Modelling Slug Flow in ANSYS Fluent VOF.

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March 2, 2015, 10:39
Modelling Slug Flow in ANSYS Fluent VOF.
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Oliver Stewart
Join Date: Mar 2015
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Hey guys,
Im a fourth year mechanical engineering student and im conducting some CFD analyses on two-phase air-water flow as part of my final project. Im having some problems simulating slug flow and I was hoping someone with a little more experience than me would be able to offer some much appreciated advice.

I currently have 2 configurations of pipe, with the possibility of a third if I manage to complete these two. The two are a horizontal and a vertical pipe, however the main problem lies with getting the slugs to develop in the horizontal pipe, as so far, the flow remains stratified no matter the difference in superficial velocities of the two phases. My configuration is currently set up as follows:
Geometry and Mesh:
Pipe diameter - 50.8mm (2 inches)
Pipe length - 3500mm
Main Body Element size - 7.5mm
Pipe has two inlets shown in picture attached - one for gas [top] one for liquid [bottom] - this set up was chosen so that the flow started off as stratified as opposed to mist - I was unable to patch in seperated without this configuration but i am open to suggestions.
Element size where phases meet - 3.25mm

Setup and Solution:
Double Precision, 3D, Parallel (4)
General - Type: Pressure-Based, Time-Transient, Velocity Formulation-Absolute. Gravity on, -9.81 Y.
Models - Volume of Fluid, Scheme-Implicit, Implicit Body Force ON. # of phases 2. Volume Fraction Cutoff- default, 1e-6. Viscous, K-omega, SST.
Materials - Water-liquid, air
Phases - Primary = air, Secondary = water. Interaction - Default [Options available: Discretization, Surface Tension, Mass.
Boundary Conditions - Gasinlet = mass-flow-inlet, Liquidinlet = mass-flow-inlet, outlet = pressure-outlet. Mass flows are such that the superficial velocities of the gas and liquid are 10m/s and 4.7m/s respectively. I have tried other velocities both faster and slower to see if this allows the slugs to form - it did not. Wall - default, stationary, no slip. Operating Conditions - specified operating density ON, 1.225 kg/m3.
Solution Methods - Scheme = SIMPLE, Gradient = Least Squares Cell Based, Pressure = PRESTO!, Momentum = Power Law, Volume Fraction = QUICK, Turb Kin Energy = 2nd Order, Spec Dissipation Rate = 2nd Order. Transient Formulation = 1st Order.
Hybrid initialization, Running 200 time steps at 0.02s

The simulation runs no problem at all - however I am not seeing the slug flow developing at the speeds in which it develops in experiments. Does anyone have any advice on how to begin setting up the slug flow? Is there some fundamental thing I'm missing? Any advice would be massively appreciated - I've exhausted all else and there's not much help online for this specific type of problem.

Thank you to anyone who took the time to read this and manages to reply.

I've also attached the type of result i'm getting with the current set-up. The flow remaining Stratified.

Too long, didnt read? Need help setting up two-phase slug flow in ANSYS Fluent
Attached Images
 Horizontal Inlets Highlighted.jpg (59.3 KB, 85 views) Horizontal Result.jpg (34.9 KB, 103 views)
__________________
Fourth year Computer Aided Mechanical Engineering student in Scotland.
Dissertation topic: A CFD investigation on the effect of pressure on flow behaviour in a two-phase flow.

 March 18, 2015, 05:45 #2 New Member   Join Date: Mar 2015 Posts: 3 Rep Power: 11 Hello Oliver, I am actually working on modeling slug flow(I work on segmental flow). I have problem create slug flow inside my model as well. I tried your parameters on my case and I can't get any slug flow like you. Here are some pictures: Then I tried different parameters: My pipe diameter is 10 mm for all inlets and outlets. General: Type: Pressure Based Velocity Formulation: Absolute Time: Transient Models: VOF : Explicit 1e-06 0.25 Implicit Body Force Viscous: Laminar Materials: Water-liquid, air Phases :air, water. Interaction: Surface tension Force modeling air/water CSF 73.5 dyn/cm Boundary Conditions: Velocity inlet air : 0.0237 m/s Velocity inlet water : 0.0168 m/s Solution Method: Simple: Scheme: SIMPLE Gradient: Least Squares Cell Based Pressure: PRESTO! Momentum: Power Law Volume Fraction: QUICK Transient Formulation: 1st Order Calculation: Time Step Size: 0.02s Number of time steps: 200 With those parameters I think I got some waves at the beginning of the pipe. I tried as well to see if in a "T" pipe I could get the same. Here are some pictures on the "Y" and "T" pipe simulation: Hope someone could give us some advice about slug flow simulation. Thanks in advance, Piba

 Tags fluent, multiphase, slug flow, transient, two-phase