## Fluids

#### Temperature increase in a Two-Phase simulation in FLUENT (VOF)

• abhnv_01
Subscriber

hello,

I am working with 2-D transient simulation with compressible flow, gravity assisted.

I am using VOF- multiphase model and evaporation condensation model as the phase change model, with saturation temperature as a function of saturation pressure.

Liquid is primary phase and Vapor is secondary phase.

liquid properties - piece-wise polynomial function of temperature

vapor density - Ideal gas

vapor viscosity and thermal conductivity - piece-wise poly function of temperature.

Operating pressure - (Floating) 0 Pa, gauge pressure - 76043 Pa

Operating density - 0 kg/m3

Initial temp - 75K

CONVERGANCE criteria -

(continuity - 1e-05), (velocity - 1e-04) and (energy - 1e-06)

SOLUTION METHODS

Pressure-velocity coupling - SIMPLE

Gradient - Least squares cell based

Pressure - PRESTO

Density - second order upwind

Momentum - Quick

Volume fraction - Conpressive

Energy - first order upwind

All the URFs are default.

The temperature of the evaporator section is increasing. I do not understand why..

Any help will be highly appreciated.

• Rob
Ansys Employee

What are the boundary conditions?

• abhnv_01
Subscriber

So, basically I have three sections of the PHP.

BCs are Thermal wall boundary conditions

Evaporator section - Constant heat flux

Condenser section - Constant temperature

Adiabatic section - Zero heat flux

• Rob
Ansys Employee

Evaporator section will increase in temperature because the wall MUST push in the defined heat flux. So, that wall will heat up to infinity if you don't extract sufficient energy from the condensor section.

• abhnv_01
Subscriber

Rightly said Rob.

The boundary conditions are set according to experimental values.

Regarding the sufficient energy generated on the condensor section, I have two queries as following:

Say, if I am not being able to get sufficient cold from the condenser section, will it be the right way to decrease the condensor temperature to get enough cold?

Or is there any way to check whether enough cold is generated on the condensor side or not?

• Rob
Ansys Employee

That depends on what you want to adjust: I can only advise on what the solver is doing, not what boundary values you ought to pick.

I also assume that the mesh is sufficiently well refined to pick up the wall & free surface boiling & flow.

• abhnv_01
Subscriber

Yes, my mesh is well refined.