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March 20, 2018, 15:54 
Mass fraction source term in CFX

#1 
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David
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Dear All,
I'm now trying to do some simulation of mass transfer from air to water using Algebraic Slip Model (ASM). There is a sinksource for air due to mass transfer, so I want to add this source term in Continuity equation of air. However, I can only find Air.mf (mass fraction of Air) Source in SubdomainSource for ASM. My question is the source term for Mass Fraction is the same as the source for Continuity equation? Or the mass fraction equation is the same as continuity equation? Thank you. 

March 20, 2018, 17:29 

#2 
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Glenn Horrocks
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Make sure you have the basics under control before you proceed:
The mass fraction equation and continuity equations are different fundamental equations. You mention the ASM model, which is a multphase model which uses a volume fraction equation, but then say you see mass fraction variables. Is this a mass fraction or a multiphase model? Make you sure have the right basic equations. Or do you have both a mass fraction equation to track water vapour+air, and multiphase to track liquid water drops to gaseous air?
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March 20, 2018, 18:04 

#3 
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David
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Thanks Glenn for your kind reply.
The background of my simulation is air bubbles would dissolve into the water due to high pressure and turbulence. I combine air and water as a Variable Composition Mixture. I set air as dispersed, water as continuous flow based on the help document. I think this should be mass fraction model rather than multiphase model in CFX. Do you know how to find the mass fraction equation? Or it has some relevance to continuity eqaution? 

March 20, 2018, 18:12 

#4  
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Glenn Horrocks
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Quote:
Quote:
Quote:
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March 20, 2018, 20:48 

#5 
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As I understand the Algebraic Slip Model (from the Theory documentation), it is a simplification of a true multiphase model where one of the phases is fairly diluted, and its flow field can be decomposed as the flow field from the continuous phase and a slip velocity; therefore, no need to solve the momentum equations for the dispersed phase.
After the algebra of decomposing the dispersed phase velocity, the continuity equation for the dispersed phase resembles the mass fraction equation; therefore, the ASM in ANSYS CFX uses the variable composition mixture to setup a multiphase model. The implementation does not account for mass transfer between the phases (which are represented by mass fraction in the simulation). If there is mass transfer between the phases, it needs to be included manually by adding appropriate source terms in the corresponding equations. To include the mass into the continuity equation of the continuous phase, you must use a "Mass Source" option for continuity and the amount must be equal to the amount leaving the dispersed phase. Hope the above helps, 

March 20, 2018, 22:03 

#6  
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David
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Thanks for your help Opaque, it gives me a better understanding of ASM.
Quote:
Thank you. 

December 10, 2019, 01:49 

#7  
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Lucas
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December 10, 2019, 01:54 

#8 
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Glenn Horrocks
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They are both scalar transport equations, so in that sense they are similar. But temperature, turbulence, volume fraction, mass fraction and so on are scalar transport equations as well, so a lot of things which are very different are covered in that class of equation.
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continuity source, mass fractions, mass transfer 
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