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Old   August 16, 2015, 10:33
Default Turbine Operating Point
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Stephen
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Hello all,

First post! This seems like a very active forum with people that know what they're doing, so I am very much hoping for some expertise I'm trying to design an axial hydraulic turbine. I created a blade and am currently simulating this in CFX.

I have read on here, a few times, that the operating point of a turbine is when torque goes to zero. Is this the same as what I can expect from calling the (torque_z()@R1 Blade function? What I don't understand is, how is the blade actually generating power without generating torque? If I could load the blade with a counter-torque (from the electric motor), can I expect the turbine to try to maintain speed and generate a torque?

Currently, to find my operating point, I put a total pressure on the inlet and static on the outlet. Then, I do a parameter sweep of RPM. This kind of gives me a mass flow rate to work with. I plan to use these around my 'operating point', then keep rpm constant and sweep mass flow rate for a more accurate solution. It seems that a CFD analysis likes to have a mass flow at either inlet or outlet, as opposed to pressures.

Any input or examples would be highly appreciated! I'm very new to this..

thanks -

Stephen
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Old   August 18, 2015, 08:14
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It seems that I am definitely not the only confused person on this subject, after my many google searches.

My current thinking is,

- Fix RPM, vary mass inflow rate, until torque goes to zero. Do this for as many points as you would like, in order to give your operating curve of the device.

- to find your peak power at these points, you can set the flow to zero and apply pressure boundary conditions. Then, spin at your operating points and see how much torque you require to get the mass flow rate from the first part

Although, this doesn't seem ideal as I am not sure that the problem is completely inverse..

Any thoughts?-

Stephen
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Old   August 19, 2015, 10:40
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My understanding is at a constant rotational speed the NET torque is zero. The blade is still generating a torque but for a steady state point this is balanced by a load.

As for the approach have a look at this post - How to know turbine angular velocity under wind load
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Old   August 20, 2015, 12:26
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Thanks for the response!

I think the confusion with me is that the turbine doesn't seem to work as the inverse of the pump, which I found odd.

For example, I used CFTurbo to design a pump with the following specifications: 150gpm, 4m head, 3200 RPM. Then, I simulated with a varying flow rate and constant RPM.

Indeed, it's working as a pump at around 8.7l/s (around 150gpm).. Then, at higher inlet mass flow, it's being more of a turbine. I get around ~68% peak efficiency.. This is without an inlet stator. I'm planning on making a counter-rotating turbine to capture the swirl and create a more efficient design.

Any input on the results or approach?

Stephen
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Old   August 21, 2015, 06:21
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Have you got any other plots? Anything that shows the flow field?
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