How to check size scale effect in a butterfly valve

fresty · November 4, 2015, 14:35

I believe yes the next logical step would be to apply compressibility on both water and water vapor..
Though i wouldn't be able to summarize it all but yes this is pretty much doable in ansys and you could browse the forum and conveniently come across many similar/ analogical discussions..

bejanyar · November 4, 2015, 14:56

I tried to find some topic about bubble transfer variable in CFX in forums.
But nothing special i found.
I will continue to search but if you know it i'll be happy if you tell it.

By knowing that i am not even sure that is there something about bubble transfer exist .

fresty · November 6, 2015, 07:28

Not sure if I could find something precisely for your intended simulation but I could bet on this being doable in CFX.. have you acquired some results using compressibility for your simulation? There are density changes necessary to account for whenever we think of cavitation phenomena per my understanding..

bejanyar · November 6, 2015, 15:28

If you want the truth , No , I couldn't apply compressibility to my model.
I dont how to do it.

In fluent there is an option for compressibility , but i couldn't find it in CFX.

fresty · November 6, 2015, 16:21

You could probably use CEL expression to alter the constant density of the material (create a copy of the material and then in material properties input the density as an expression) as a function of pressure.. something like below which i learnt from CFX forum for a water hammer concept simulation i carried out...
1000 [kg m^-3]/(1-(p-101325[N m^-2])/2150000000[N m^-2])
where 2.15E9 is the bulk modulus fluid elasticity of water..

bejanyar · November 7, 2015, 16:09

Ok dear fresty.

It worked.
The expression you told for change density by pressure give some error to me.
I replaced "p" with " Pressure" and it worked.

997 [kg m^-3]/(1-(Pressure -101325[N m^-2])/2150000000[N m^-2])

I changed my water to compressible.
In a fixed pressure drop the cavitation in a bigger valve in bigger.
This is acceptable duo to the researches and papers.

But the grow of the cavitation in lower than the value estimated by experience.
I added the compressibility only to water.
Should i add it to water vapor too ???
I attached 3 picture from 12 - 36 - 108 inch valve in fixed pressure drop.

fresty · November 8, 2015, 00:23

That's good to know.. yeah the P was just symbolic.. The results seem pretty reasonable..
Regarding lower value, you may have to refine mesh further to get rid of grid induced deviance, apply compressibility to both fluids (which you would need to try as i am not quite sure about it), apply double precision etc...

cheers.

bejanyar · November 8, 2015, 01:38

Thank you.
In case of adding compressibility to vapor i used similar formula to liquid phase.
By knowing that the density of water vapor is 0.023865 and the Bulk Modulus of water vapor is 3011 .
my ref is this page : http://www.roymech.co.uk/Related/Flu...ter_Props.html

But when you use this values by previous formula the density became negative !!!
assume Pressure = 150000 pa , since 150000-101325 is grater than 3011 result is negative.
What other formula for gas should i use ??

fresty · November 8, 2015, 04:01

Not really sure if the same formula could be applied on vapor phase water.. however, as we know from adiabatic conditions and from mathematical as well as physical point of view the density change for a vapor phase should probably simply be the ideal gas relation p2/p1=rho2/rho1..(your example: (200000/101325*0.023865 = 0.04710) this directly indicates increased pressure causing increase in density of the gas...
to reiterate, this is just from top of my head..

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November 4, 2015, 14:35		#21
fresty Senior Member Join Date: Aug 2014 Location: UK Posts: 213 Rep Power: 12	I believe yes the next logical step would be to apply compressibility on both water and water vapor.. Though i wouldn't be able to summarize it all but yes this is pretty much doable in ansys and you could browse the forum and conveniently come across many similar/ analogical discussions..

November 4, 2015, 14:56		#22
bejanyar Member Hamidreza Bijanyar Join Date: Jun 2015 Posts: 40 Rep Power: 10	I tried to find some topic about bubble transfer variable in CFX in forums. But nothing special i found. I will continue to search but if you know it i'll be happy if you tell it. By knowing that i am not even sure that is there something about bubble transfer exist .

November 6, 2015, 07:28		#23
fresty Senior Member Join Date: Aug 2014 Location: UK Posts: 213 Rep Power: 12	Not sure if I could find something precisely for your intended simulation but I could bet on this being doable in CFX.. have you acquired some results using compressibility for your simulation? There are density changes necessary to account for whenever we think of cavitation phenomena per my understanding..

November 6, 2015, 15:28		#24
bejanyar Member Hamidreza Bijanyar Join Date: Jun 2015 Posts: 40 Rep Power: 10	If you want the truth , No , I couldn't apply compressibility to my model. I dont how to do it. In fluent there is an option for compressibility , but i couldn't find it in CFX.

November 6, 2015, 16:21		#25
fresty Senior Member Join Date: Aug 2014 Location: UK Posts: 213 Rep Power: 12	You could probably use CEL expression to alter the constant density of the material (create a copy of the material and then in material properties input the density as an expression) as a function of pressure.. something like below which i learnt from CFX forum for a water hammer concept simulation i carried out... 1000 [kg m^-3]/(1-(p-101325[N m^-2])/2150000000[N m^-2]) where 2.15E9 is the bulk modulus fluid elasticity of water..

November 8, 2015, 00:23		#27
fresty Senior Member Join Date: Aug 2014 Location: UK Posts: 213 Rep Power: 12	That's good to know.. yeah the P was just symbolic.. The results seem pretty reasonable.. Regarding lower value, you may have to refine mesh further to get rid of grid induced deviance, apply compressibility to both fluids (which you would need to try as i am not quite sure about it), apply double precision etc... cheers.

November 8, 2015, 01:38		#28
bejanyar Member Hamidreza Bijanyar Join Date: Jun 2015 Posts: 40 Rep Power: 10	Thank you. In case of adding compressibility to vapor i used similar formula to liquid phase. By knowing that the density of water vapor is 0.023865 and the Bulk Modulus of water vapor is 3011 . my ref is this page : http://www.roymech.co.uk/Related/Flu...ter_Props.html But when you use this values by previous formula the density became negative !!! assume Pressure = 150000 pa , since 150000-101325 is grater than 3011 result is negative. What other formula for gas should i use ??

November 8, 2015, 04:01		#29
fresty Senior Member Join Date: Aug 2014 Location: UK Posts: 213 Rep Power: 12	Not really sure if the same formula could be applied on vapor phase water.. however, as we know from adiabatic conditions and from mathematical as well as physical point of view the density change for a vapor phase should probably simply be the ideal gas relation p2/p1=rho2/rho1..(your example: (200000/101325*0.023865 = 0.04710) this directly indicates increased pressure causing increase in density of the gas... to reiterate, this is just from top of my head..