Learning Forum

[radomir.filip]

Dear all,

I am trying to simulate the water evaporation driving the natural convection flow within a single vertical hellical coil using the Eulerian boiling model. I have used folliwing settings:
 

• boiling model : tried RPI and non equiblirum boiling with similar results
• primary phase - water liq., secondary phase water vap.
• water properties and saturation temperature given by rgp tables (operating pressure 9 MPa)
• mass transfer by default settings
• heat transfer coefficient given by tomyiama
• forces using pretty all recommanded settings including virtual mass coefficient = 0.5
• accounting for buoyancy effect by setting operation density =0
• using const. Temperature profile > Tsat (303°C) at the tube outer wall (as show in the attached figure)
• Inlet conditions - specified mass-flow for liquid phase and temperature slightly undercooled T = 300°C

The solution converges nicely, however the resulted steam volume fraction at the outlet is way higher than the corresponding heat transfer rate.

For example I would expect for transfered heat of 2kW at given mass-flow a volume fraction about 0.3, but the solution shows 0.8.

You can imagine how the diferrence in the outlet density impacts the whole naturally driven flow.

I tried to tweak these liquid/vaport interface transfer coefficiens within the boiling model options, to lower the outlet volume fraction and match it with the corresponding heat transfer rate, but without any major influence on the overall solution.

Does anyone have a similar experience or am I missing something?

Thank you.
R.

[Rob]

Check volume integrals too. Are you expecting pure vapour at the outlet? Multiphase models are often very transient, so you may find you've got mass & energy storage to account for too. 

[radomir.filip]

I am expecting the vapor fraction at the outlet would correspond to the transfered heat, so in this particular case should be around 0.3, but for some reason it is way higher (0.8). I dont understand how checking volume integrals would be helpful. 

[Rob]

It's to check you've got a stable domain energy too - ie heat in may leave, cause boiling or heat up the liquid.