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-   -   Difference of multicomponent and multiphase homogenous flows (http://www.cfd-online.com/Forums/cfx/69181-difference-multicomponent-multiphase-homogenous-flows.html)

 Luk_Fiz October 14, 2009 10:35

Difference of multicomponent and multiphase homogenous flows

Hi all - I have partly theoretical partly practical question.
1) What is a difference between multicomponent and homogenous multiphase flows? Assume I have 2 fluids of different densities (fe. water and methanol), no mass nor energy transfer between. Looking through the CFX documentation I see that simulating it as multiphase homogenous flow it solves for volume fractions, after that calculates density (where? mean density?) and than solves momentum eqations.
If it is multicomponent than it solves for mass fractions, after that calculates mean density and momentum and so on in the end.

Am I right? Where is difference? Is in "multiphase version" density taken from material properties and than the momentum equations are solved with these densities without calculating mean value?
2) Assume these two luquids are in the long pipe with gravity. In case of simulating it as multiphase homogenous I see fast stratification. In case of simulation as multicomponent I see no stratification, OR VERY SLOW. Does it mean that in case of multicomponent treatment there is only signle "mean" (mass fraction-weighted) density and because of that it is non possible to stratify mixture?

Thanks for all answers because it makes me confused since quite long.

Luk

 ghorrocks October 14, 2009 17:31

Quote:
 Am I right?

Quote:
 Where is difference?
The difference is the multi-component model assumes they mix into a single phase, which can be represented by a bulk density, viscosity etc. The components are mixed on a microscopic scale. Multi-phase homogeneous means you have multiple phases (eg gas and liquid) and they are separated on a resolvable scale.

Yes, a multiphase model (eg air and water) are likely to separate very quickly. I am pretty sure buoyancy also works with multi-component models - but keep in mind the separation occurs at a microscopic length scale so is likely to be slower than the multiphase approach.

 Luk_Fiz October 15, 2009 03:53

Thanks Glenn,
Now it seems for me that I understand this:)
Assume that mentioned flow in the pipe. It seems to be like this:
1) If we define it as multiphase homogenous mixture and we set volume fractions of inlet as 50%/50% than the gravity acts in the moment and stratifies mixture because 50% of volume (fractions) are occupied by ingridients of different densities. Thats why multiphase homogenous stratifies in the moment.
2) If we define it as multicomponent and we set mass fractions of inlet as 50%/50% than the fluid has equal (mass-weighted) properties and will not stratify or stratify very slowly in case of small inhomogeneities.

Thanks,
Luk

 ghorrocks October 15, 2009 06:21

Be careful comparing the separation rates. The density difference between air and water is around 1000, such as is commonly modelled in homogeneous multiphase models. Multicomponent mixtures are commonly used for mixtures of gases (eg O2+CO2+N2), and the density differences between these components is much smaller.

So the separation speed of a water/air mixture is much faster than a O2/N2/CO2 mixture simply because of the larger density difference.

 Luk_Fiz October 15, 2009 08:04

Yes, of course.

Thanks,
Luk

 prishor September 30, 2012 03:21

hi all,
i am doing the simulation of gasification in Fluidized bed reactor. there are four components air, steam, biomass and sand..
my doubt is that can i consider this problem as 4 phase flow or consider as multi component 2 phase flow (gas-solid flow).??.
prishor

 ghorrocks October 1, 2012 07:41

How can this be a 4 phase model when there are only 3 phases (solid, liquid, gas)?

This should suggest the answer - the air and steam are both gases, so these are probably bets modelled as a gas phase multicomponent mixture. The two solids are modelled as two different solid phases.

 prishor October 1, 2012 11:37

thanks Glenn.
you are saying the two solids can be treated as different phase even if they both are solids rite.??
how can i input both air and steam into FLUENT as components of gas phase.

 ghorrocks October 1, 2012 19:00

If you are using Fluent then you are on the wrong forum. This is the CFX forum.

 srinath_cfx April 2, 2013 10:35

shock tube problem

Hi,

If I have a shock tube of Driver gas 'helium' and Driven gas 'air', is it multi-component flow. If this is the case will mixing take place at large time scales and contact surface disintegrate?

How to define a multi component gas?

 ghorrocks April 2, 2013 18:43

Have a look at the CXF tutorials on how to specify a multi-component gas.

 srinath_cfx April 4, 2013 05:29

@ ghorrocks, Thanks

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