Stator blades in turbine engines
 

I was told the stators stop compressed air from slipping through the gap between the rotors and the engine case. But how does it works actually?
Also learnt that the clearance between the rotors and the case is controlled by bleed air driven devices, can someone explain please?
Many many Thanks.

cx007

In my experience, the reason for having stators is to straighten or align the airflow ready for the next set of rotating blades, be they compressor or turbine. With reference to the gaps around the base of the disks, there are seals on some engines to prevent flow to the lower pressure stages. The design of the static structure in a gas turbine is extremely complex since all the services required to run an engine need to pass through - such things as oil pressure and scavange, seal pressure balance air etc etc Not as obvious as the rotating machinery but equally important.

As for clearance control on turbine blades, I have seen systems that flow air externally along the case to give "active clearance control" i.e. this system keeps the growth of the case less than the growth of the blades thus closing the tip to case gap - clear as mud eh? I'm sure there are many other ways of acheiving the goal of leakage reduction and each manufacturer probably has their own favorite method.

Hope my ramblings make sense!

Cheers

The bleed air thingy is for the turbine and essentially controls the thermal expansion of the cases vs the rotor by cooling the cases in the turbine until the rotor has a chance to heat up the thick disks.

The stators themselves are vanes and are there to straighten out the swirl coming off the blades. The stators however are part of a case structure which does control the clearance between the blades and case. How it controls is pretty complicated since there are build clearances (blade to blade length), Thermal growth differences like the turbine and structural distortion caused by engine loads being unequal passing between the mount structures along the engine.

The most effective way to minimize some of these clearances is to allow them to be negative under some conditions and let the blades cut a swath through an abradeable material.

You wouldn't go far wrong in investing in a copy of the Rolls-Royce book of the jet engine:ok:

boeing has some trick inlet guide vanes that the use in internal engine ducts to suppress noise - Ive heard that the oil companies use the same concept oil on distribution pipelines to control eddy current erosion .. only difference is the fluid density is different ...:}

Some additional info of which I'm sure you're aware.

Inlet guide vanes are parallel (leading edge vs. trailing edge). They guide the flow of air onto the first rotor stage.

The rotors and the stators have divergent blades (the leading edge and trailing edges diverge). This creates an increase in pressure re. the purpose of the compressor. (We are talking subsonic airflow here)

blades vs vanes
 

In the compressor (including the fan), the rotor blades impart kinetic energy to the air, which it evidenced by a swirl about the rotor axis (ie circumferential velocity of the departing air).

The stator vane row behind this rotor is configured as a diffuser to slow the airflow down again by turning it back parallel to the rotor axis. In so doing, it converts that excess velocity into a rise in static pressure. Modern engines can achieve a pressure rise of up to 40-50% (absolute pressure) per stage.

Essentially the turbine behaves the opposite; turbine stator vanes (aka nozzle guide vanes, aka turbine diaphragm) increase velocity and turn it in the direction the rotor's going, so the rotor blades can catch some of that energy to drive the compressor & accessories.

OK?

:ok:

Thanks everyone for sharing your knowledge. Have learnt a lot here.
Cheers.

Hang on cx007, we haven't finished with you yet!!!!.

The G.E. CF-6's, CFM-56 and PW4000 (Guess who's just finished an A330+A340 course!!!!) use cooler air bled from the compressor to cool the turbine case, causing it to contract and reduce the clearance between it and the turbine. This reduces the "tip losses" you discribe.

Rgds Dr I

Still not finished... whilst the RR book is good, a better bet IMHO is 'Gas Turbine Engine Technology' by Treager - go to Amazon and search on 'Treager'.

R1

This is PPRuNe at its best...

Intelligent questions, informative replies, and a generous sharing of expertise.:ok:

Shamrock 602

U guys are incredible
 

Excellent!
You guys are really helpful, will look at that book and see what questions can i dig out.

