How to add total pressure in state equation in CFX

Goenitz · July 30, 2019, 08:56

Hi all,

I have a problem regarding initial condition.
I have velocity, temperature and pressure (Pin) as inlet conditions. At a time, I can assign velocity or pressure so i assign velocity. Now how to add pressure in equation of state?

I am using CEL expression for chemical reaction and in rate equation I put the Pin= 101.235 [kPa] but still the pressure in post-CFX is based on the inlet velocity and temperature (0.1m/s, 793k) and turns out to be max 0.25 Pa. Inlet area (1mm by 6.1mm).

ghorrocks · July 30, 2019, 22:15

You can define pressure, temperature and velocity as initial conditions. You can define pressure or velocity at a subsonic inlet boundary, in CFX you cannot define both. The normal approach is to define velocity at the outlet and pressure at the inlet.

If this is not possible you better explain what you are doing more fully so we can help you.

Also, I do not understand what you are asking in your second paragraph.

Goenitz · July 31, 2019, 08:22

Sure man,

I have extracted velocity profile from one session at outlet. Then used this profile (for next session) at outlet and pressure inlet as Pin=1 atm. However, the results deviated a lot from original work.

I am using chemical reaction. So the rate equation is

Rate=Ae^(-E/R/T) *P_H2O* P_CH4
So when I use velocity as 0.1 m/s and I cannot assign pressure then what I do is: P_H2O = Pin *H2O.molconc
1 atm * water mole concentration

I thought as I assign pressure in chemical reaction so it will reflect in static pressure. But that is limited to < 0.25Pa in post.

If i calculate in post chart/table the Pin*H2O.molconc quantity then that value exist (2..e4 Pa)

Opaque · July 31, 2019, 08:41

I would assume that P_H2O, and P_CH4 are partial pressures; therefore, you only need to write as

P_H2O = Absolute Pressure * H2O.molconc
P_CH4 = Absolute Pressure * CH4.molconc

Those two expressions above are generic; therefore, you can use it in the interior of the domain, and boundaries.

Pin can only be used as a boundary condition if StaticPressure option used, or to verify the model if velocity option used at the corresponding location

Goenitz · July 31, 2019, 08:55

Quote:

Originally Posted by Opaque

I would assume that P_H2O, and P_CH4 are partial pressures; therefore, you only need to write as

P_H2O = Absolute Pressure * H2O.molconc
P_CH4 = Absolute Pressure * CH4.molconc

Those two expressions above are generic; therefore, you can use it in the interior of the domain, and boundaries.

Pin can only be used as a boundary condition if StaticPressure option used, or to verify the model if velocity option used at the corresponding location

You suggested this before but then I was not sure. Now, I know that I have to use 1 atm as 'total' pressure of Dalton's law so I can use Absolute pressure as i have set my reference pressure as 1 atm.
and I am clear that Pabs= Pref+Pstatic =1atm+Pstatic (<0.25 pa)
Ptot=Pstatic+Pdyamic=0.25 Pa (my case)

I will try that now.

Opaque · July 31, 2019, 09:09

Not sure, but I think there is confusion here.

Dalton's law never talks about total pressure, but absolute pressure instead. The textbook may have intended to say "not gauge pressure" since the law applies to gases.

Total pressure implies isentropic flow from stagnation conditions to local conditions.

July 30, 2019, 08:56	How to add total pressure in state equation in CFX	#1
Goenitz Member Abdullah Arslan Join Date: Apr 2019 Posts: 94 Rep Power: 7	Hi all, I have a problem regarding initial condition. I have velocity, temperature and pressure (Pin) as inlet conditions. At a time, I can assign velocity or pressure so i assign velocity. Now how to add pressure in equation of state? I am using CEL expression for chemical reaction and in rate equation I put the Pin= 101.235 [kPa] but still the pressure in post-CFX is based on the inlet velocity and temperature (0.1m/s, 793k) and turns out to be max 0.25 Pa. Inlet area (1mm by 6.1mm). Last edited by Goenitz; July 30, 2019 at 09:03. Reason: correction

July 30, 2019, 22:15		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,870 Rep Power: 144	You can define pressure, temperature and velocity as initial conditions. You can define pressure or velocity at a subsonic inlet boundary, in CFX you cannot define both. The normal approach is to define velocity at the outlet and pressure at the inlet. If this is not possible you better explain what you are doing more fully so we can help you. Also, I do not understand what you are asking in your second paragraph. __________________ Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.

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July 31, 2019, 08:22		#3
Goenitz Member Abdullah Arslan Join Date: Apr 2019 Posts: 94 Rep Power: 7	Sure man, I have extracted velocity profile from one session at outlet. Then used this profile (for next session) at outlet and pressure inlet as Pin=1 atm. However, the results deviated a lot from original work. I am using chemical reaction. So the rate equation is Rate=Ae^(-E/R/T) P_H2O P_CH4 So when I use velocity as 0.1 m/s and I cannot assign pressure then what I do is: P_H2O = Pin H2O.molconc 1 atm water mole concentration I thought as I assign pressure in chemical reaction so it will reflect in static pressure. But that is limited to < 0.25Pa in post. If i calculate in post chart/table the Pin*H2O.molconc quantity then that value exist (2..e4 Pa)

July 31, 2019, 08:41		#4
Opaque Senior Member Join Date: Jun 2009 Posts: 1,880 Rep Power: 33	I would assume that P_H2O, and P_CH4 are partial pressures; therefore, you only need to write as P_H2O = Absolute Pressure * H2O.molconc P_CH4 = Absolute Pressure * CH4.molconc Those two expressions above are generic; therefore, you can use it in the interior of the domain, and boundaries. Pin can only be used as a boundary condition if StaticPressure option used, or to verify the model if velocity option used at the corresponding location

July 31, 2019, 09:09		#6
Opaque Senior Member Join Date: Jun 2009 Posts: 1,880 Rep Power: 33	Not sure, but I think there is confusion here. Dalton's law never talks about total pressure, but absolute pressure instead. The textbook may have intended to say "not gauge pressure" since the law applies to gases. Total pressure implies isentropic flow from stagnation conditions to local conditions.