December 11, 2022 at 4:01 amabhnv_01Subscriber
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?
December 12, 2022 at 12:08 pmNikhil NaraleAnsys Employee
Are the residuals (specifically the energy) converging at every timestep? You can also check energy imbalance in the system.
Let me know.
December 12, 2022 at 1:21 pmabhnv_01Subscriber
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.
December 12, 2022 at 1:24 pmNikhil NaraleAnsys Employee
Is this a 2D case?
December 12, 2022 at 1:25 pmabhnv_01Subscriber
Yes it is a 2D case
December 12, 2022 at 1:40 pmNikhil NaraleAnsys Employee
Okay, can you check the total heat (in watts) and not the heat flux at the evaporator and condensor section? Let me know.
December 12, 2022 at 1:42 pmabhnv_01Subscriber
Yes, I checked total heat in watts and Net heat transfer in watts is positive.
December 12, 2022 at 2:40 pmNikhil NaraleAnsys Employee
Can you share the temperature contours?
December 13, 2022 at 4:18 am
December 13, 2022 at 6:08 amNikhil NaraleAnsys 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.
December 13, 2022 at 6:11 am
December 13, 2022 at 6:13 amabhnv_01Subscriber
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.
December 13, 2022 at 6:47 amNikhil NaraleAnsys 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.
December 13, 2022 at 6:55 amabhnv_01Subscriber
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.
December 13, 2022 at 7:02 amNikhil NaraleAnsys Employee
Then I suspect you must have frozen the VF equation. You can check that here: Solutions -> Controls -> Equations. Send me a screen grab.
December 13, 2022 at 8:28 am
December 13, 2022 at 2:45 pmNikhil NaraleAnsys Employee
Was that already selected or did you do that now, before taking the screenshot?
December 14, 2022 at 1:43 amabhnv_01Subscriber
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?
December 14, 2022 at 8:13 amNikhil NaraleAnsys 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)
December 14, 2022 at 8:16 amabhnv_01Subscriber
How to include the VOF equation in equations, if not available?
December 14, 2022 at 10:00 amabhnv_01Subscriber
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 ?
December 14, 2022 at 10:10 amabhnv_01Subscriber
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.
December 14, 2022 at 11:48 amNikhil NaraleAnsys 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."
December 14, 2022 at 12:58 pmabhnv_01Subscriber
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 ?
December 15, 2022 at 5:12 amabhnv_01Subscriber
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.
December 16, 2022 at 11:16 amSRPSubscriber
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?
December 16, 2022 at 12:50 pm
December 16, 2022 at 12:52 pm
December 19, 2022 at 5:58 amNikhil NaraleAnsys 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?
December 19, 2022 at 6:05 am
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