Refer the book "Gas Turbine Powerhouse: The Development of the Power Generation Gas Turbine at BBC-ABB-Alstom" by Dietrich Eckardt (available at https://books.google.com/). On page 275 a reference of GT8 rotor damage due to high slenderness ratio is given. I could not find much about such effect in open literature. Does anybody know what is effect of rotor slenderness ratio (L/D) (or rotating body in general-any papers/references)? Few more specific questions:
- What will be the effect of slenderness ratio (other than regular rotor dynamics parameters i.e. critical speed, unbalance response and catenary) on the behavior of steam turbine rotor?
- Is there any allowable limit of slenderness ratio for a steam turbine rotor?
Regards
Naveen Bhatt
Hi Naveen,
I can't specifically answer your question, but I just finished my PhD in a related area, looking at buoyancy-driven thermal rotor bow post-shutdown, in which I performed a parametric study using simplified models with CHT CFD coupled with FEA.
I found that L/D ratio is a pretty important driver, with increased L/D resulting in more severe thermal bow response severity and duration. Interestingly, increasing the length of the rotor will clearly reduce stiffness, resulting in a more severe thermal bow response; however, the diameter of the rotor is not only linked to the stiffness, but also to the fluid-dynamics which drives the buoyancy-driven heat transfer, such that an increase in diameter results in a stiffer rotor, but also a more severe temperature difference due to buoyancy. As it turns out, mid-range diameters are the worst, as they are big enough to generate a large temperature difference due to buoyancy, but small enough so as not to be too stiff to deform under the asymmetric thermal load.
You can read my thesis and my published works via my researchgate profile. If you have any questions, please let me know.
Cheers,
Evan Smith
Dear Evan,
Thank you for replying and sharing your thesis.
You gave a very important and valid point, but as per my understanding not much applicable to GROUND BASED turbine rotors.
This will be a very severe problem in the rotors of reaction steam turbines where rotors are of around 700-1000mm diameter and 5-8 meters in length. In super critical thermal power plants rotor inlet temperature of HP rotor is around 565-600 deg C. If after shut down these rotor stop, these will bow due to several reasons (including the problem mentioned by you). To avoid this, in ground base thermal power plants rotors are always rotating, they never stop (I think not possible in aero engines). In India grid is at 50HZ and hence the turbine during operations run at 3000 RPM. During no load conditions they rotate at 60-100 RPM with the help of a hydraulic turning motor.
But we have found the deformation problems in casings during shut down (we call it top-bottom temperature difference).
Regards
Naveen
Dear,
you can see the following doc.
Article The effect of shear and rotary inertia of a rotor at its cri...
Dear Dr. Ali,
Thank you very much for sharing the information. Got soft copy...will go through it.
-Naveen
Hi Naveen, thanks for your feedback.
I discuss barring or geared motor operation at length in my thesis, and my recent Aeronautical Journal publication. I also discuss case bow (case thermal distortion). I am confident that the results of my thesis are applicable to all turbomachinery (air and ground), as the study was intentionally simple to ensure that the results would be broadly applicable to all gas turbines.
I understand your point that the use of geared motor operation or barring prevents the rotor from bowing; however, I would consider this to be a procedural control, and doesn't change the fact that without this method, the rotor would still bow, and the bow behaviour of the rotor would be different depending on the parameters that I discuss in my research. Perhaps it is a moot point, as the barring is clearly a reliable way of preventing rotor bow, and this is why my research focused on aeronautical gas turbines, where this method is not currently in use.
With respect to case distortion, you might be interested in some work done recently by Dr Monika Topel. Article Operational Improvements for Start-Up Time Reduction in Sola...
Thanks for the interesting discussion.
Cheers,
Evan Smith
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