Difference of multicomponent and multiphase homogenous flows

Luk_Fiz · October 14, 2009, 11:35

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, 18:31

Quote:

Am I right?

Yes, I think your explanation is about 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, 04:53

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, 07:21

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, 09:04

Yes, of course.

Thanks,
Luk

prishor · September 30, 2012, 04:21

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).??.
please help me.
thanks in advance,
prishor

ghorrocks · October 1, 2012, 08:41

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, 12:37

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, 20:00

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

srinath_cfx · April 2, 2013, 11:35

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, 19:43

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

srinath_cfx · April 4, 2013, 06:29

@ ghorrocks, Thanks

October 14, 2009, 11:35	Difference of multicomponent and multiphase homogenous flows	#1
Luk_Fiz Member Lukasz Join Date: Mar 2009 Posts: 67 Rep Power: 17	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

October 1, 2012, 20:00		#9
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,870 Rep Power: 144	If you are using Fluent then you are on the wrong forum. This is the CFX forum. Danial Q and Mfaizan like this.

April 2, 2013, 11:35	shock tube problem	#10
srinath_cfx New Member srinath Join Date: Apr 2013 Posts: 3 Rep Power: 13	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?

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October 15, 2009, 04:53		#3
Luk_Fiz Member Lukasz Join Date: Mar 2009 Posts: 67 Rep Power: 17	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

October 15, 2009, 07:21		#4
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,870 Rep Power: 144	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.

October 15, 2009, 09:04		#5
Luk_Fiz Member Lukasz Join Date: Mar 2009 Posts: 67 Rep Power: 17	Yes, of course. Thanks, Luk

September 30, 2012, 04:21		#6
prishor New Member prishor p k Join Date: Jul 2012 Posts: 29 Rep Power: 14	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).??. please help me. thanks in advance, prishor

October 1, 2012, 08:41		#7
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,870 Rep Power: 144	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.

October 1, 2012, 12:37		#8
prishor New Member prishor p k Join Date: Jul 2012 Posts: 29 Rep Power: 14	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.

April 2, 2013, 19:43		#11
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,870 Rep Power: 144	Have a look at the CXF tutorials on how to specify a multi-component gas.

April 4, 2013, 06:29		#12
srinath_cfx New Member srinath Join Date: Apr 2013 Posts: 3 Rep Power: 13	@ ghorrocks, Thanks