[arunraj]

Hello all,

I would like to know the application of Lee model in heat pipes.

As I am new to this field, I started with a simple problem. Two dimensional geometry of heat pipe with wick and wall. There are two phases, liquid and vapor. The liquid is in the wick and vapor is in the core region. The heat pipe is made of copper. I have attached the domain details. I have used the following work as the reference to simulate the problem.

Poplaski, L.M., Benn, S.P. and Faghri, A., 2017. Thermal performance of heat pipes using nanofluids. International Journal of Heat and Mass Transfer, 107, pp.358-371.

https://scholarcommons.sc.edu/etd/4026/


It is clear case of phase change heat transfer.

The boundary conditions are heat flux at evaporator and condenser and adiabatic at center region. In addition, we need to specify interfacial velocities and temperature at the wick-vapor interface. However, the above thesis has implemented several UDFs and UDS citing that the interfacial temperature is not taken properly by ANSYS Fluent V16. I would like to know

1. If Lee model with VOF can solve this problem without any UDFs in ANSYS Fluent v20.2?
2. How to treat the interfacial velocities and temperature at the condenser-wick and evaporator-wick. Whats the best way?
3. Does any sort of UDF or UDS is required for this problem?

Thank you for your effort and time. Looking forward for a reply.

[vinerm]

Fluent does not consider any blockage to the flow in a porous zone; whole of the area is open. So, as such, the interface between porous and fluid zones are simple interior boundary conditions. Flux is conserved for all fields. As far as thermal energy is concerned, default condition is equilibrium heat transfer between the wick and the fluid in the pores. In a heat pipe, wicks usually adhere to thermal equilibrium. However, if required, Non-Equilibrium heat transfer can be enabled within porous zone. This would require more information, such as, surface area density of wick and material properties for the wick. The advantage is that wick and fluid can be at different temperature.

[arunraj]

Hi Vinerm,

Thank you for your kind reply. I have edited the post since I am able to simulate successfully. However, it is taking lot of computational time and less accurate. So, I will slightly modify my clarifications too.

I have few clarifications and questions regarding simultaneous evaporation-condensation problem considering the porous wick region. I divided the domain into three zones, solid zone, porous zone with water liquid and vapor zone with water vapor. I agree that the interface between the porous and vapor zone is a simple interior.
Q1. But how to ensure mass and phase transfer happens only in the wick-vapor interface of evaporator and condenser and not adiabatic section as shown in the domain figure.
Q2. This problem does not have any inlet velocity as it is a close cycle and only mass/phase change happen.

(I). How to compute Courant number before simulation for such application ?
(II). Is there any way other than time-step to limit the Courant number within 3 for simultaneous evaporation-condensation problem mentioned here for explicit VOF?
I have seen researchers using time-step size of 1s for similar condition with implicit VOF. Also, they have used UDF based on lee model, not the one available in ANSYS Fluent.
(III) Please shed some light on the best practices for accurately simulating simultaneous evaporation-condensation problem with optimal computational time.

Thank you for your effort and time.

Regards
Arun Raj

[arunraj]

Attchments

[arunraj]

Attachement1

[arunraj]

Attachment2

[vinerm]

What's the objective behind using two mass-transfer mechanisms? Is it to model mass transfer in evaporator and condenser sections? That is not required then. A single mechanism will take care of all the mass transfers throughout the domain. Secondly, there is no inbuilt mechanism to specify or control the mass transfer to take place only at the interface of fluid and porous zone. Mass transfer is purely based on thermal conditions. The job of the wick is to transfer the liquid.

[siddiquesil]

What types of mesh interface should be define along vapor liquid common zone where there is no condensation or evaporation take place? At the interface, velocity component should be zero, but temperature and pressure information should pass from liquid zone to vapor zone. Kindly respond.