CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > FLUENT

Mass Flow Rate in Fluent

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

Like Tree1Likes
  • 1 Post By LuckyTran

Reply
 
LinkBack Thread Tools Display Modes
Old   April 21, 2016, 23:39
Default Mass Flow Rate in Fluent
  #1
New Member
 
hakim
Join Date: Mar 2016
Posts: 2
Rep Power: 0
mhakimakbar is on a distinguished road
I'm doing a simulation in fluent for a radial outflow turbine. I use n-pentane as a working fluid.
I set the inlet BC with pressure-inlet type p=12 bar T=136C and the outlet BC with pressure-outlet type p=0.3 bar T=52C.

After the simulation I get the mass flow rate 1.039 kg/s from Reports>Fluxes>Mass Flow Rate for both inlet and outlet.
Then I calculate the pressure and temperature for both inlet and outlet from Reports>Surfaces Integral>Area-weighted Average
After that I calculate the turbine power, W = m*(h inlet - h outlet) = 25.4 kW

But when I calculate the power from moments and turbine angular velocity, I just get 3.285 kW.
I get the moment from Reports>Forces>Moments is 5.22 Nm and the angular velocity from my cell zone conditions 6000rpm.
W = moment*rpm = 3.285 kW

I expect the turbine power to be 3-5 kW, so I assume fluent reports a wrong mass flow rate.
What does mass flow rate mean in fluent?
mhakimakbar is offline   Reply With Quote

Old   April 22, 2016, 10:47
Default
  #2
Senior Member
 
Lucky Tran
Join Date: Apr 2011
Location: Orlando, FL USA
Posts: 1,372
Rep Power: 20
LuckyTran will become famous soon enough
For moments, make sure you have specified the correct axis. But it looks like the moment calculation is correct,3-5 kW is the correct answer?

Are you calculating the exit temperature and using that in your calculation or are you using the 52 degrees? Make sure you are using the calculated exit temperature (you cannot specify temperature at an outlet, you only specify the backflow temperature). In either case, it's better to just calculate enthalpy rather than temperature.

You should be using a mass-flow-weighted average enthalpy integral if you want to calculate the work output. This is probably not the actual problem though. Usually the difference between an area-weighted average and mass-flow weighted average is small unless there are large property changes.
mhakimakbar likes this.
LuckyTran is offline   Reply With Quote

Old   April 22, 2016, 13:12
Default
  #3
New Member
 
hakim
Join Date: Mar 2016
Posts: 2
Rep Power: 0
mhakimakbar is on a distinguished road
Thank you VERY MUCH for your answer, really!
I've re-checked the axis and yes I expect 5kW

I'm not using the backflow pressure and temperature to calculate the enthalpy. I use pressure and temperature from result then calculate enthalpy with my table properties.
How to calculate enthalphy in fluent?

I try to change my boundary condition to mass flow and specify it to be 0.1 kg/s. I get a right mass flow and my power calculation with m*(h_in - h_out) become 4 kW.
BUT my momen calculation become totally wrong, it change to 0.4 Nm, so my power is 0.2kW
Can you explain more how to calculate the work output?

*Sorry for my bad english
mhakimakbar is offline   Reply With Quote

Reply

Thread Tools
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 On
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
mass flow rate not conserved in turbomachine, interface defined wrong? wildli FLUENT 2 July 8, 2015 11:38
rotor 37 choking mass flow rate fluent simulation sehs15 FLUENT 1 January 15, 2015 07:07
Mass flow rate boundary condition with negative values ashtonJ CFX 3 November 26, 2014 06:21
mass flow rate computation in fluent Kishore FLUENT 5 June 27, 2007 03:53
particle, parcel and mass flow rate balance flybird FLUENT 0 May 24, 2007 10:44


All times are GMT -4. The time now is 07:02.