Understanding Repair Limits for Air Seal Serration in Gas Turbine High Pressure Rotor Sections 3

[HighPressureProfessor]

Hello,

I am looking for some general insight on the function of “knife” edge Air Seal Serrations in Gas Turbines.

I have recently stumbled upon two components in the same High Pressure Rotor Section of a Gas Turbine engine, both of which are capable of maintaining engine service parameters while missing material (blends) from their Air Seal Serrations.

The difference is one component has a limitation for the number of serrations that can have missing material in line axially, while the other does not.

There are a total of 12 Seals with axial limitations that control the CPD Air Flow through the High Pressure Rotor Section. There are a total of 2 Seals without axial limitations which prevent leakage of cooling air needed for the rotor blades that receive the heated gas directly from the combustion system.

My questions are:
If the seals are meant to cut into an abradable material to effectively act as a narrow labyrinth to slow air flow, why do some components allow for axial in line blends while others do not?

Is there a way to quantify the amount of “leakage” for 2 serrations with blends axially in line versus 2 serrations with blends separated by X amount of circumferential distance?

This is my first post and I have only been in in the Gas Turbine Analysis field for a little over a year. Any general concepts or mentoring on Seal Serration Air flow would be IMMENSELY appreciated.

Cheers,
HPProfessor

 

[WKTaylor]

This all sounds like the science and art of the engine OEM... and likely highly Proprietary.

As You are likely aware...

When the engine is run for the first time [new/overhaul]... the extremely close fit of the blade tip to the surface of the seal material will be grooved when the blades stretch and bend slightly forward by centrifugal and blade thrust forces and thermal strains. This forms the seal groove track. I am sure the OEM has the entire circumference of the flow-path mapped with high precision. I have to stop here in my description.

Regards, Wil Taylor
o Trust - But Verify!
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation, Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", HBA forum]
o Only fools and charlatans know everything and understand everything." -Anton Chekhov

 

[HighPressureProfessor]

Hello Wil!

Thank you so much for the response.

I hope I didn’t cause any uneasiness about proprietary information. This example of two components in the High Pressure Turbine Section is seen across many different Gas Turbine Engine Models and OEMs. The very vague description I gave can be found in descriptions and operating manuals for many different Models / OEMs which are free to the public online and non-proprietary.

The truth of the matter is, I work with many different components across many different OEMs which all have different limitations for seal serration repair limits. I felt the example I provided would give a good illustration and provide some general information on function while limiting some of the variables another engineer might question such as engine rating or engine section.

That being said the question I have is not specific to one model nor is it relevant to proprietary information. I have found similar components in the same engine, and similar components across different OEM models all having substantially different repair limits.

Unfortunately the date on the repair revision seems to be a large factor for why the repairs are worded and restrained so differently (i.e. repairs last revised on 19XX are closely defined to other repairs from 19XX, but are wildly different than repairs last revised in 20XX).

That is why I am looking for general insight on the function of seal serrations and how missing material across the same axial band might affect their function.

There are plenty of other free non-proprietary sources of information about the function of “knife” edge seal serrations in Gas Turbine Engines, for example, U.S. Patents. However, I have had a very difficult time finding technical data on my particular “Axially in-line” question, hence why I am here.

I can say with confidence that almost all OEM components I have seen, allow for some deviation from new make for repair/overhaul that allows for material to be missing. The true numbers for the amount varies which I understand is in the art and design of the engine.

What I don’t understand conceptually and what leads me back to my original question, is how air flow differs when two serrations are missing material axially in-line versus two serrations missing material X circumferential distance apart.

Again, the seal serrations are meant to restrict airflow by creating a slight gap between the serration and the abradable coating. Therefore, the flow path is always continuous through this gap, 360 degrees around that seal. So wouldn’t the amount of “extra” air flow through two serrations with missing material axially in-line, be the same as two serrations with missing material x circumferential distance apart?

Cheers,
HPProfessor

 

[3DDave]

When you ask "how airflow differs," what form would than answer take? That's a tremendously complex flow field to describe that will vary considerably over the operating envelope.

