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Laminar flow and heat transfer

Posted Jul 10, 2017, 4:32 a.m. EDT 4 Replies

Fabien Dhainaut
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Hello everyone!

My question may sound trivial but I'm really blocked.

Take a tube at 293K with an flowrate of 10ml/min of an ideal gas.

Protract this tube with a surface temperature of 586K (293*2) long enough to reach this temperature in all the gaz at the end.

Normally you should get a flowrate of 20 ml/min.

I tried that with Comsol: laminar flow and heat transfer with 2 multiphysics to link the convection to the temperature and also put the density to ht.rho because I saw that it didn't evolve in the spf.rho. Still 10 ml/min at the end!

Any advice?

Regards
4 Replies Last Post Jul 16, 2017, 9:06 p.m. EDT
Steve Conrad
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Posted: 7 years ago Jul 10, 2017, 3:30 p.m. EDT
I'm not clear on your model setup, but if you applied an inlet temperature of 293K on the gas and a temperature of 586K on the cylinder walls, you should get what you intend.

It sounds like you have not coupled your 2 physics properly. Make sure the convective velocity in the heat physics is set to your gas flow velocity from the CFD physics. Also, make sure you are specifying Kelvin for your temperature scale at all relevant boundaries.

This would also be a good problem for the optimization solver, to optimize the length of the cylinder to your desired outlet temperature.

--
Steven Conrad, MD PhD
LSU Health
I'm not clear on your model setup, but if you applied an inlet temperature of 293K on the gas and a temperature of 586K on the cylinder walls, you should get what you intend. It sounds like you have not coupled your 2 physics properly. Make sure the convective velocity in the heat physics is set to your gas flow velocity from the CFD physics. Also, make sure you are specifying Kelvin for your temperature scale at all relevant boundaries. This would also be a good problem for the optimization solver, to optimize the length of the cylinder to your desired outlet temperature. -- Steven Conrad, MD PhD LSU Health
Mohammad Amin Kazemi
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Posted: 7 years ago Jul 11, 2017, 1:06 p.m. EDT
Fabien,
How do you calculate the volumetric flow rate?
I run a model according to what you suggested and got a correct result.
I used this expression for the volumetric flow rate : (spf.out1.Mflow/ht.rho)
Amin,

--
Mohammad Amin Kazemi
Fabien, How do you calculate the volumetric flow rate? I run a model according to what you suggested and got a correct result. I used this expression for the volumetric flow rate : (spf.out1.Mflow/ht.rho) Amin, -- Mohammad Amin Kazemi
Fabien Dhainaut
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Posted: 7 years ago Jul 12, 2017, 3:39 a.m. EDT
In fact the first answer was very helpful.

I didn't chose the coupled heat transfer with a laminar flow directly: I didn't see it at the first glance. There is only one multiphysics node with this option.

With this feature it works, however i still don't know why it doesn't work with the 2 physics separetly with 2 multiphysics.

thanks a lot.

I think I will come back with an absorption problem where a compound of a gas dilutes and the volumetric flowrate decreases...

thanks again!
In fact the first answer was very helpful. I didn't chose the coupled heat transfer with a laminar flow directly: I didn't see it at the first glance. There is only one multiphysics node with this option. With this feature it works, however i still don't know why it doesn't work with the 2 physics separetly with 2 multiphysics. thanks a lot. I think I will come back with an absorption problem where a compound of a gas dilutes and the volumetric flowrate decreases... thanks again!
Steve Conrad
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Posted: 7 years ago Jul 16, 2017, 9:06 p.m. EDT
Updated: 7 years ago Jul 16, 2017, 9:07 p.m. EDT
Fabien,

The Multiphysics node is a relatively new feature of COMSOL. Unfortunately it is not available for all couplings (perhaps it will be in future releases). But you don't need this node to do what you need.

Any two or more physics can be coupled by setting the appropriate independent variable(s) of one physics to the dependent variable name of another physics. This is how we did it before the Multiphysics node was introduced.

--
Steven Conrad, MD PhD
LSU Health
Fabien, The Multiphysics node is a relatively new feature of COMSOL. Unfortunately it is not available for all couplings (perhaps it will be in future releases). But you don't need this node to do what you need. Any two or more physics can be coupled by setting the appropriate independent variable(s) of one physics to the dependent variable name of another physics. This is how we did it before the Multiphysics node was introduced. -- Steven Conrad, MD PhD LSU Health

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