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Thrust in Axial flow fan for different inlets

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Old   April 19, 2014, 06:31
Default Thrust in Axial flow fan for different inlets
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Hello Dear CFX users,
I am simulating axial flow fan for different inlet geometries viz.sharp inlet(zero camber radius) and smooth inlet (bell mouth) . The inflow conditions are non-symmetric i.e I have certain cross flow velocity. This means I have large flow separation region at the fan inlet for sharp inlet and no flow separation at the bell mouth inlet region.
While calculating the thrust generated by fan I am getting higher values. And also surprisingly thrust values for sharp inlet is higher than the smooth inlet.
I tried of monitoring the force_z@single blade , which is reaching to maximum value at the separation region. And also for the total thrust I used cfx expression as

Thrust=force_z@rotorblades+force_z@statorblades +force_z@hub

what is the right expression for the thrust generated by the axial flow fan in CFX. do I need to subtract any momentum force . please help with the right expression .

Thanks you
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Old   April 20, 2014, 06:42
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The force acting on a body has to be transmitted to the body through the surfaces. Assuming you have enough of the surfaces of the body that summing the forces over it is meaningful - then no, you do not have to include momentum differences in the fluid.

How have you modelled this? Can you show an image?
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Old   April 20, 2014, 06:48
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I used sliding mesh method to model the rotation. modelling is done by using multi domain method.
Whole model is divided into three domains likely

Rotor --- Rotorblades, rotorhub and rotor duct
Stator --- statorblades, statorhub and stator duct
Domain -- Inletlip, and out domain

I used GGI interface to connect the domain -rotor , rotor-stator which is rotating. and stator -domain is stationary.
Axis of rotation is around z axis.

I used the thrust expression

Thrust=force_z@Rotorblades+force_z@rotorhub+force_ z@rotorduct +force_z@statorblades+force_z@statorhub+force_z@st atorduct

is this right expression to find the Axial thrust. or if I calculate the force acting on exit plane of plane
i.e.
Thust=force_z@exit plane at stator outlet

help me in solving this issue ..

Thanks regards
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Old   April 20, 2014, 07:37
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I attached the model ..pic

Last edited by acoustica; April 22, 2014 at 06:46.
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Old   April 20, 2014, 23:26
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If this is something like an engine in a nacelle on a wing (like an aircraft) then you really should include a domain around the engine to allow air to bypass the engine if it wants to. This is especially for cases where the incoming air is not aligned with the engine - this will change the amount of air passing through the engine.
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Old   April 21, 2014, 05:51
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You mean my domain modelling is wrong or do I need to include the bodies around it to get the right value of Thrust ?
Actually I have the domain on top , which span 5 fan diameters in all three directions.
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Old   April 22, 2014, 06:04
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No, it is the air flow which bypasses the engine I am referring to. So some air can bypass the engine, and some goes through the engine. And the amount which takes either path will depend on the flow conditions, engine operating point and many other things - including angle of incidence.
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Old   April 23, 2014, 19:29
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what is the effect of the inlet duct shape on the generated thrust.
let if I have two types of inlets
1.First one with Zero camber radius
2.Second with specific camber radius.

especially in the presence of crossflow ..
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Old   April 24, 2014, 03:44
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I have no idea. This is your area of research, not mine.

But sharp inlet corners cause separations and cross flow at the inlet will also cause separations and I know that a fan operating in a partially separated/partially attached flow is not going to be running very well.
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