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Gas turbine in CFX

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Old   April 11, 2014, 06:13
Default Gas turbine in CFX
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Rahat
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I am simulating a single stage axial flow gas turbine for various rotor speeds in CFX. The operating conditions of axial flow gas turbine include inlet temperature of 800 K, inlet pressure of 3.036 bars, speed of rotor 6872 RPM and mass flow rate 3.196 kg/s. In ANSYS Workbench Release 14.0, ANSYS BladeGen is used for modeling stator and rotor. ANSYS TurboGrid is used for meshing stator and rotor and ANSYS CFX is used for analysis of single stage axial flow gas turbine. To check the effect of rotor speed, the speed of rotor is varied from 6872 RPM to 20000 RPM. I unique behavior is found in the velocity vectors diagram shown in attached figure.

Beyond 12500 RPM ie (d) the velocity vectors that are leaving the stator blade is not parallel to velocity vectors that are entering the rotor.
It is found that beyond 12500 RPM the rotor power is more than the available power given by the enthalpy drop across the gas turbine. It means that the results are not correct beyond 12500 RPM. So my question is that why CFX calculates the results when even we give wrong value of rotor RPM. In other words the rotor RPM should not be more than a value which generate more power than the maximum available power to turbine (ie the enthalpy drop along turbine).
If any one can help me in this issue, I shall be thankful.
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Old   April 11, 2014, 07:07
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Glenn Horrocks
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I am not exactly sure what you are asking, so let me make a few guesses:

Why is the velocity vectors not straight across the domain interface - this is an FAQ: http://www.cfd-online.com/Wiki/Ansys...f_reference.3F

If you have entered a rotating speed faster than the steady state speed CFX should still converge to a value, it is just that it will have a negative net torque. This means the rotor will slow down, so the equilibrium speed is slower than this speed.

Alternately, if you are asking why is my result in accurate: this is also an FAQ: http://www.cfd-online.com/Wiki/Ansys..._inaccurate.3F
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Old   April 11, 2014, 08:21
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Quote:
Originally Posted by rahat69 View Post

Beyond 12500 RPM ie (d) the velocity vectors that are leaving the stator blade is not parallel to velocity vectors that are entering the rotor.
The presented velocity vectors in the figure have not the same meaning in the two component of the turbine:
In the stator: velocity means the absolute velocity
In the rotor: velocity means the relative velocity.
If you plot the velocity at str frame you will find that the vectors are parallel at the interface.


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Originally Posted by rahat69 View Post
It is found that beyond 12500 RPM the rotor power is more than the available power given by the enthalpy drop across the gas turbine. It means that the results are not correct beyond 12500 RPM.
Tray to use the turbo-Post to calculate these quantities
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Old   April 11, 2014, 23:38
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Thank you Glenn Horrocks. Your answer solved my issue.
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Old   April 11, 2014, 23:44
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Thank you Abou Ali. Your answer is also helpful
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