## Fluids

#### Why CFL exceeds 250 for constant heat flux bcs but works well with constant T ?

• abhnv_01
Subscriber

I am trying to simulate a heat pipe in (2D) 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 first 10000 time steps (1 seconds of flow time) and then this error where global courant number exceeds 250.

But here is a mystery, the simulation runs well with constant temperature boundary condition at the evaporator.

I want results with constatn heat flux, I am not able to track the error, any help will be highly appreciated.

• Rob
Ansys Employee

Report the surface temperature around the point the solver fails. I assume you've read and understood the implications of the heat flux and temperature wall boundary?

• abhnv_01
Subscriber

But if you can enlighten about it, then that would be great.

• Rob
Ansys Employee

In what way? What wasn't clear from reading the documentation?

• abhnv_01
Subscriber

from the documentation, I have read that both the boundary conditions use the same heat transfer equation.

My simulation works well with constant temperature but fails with constant heat flux.

So, I cannot detect the reason for the difference here.

• Rob
Ansys Employee

OK. With a constant temperature the heat flux floats to fix the wall temperature. With a fixed flux the temperature floats. But.... In regions with poor heat transfer the wall temperature may become high to force the fixed heat flux. If that alters the material density in any way the solution may become unreasonable, ie buoyancy forces are excessive.  A better boundary option may be convection, but that's for you to decide.