Learning Forum

[abhnv11295]

Hello !

I am trying to simulate a heat pipe in (3D) which has three sections:

Evaporator - Constant heat flux (boundary conditions)
Condensor - Constant temperature (boundary conditions)
Adiabatic section - Zero heat flux (boundary conditions)

I am using VOF multiphase model -> Evaporation - condensation model -> Lee model.

Transient simulation with gravity and implicit body force formulation.

Time step size = 0.0001 (1E-04).

The simulation works well for smaller heat flux values.

When I go for higher heat flux values, I come across an error that global courant number exceeds 250.

 Any help will be highly appreciated. 

[Rob]

What does increasing the heat flux do to the evaporation rates etc? 

[abhnv11295]

 

By evaporation rate you mean liquid mass evaporation to vapor mas ?

I have not checked that, but I have monitored evaporator temperature, pressure and liquid volume fraction. All of them increase with increase in heat flux. 

 

[Rob]

Liquid volume fraction increases with increased heat flux? And velocity? 

I mean evaporation rate, ie how quickly the liquid boils and leaves the hot region. 

[abhnv11295]

Velocity increases with increase in heat flux.

Also to note: After shifting from lower heat flux to higher heat flux. the simulation runs well for about 4-5 seconds (40,000 time steps) and then the courant numbers goes beyond 250.

[Rob]

OK, and if you monitor liquid fraction near the hot part of the domain?  If you're using a fixed heat flux the heat has to go into the model, if that is heating a gas rather than a liquid the temperature will increase until that much heat is pushed into the system. 

[abhnv11295]

I understand your point. 

I have monitored the heat flux released from the system and the heat flux released is of the same order of the heat flux given inside the system. 

So, I don't think there is any concerned about the heat released from the system. 

My major concern is why the global courant number goes beyond 250. 

What can be the reason for this and how can I rectify this error ? 

[Rob]

The global courant number will increase as the flow speed (or other phenomena) moves too quickly for the time step. 

[abhnv11295]

Yes. 

Why for higher heat flux and how to rectify the issue?

[abhnv11295]

what is wrong with my model?

anything to look into?

 

[Rob]

Decrease the time step, that'll improve the Courant Number issues. Then look to see how sensible the heat addition is if that part of the heat pipe is dry. Remember, in Fluent a heat flux boundary will increase the boundary temperature until that flux is acheived, even if it's hotter than the melting point of the materials.