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Old   September 23, 2014, 06:02
Question Additional Variable
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Additional variable in multiphase flow is really confusing. Although I have already post several threads about this, there are still some questions. Overview of my case: a simulation of argon gas injected in a ladle full of molten steel and tracer variation with space and time in steel.
Steps:
1)Start the simulation with inhomogeneous model. Select steel as continuous fluid and argon dispersed fluid with a constant diameter. After several time, the flow field is seen as steady.
2)Insert a specified additional variable named “Tracer” with no units. Set it as Fluid Dependent in Fluid Models panel. In Fluid Specific Models panel, set AV for steel phase with a Transport Equation without a Kinematic Diffusivity. Keep the default setting for Ar phase.
3)Insert a Source Point for steel phase. Enter an expression of “if(t<=1[s],1 [kg s^-1],0 [kg s^-1])” for total source. Set the value of AV as 0 for boundary and initial condition. Monitor the total mass of Tracer with an expression “volumeInt(Steel.Tracer)@Default Domain*7[g cm^-3]”.
4)Turn off the fluid & volume fraction & turbulence equation with expert parameter. Start the solution with previous steady flow field as initial condition.
Parts of CCl is as below:
Quote:
FLUID MODELS:
ADDITIONAL VARIABLE: Tracer
Option = Fluid Dependent
END
FLUID: Ar
FLUID BUOYANCY MODEL:
Option = Density Difference
END
TURBULENCE MODEL:
Option = Dispersed Phase Zero Equation
END
FLUID: Steel
ADDITIONAL VARIABLE: Tracer
Option = Transport Equation
END
FLUID BUOYANCY MODEL:
Option = Density Difference
END
TURBULENCE MODEL:
Option = k epsilon
BUOYANCY TURBULENCE:
Option = None
END
TURBULENCE MODEL:
Homogeneous Model = False
Option = Fluid Dependent
END
FLUID PAIR: Ar | Steel
INTERPHASE TRANSFER MODEL:
Option = Particle Model
END
MASS TRANSFER:
Option = None
END
MOMENTUM TRANSFER:
DRAG FORCE:
Option = Schiller Naumann
END
MULTIPHASE MODELS:
Homogeneous Model = False
FREE SURFACE MODEL:
Option = None
END
SOURCE POINT: Source Point 1
Cartesian Coordinates = -0.6 [m], 0 [m], 2.45 [m]
Option = Cartesian Coordinates
FLUID: Steel
SOURCES:
EQUATION SOURCE: Tracer
Option = Total Source
Total Source = Expression 1
END
Results:
Fig.1 is the volume fraction distribution of steel. Fig.2 is the tracer distribution. Fig.3 is the total mass(kg) of tracer added in.
Combining fig.1 and fig.2, it seems that tracer spreads into the argon phase from steel phase. From Fig.3, it can be seen that the mass of tracer is not conserved.

Questions:
1)Tracer is set as steel dependent and no mass transfer mechanism is given between this two phases. How can tracer spread into the Ar phase?
2)Tracer injection is controlled by the expression “if(t<=1[s],1 [kg s^-1],0 [kg s^-1])”. So the total mass of tracer should be 1 kg and keep constant after 1 s. Why does the quantity of tracer still increase after 1 s?
Attached Images
File Type: png Steel volume fraction.png (30.1 KB, 18 views)
File Type: png tracer distribution.png (30.9 KB, 12 views)
File Type: jpg total mass of tracer.jpg (29.3 KB, 9 views)
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Old   September 23, 2014, 18:50
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Glenn Horrocks
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The tracer spreads into the other phases as:
* There is diffusion, both real and and artificial (ie additional diffusion because of the simulation method) which causes the tracer to diffuse.
* In a multiphase simulation both phases exist at all locations, and the volume fractions tell you how much there is. The volume fraction usually does not go to exactly zero, so this means the simulation assumes there is at least tiny amount of all phases everywhere, and the tracer goes with this.

Why does the tracer keep increasing?
This is a conservation problem, or maybe your function is not stopping as you intend. To fix conservation try converging tighter or using double precision. Also check that your function is working as you intend.
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Old   September 24, 2014, 08:36
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Thanks for your reply.
Quote:
Originally Posted by ghorrocks View Post
* There is diffusion, both real and and artificial (ie additional diffusion because of the simulation method) which causes the tracer to diffuse.
* In a multiphase simulation both phases exist at all locations, and the volume fractions tell you how much there is. The volume fraction usually does not go to exactly zero, so this means the simulation assumes there is at least tiny amount of all phases everywhere, and the tracer goes with this.
I know what you mean. But the fact is the diffusion of tracer can't be neglected here.
Quote:
Originally Posted by ghorrocks View Post
Why does the tracer keep increasing?
This is a conservation problem, or maybe your function is not stopping as you intend. To fix conservation try converging tighter or using double precision. Also check that your function is working as you intend.
The expression does work. Once I use a mixture composed of steel and tracer and then calculate the species transport, the result is nearly the same with that of AV. I'm quite puzzled over this phenomena.
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Old   September 24, 2014, 18:29
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I do not understand "I know what you mean. But the fact is the diffusion of tracer can't be neglected here." I am not neglecting the diffusion, I am saying it is inevitable. Your question was why do you see tracer in the argon phase so I gave two reasons why it would be so.
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