[adilsyyed]

I am performing analysis on a double pipe heat exchanger with the hot fluid in the outer shell and cold fluid in the inner tube.

Now I can model a solid domain between the two fluid domains and specify the material, or I can simply define a fluid-fluid interface and specify the interface thickness along with material.

My question is, if I specify a fluid-solid interface after modeling the metal tube, do I still need to give this interface the thickness and material? I already defined a solid domain of 1 mm thickness between the two fluid domains.

[urosgrivc]

You do not need to model the solid domain (tube).

1. First you create 2 domains (outer fluid, iner fluid) and under heat transfer you check either thermal or total energy.

2. create an Interface (pick the pairing surfaces) and under fluid models
check no slip wall this means that no fluid can flow through the interface than Heat transfer options ocure and you can select
conservative interface flux (this will be thermal energy) and under interface model you have an option to select
thin material than select the material and its thicknes.

that is it... You will not be able to see volume temperatures of the tube in cfx post, only
on both sides (wals of the tube) and in the fluid domains ofcourse.

______________
The other option is to do a CHT simulation If you would do that you would need a third solid domain but why complicate if it is not nesecary

And the ansver for the last question is, no, if you model the solid tube domain than you dont have any additional interface material,
unles the tubes have let say a lot of lime stone on them that could be the interface material
, but you do hawe two (fluid solid) interfaces,

[adilsyyed]

Originally Posted by urosgrivc

You do not need to model the solid domain (tube).

1. First you create 2 domains (outer fluid, iner fluid) and under heat transfer you check either thermal or total energy.

2. create an Interface (pick the pairing surfaces) and under fluid models
check no slip wall this means that no fluid can flow through the interface than Heat transfer options ocure and you can select
conservative interface flux (this will be thermal energy) and under interface model you have an option to select
thin material than select the material and its thicknes.

that is it... You will not be able to see volume temperatures of the tube in cfx post, only
on both sides (wals of the tube) and in the fluid domains ofcourse.

______________
The other option is to do a CHT simulation If you would do that you would need a third solid domain but why complicate if it is not nesecary

And the ansver for the last question is, no, if you model the solid tube domain than you dont have any additional interface material,
unles the tubes have let say a lot of lime stone on them that could be the interface material
, but you do hawe two (fluid solid) interfaces,

Thanks for the guidance. I am clear about it now.

PS: I may use solid domain IF the geometry i.e diameter of the outer fluid shell effects other parts of the geometry. Then to preserve the geometry this approach might be used.

[adilsyyed]

The two fluids does not mix, only heat transfer across the interface occurs.

What if I use water as first fluid and steam as second. There is no physical interaction, no mass transfer occurs. But still it becomes a two phase flow and requires two phase flow modeling.

How can one get around that problem.

PS: I asked a similar question in another thread, hope that's not a problem.

[adilsyyed]

Originally Posted by adilsyyed

The two fluids does not mix, only heat transfer across the interface occurs.

What if I use water as first fluid and steam as second. There is no physical interaction, no mass transfer occurs. But still it becomes a two phase flow and requires two phase flow modeling.

How can one get around that problem.

PS: I asked a similar question in another thread, hope that's not a problem.

This question has already been answered in the other thread. Thanks

[soumitra2102]

What if I want both Mass and Heat Transfer to occur between two domains?
What should I select? Can you please elaborate what does Conservative Interface Flux mean?

[ghorrocks]

Conservative interface flux means exactly what it says - the flux of the variable over the interface is conserved. This might sound obvious but a simple interpolation at an interface is not always conservative - meaning material is created or destroyed by the numerical approach of the interface.

By mass transfer do you mean your flow passes through it, or do you mean a mass fraction passes through it, such as water diffusing into a solid?

[soumitra2102]

Thanks.
And by mass transfer I mean fluid flow passing through the interface without any loss.

In my case, I have two exactly same fluid domains but with different geometries and with common cross section at the interface. I want to simply connect the two domains and allow fluid flow (carrying mass and heat) in-between the two domains as if there is no interface boundary; which option should I use?

[ghorrocks]

If you set up a interface between two fluid domains the default condition is that it allows fluid flow and all scalar variables with it (heat, volume/mass fraction, other scalars) to pass through the interface. No special handling is required.

[IronLyon]

Originally Posted by soumitra2102

Thanks.
And by mass transfer I mean fluid flow passing through the interface without any loss.

In my case, I have two exactly same fluid domains but with different geometries and with common cross section at the interface. I want to simply connect the two domains and allow fluid flow (carrying mass and heat) in-between the two domains as if there is no interface boundary; which option should I use?

I'm doing somethig similar with your case, fluid fluid but one is melt (liquid) metal (hot) and the other is the gas (forced convection). The simulation is the cooling process for the metal, and in the physical conditions there is no others (means the interface is just the gas and the metal). I think, once you choose the conservative interface flux the heat transfer just depends on the temperature of each fluid and the materials (including the physical properties).