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how to know the turbine rotating speed? |
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July 1, 2014, 00:36 |
how to know the turbine rotating speed?
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#1 |
New Member
Cafard
Join Date: Jun 2014
Posts: 29
Rep Power: 12 |
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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July 1, 2014, 06:06 |
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#2 |
Senior Member
Mr CFD
Join Date: Jun 2012
Location: Britain
Posts: 361
Rep Power: 15 |
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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July 1, 2014, 06:30 |
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#3 |
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Thomas
Join Date: May 2014
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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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July 1, 2014, 08:29 |
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#4 |
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Cafard
Join Date: Jun 2014
Posts: 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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July 1, 2014, 08:33 |
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#5 |
New Member
Cafard
Join Date: Jun 2014
Posts: 29
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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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July 1, 2014, 09:01 |
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#6 |
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Thomas
Join Date: May 2014
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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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July 4, 2014, 07:23 |
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#7 |
Senior Member
Blanco
Join Date: Mar 2009
Location: Torino, Italy
Posts: 193
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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. Regards |
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July 8, 2014, 03:36 |
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#8 |
New Member
Cafard
Join Date: Jun 2014
Posts: 29
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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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July 8, 2014, 04:00 |
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#9 |
Senior Member
Blanco
Join Date: Mar 2009
Location: Torino, Italy
Posts: 193
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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 "velocità 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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July 8, 2014, 04:07 |
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#10 | |
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Thomas
Join Date: May 2014
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Quote:
Regards |
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July 9, 2014, 03:34 |
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#11 |
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Marcin
Join Date: May 2014
Location: Poland, Swiebodzin
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On the outlet You can define opening - i think that will be a good solution
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July 14, 2014, 04:25 |
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#12 |
New Member
Cafard
Join Date: Jun 2014
Posts: 29
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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 04:11. |
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