diffusion coefficient

mamalili · February 19, 2015, 07:43

hi
i am trying to simulate hydrogen release in a closed vessel with cfx
Hydrogen was released vertically upward during 60 s. the sensors were measuring hydrogen concentrations during 250 min after the end of release.
my volume fraction in 2 min after end of release at the top sensors is very overestimate and at the bottom sensors is underestimate.
the authors mentioned that diffusion coefficient is 0.74 cm2/s.
i think diffusion coefficient can solve my problem estimate.
but i dont know how and where i can define diffusion coefficient in cfx.
i am using of (fluid pair model>interface transfer>mixture model)
and (fluid model>free surface model) is set to "none"
please help me, my friends.
thank U

evcelica · February 19, 2015, 10:42

You should be doing this as a variable composition multi-component mixture. Look in the tutorials, and search multi-component. There is one on a reacting mixture that shows you how to set this up.

ghorrocks · February 19, 2015, 16:59

If all the fluids are the in a single phase then it is not multiphase, is it?

Erik is right, you should model this as a multi-component mixture. If the fluid property changes due to hydrogen concentration are not important then consider a simple additional variable approach.

evcelica · February 21, 2015, 10:57

Since this is hydrogen, any other fluid will at least be double the density, so additional variable would not be suitable of course.

ghorrocks · February 21, 2015, 23:46

If the concentration of hydrogen is low the additional variable approach is suitable

.

mamalili · February 22, 2015, 03:41

what do U mean of additional variable my friends?

ghorrocks · February 22, 2015, 04:23

Have a look in the documentation about additional variables. It allows you to add an additional variable to the equations to solve. The additional variable can be an algebraic, diffusion, convection or diffusion+convection equation. I am talking about a diffusion+convection equation additional variable.

To quote the documentation: "Additional Variables are non-reacting, scalar components that are transported through the flow, or through a solid (including the solid portion of a porous domain). They can be used to model, for example, the distribution of dye through a liquid, or smoke from a fire. ANSYS CFX typically interprets Additional Variables as concentrations within the fluid domain."

mamalili · February 24, 2015, 10:04

thank U every one
hey
i solve my prob successfully with your guides

now in cfd post i draw a chart
i interested to have the charts data in a txt or excel file

but in reports chart is exist as a pic

can U tell me about this???

ghorrocks · February 24, 2015, 16:51

Go file/export in CFD_post and select the object locator you wish to export, and the variables you want to include.

February 19, 2015, 07:43	diffusion coefficient	#1
mamalili New Member Join Date: Jul 2013 Posts: 16 Rep Power: 11	hi i am trying to simulate hydrogen release in a closed vessel with cfx Hydrogen was released vertically upward during 60 s. the sensors were measuring hydrogen concentrations during 250 min after the end of release. my volume fraction in 2 min after end of release at the top sensors is very overestimate and at the bottom sensors is underestimate. the authors mentioned that diffusion coefficient is 0.74 cm2/s. i think diffusion coefficient can solve my problem estimate. but i dont know how and where i can define diffusion coefficient in cfx. i am using of (fluid pair model>interface transfer>mixture model) and (fluid model>free surface model) is set to "none" please help me, my friends. thank U

February 19, 2015, 10:42		#2
evcelica Senior Member Erik Join Date: Feb 2011 Location: Earth (Land portion) Posts: 1,140 Rep Power: 22	You should be doing this as a variable composition multi-component mixture. Look in the tutorials, and search multi-component. There is one on a reacting mixture that shows you how to set this up. mamalili likes this.

February 22, 2015, 04:23		#7
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,365 Rep Power: 139	Have a look in the documentation about additional variables. It allows you to add an additional variable to the equations to solve. The additional variable can be an algebraic, diffusion, convection or diffusion+convection equation. I am talking about a diffusion+convection equation additional variable. To quote the documentation: "Additional Variables are non-reacting, scalar components that are transported through the flow, or through a solid (including the solid portion of a porous domain). They can be used to model, for example, the distribution of dye through a liquid, or smoke from a fire. ANSYS CFX typically interprets Additional Variables as concentrations within the fluid domain." mamalili likes this.

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February 19, 2015, 16:59		#3
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,365 Rep Power: 139	If all the fluids are the in a single phase then it is not multiphase, is it? Erik is right, you should model this as a multi-component mixture. If the fluid property changes due to hydrogen concentration are not important then consider a simple additional variable approach.

February 21, 2015, 10:57		#4
evcelica Senior Member Erik Join Date: Feb 2011 Location: Earth (Land portion) Posts: 1,140 Rep Power: 22	Since this is hydrogen, any other fluid will at least be double the density, so additional variable would not be suitable of course.

February 21, 2015, 23:46		#5
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,365 Rep Power: 139	If the concentration of hydrogen is low the additional variable approach is suitable .

February 22, 2015, 03:41		#6
mamalili New Member Join Date: Jul 2013 Posts: 16 Rep Power: 11	what do U mean of additional variable my friends?

February 24, 2015, 10:04		#8
mamalili New Member Join Date: Jul 2013 Posts: 16 Rep Power: 11	thank U every one hey i solve my prob successfully with your guides now in cfd post i draw a chart i interested to have the charts data in a txt or excel file but in reports chart is exist as a pic can U tell me about this???

February 24, 2015, 16:51		#9
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,365 Rep Power: 139	Go file/export in CFD_post and select the object locator you wish to export, and the variables you want to include.