey guys,

I'm trying to include a thermal analysis into my problem.

I'm modelling the thermal comfort in my solar car. I have the interior airflow already and I want to turn on the energy equation and add some heat into the car cabin. I will be setting the inlet temperature (natural air) at 293.15K (20 degrees celcius). The problem I am having is to know which Wall thermal boundary to set. I am thinking between heat flux and temperature. The main heat will be coming from the solar radiation actually...but I just want to keep it simple and assume that the inside temperature of the cabin reaches up to 323.15K (50 degrees celcius, travels through the desert). The car is made from Carbon Fibre (I will need to assume a heat flux for this).

So If i choose the heat flux option, i get heat flux W/m2 and Heat Generation Rate. I am confused what is the heat generation rate w/m3. Actually, does anyone mind explaining what the heat flux option does? So at the wall boundary, the first cell, heat (at the heat flux and generation rate) is generated into the car interior?

If i choose the temperature option, I get a "Temperature" and heat generation rate. How would this be utilized?

Also for Material Name: It is currently set to aluminium, with it's properties. I'm assuming Ill have to change this aswell.

Cheers Guys, this is quite urgent. My thesis is due in less than 2 weeks. Appreciate the help! Sorry for the broken english, panicking here!

Thanks!

[ghost82]

Hi Michael,
If you are studying the air temperature, streamlines, etc into the cabin, I think there's no need to model the thickness of the walls.
I think you can set the temperature of the walls to a constant temperature (an equilibrium temperature, 50 ˚C, or what you want..).

If you model also the wall thickness then you need to change solid material properties, but if you are modeling zero tickness walls there's no need to change solid properties.
I think you can set heat generation rate to 0, if you assume the wall temperature constant.

I hope this helps a bit.

Daniele

[macfly]

Hi Michael,

Where are you doing your PhD?

A simple approach could be to look for irradiation flux data in the desert and apply this heat flux at the boundary. Or look for data of typical solar irradiation entering a car (there is probably a non negligible part of the solar irradiation that is reflected by the car surface and the windows...).

A heat generation rate would be specified if you considered a thermal resistance at the wall. You would have to specify a wall thickness.

[mjkey]

Originally Posted by ghost82

Hi Michael,
If you are studying the air temperature, streamlines, etc into the cabin, I think there's no need to model the thickness of the walls.
I think you can set the temperature of the walls to a constant temperature (an equilibrium temperature, 50 ˚C, or what you want..).

If you model also the wall thickness then you need to change solid material properties, but if you are modeling zero tickness walls there's no need to change solid properties.
I think you can set heat generation rate to 0, if you assume the wall temperature constant.

I hope this helps a bit.

Daniele

Thanks Daniele!

I've just set a constant temperature and it's looking pretty fine for now!

[mjkey]

Originally Posted by macfly

Hi Michael,

Where are you doing your PhD?

A simple approach could be to look for irradiation flux data in the desert and apply this heat flux at the boundary. Or look for data of typical solar irradiation entering a car (there is probably a non negligible part of the solar irradiation that is reflected by the car surface and the windows...).

A heat generation rate would be specified if you considered a thermal resistance at the wall. You would have to specify a wall thickness.

Hi macfly,

I'm not doing my PhD, I'm doing my undergraduate honours thesis for my mechanical engineering degree at UNSW, Australia.

Thanks for the tips!

[mjkey]

Hey Macfly,

Would it make sense to set a Heat Flux boundary AND a temperature boundary on the wall?

I'm still a bit confused as to what the boundaries are really doing.
I know the heat flux is probably transferring heat into the car from the wall, but what temperature is the wall at? Is this then achieved by setting the temperature boundary?

If you only set a temperature boundary on the wall, does FLUENT then calculate the heat transfer from the wall into interior car space? Would you also happen to know how to set the operating conditions? How should I set up the boussinesq model (should i?)

Cheers

[macfly]

Originally Posted by mjkey

Would it make sense to set a Heat Flux boundary AND a temperature boundary on the wall?

No, you apply either a heat flux or a temperature boundary condition.

Originally Posted by mjkey

I'm still a bit confused as to what the boundaries are really doing. I know the heat flux is probably transferring heat into the car from the wall, but what temperature is the wall at? Is this then achieved by setting the temperature boundary?
If you only set a temperature boundary on the wall, does FLUENT then calculate the heat transfer from the wall into interior car space?

If you apply a heat flux, Fluent will calculate the temperature at the wall and vice versa.

Originally Posted by mjkey

Would you also happen to know how to set the operating conditions? How should I set up the boussinesq model (should i?)

- Operating conditions: I think you only have to worry about specifying atmospheric pressure and gravitational acceleration.
- Boussinesq: I think it's the appropriate natural convection approach to use in your case, but I'm not familiar with the settings. I know it's typically hard to converge. There is tons of information on the forum about how to set the Boussinesq model.