Mass Flow Rate in Fluent

mhakimakbar · April 21, 2016, 23:39

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?

LuckyTran · April 22, 2016, 10:47

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 · April 22, 2016, 13:12

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

April 21, 2016, 23:39	Mass Flow Rate in Fluent	#1
mhakimakbar New Member hakim Join Date: Mar 2016 Posts: 2 Rep Power: 0	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 = momentrpm = 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?

April 22, 2016, 10:47		#2
LuckyTran Senior Member Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,665 Rep Power: 65	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.

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April 22, 2016, 13:12		#3
mhakimakbar New Member hakim Join Date: Mar 2016 Posts: 2 Rep Power: 0	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