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How does the CFX distinguish between Turbine and Compressor?

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Old   August 6, 2016, 02:10
Default How does the CFX distinguish between Turbine and Compressor?
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There is a option only to specify the Speed of rotation.
How does the CFX knows whether the work is added or extracted?
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Old   August 6, 2016, 06:39
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CFX is a Navier Stokes solver, and uses a mesh to discretise the volume. This means the flow is evaluated at each control volume in the domain based on the conditions which apply to it. The work done or absorbed by the rotors is calculated by the integral of the forces acting on the body applied at the control volumes.

When you sum up all the contributions by the control volumes you get quantities like net torque, power and flow rate. These are global quantities which engineers use to describe a flow, but CFX only sees it a zillions of little control volumes.
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Old   August 6, 2016, 06:54
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For example let's take a 2 similar rotor blade model..
One should be analysed like a compressor, that is, it is rotated by the mechanical energy
Another should be analysed like a turbine, that is, the flow rotates the rotor..

How we will distinguish these two model in CFX pre setup?
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Old   August 6, 2016, 07:49
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The same way as a turbine and a compressor are different in real life. It has to be designed right and the flow conditions applied correctly such that it is in the flow regime where it behaves as expected.

This means that the conditions the flow sees are such that its pressure increases as the flow progresses (ie a compressor), or the fluid does work on the blade and spins it (ie a turbine).

If you don't know what design features and flow conditions are required for a compressor or turbine then you better consult a turbomachinery design textbook (such as The Thermodynamics of Turbomachinery by Dixon).
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Old   August 6, 2016, 08:00
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Thanks for your suggestion ghorrocks!

If we give high pressure at the outlet and low pressure at the inlet, then the CFX may understand that the pressure is increasing downstream and it is a compressor!
But what if I apply a low pressure at the inlet and mass flow rate at the outlet as Boundary condition??
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Old   August 6, 2016, 08:28
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then the CFX may understand that the pressure is increasing downstream and it is a compressor!
No, that is not what I am saying. CFX does not "understand" anything, it only works out what the Navier Stokes equations says will happen in each of the little control volumes.

Let me explain it differently. The angle of attack of the fluid on the blade determines the force on the blade, and the force on the blade determines the power input or output. So simply changing the AOA from positive to negative will change from positive to negative torque, and will mean the device changes from being a compressor to a turbine (or possibly to just a dead weight .
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Old   August 6, 2016, 08:30
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Understood thanks a lot
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