 ## Fluids

• abhnv_01
Subscriber

I am trying to simulate a heat pipe which has three different section:

Evaporator section : Constant wall heat flux

Adiabatic section : Zero heat flux

Condenser section : Constant temperature

I am using VOF model as my multiphase model and Lee model for phase interaction.

All properties are functions of temperature except vapor density (Ideal gas equation)

I am observing increase in my evaporator section temperature.

I also observed that my condenser is not able to extract the amount the heat flux I am supplying at evaporator end.

What can be the reason for this behaviour?

• Nikhil Narale
Ansys Employee

Hello,

Are the residuals (specifically the energy) converging at every timestep? You can also check energy imbalance in the system.

Let me know.

• abhnv_01
Subscriber

There is energy imbalance inside the system.

Evaporator = +Q1 watt

Condensor = -Q2 watt

lQ1l > lQ2l

Net heat is positive. That means condenser is not releasing the amount of energy given at evaporator section.

For residuals below is the attached screenshot, it looks like it is converging. Kindly put your opions on this. • Nikhil Narale
Ansys Employee

Is this a 2D case?

• abhnv_01
Subscriber

Yes it is a 2D case

• Nikhil Narale
Ansys Employee

Okay, can you check the total heat (in watts) and not the heat flux at the evaporator and condensor section? Let me know.

• abhnv_01
Subscriber

Yes, I checked total heat in watts and Net heat transfer in watts is positive.

• Nikhil Narale
Ansys Employee

Can you share the temperature contours?

• abhnv_01
Subscriber
• Nikhil Narale
Ansys Employee

Assuming that the bottom-most section is the evaporator section, why don't I see a rise in temperature there; rather, the temperature variation can be observed in the vertical adiabatic section. Please correct me if I am incorrectly pointing out the sections. If so, can you highlight the evaporator, adiabatic and condensor sections? Perhaps some illustration will be helpful.

• abhnv_01
Subscriber
• abhnv_01
Subscriber

The increase in temeprature is very high as expected.

The critical temperature for Nitrogen (working fluid for my simulation) is about 126K but the temperature rise in my system is very high. And that too for a small heat input.

• Nikhil Narale
Ansys Employee

By the way, why there is no vf-residual in the residuals plot? Also, have you set the mass transfer and saturation settings properly? Please check.

• abhnv_01
Subscriber

In VOF multiphase model, I did not see any VF- residual in RESIDUALS.

In Mass transfer mechanism, Evaporation-Condensation model is chosen. Then Lee model for Phase interaction.

Accomodation coefficents have been tried to fine tune with experimental results, Saturation temperature is set as a polynomial function of pressure. • Nikhil Narale
Ansys Employee

Then I suspect you must have frozen the VF equation. You can check that here: Solutions -> Controls -> Equations. Send me a screen grab.

• abhnv_01
Subscriber
• Nikhil Narale
Ansys Employee

Was that already selected or did you do that now, before taking the screenshot?

• abhnv_01
Subscriber

All the three equations were selected before taking the screenshot.

I have one doubt to ask, isn't the "Continuity" residual represented by the "Volume fraction" equation in VOF?

• Nikhil Narale
Ansys Employee

In VOF, for two phase flows, an additional transport equation is solved for the VF of the secondary phase. For primary phase, volume fraction is simply calculated by: VF = 1 - VF of secondary phase. For more details, please check this section of the theory guide: 14.3.4. Volume Fraction Equation (ansys.com)

If you are not able to access the link, please refer to this forum discussion: Using Help with links (ansys.com)

Thanks!

Nikhil

• abhnv_01
Subscriber

How to include the VOF equation in equations, if not available?

• abhnv_01
Subscriber

I think I should reframe my question here:

How to get the residual plot for the volume fraction equation ? What other things should I look for ?

• abhnv_01
Subscriber

In the models section > Volume fraction paramters > (Explicit was chosen earlier, I changed it to Implicit), and now I can see the residual for VF-secondary phase.

• Nikhil Narale
Ansys Employee

Here are some excerpts regarding Explicit Volume Fraction Formulation from the Ansys Help. Hope this clears your doubts!

"Since the volume fraction at the current time step is directly calculated based on known quantities at the previous time step, the explicit formulation does not require and iterative solution of the transport equation during each time step."

"The explicit formulation is non-iterative and is time-dependent."

• abhnv_01
Subscriber

Thank you for your reference. I myself went through the Ansys theory guide regarding this.

But, I also went through some sources which said Explicit scheme is usually chosen for trainsient cases, because it gives clear and crisp boundaries or interfaces.

So, which scheme should be chosen for a two-phase transient case : Explicit or Implicit ?

• abhnv_01
Subscriber

There is not much change in temprature when using implicit scheme instead of explicit scheme.

So, the reason for increase in temperature of the system is still unresolved.

• SRP
Subscriber

Hi,

1) Did you activate the term "implicit body force"? (It is activated as follows: multiphase model>Body force formulation>implicit body force.
2) Did gravity point in the proper direction, or not? Just make sure
3) Would you kindly send a screenshot of the volume fraction after patching, as well as a screenshot of the phases you specify and their interaction?

Thank you

• abhnv_01
Subscriber
• abhnv_01
Subscriber
• Nikhil Narale
Ansys Employee

Make sure you set the Standard State Enthalpy for the liquid and the vapor correctly. Can you share the screenshots of the material properties window for both, the liquid and the vapor?

• abhnv_01
Subscriber 