## Fluids

Topics relate to Fluent, CFX, Turbogrid and more

#### 2D axisymmetric solution for Helical Tube in a Tank

• sinan ozbolgili
Subscriber

Hi all,

I would like to ask a technical question regarding to helical tube.

I need to analyze helical tube in tank and it the tube length is around 12m.
Even meshing time is taking 30 min.
Solving time will be more than 12 hours.

I checked some articles and realized many researchers did 2D axisymmetric helical tube in tank ( Pls see pic.)
Is there any one support how to give boundary conditions ?

I confused about defining liquid line in helical tube .
Normally in 3D solution it is certain see liquid domain in helical tube but in 2D axisymmetric I can see from below pic there are many circles. In attached picture hot to connect each circles?

• NickFL
Subscriber
Is the helix a solid body, or do you have a second fluid domain inside the coil that you are trying to heat/cool? If it is the former, then the 2D quasi-"axisymmetric" model may very well be sufficient for what you are looking for. You could even create a nice pure quad mesh with an o-grid looking block around each tube. Boundary Conditions: Do you know the temperature at the surface of the helix? Then just apply this. Or does the coil have heat generation? Then you can divide out what the heat generation would be and apply the flux on the surfaces.
• sinan ozbolgili
Subscriber

Hi,

Actually the tank is filled with ethanol and  water is passing through the helical tube.
The temperature of  inlet water is 20° and  ethanol temperature  is 5°C. i need to check when the outlet temperarure of water will be 7°C.

For this one I think ( I am not sure ) 2-D  axisymmetric will be better to analyze.  When I checked the picture that I shared before I can see many circles .So I can define first circle as inlet and the last one as otutley. The issue is  I dont know to how connect other ones to eact other.

• NickFL
Subscriber

I think there is going to be many opinions on how to solve this by going to 2D. I would approach this in stages. The first would be to determine what is an appropriate heat transfer coefficient inside (water side) of the pipe. We will use this value later on. On to your model, I image your boundary conditions will be like this: (Note I always get tripped up by this, but for axisymmetric models in Fluent put the axis on x-direction.)

Notice the tubes do not have any flow going through them. What we are modeling here is simply the ethanol side. The inlet and outlet for the ethanol are pretty straightforward. What I want to draw your attention to is the tubes. Here, I would set the boundary condtions using the aforementioned heat transfer coefficient. Here you can even include the thermal resistance for the thickness of the tube. A Named Expression can be used to define what is the water temperature.

The next stage would be to go back and adjust some of the assumptions and improve them. For example, the water temperature that is give above is linear, but this is probably not the case. It may be useful to even have a 3D model of just the water side that you can iterate back and forth to get to a converged solution.

• Rob
Ansys Employee

Looks reasonable. I'd check the tube side HTC, but that's for the OP to do: text books have their uses.

The only question then is the tube side temperature. Question to both, can you assume a linear relationship with position? Ie does the temperature increase linearly between passes, and is the temperature variation over a single pass permissible?

• sinan ozbolgili
Subscriber

sorry to ask some easy questions but tring to learn and solve the problem .

If I had solved the problem in 3D, I would not have given inlet and outlet for, do we need give for 2D ?
Also in attached picture, I think reseracher did not give inlet and outlet for ethanol .

IN second picture you specfy convection outer heat transfer coeff, which is unknown actually, so cant spacfy this value.  I just know ethanol tempeature is 5° and water inlet temparature is 20°- outlet temp is 7°C.

Another question is, still cold not understand  how to cinnect these circles in 2D ? How or what kind of boundry conditions do I need to give these circles. Even how software will understand these are contuniues circles and water passing through them.

• NickFL
Subscriber

If I had solved the problem in 3D, I would not have given inlet and outlet for, do we need give for 2D ?
Also in attached picture, I think reseracher did not give inlet and outlet for ethanol .

If there is no inlet and outlet for the ethanol, where is the energy going? There would be no heat sink. The bath of ethanol would slowly heat up. Are the walls of the ethanol tank cooled? There needs to be a place for the heat to go. Otherwise, the steady answer is just the incoming water temperature.

IN second picture you specfy convection outer heat transfer coeff, which is unknown actually, so cant spacfy this value.  I just know ethanol tempeature is 5° and water inlet temparature is 20°- outlet temp is 7°C.

On the inside of the helical pipe, I recommended you specify the heat transfer. What is this value? As you mentioned, this is an unknown. But a simple hand-calc can give us an estimate. Look in your heat transfer book or in the literature for a helical specific value. We could easily create a 3D mesh for this body can obtain it through CFD computation. (This is the iterative approach I mentioned.)

Another question is, still cold not understand  how to cinnect these circles in 2D ? How or what kind of boundry conditions do I need to give these circles. Even how software will understand these are contuniues circles and water passing through them.

In the modeling approach I mentioned, I would not even model the water side. If you were to model the water side, you would likely need to create a UDF that would couple the faces together. And then you would have to develop an approximation for the heat lost as it flows in the "pie slices" you are not modeling. Given the added complexity and uncertainty, I personally would avoid it. Can it be done? Yeah, sure. But why? I promise with minimal temperature change, the iterative approach I mentioned would converge in one step.

From Rob: Question to both, can you assume a linear relationship with position? Ie does the temperature increase linearly between passes, and is the temperature variation over a single pass permissible?

This is why I recommend the iterative approach. For a first "guess" it is good.

• Rob
Ansys Employee

Just a note on the 2d axi-symmetric model: read the documentation on how the axis is defined.

For Nick - yes. Separate surfaces will allow more fine tuning, but that can be done in the solver after the first run.  Separate>Faces by (I think) Region.