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Old   December 4, 2013, 13:36
Question Turbine Angular Velocity Problem
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Hi,

This is a question of general interest. When we model a turbine in CFX we must supply the angular velocity with which the turbine will rotate. Now I know the mass flow rate. If the mass flow rate is variable the angular velocity will also vary. Can anyone help me with a relation how we could establish the angular velocity for different situations? I have tried with the velocity triangles for an axial flow turbine but couldn't solve it. Evidently to solve the triangle we need to have the actual flow velocity to the inlet, blade inlet angle and blade velocity or relative velocity.
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Old   December 4, 2013, 17:42
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What you are asking is what is the operating curve of the device - the RPM versus flow rate curve where the device is running at steady state.

To determine this in CFX you choose a fixed flow rate and run a series of RPM. When the rotor is running faster than the operating point the net torque on the rotor will be negative, if slower it will be positive. Where there is no net torque on the rotor you have your operating point at that flow rate.

You then repeat this at enough flow rates to describe the curve.
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Old   December 7, 2013, 11:14
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Thanks for the suggestion.

I tried simulating an axial turbine with an rpm of 15000. I am attaching a picture of the Gas Turbine Calculator Macro. The power output shown is negative. Doesn't sound right.

Also ,when I loaded the Turbine Rotor Template Report, the performance table was titled Compressor Performance Results Table, of which I am attaching another snapshot. I am not simulating a compressor though, just a single row of turbine blades.
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Old   December 8, 2013, 06:04
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I do not think you understood my last post. If the net torque (or power) is negative then the rotor is spinning too fast for the current conditions and you must do another simulation at a slower speed. This is simply following Newtonian mechanics - the net torque is against the motion so the motion will slow down.

So you need to redo the simulation at a slower speed. In fact you need to sweep a range of speeds so you can accurately work out the zero net torque point.

But if you know this speed should work OK then you have a simulation accuracy issue. In this case, read the FAQ: http://www.cfd-online.com/Wiki/Ansys..._inaccurate.3F
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