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how to know the turbine rotating speed?

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Old   June 30, 2014, 23:36
Default how to know the turbine rotating speed?
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Cafard
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Hi everybody

I want to simulate a turbine performance
but i know the inlet condition only (pressure、velocity)
because turbine is not the same as compressure that could set a rotating speed,I want to know the rotating speed due to inlet condition

how should I set the outlet area and turbine wall condition?
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Old   July 1, 2014, 05:06
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If you know the inlet conditions you can do back-of-the-envolope calculations by hand to roughly determine the speed.

Once you know this then you should know what kind of solution to expect from CFD.

I don't have an answer to your original question.
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Old   July 1, 2014, 05:30
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I would set the wall as no slip (smooth wall) in a rotating domain for first simulations. But the keep in mind that torque is dependent to the surface.
For the bc you should know the characteristic (at least the power) of your turbine and simulate with that: Guess a initial rotating velocity and calculate the resulting power (with the measured torque). For Inlet and Outlet you can use pressure-pressure or massflow Inlet and pressure Outlet. If you dont have the bc just guess it and see how it fits to your characteristic.
As far as I know there is no option that cfx calculates the rotation velocity of a domain but itself.
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Old   July 1, 2014, 07:29
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hi Mester
thx for your recommend
I think I can set pressure-massflow as in-out
but I still don't understand the goal of guess a rotating speed

as I know
a fluid after turbine,the total energy would reduce because part energy transfer to turbine rotating energy
but if I set turbine a initial rotating speed,the total energy maybe not reduce
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Old   July 1, 2014, 07:33
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someone give me some recommend that I could use move grid (or FSI) to simulate the rotating blade
did it work?
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Old   July 1, 2014, 08:01
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Hi cafard

there is no need for FSI. I described is the proper way to simulate a turbine and you will see that the Total Pressure after the Turbine is less than before. It depends on the rotating direction (you can use your turbine also as a compressor!). Dont make it more complicated than it is.
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Old   July 4, 2014, 06:23
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Hi all,

I think Thomas is right, you have to guess an initial rotating velocity for your turbine, then you can perform your simulation with boundaries mass-flow inlet and pressure outlet (you obviously know the values to be used here). Then you can verify if the pressure difference between inlet and outlet at the end of the simulation will match your desired values (actually you should have an info similar to "from this turbine you get XXX W if mass flow is YYY and pressure difference between inlet and outlet is ZZZ". You have to obtain that pressure diff. is ZZZ when mass flow inlet boundary is set to YYY). If not, then you have to modify the rotational speed until you obtain the desired pressure difference. At that point, you have found the correct rotational speed.

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Old   July 8, 2014, 02:36
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HI Meister & Blanco
after your recommends,I refered to a real turbine to design
then I simulate in this BC:
1.pressure-in(12 MPa) & mass flow-out(1 kg/s)
2.set rotating speed : 3600 rpm

then I found that the out pressure(11MPa) don't low enough,then I increase the rotating speed to 18000 rpm
the result became 10MPa,and that is what I want

but I still have some hesitation:
if I don't design miscount ,then I can change the rotating speed to get any target out pressure,but it's reasonable!?

in real situation,give constant preaaure and mass flow,the turbine will have only one rotating speed (don't consider the orther contditions).if the design can't achieve the target ,representing the design with the problem.
but in simulation as the opposite,if doesn't achieve target,just change the ratating speed??
if so,I don't need to design again??
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Old   July 8, 2014, 03:00
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Hi Cafard,

well good news for your sim.
Coming to your question, let's consider that you impose a specific working point (pressure, mass flow) to the turbine. In real conditions the aeraulic power at your inlet (mass flow*pressure) minus the aeraulic power at your outlet, minus the power losses (friction, shear, etc.) gives you the mechanical power on your turbine shaft. Then you will have something connected to that shaft, and this "something" will exert a resistance to the rotation, so you have a resistance torque. In steady-state conditions the torque coming from the turbine shaft will be equal to the resistance torque of the user mechanism, so the rotating speed is established from mechanical power AND torque on the turbine, not only aeraulic power... If you doesn't connect anything to the turbine shaft, than it will speed up until it reaches its "escape speed" (don't know if this is the exact name in english, in italian it is "velocitÓ di fuga"), the speed where you have useful torque = 0 (all the torque is wasted in friction inside the turbine bearings and losses).
This is why you need to impose a speed in you CFD sim...it's like you are imposing a torque on the turbine shaft, considering also friction in bearings and so on.
When you design a turbine you want it to have a certain efficiency in a specific (pressure,mass flow) point, so you have aeraulic power/mechanical power as a target. But you have to consider the torque that your user mechanism will require, and you can derive target speed for your turbine. So, you will simulate the turbine rotating at your target speed, and design the blades in such a way that you achieve your target efficiency when you have the design (pressure, mass flow) working point.
I hope this can solve your doubts about this topic.
Regards!

Andrea
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Old   July 8, 2014, 03:07
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Quote:
Originally Posted by Cafard View Post
then I can change the rotating speed to get any target out pressure
Its not. Please look for surge and stall in turbines - there is plenty of literature about turbine characteristics. As Andrea already mentioned - it is not only one single working point it is a whole characteristic.
Regards
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Old   July 9, 2014, 02:34
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On the outlet You can define opening - i think that will be a good solution
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Old   July 14, 2014, 03:25
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hi all

First:
I found some questions in the result
I want to know where's the max stress(or pressure) happen to prevent the blades breaken.
but in my simulate,the force direction on the blade is different from reality
beacuse one is initiative and one is passive

Second:
When I use a new centrifugal turbine model that is changed some design to simulate,
I found some phenomenon:
1. the outlet T is almost the same as the inlet T (it's unreasonable)
2. when the iteration is large enough , the RMS will happen virbration (may the grid quality not good enought?)
3. the outlet velocity is smaller then the inlet velocity (is it unreasonable???)
4. I set the inlet direction and turbulence as zero gradient,is it appropriate?
5. the important is that the massflow of inlet and outlet are not the same (error bigger than 80% !!!)

so,how should I solve these question??

Last edited by Cafard; July 16, 2014 at 03:11.
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