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Axial fan compressor - Mass flow not matching

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Old   December 2, 2017, 09:41
Default Axial fan compressor - Mass flow not matching
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Miguel Mazzucchelli Bayo
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Hello Engineers,
I am analysing the performance of an axial aero-fan for my MSc thesis using ANSYS CFX 17.1. The geometry of the fan is detailed below:

Fan diameter = 3.3 m
Fan area = (PI/4)*(3.3 m)^2 = 8.553 m2
Fan inlet Mach number = M0.6
Total Inlet Temperature: P = 248.02 K
Total Inlet Pressure: T = 29787 Pa

The static pressure and temperature are:
Static inlet pressure: p= 23353 Pa
Static inlet temperature: t = 231.36 K

The air velocity is:
v = Mn*sqrt(Gamma*R*t) = 0.6*sqrt(1.4*287.05*231.36) = 183 m/s
Applying the ideal gas law, the density is:
rho = 0.3516 kg/m3
Hence, the mass flow that I need at the fan inlet should be:
W = rho*v*Area = 0.3516*183*8.553 = 550.32 kg/s


I've calculated that the fan has 18 blades, hence the mass flow per blade section should be 550.32/18 = 30.573 kg/s

The boundary conditions and solver requirements I am using, are detailed in the attached images.

The simulation converges. And the residuals are acceptable (also shown on the attached file).

However, when I verifying the inlet mass flow with CFX-Post, the mass flow I am obtaining is 26.435 kg/s (475.83 kg/s overall) and the Mach Number in Stn Frame using massFlowAveAbs is M0.50


All the further analysis will depend on the consistency between the calculated value and the CFD value, so I need to understand the reason for the difference. This will determine if the numerical model is coherent with the reality.

Is the difference due to the BC I am setting?
I've read all the CFX manual, and I cannot understand why the gap between this values is so large.

Is it because of CFX post is presenting the results in a different way? (e.g. corrected mass flow instead of actual mass flow) If this is the case, I'll appreciate if you may indicate me which formula is used on CFX.

Very thanks in advance for your kind help.
Regards,

Miguel
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Old   December 2, 2017, 19:42
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Glenn Horrocks
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Please attach the images directly on this post, not in zip files. Images of your mesh, geometry and results are good. Please do not post screen dump images of your setup, post your setup by attaching either the CCL file or the output file. If you attach the output file just leave the setup information and the first and last few iterations. You can remove all the iteration in between.

I have not looked in detail at your information (zip files are a bit painful to manage), but are you sure that your stated operating point leading to 550 kg/s is correct? For instance, this sounds like a turbofan aero engine. If the aircraft is going at Mach 0.6 that does not mean the air through the fan is Mach 0.6. The engine's operating point will accelerate or decelerate the air. I am not an aero engine expert so I might not have used the right terminology to explain this, but I think you need to look at your expected operating point carefully.
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Old   December 3, 2017, 19:18
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Miguel Mazzucchelli Bayo
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Hi ghorrocks, thank you for your quick answer. As you mentioned correctly, it is for an aerospace application. I am trying to upload the images directly to the post, but I don’t know how to do it with.

Regarding your comment, the aircraft is flying at M0.85 but the intake (which is not modelled) has a diffusive effect that reduces the Mach number and raises the static pressure from 18753.76 Pa (ISA @ 40,000 ft) to 23353 Pa (at the rotor fan face).

Yesterday afternoon, I realized the mass flow reduction is because the inlet Mach N that ANSYS is using, is not 0.6 but 0.5

I tried to modify the boundary conditions to achieve the desired flow regime. However, if I replace the BC settings, the solver outputs wrong conditions, errors or directly does converges.

I’ve already tried all the options available in the BC settings.

If I over-impose the mass flow, the calculated total inlet pressure is above the actual value.
If I change the BC settings – for example, using normal speed or static pressure- the solver is either not converging or outputting errors.

On the other hand, if I change the output conditions to define the actual mass flow of 30.573 kg/s, I obtain no convergent results or solver errors.

I’ve also tried to change from the subsonic, to supersonic or mixed. The results are still not good.

Can you tell me how to increase the Mach number without violating the design conditions?

Thank you in advance.
KR

miguel_mazzu
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Old   December 3, 2017, 19:33
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Posting images: https://www.cfd-online.com/Wiki/Ansy...n_the_forum.3F

General FAQ on accuracy: https://www.cfd-online.com/Wiki/Ansy..._inaccurate.3F

Have you done all the normal checks on mesh, convergence, boundary condition proximity and so on? You need to check your mesh is fine enough, your convergence is tight enough and your boundary conditions are far enough away.
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