[Martin_Sz]

Hello everyone
I have problem with my simulation. I have multiphase flow air and nh3 at stp.
If I have 50 mm diameter of outlet (opening boundary) my simulation goes on. If I have 20 mm diameterr of outlet I have error overflow. What can I change to run this simulation with smaller outlet (diameter)??
I need to have opening because I need to have fluid flow to domain by opening boundary (I have pressure difference between opening and domain - 1 bar).

[monkey1]

What is the exact error you are getting?
What are you exactly simulating? Transient? Steady state?

If your errore is

"Floating point exception: Overflow"

maybe this might help you already:
http://www.cfd-online.com/Wiki/Ansys...do_about_it.3F

[Martin_Sz]

transient simulation
840 s with step 1 s

[Martin_Sz]

If i define outlet on outlet simulation goes on.

[ghorrocks]

You have answered 1 of Monkey1's questions. The others are "What is the exact error you are getting? What are you exactly simulating?". We can't help you until you answer these questions.

[Martin_Sz]

Multiflow
On inlet I define nh3 gas. On domain I have air. On outlet I defined opening boundary with 1 bar relative pressure. If i decrease diameter of outlet from 50 mm to 20 mm I get error overflow c fpx handler nothing else.
I define isothermal/sst/ solvers.

[Martin_Sz]

FLOW: Flow Analysis 1
&replace DOMAIN: Default Domain
Coord Frame = Coord 0
Domain Type = Fluid
Location = B23
BOUNDARY: Default Domain Default
Boundary Type = WALL
Create Other Side = Off
Interface Boundary = Off
Location = F24.23,F26.23,F27.23,F28.23,F30.23
BOUNDARY CONDITIONS:
MASS AND MOMENTUM:
Option = No Slip Wall
END
WALL ROUGHNESS:
Option = Smooth Wall
END
END
END
BOUNDARY: Opening
Boundary Type = OPENING
Interface Boundary = Off
Location = Outlet
BOUNDARY CONDITIONS:
FLOW DIRECTION:
Option = Normal to Boundary Condition
END
FLOW REGIME:
Option = Subsonic
END
MASS AND MOMENTUM:
Option = Opening Pressure and Direction
Relative Pressure = 1 [bar]
END
TURBULENCE:
Option = Medium Intensity and Eddy Viscosity Ratio
END
END
FLUID: Fluid 1
BOUNDARY CONDITIONS:
VOLUME FRACTION:
Option = Value
Volume Fraction = 1
END
END
END
FLUID: amoniak
BOUNDARY CONDITIONS:
VOLUME FRACTION:
Option = Value
Volume Fraction = 0
END
END
END
END
BOUNDARY: inlet
Boundary Type = INLET
Interface Boundary = Off
Location = Inlet
BOUNDARY CONDITIONS:
FLOW REGIME:
Option = Subsonic
END
MASS AND MOMENTUM:
Normal Speed = 4.78 [m s^-1]
Option = Normal Speed
END
TURBULENCE:
Option = Medium Intensity and Eddy Viscosity Ratio
END
END
FLUID: Fluid 1
BOUNDARY CONDITIONS:
VOLUME FRACTION:
Option = Value
Volume Fraction = 0
END
END
END
FLUID: amoniak
BOUNDARY CONDITIONS:
VOLUME FRACTION:
Option = Value
Volume Fraction = 1
END
END
END
END
DOMAIN MODELS:
BUOYANCY MODEL:
Option = Non Buoyant
END
DOMAIN MOTION:
Option = Stationary
END
MESH DEFORMATION:
Option = None
END
REFERENCE PRESSURE:
Reference Pressure = 1 [bar]
END
END
FLUID DEFINITION: Fluid 1
Material = Air at 25 C
Option = Material Library
MORPHOLOGY:
Option = Continuous Fluid
END
END
FLUID DEFINITION: amoniak
Material = NH3 at STP
Option = Material Library
MORPHOLOGY:
Option = Continuous Fluid
END
END
FLUID MODELS:
COMBUSTION MODEL:
Option = None
END
HEAT TRANSFER MODEL:
Fluid Temperature = 360 [C]
Homogeneous Model = On
Option = Isothermal
END
THERMAL RADIATION MODEL:
Option = None
END
TURBULENCE MODEL:
Option = SST
END
TURBULENT WALL FUNCTIONS:
Option = Automatic
END
END
FLUID PAIR: Fluid 1 | amoniak
INTERPHASE TRANSFER MODEL:
Interface Length Scale = 1. [mm]
Option = Mixture Model
END
MASS TRANSFER:
Option = None
END
END
INITIALISATION:
Option = Automatic
FLUID: Fluid 1
INITIAL CONDITIONS:
VOLUME FRACTION:
Option = Automatic with Value
Volume Fraction = 1
END
END
END
FLUID: amoniak
INITIAL CONDITIONS:
VOLUME FRACTION:
Option = Automatic with Value
Volume Fraction = 0
END
END
END
INITIAL CONDITIONS:
Velocity Type = Cartesian
CARTESIAN VELOCITY COMPONENTS:
Option = Automatic with Value
U = 0 [m s^-1]
V = 0 [m s^-1]
W = 0 [m s^-1]
END
STATIC PRESSURE:
Option = Automatic with Value
Relative Pressure = 0 [bar]
END
TURBULENCE INITIAL CONDITIONS:
Option = Medium Intensity and Eddy Viscosity Ratio
END
END
END
MULTIPHASE MODELS:
Homogeneous Model = On
FREE SURFACE MODEL:
Option = None
END
END
END
END