I was reading RR website, found that the high pressure stage of the turbine and compressor of Trent 900 turn in different direction from the rest. How is it possible? I know they have 3 independent shafts, but how can 1 of them turn in another direction? Is it because the direction of the blades of that stage is different?
The site says this configuration can straighten the air flow and thus increase the efficiency. It seems similar to the aim of the stator.
Thanks.

Stator vanes, or to use the more accurate term " guide vanes " have two fundamental functions.

At the cold end, they angle the airflow at the optimum setting onto the face of the rotating blades for both the linear velocity of the airflow, and the velocity with respect to the speed of sound. This prevents compressor stall, and in multi spool engines a la RB211 ( ah, Rolls Royce ) they - along with bleed valve scheduling - prevent compressor mismatch and thus make the stall or surge margines more user friendly.

These cold end guide vanes ( called inlet guide vanes right at the front, or interstage guide vanes further back along the gas path ) are variable in angle, dependent upon spool RPM and a few other things, and there can be many stages of variable guide vanes in the compressor. It is because these vanes are moveable that the term "stator vanes" fell into disuse.

A secondary, but none the less important function is to reduce the velocity of the gas stream and increase the pressure, thus making the compressor stage more efficient.

At the hot end the function is much the same primarily as it is for the cold end, except that the secondary function is to increase the linear velocity of the hot gases to keep them as near as possible to mach 1, thus making the multi stage turbines more efficient and closer matched in effort to each other.

Hot end guide vanes ( called nozzle guide vanes ) are usually fixed, although some high performance military applications are being developed with variable hot end vanes as well.

The overall effect is a much more efficient engine which is also much quieterand less susceptible to surging.

Hope this helps.

NOT anti-semantic!!
 

I don't want to get into a war of words, but some engine people DO still use "stator vanes" - GE and CFMI for example.

Some stators (generally on aft compressor stages) are fixed, while upstream stators are more likely to be variable. The variable stator vanes (VSV's) probably include variable inlet guide vanes (IGV's).

It is the vector analysis of airflow in and out of each stage, at both high and low RPM, and high and low temperatures, that is the key to making engine operation troublefree and responsive.

Barit 1:

I think you'll find that I made those very points if you read the post. Guide vane design is one of the reasons why an RB211 is much less susceptible to a surge than the JT9 for example, despite what GE, P&W or the Franch lot call them. Still, what would I know. I only designed airflow control systems for four years of my life.

Thanks for the correction.

PJ

cx007 wrote:
I don't fly an aircraft with Trent 900s (sadly), but my little RB199s also have the LP and IP turning one way, and HP the other. According to Mr R-R this is "reduces gyroscopic coupling effects and resonant vibration".

I have a picture of the offending article here, and it sure looks like the HP blades are the other way around. Makes sense I guess. Saying that, I'm sure someone who has built / invented / designed these things will correct me!

Hope that's of some help,

Psy

counter-rotation
 

There's a clever synergy in the counter-rotating turbine stages. The swirl leaving the HP stage is just what what the LP rotor stage likes to see, and thus no need for a nozzle guide vane for the LP stage. Better efficiency, simpler, less weight, all that good stuff.

Other benefits are reduced gyro reaction in the engine mounts, plus lesser rotor seizure loads (must design for simultaneous seizure of all rotors without mount failure).

Biggest problem though is the extremely high differential speed of any intershaft bearings.

Now where on earth does it say that is a requirement:confused:

Lompasa,
The bleeds are there to match the compressor to the the turbine
The air from the compressor is only at optimum above cruise power. Below this the compressor is producing more air then the turbine can handle.
Hence at low power they bleed away unwanted air and close as the power ( and turbine demand) increases
:cool:

rotor seizure?
 

Right now I cannot find it in FAR33, but it's been a design requirement on every engine (repeat: Engine) I can recall. It may be a US military requirement carried over as good practice for civil engines.

Don't confuse this with the structural fuse usually seen in engine pylons, which has resulted in some grief on 747's. It is intended to protect the wing spar - maybe not too bad an idea.:ooh: