Learning Forum

[Kevin Julian]

Hello, everyone.

I am currently trying to simulate condensation phenomenon in an evaporator (shell and tubes model, with baffles).

I am using ANSYS Fluent 18.1 for my simulation. Here are some information about my simulation :

1. 3D, Pressure-based, Transient Simulation, gravity on (y=-9.81 m/s2)
2. Multiphase - MIXTURE - 3 eulerian phases : air, water-vapor, water-liquid
3. Energy (ON), and Species Transport (ON)
4. Viscous Model : Realizabe k-eps with enhanced wall treatment
5. Boundary condition : 1 inlet and 2 outlets (top and bottom).

The thing is, I have tried so many times, but the water(liquid) phase keeps coming out of the TOP outlet, which for me does not makes sense since the actual evaporator (which I personally run on the field) did not spray water from its top outlet.

I have tried restricting this by using backflow (0.99) for the water(liquid) phase at the top outlet, but it still did not work. Mass flow rate flux still indicates that water on the top outlet is greater than that of the bottom one.

I'm hoping that some of you might have an insight on what I should be doing.

Thank you for your assistance. I will await for any of your insights.

[Rob]

You only need two phases, air and vapour are both gases. You then have a droplet size, that's set in the solver (generally). I assume gravity is pointing down, common mistake! 

What is the bottom inlet? Just wondering how liquid is supposed to leave the domain. 

I'd also look to update, R18 is 5-6 years old and there have been significant improvements in both the general code and more interest here to multiphase modelling. We're on 2023R1 now, which I think is equivalent to R24 in old numbering. 

[Kevin Julian]

Hello, Rob.

Thank you for answering.

So, I have already made sure that my gravity is set on the correct direction.

However, I'm still confused about what you mentioned,

Why do I only need two phases (air and vapor) ? 

Don't I need water-liquid as a third phase, so that I can model the mass transfer through the mixture(multiphase) settings ?

And also, I would like to know further about these droplets size, if you would kindly help me with this.

Thank you very much.

[Rob]

Air and vapour are one phase - gas. You then model the gas phase as a species mixture of air and vapour. 

Second phase is then the liquid water. 

Droplet size is more difficult if you're using the single size options. With 2021Rx and onwards I'd look at the wall film and DPM model rather than a full multiphase approach. For Mixture, I'd pick a single droplet size but then look at upwards velocity and see what should fall (look up turbulent Stokes Law).