CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > CFX

very confused problem,Please help me

Register Blogs Members List Search Today's Posts Mark Forums Read

Reply
 
LinkBack Thread Tools Display Modes
Old   March 24, 2004, 11:39
Default very confused problem,Please help me
  #1
jiaye gan
Guest
 
Posts: n/a
Hi everyone: (1)I got a difficult problem while learning CFX5.5 tutorial12.The problem as follows: For a rotor/stator (such as hydraulic turbine) problem,the rotation velocity is not known first.Why in the example gave a known velocity? (2)For a hydraulic turbine problem,We all know that the rotor is rotated by the water.Then the velocity is not known first.Could the CFX5.5 solve this problem? How do we treat the rotation reference frame without specified a rotated velocity?

All your returns would be appreciated.

  Reply With Quote

Old   March 24, 2004, 17:45
Default Re: very confused problem,Please help me
  #2
Glenn Horrocks
Guest
 
Posts: n/a
Hi Jiaye,

It is possible to link the rotational speed to the solution. You would use a CEL expression to get the net torque on the blades and would adjust the rotational speed to get zero net torque. This would be quite tricky to get running reliably so is not a simple thing to do.

If you run a series of fixed speeds with given inlet conditions there will be some with negative torque and some with positive torque. From there you should be abel to interpolate to get the steady state zero net torque speed.

Regards, Glenn
  Reply With Quote

Old   March 25, 2004, 07:10
Default Re: very confused problem,Please help me
  #3
jiaye gan
Guest
 
Posts: n/a
Hi Glenn Horrocks :Thanks for your returns indeed. (1)I'm a beginner in learning CFX5.5. I got confused for your returns at that why we need to get the zero net torque. Could it explain that we can specified a rotational speed first in hydraulic problem? (2)Could you tell me how can I use CEL to get the net torque?

Thanks a lot.
  Reply With Quote

Old   March 25, 2004, 11:25
Default Re: very confused problem,Please help me
  #4
BAK_FLOW
Guest
 
Posts: n/a
Hi,

yes it looks like you need to take a step back to the basics. The reason we have to specify the rotational speed first is because we solve the rotor flow field in a coordinate frame which rotates at the rotor rotational speed. This greatly simplifies the problem because (in the limit of being far away from any stationary components) the problem becomes steady in that frame. Since this is a non-inertial frame there must be some pseudo-forces added (in the steady rotating case they are Coriolis and Centripetal forces).

The only way to solve a problem in this frame (to date anyhow) is to guess the rotational rate, set up the equations in that frame, do the calculation to determine the forces or flow rates. From this calulation we can then adjust the rotational rate, do the calucation again, and so on....see.

For some of the fundamentals a good place to start is some of the papers written by Bill Dawes or John Dneton in the 1980's in ASME Journal of Turbomachinery. I don't have the exact reference handy, however. Maybe someone else on here has the detials.

I hope this is a good starting point.

Regards,

Bak_Flow
  Reply With Quote

Old   March 25, 2004, 17:23
Default Re: very confused problem,Please help me
  #5
Glenn Horrocks
Guest
 
Posts: n/a
Hi Jiaye,

When the impeller has zero net torque it will not accelerate or decelerate but rotate at constant velocity. Hence this will be the steady state speed.

If you are a beginner at CFD then I would have a look at BAK_FLOW's posting as he summaries why it is far easier to use a constant speed (even if guessed).

A question for the CFX gurus out there: If you wanted to run this simulation where it found the steady state speed by converging to zero net torque on the impeller, how could it be done? You could do it in fortran, but can anybody think of a way of doing it in CEL? You would need the rotational speed in the last iteration so you could apply a correction to it and converge to the steady state value. Any ideas how you would get this variable from the last iteration in CEL?

Regards, Glenn
  Reply With Quote

Old   March 26, 2004, 12:02
Default Re: very confused problem,Please help me
  #6
jiaye gan
Guest
 
Posts: n/a
Hi Glenn I also have no idea about questions. I hope somebody can help us.
  Reply With Quote

Old   March 26, 2004, 12:02
Default Re: very confused problem,Please help me
  #7
jiaye gan
Guest
 
Posts: n/a
Hi BAK_FLOW: Thanks for your information.Now I get some rough idea that how to solve my problem.And I also know that it is a difficult job.
  Reply With Quote

Old   March 27, 2004, 09:54
Default Re: very confused problem,Please help me
  #8
gg
Guest
 
Posts: n/a
Hi Glen,

I think this is not right way because turbine power equals to P=T*omega (or torque times rotational speed). So if torque goes to zero then power also goes to zero and hydraulic machines are not able anymore to produce electricity.

Right way is to determine total pressure at the runner inlet or even better at the spiral-casing inlet and define static pressure at the draft tube outlet.

Regards, gg
  Reply With Quote

Old   March 28, 2004, 11:06
Default Re: very confused problem,Please help me
  #9
jiaye gan
Guest
 
Posts: n/a
Hi BAK_FLOW:

Here I am again.When I got the forces or flow rate form the calculation,how to ajust the the rotational rate according to it.Could you explain it detailly.Should I follow Glenn Horrocks' introduction?
  Reply With Quote

Old   March 29, 2004, 00:42
Default Re: very confused problem,Please help me
  #10
Glenn Horrocks
Guest
 
Posts: n/a
Hi gg,

Of course you have to include the shaft torque in summing the torques. Then the shaft power is included. When there is zero net torque on the impeller then the impeller will not change velocity.

Glenn
  Reply With Quote

Old   May 7, 2004, 16:07
Default Re: very confused problem,Please help me
  #11
Robin
Guest
 
Posts: n/a
Actually, in a hydraulic turbine the speed is NOT determined by the flow, but rather by the generator. A three-phase motor will operate at a fixed speed based on the number of poles and the frequency of the grid. What will vary is the flow rate and power output.

Regards, Robin
  Reply With Quote

Old   May 8, 2004, 03:06
Default Re: very confused problem,Please help me
  #12
Mangnan
Guest
 
Posts: n/a
Hi Robin:

My model is similar to the hydraulic turbine,and it has no generator to output.So the constant speed cann't be got.Is there another way to get it?It might be get from torque calculation.But to get the zero net torque is not an easy job.It comes back to my problem again.I think the design of hydraulic turbine has been developed for decades.There might be some ways to solve this problem.

Regards, Mangnan
  Reply With Quote

Old   May 11, 2004, 11:10
Default Re: very confused problem,Please help me
  #13
Robin
Guest
 
Posts: n/a
Hi Mangnan,

You could add expressions for torque to your solver run and compute the rotation rate accordingly. However, what one would typically do is run a speed line and determine where the operating point falls.

Regards, Robin
  Reply With Quote

Reply

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
Confused with solvers / Radiaton model mixanologos OpenFOAM Running, Solving & CFD 3 October 25, 2011 06:58
Confused about how these variables are used gchnhn Main CFD Forum 0 October 3, 2011 03:44
Questions about tutorial from User Guide :confused: inginer OpenFOAM 4 May 25, 2010 15:10
confused on messages after solver has terminated mactech001 CFX 1 April 21, 2010 16:58
confused by "periodic Non-Conformal Grids" turbinesv FLUENT 1 February 12, 2009 10:00


All times are GMT -4. The time now is 06:40.