[monkey1]

First: Why are you using a multiphase model?
At 360°C NH3 and Air are gaseous and could just be treated as "variable composition mixture" single phase.

Second (although maybe not that relevant): you define Air at 25°C as your substance but you want to use it for a domain temperature of 360°C? You should switch to air ideal gas.

Third: You are mixing 2 gases at quite high temperatures, shouldn't you take into account the bouyancy?

Fourth:
Glenn may correct me if I'm wrong but you define at your opening

Option = Opening Pressure and Direction
Relative Pressure = 1 [bar]

Isn't that meaning that you have an overpressure OUTSIDE of your domain of 1 bar?
Meaning if your pressure inside is below 1 bar + reference pressure you will get no outflow?!?

[ghorrocks]

You have a fundamental error. You are modelling a mixture of 2 gases with a multiphase model. But you only have one phase as everything is gas! So you are using the wrong physical model. You should be using a multicomponent model which is the appropriate model for a mixture of gases.

Also you should make your reference pressure 2 bar and the outlet pressure 0 bar. This will minimise round off errors.

[Martin_Sz]

OK So I use multiphase flow to determine nh3 fraction in domain
on initialization on domain is only air
on inlet only nh3
On outlet i give higher pressure than on domain because I need to have nonlinear flux of air from opening into domain.

So my question is any tutorial similar to my case if not which solvers do i need to choose to determine nh3 fraction on domain over time??
Best regards

[monkey1]

You do not need multiphase to get the repartition of the substance in the domain.
You have only ONE phase = the gaseous phase
This one is compsed of several gases of varying fraction = variable composition mixture
And for each gas you can define separate Initial conditions, boundary conditions and will get for each substance the fractions in the domain!

For a first idea what I am talking about have a look at the "Reacting Flow in a mixing tube" Tutorial, and just ignore the reaction part, to understand how to to define such a variable composition mixture.

[Martin_Sz]

Hey
I change to single phase flow with mixture of air and nh3 ideal gases.
So I have the same problem like before. Overflow If I give higher pressure on opening boundary that on the domain . Difference must be 1 bar between outlet and domain.

I need (on overall description of the problem) pump up tank from 0.01 bar to 1 bar with amonium nh3 with little holes (leaks) of air for example 20 mm diameter hole. Tank has 1300 mm doiameter and 1000 mm long.,