Learning Forum

[Abdullah Hassan]

Dear Sir,

I am new to ANSYS and doing an Implicit VOF transient simulation on single tube parrallel heat exchanger. 

Viscous Model:

K-Epsilon with realizable and Enhanced wall function 

VOF Model: 

Primary phase = water liquid (as I don’t need it to put in boundry condition volume fraction)

Secondary phases = water vapor, ammonia liquid, ammonia vapor.

 Material:

Tube side = Hot fluid =water that converts from vapor to liquid with VOF Lee’s Model at frequency 180/s.

Shell side = Cold fluid = ammonia that converts from liquid to water with Lee’s model frequency 89/s.

Boundry conditions: 

Velocity inlet and pressure outlet.

Tube side = I put water vapor Volume fraction = 1

Shell side = I put ammonia liquid volume fraction = 1

Method:

Piso Scheme with all second order upwind

Control: 

Initially all are at 1 UR factor and then reduced the convergence UR till 0.7.

Initialization: 

Hybrid.

Patch: 

Water vapor VF = 1 in tube domain

Ammonia liquid VF = 1 in shell domain.

Problem:

I am facing two big issues.

1- Continity residual is showing a straight line. It is not converging to 1e-3 or 1e-4. it stays at 1e-1 as a straight line. All other residuals are below 1e-6.

2- As water liquid is primary phase so I can’t assign it to any domain. I did the patch to avoid any water liquid in shell domain. After the simulation. What I find out through volume fraction surface and volume intergrals is that “the water liquid is present in both the tube and shell domain”. In tube domain it is condensed from water vapor but from where it comes to shell domain.

 

Can you guide me on this?

[Abdullah Hassan]

Hello, Can you please suggest me some solution?????

[Rob]

I suspect you need two phases, one for the liquid and a second for gas. Each is a mixture. You then need to transfer ammonia and water from/to their respective phases. 

[Abdullah Hassan]

Yes, 

I have chosen four phases. 

And in boundary conditions, from phases, I assign Water Vapor volume fraction =1 to Tube inlet, 

Ammonia liquid volume fraction = 1 to Shell Inlet. and then after initialization,

I patch the water vapor VF = 1 at tube domain and Ammonia liquid VF =1 at shell domain.

Above is the geometry. 

And if you want to see the traces of water liquid, please refer below image.

[Rob]

Did you set all of the boundary conditions, including outlet and patch the zones?

[Abdullah Hassan]

Yes, I have put an outlet guage pressure = -0.15 

and patch volume fraction of ammonia liquid = 1 in Shell domain. 

and patch volume fraction of water vapor = 1 in tube domain. 

 

[Rob]

Assuming all bc's are set correctly, if convergence is initially poor you could be getting some of the wrong material in the channel.