Low Pressure Turbine vs. High Pressure Turbine
 

Hello!
I have a general question regarding about the number of stages in Low Pressure Turbine (LPT) and High Pressure Turbine (HPT).

Why are there more number of stages in LPT than HPT?

As an example, General Electric CF34 Engine, there are:
1 stage of Low Pressure Compressor (FAN)
10 stages of High Pressure Compressor

4 stages of Low Pressure Turbine
2 stages of High Pressure Turbine

Can anyone explain why LPT have more stages than HPT?
Thank you.

In simple terms, although purists will describe it in a better manner:

The HPT has taken off a big chunk of the kinetic energy (to drive the High Pressure Compressor).

In order to have sufficient/more energy to drive the Low Pressure Compressor/Fan, you need additional set of blades to increase the surface area, since there is less kinetic energy available past the HPT.

Makes sense? :)

Because the pressures are lower and it spins slower while still fitting in a fixed pod area.

I wonder just what dimension fixes the stage (blade hub-tip ratio) size that prevent a single stage from producing the same output? in a LPT

anybody?

Just for the record, there have been several versions of the CF-34, with differing numbers of discs in each compressor or turbine stage. Current CF34-10 has:

fan - 3 LP compressor discs - 9 HP compressor discs - 1 HP turbine - 4 LP turbines.

However, your question still applies.

Basically, you are seeing the Law of Diminishing Returns in action.

As air is compressed coming into the combustion chamber, each compressor disc is having to add more and more compression to already compressed air. So it takes a lot of stages to produce the last few percent of "squeeze" before combustion.

Once the fuel/air burns, the hot flow out of the combustion chamber is at its peak pressure, so it takes only one (or two) HP turbines to extract enough power to drive the HP compressor.

By the time the air gets to the LP turbines and the pressure is dropping, it takes 4 stages to extract every last drop of power from that low(er)-pressure exhaust.

Which is the important power from the point of view of flight thrust, since it is what drives the fan, which is producing 75-85% of the total thrust (5:1 bypass ratio).

In other words, the LP turbines have to do most of the work, with less pressure available. So more turbines.

The HP turbine has less work to do (it is essentially just a "gas generator", to use the turboprop term) with more pressure available, so you only need 1 or 2 discs.

pattern_is_full,
Great summary!

Lomapaseo,
The expansion of the hot flowpath gas passing through the HP turbine and then entering and passing through the LPT pretty much sets blade hub to tip size of the LPT. The trend is towards higher LPT temperature capability, higher stage loading, fewer airfoils and fewer stages, but that is not always possible to achieve. Higher bypass ratios, i.e. larger diameter fans, require more LPT stages to fully extract the energy from the flow path gas. Aerodynamic limitations prevent a one stage LPT from consideration in a high performance LPT that drives a high bypass fan and booster.

Turbine D

I could easily assume that as well, but just what aerodynamic limitation that can't be accomodated by design as a single stage?

I can make up a word or two, but I would rather hear it from an expert :)

Thank you everyone for the explanation.
I now have much clearer understanding of it.

And thank you pattern_is_full
That is a one awesome explanation!.

Lomapaseo,
Err, think about it. What single LPT turbine blade could aerodynamically convert all the gas path energy exiting from the HPT into torque powering a 115" diameter fan and a three stage booster? What would the airfoil look like? How big would it be? What material would it be made of to withstand the heat? What would be the efficiency compared to a 4 or 5 stage LPT? Why don't we have single stage LPTs on high bypass engines after all these years? Let me know what you come up with.:hmm:

Oops, I was not thinking about a large Fan engine since the OP only introduced a small engine multi-stage LPT as the subject.

I still don't know what the limiting dimension is regarding my question. Obviously it hasn't been overcome by the manufacturers, I just want to know what part is key. I would of course accept that an assumption of a current HPT exit temperature and pressure should be presumed.

really, it is quite similar to the triple expansion steam engine used in some naval ships (battleships of the WW1 time frame for example).

Try too understand and expand upon this as your homework.

I reckon, it has a lot to do if we are talking about a fan engine or a pure turbojet engine. As mentioned, the fan of a high bypass fan engine produces most of the thrust at take off/low altitude. That ratio reverses somewhat at high altitude where the core produces relatively more thrust than the fan.

Looking at a pure twin spool turbojet like the Olympus, the compressor has 6 LP pressure stages, 8 HP stages and single stage HP and LP turbines. Quite a few LP stages when compared to modern fan engines.

On the side, technically, the rotating compressors add speed to the incoming air and the stators convert this speed to pressure.

Really? I would have thought that the thrust per pound of air is constant across hot and bypass streams for optimum propulsive efficiency.

Glendalegoon's triple expansion steam engine comparison is the first thing that occurred to me too, however another analogy would be a wing with a multi-stage flap.

Each aerofoil can only deflect air down (thus generating lift) by so much before it experiences separation (stalls). If you want maximum lift (on the wing & flaps) or power extracted from the turbine, you need more stages.

A four stage LP Turbine could be compared to, say, an A321 wing with a three stage flap (along the lines of the bottom image).

As the air leaving the aerofoil ahead is deflected, the following one needs more incidence or pitch to extract lift from the downward moving airflow.

http://static.howstuffworks.com/gif/airplane-flaps.gif

I was just thinking that turbines' response* might have an effect on the design as well. Its easier to 'spin up' the HP compressor and turbine shafts independently compared to the large diameter fan, compressor stages and all the turbines at once.

*A significant issue when starting.

@Gysbreght,

I rekon, I should habe been more specific. I was not talking about bypass ratio but thrust ratio.

Flying magazin May 2003, page 56: Flying Magazine - Google Books

The stages, of course have stators in between to straighten the flow out. Perhaps flow over a large single stage turbine would be swirling too much opposite the direction of rotation before it exited the blades.

The lift to drag ratio on the blade section for a single stage turbine may be worse than the cumulative lift to drag ratio of the series of blades in a multi stage turbine. I would guess that a single stage blade would need to be heavily undercambered to be effective, which may make it fairly draggy.

lomapaseo mentioned it early on -

Because of the large high-bypass fan (large relative to the core diameter), and its tip speed limits (acoustics, etc.), the LP shaft must turn a lot slower than the HP. This limits the amount of work a turbine stage can extract from the gas stream - ergo multiple stages are needed.

UNLESS - someone introduces a reduction gear in the LP shaft. Now the turbine can spin up to a speed where only 1 or 2 stages can do the work, still keeping the fan at a happy slower RPM.

Then, the only problem is the weight & complexity of the gears, and just one more "feature" to go wrong. :ooh: