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Mass imbalance problem in multiphase water and steam CFX case

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Old   February 19, 2014, 06:46
Default Mass imbalance problem in multiphase water and steam CFX case
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Andrew
Join Date: Feb 2014
Location: Russia
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Hi everyone.
Sorry for language (my native is russian).

I'm doing a CFX simulation for power industry and I've encountered a problem with mass imbalance in multiphase case.

The object is a steam boiler drum that is supplied with water-steam mixture througt two inlets at the bottom. Steam is separated from liquid phase and leaves drum volume through two outlets at the top. Water sinks through two outlets at the bottom. See first attachment for bief case geometry and process flow overview. Target: investigation of irregularity of water and steam flow along the drum length.

The model only includes half of real object to reduce resource requirements. Mesh (attachment 2) consists of 61+ millions of tetrahedral cells (min size 1.5 mm, max size 20 mm, growth rate 1.2, built with Ansys Meshing).

Main case features are as follows:
- Dimain volume = 11.0 m^3;
- Operating pressure = 10.1 MPa (set as reference);
- Mass flow at mixture inlet = 100.3 kg/s with volume fraction of liquid = 0.30;
- Mass flow (or velocity) at water outlet (at the bottom) set to zero for test purpose.

Main model features:
- Ansys 14.5, CFX;
- Steady state, physical timescale = 1 s, more than 600 iterations;
- Two continuous Eulerian fluids: saturated water and steam (IF-97);
- Multiphase: inhomogenous + free surface, isothermal (312 *C);
- Turbulence: k-epsilon
- Buoyancy + gravity 9.806 m/s2 (density difference);
- Surface tension = 0.0117 N/m
- Interphase transfer option: Free surface;
- Momentum transfer: Drag coefficient 0.44;
- Initialization: lower half of the domain filled with water, upper - with steam.

What is the problem? Mass imbalance in liquid phase (water). Under TEST conditions water outlet was considered a wall (so no any mass flow of any phase through it occured), volume fraction of liquid at steam outlet was about zero (CFD-Post calculator) and water was fed through inlet at 84.5 kg/s (along with steam at 15.7 kg/s). Steam mass balance is not very good (22.8 kg/s at outlet), but it may be just a iteration-to-iteration fluctuation. More relevant problem is water balance. Water does not outcoming from the domain, it is only fed through the mix inlet (verified with CFD-Post). And it's volume (and level) not increasing but slightly decreasing with iterations (see attachment 3).
What can be the reason for such an imbalance? I have performed a test in CFD-Post with spherical surface immersed in water volume of the domain. Mass flow (massFlow function) through this surface was -0.15...0.04 kg/s depending on position and radius (up to 0.3 m). I don't think this is a high imbalance concerning inlet flow 84.5 kg/s of water and variation of massFlow sign (through the sphere). So no any "buggy" volume water sources/sinks. Can anyone suggest how to cope with this?

Thanks for your attention.

Update 20.02.2014
I've calculated another test case. Initials are the same (water in the lower half of the drum) but there are no any inlet or outlet flows, just static conditions. Water (liquid) phase volume in the domain decreases with iterations! Another thing I've discovered in this "static test" is that after 356 iterations there is a thin layer of liquid on the horizontal perforated plate in the upper part of the drum although there is no any inflow/outflow on boundaries and at initial conditions the water (liquid) was set to reside only in lower half.

Update 26.02.2014
The problem was too big physical timescale (1 s). Experiments with different timescales showed that for our case steps about 0.01...0.02 s is required in order to satisfy the mass balance of water inside domain.
Attached Images
File Type: jpg Geometry scheme.jpg (35.6 KB, 47 views)
File Type: jpg Mesh.jpg (97.5 KB, 49 views)
File Type: jpg Monitors.jpg (69.0 KB, 53 views)
File Type: jpg Isosurface Liquid.jpg (57.2 KB, 48 views)

Last edited by Antech; February 26, 2014 at 05:36. Reason: Update
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