CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > Software User Forums > ANSYS > CFX

In CFX and Fluent, is "mass flow" set under normal or operating conditions?

Register Blogs Members List Search Today's Posts Mark Forums Read

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   September 28, 2020, 09:37
Default In CFX and Fluent, is "mass flow" set under normal or operating conditions?
  #1
New Member
 
Ilya
Join Date: Oct 2019
Posts: 5
Rep Power: 4
offroader10052 is on a distinguished road
Hello.

Please tell me

in CFX and Fluent "mass flow" (kg / s)

on the input boundary conditions tab

is asked under normal or operating conditions?



For a liquid, this is not usually important.

It is very important for gas.
offroader10052 is offline   Reply With Quote

Old   September 28, 2020, 09:42
Default
  #2
Senior Member
 
Join Date: Jun 2009
Posts: 1,471
Rep Power: 27
Opaque will become famous soon enough
Not clear what you are asking.

You can select several options at inlet boundaries. It is your decision. The first option is just a default, not a requirement.
__________________
Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.
Opaque is offline   Reply With Quote

Old   September 28, 2020, 09:43
Default
  #3
Senior Member
 
Gert-Jan
Join Date: Oct 2012
Location: Europe
Posts: 1,388
Rep Power: 21
Gert-Jan will become famous soon enough
Mass is the amount of kilograms per second. In either normal or operating conditions.
Since the software cannot read your mind, it is you to tell what the conditions are, by setting the reference pressure and inlet temperature.
Gert-Jan is offline   Reply With Quote

Old   September 29, 2020, 02:50
Lightbulb
  #4
New Member
 
Ilya
Join Date: Oct 2019
Posts: 5
Rep Power: 4
offroader10052 is on a distinguished road
Thanks.
I understood like this:

Given:
Volumetric gas flow rate under standard conditions (temperature 25 C, pressure 1 atm).

Task 1:
Determine the mass flow rate at a temperature of 50 C and a pressure of 10 atm in the apparatus.

Task 2:
Enter the obtained mass flow rate into the ansis.

Solution step by step:

Step 1.
Recalculate the volumetric flow rate (m3/s) in Excel under operating conditions (temperature 50 C, pressure 10 atm).

Step 2
Recalculate in Excel the mass flow rate (kg/s) according to the obtained volumetric flow rate under operating conditions in the previous step.

Step 3:
Enter the obtained mass flow rate into the ansis.

Let it be useful to someone.
offroader10052 is offline   Reply With Quote

Old   September 29, 2020, 04:29
Default
  #5
Senior Member
 
Gert-Jan
Join Date: Oct 2012
Location: Europe
Posts: 1,388
Rep Power: 21
Gert-Jan will become famous soon enough
I can't follow your thoughts. I'm blinded by 25 years of CFD experience.

In CFD, you either set massflow and temperature (or velocity) on inlet and zero pressure on the outlet. Absolute pressure is determined by the sum of pressure in your domain and reference pressure. Using these settings, you get the pressure on the inlet required to get the massflow through you geometry.
Alternatively you set an inlet pressure (and temperature) and CFD will give you the massflow in line with the pressure difference you set.

So, if you have standard conditions, you set:
- reference pressure on 1 atm,
- outlet pressure 0 Pa
- inlet mass flow and temperature
The outcome will be pressure required to get the massflow through at that condition.

You can repeat the calculation at elevated conditions by increasing the reference pressure and elevated inlet temperature. Then with the same mass flow, the outcome will be pressure required to get the massflow through at that condition.

If you are not satisfied with the outcome, you can increase or decrease the massflow to get the inlet pressure you want. Alternatively, you set the inlet pressure and you get the massflow as a result.

It is just a matter of trial and error.
Gert-Jan is offline   Reply With Quote

Old   September 29, 2020, 09:48
Default
  #6
Senior Member
 
Join Date: Jun 2009
Posts: 1,471
Rep Power: 27
Opaque will become famous soon enough
I think I now understood the question. Let me rephrase it as

Are the inlet conditions using local conditions, or the operating conditions as stated in Fluent (CFX does use such vocabulary)?

The concept of operating conditions in Fluent, or Reference Pressure in CFX, is used to determine a level for shifting values in order to control numerics. In theory, the engineering solution should be independent of any operating conditions settings used.

The answer for the original question is: the boundary conditions is always evaluated using local conditions. Just be careful that the input values must be consistent with the operating conditions.

Example using CFX, if you set your domain Reference Pressue to 1 atm, all your boundary absolute pressure values must be expressed as gauge pressure, i.e. Relative Pressure = MyTrueAbsPresValue - Reference Pressure

If your Reference Pressure is set to 0 [any], your Relative Pressure is your Absolute Pressure.

There is no need to recompute your inlet conditions using the algorithm you described.

Hopefully I understood the question correctly this time around.
__________________
Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.
Opaque is offline   Reply With Quote

Reply

Tags
cfx, conditions, fluent, mass flow

Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
CFX vs. FLUENT turbo CFX 4 April 13, 2021 09:08
Xeon Gold Cascade Lake vs Epyc Rome - CFX & Fluent - Benchmarks (Windows Server 2019) SLC Hardware 18 June 13, 2020 17:48
Comparison of fluent and CFX for turbomachinery Far CFX 52 December 26, 2014 19:11
Different result in CFX and fluent for mass trans.? is segregated better? ftab CFX 7 September 27, 2012 08:57
CFX or Fluent for Turbo machinery ? Far FLUENT 3 May 27, 2011 04:02


All times are GMT -4. The time now is 00:29.