## Fluids

#### VOF two phase thermosyphon heat pipe. “No pressure Rise”

• AmitDobal
Subscriber

I am simulating two-phase thermosyphon heat pipe with VOF model, k-e Realizable, primary phase water vapor, secondary phase is water liquid,

I have tried both SIMPLE and PISO with PRESTO solution methods.

Three sections - Evaporator, condenser and adiabatic,

Constant heat flux is given at evaporator and convection at condenser section.

I am doing transient analysis with Explicit formulation. with very small variable time steps.

Everything is fine, my temperature contour velocity profile, residuals are also good, but there is no significant change in pressure. The pressure is the same as what I give as the operating pressure. As the temperature is increasing inside the thermosyphon, there should be a rise in pressure also. But it is not rising, Please can anyone help?

• AmitDobal
Subscriber
Evaporator section is filled half with water
• DrAmine
Ansys Employee
EOS of the vapor phase?
• DrAmine
Ansys Employee
Also check the mass of the whole system: it should not lose mass during the time it is heated up.
• AmitDobal
Subscriber
Thankyou for the reply, EOS means? I have used lee condensation evaporation model. And used vapor and water as phases. Standard state enthalpies are default.
And I have given saturation temperature as a function of pressure. As pressure is not changing so saturation temperature was around 373K. As I have given operating pressure 101 kPa and initial gauge pressure 0 Pa.
DrAmine, could you please tell me how I can calculate mass ? As I can calculate avg volume fraction in Ansys post.
DrAmine and what could be the reason of mass loss? As my container is closed.? Please elaborate. It would be a great help.
• DrAmine
Ansys Employee
EOS: Equation of State.
There is mass report under Volume Integral available for creation: use it to get mass of water and vapor. The sum should be ideally constant Alternative you create a volume integral of mixture density using Expression or directly.
• AmitDobal
Subscriber
I have used report definition and from there I chose volume report then mass.
It is showing mass of steam is 0, and mass of water is 4.5 kg (and decreasing).
How can I fix this? Why vapor mass is zero?
• Rob
Ansys Employee
What is the vapour density?
• AmitDobal
Subscriber
I have given polynomial as a function of temperature from steam table.
I had also given contant 0.5kg/M3 to check.
And I also used ideal gas density.
But there was no pressure rise.

• DrAmine
Ansys Employee
To monitor pressure please create a volume Avreage of static pressure. Regarding the mass reports: mass of vapor + mass of liquid = initial mass (ideal) if you keep losing mass than system is wrong.
• AmitDobal
Subscriber
Please see these images, I have run a pseudo transient analysis to check mass. For steam, Water And Mixture