## Fluids

Topics relate to Fluent, CFX, Turbogrid and more

#### Thermal conduction problem

• seung mook park
Subscriber

i want to observe thermal conduction in heat exchange channel using transient analysis.

if i set air density to constant, it works well.

but, if i change density constant to ideal gas, conduction does not happen.

except for density, everythings are same.

• NickFL
Subscriber

First things first, the inlet temperatures do not appear to be the same. The scale on the lower plot shows it might be 1K whereas above it is 20C.

Why, in the lower image, why is there a big change in the T? I am not sure what the colored non-blue structure is in the lower section. We don’t know the problem as well as you, so I would recommend showing the same image when comparing things.

You, also, mentioned this is a transient simulation. Is this the first time step or later on in the simulation?

What is the expected pressure drop through the U shape domain? And what will happen to the density and temperature as the pressure changes (ideal gas case)?

• C N
Ansys Employee

Hello Seung,

The Ansys fluent actually predicts a gauge pressure in the below equation and  adds it to ideal gas law (user guide -8.3.7)and considers the air as compressible flow when you choose ideal gas option for density whereas the constant option just allocates the value and the Ansys fluent does not add the gauge pressure term and treats the air as incompressible. The air although practically is compresssible by nature it can be theoretically considered incompressible for mach umber less than 0.3.So this can be the reason why you experience the difference in temperature profile and also you cannot observe conduction for density as ideal gas option. I recommend you to check with incompressible ideal gas option for density you should observe the changes. This might solve your problem.I am also attaching the tutorial for thermal conduction in heat sink.

Into to Heat Transfer in Fluids Quiz | Ansys Courses

8.3. Density (ansys.com)

Thanks,

Chaitanya Natraj