Tutorial Fan (MRF) - Energy balance - Forum-EN

#1 · October 30, 2026, 6:26 pm

Hello,

Someone has already try to compare the power Pair = Q x deltaP and Pm = C x omega in the Fan (MRF) tutorial

It seems that there is a factor 10 between the 2 values.

With defalut values of the tutorial

Pfluid = Q. delta(Pt)

Pblade = C.w

I measure Total Moment (Z) in force report = -0.001553 N.m on rotating blades
Pm = 0.001553 x 2 pi x1000 / 60 = 0.1626 W

Pair = 0.0125 [m3/s] x (101582 - 101462.85) = 1.5 W
Pt mesured with mass-weight average value

Does someone has already done this kind of verification?

Best regards

julien

#2 · October 30, 2026, 7:47 pm

Hello Julien,

This tutorial was created to demonstrate how to set up and use MRF (Multiple Reference Frame). The geometry and operating conditions were not specifically designed for evaluating fan torque or efficiency.

We reproduced the default case and found that the outlet total pressure was lower than the inlet total pressure. To calculate fan air power, the total pressure difference should be taken as outlet minus inlet, which gives a negative value in this case. Your calculation appears to use inlet minus outlet.

Although the exact cause has not been established, possible contributing factors include a high inlet velocity relative to the rotational speed, internal flow losses, and backflow near the outlet. Therefore, the default tutorial case has limitations as a basis for assessing fan performance through this power comparison.

Best regards,
In-Goo Lee

#3 · November 3, 2026, 12:22 am

Hello In-Goo,

Thank you for your answer.

Which modifications are needed in the tutorial case to have a good geometry and operating conditions for a correct estimation of fan torque and efficiency ? If the torque evaluation is correct it should give a reasonnable accuracy in the energy balance.

I tried different inlet/ outlet BC, without fixing the inlet velocity but the convergence is not possible. I am curious to know how to calculate a fan with baram and be able to measure a correct Torque and energy balance.

With the basic BC on the tutorial, the inlet pressure is larger than the outlet pressure. In a real fan it is the opposite but I don't know which BC could give a realistic behaviour.

Best regards

Julien

#4 · November 5, 2026, 12:55 pm

Hello Julien,

Thank you for trying different inlet and outlet boundary conditions and sharing your findings. We understand your goal of checking the torque and energy balance.

However, this tutorial was designed to demonstrate the setup and use of MRF and has not been validated for fan performance assessment. We are therefore unable to provide specific modifications that would ensure accurate torque and efficiency predictions at this stage.

Assessing prediction accuracy requires reviewing the mesh, turbulence model, boundary conditions, and convergence, as well as comparing the results with experimental data. We recommend using a benchmark case with published experimental data as a reference for evaluating your simulation setup.

Best regards,
In-Goo Lee