 

[SWComposites]

yeah, probably needs a very complex, detailed CFD analysis to get any sort of insight.

and repair/rework manuals are often defined very conservatively; if there isn't good data to justify a condition, then it isn't allowed.

 

[HighPressureProfessor]

Hey Dave and SW,

Perhaps my lack of experience has lead me to ask a question that general theory is not applicable to. I apologize if that’s the case.

I did not major in fluid mechanics so my background knowledge has some holes in it (no pun intended).

Let me try to simplify the question.

Let’s say I have a Red cylindrical tube with 2 Green walls in it that break it into 3 chambers of equal volume. Let’s label them Chambers A, B, and C from FWD to AFT (left to right). The middle chamber (Chamber B) is the space between the two Green Walls. There is a pipe supplying an arbitrary gas on the FWD end of the tank (intake) and a pump on the AFT end (exhaust).

The 2 Green inner walls (serrations) are centered on a rotating axis and have a diameter that is 0.001 inch smaller than the diameter of the Red inner wall of the tube. This way gas is able to fill each chamber, but the Green Walls are able to spin in tandem as gas is being pulled through the tube by the pump.

Now I duplicate this set up so I have 2 of the exact same flow systems.

In Tube 1: I punch 2 holes of the exact same size, 1 in each Green Wall. 1 hole is punched at the top of the FWD Wall and the other hole is punched in the bottom of the AFT Wall (180 degrees apart). When I look through the Hole in the FWD wall I see the AFT Green Wall because it is solid in line axially.

In Tube 2: I punch 2 holes of the exact same size as Tube 1, again 1 in each Green Wall. This time both holes are punched in the top of the FWD and AFT Walls axially in line. Therefore, if I look through the Hole in the FWD Wall I can see through the AFT Wall as well, and now I see the Pump at the Aft end of the Red Tube.

Question (“How the flow differs”):

I understand that if water were to be used instead of gas and the center axis was fixed instead of rotating, then the flow rate through Chambers A, B, and C would be identical for both Tubes.

What I am not sure of is how turbulent gas in each chamber and rotation of the Green Walls factors into this experiment.

Is the flow rate through Chambers A, B, and C in Tube 1 less than, equal to, or greater than the flow rate through Chambers A, B, and C in Tube 2?

If the flow rate through the Chambers of Tube 1 is less than or greater than the flow rate through the Chambers of Tube 2, then I would say a CFD is necessary for each engine model and this discussion can be concluded.

If the flow rate is the same however, then why would there be restrictions for axial in line holes on the Green inner walls of the tank (serrations)?

If the flow rate (leakage) is no different whether the holes are 180 degrees apart or axially in-line then “windage” would not be a factor and there should be no loss in form, fit, and function correct?

The only other factor I can think of that may play a role in limiting axially in-line holes is balance. That being said, I would like leave that variable out of the equation for now since I am only focused on how the flow path is affected.

Cheers,
HPProfessor

P.S.
I apologize for the book. This question has been haunting me for some time now and I need some form of clarification at this point to preserve my mental health (LOL).

 

[HighPressureProfessor]

Sidebar:

I am trying my best to illustrate how I am conceptualizing the serration flow path in my head.

PLEASE, feel free to point out any and all flaws in my analogies/comparisons so that I might gain a better understanding of the intent / theory behind serration flow pathing.

- HPProfessor

 

[SWComposites]

HPP - you will need to hope a very experienced engine fluids dynamist / designer is on this forum to answer these questions (its not me, I’m a structures guy).

 

[WKTaylor]

HPP... Your 1 Jul 23 17:40 post...

Your example has me confused on many elements.

As You began referring to 'water' this example broke down for a gas turbine in the flow path.... the reference to tubes and water distracted me into thinking fuel flow... more-so than air flow.

.. a picture would clarify.

Guys... is there a forum in ET that encompasses gas turbine engines or rotary compressors or turbochargers??

Regards, Wil Taylor
o Trust - But Verify!
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation, Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", HBA forum]
o Only fools and charlatans know everything and understand everything." -Anton Chekhov