PW150A Compressor/Turbine coupling

[jpilotj]

I have a simple question on the PW150A however I have had a bit of trouble finding the answer.

I understand that the PW150A constitutes A 3 Stage axial and 1 stage centrifugal compressor driven by a single stage high pressure and single stage low pressure turbine, following on to a twin stage free power turbine.

Is the single stage centrifugal compressor driven by the low pressure turbine or the high pressure turbine? Which comes first in the airstream? For what reason is the coupling designed in this manor? My thoughts would be that you could generate more mass airflow from a centrifugal compressor spinning at a high velocity than an axial compressor spinning at the same velocity???

Cheers for any help,

jpilot

[aerobat77]

ahoi !

the centrifugal hp compressor is driven by the hp turbine and comes first after the combustion chamber. then the lp turbine which drives the lp axial compressors and then the free turbine stages wich drive the prop via gearbox .

the engine is so a three shaft - free power tubine design. the goal is to provide optimum rotating speeds for each section and so improve efficency as well easy starting.

the free power tubine design, where these shaft does only drive the prop, allows for a wide range of prop rpm changing and a quiet cruise .

in theory-there are very succesful and fuel efficient single shaft turboprops like the tpe331 , the allison T56 or the massive russian NK-12 .

[barit1]

All else being equal, a single-shaft machine requires more energy for starting. OTOH an airstart is easier - the prop supplies the energy.

[lomapaseo]

All else being equal, a single-shaft machine requires more energy for starting. OTOH an airstart is easier - the prop supplies the energy

But where does the prop energy go?

[rigpiggy]

to drive the compressor fast enough to restart, .something like 75% of the energy in the fuel is used to drive the compressor

[lomapaseo]

to drive the compressor fast enough to restart, .something like 75% of the energy in the fuel is used to drive the compressor


But what compressor? since the subject thread is about a free turbine driven machine.

I believe that inflight windmill restarts are predicated on the pressure recovery from the flow ramming through the compresors and of course by any turbining that the turbines connected to these compressors can provide.

[aerobat77]

lomapaseo, barit talked singleshafts and not free turbines regarding the airstart.

of course it mixes it up since airstarts on singleshaft designs are not the question here.

[lomapaseo]

lomapaseo, barit talked singleshafts and not free turbines regarding the airstar

I missed that thread shift, like the guy going over the cliff I never saw that ewe turn.

Seeing as this is a tech forum, twists and turns are always encouraged :)

Let's continue with a similar discussion on tripple spools big fan engines. I wonder how important the windmill speed on the fan is? If only the fan was severely damaged would that have an impact on an engine windmill restart

[peter kent]

Hi jpilotj,

Please see the linked paper as the most readable account of what the PW150 is and how it came about.

It's not likely to answer anyone's specific questions but is good background on the engine.

http://ftp.rta.nato.int/public//PubFullText/RTO/MP/RTO-MP-008///$MP-008-04.pdf

Regards

pk

[jpilotj]

thanks for the link

[jpilotj]

Let's continue with a similar discussion on tripple spools big fan engines. I wonder how important the windmill speed on the fan is? If only the fan was severely damaged would that have an impact on an engine windmill restart

presumably exactly the same scenario as a free power turbine turboprop. After all the configuration is essentially the same bar a few extra turbine stages and a few extra compressor stages...and rather than continuing to a reduction gearbox it is just direct to the fan?

[pppants]

Let's continue with a similar discussion on tripple spools big fan engines. I wonder how important the windmill speed on the fan is?
If only the fan was severely damaged would that have an impact on an engine windmill restart?

From your quote above you should be asking,why would you restart an engine with a damaged fan?

[peter kent]

QUOTE]My thoughts would be that you could generate more mass airflow from a centrifugal compressor spinning at a high velocity than an axial compressor spinning at the same velocity???[/QUOTE]

I'll have a go at answering this but I'm not sure exactly what you are looking for.
Since the centrifugal and axial compressors both do the same thing to the air, ie add kinetic energy and then diffuse it to higher static pressure you can make them both pass the same flow at the same RPM. You 'just' have to dimension the impeller appropriately and choose the right number of appropriately dimensioned axial stages.

[lomapaseo]

You 'just' have to dimension the impeller appropriately and choose the right number of appropriately dimensioned axial stages.


For the same output the centifugal gives a lower length while the axial gives a lower diameter.

yet another trade are the duct losses with a multi stage centrfugal to achieve very high burner/turbine pressures.

So, if you already have a large diameter face with a prop, and moderate pressures to achieve the power to weight, than the centrifugal is attractive.

If you want very high pressures vs length than you want axial stages.

[barit1]

If you want very high pressures vs length than you want axial stages.

I'd say - If you want very high pressures vs DIAMETER then you want axial stages. This was the reason centrifugal compressors in fighters were passe by 1950.

(Well, there were a few holdovers - F9F, F-80, Mig-15...)

[peter kent]

jpilotj

Is the single stage centrifugal compressor driven by the low pressure turbine or the high pressure turbine? Which comes first in the airstream? For what reason is the coupling designed in this manor?

As already posted the centrifugal is the last compressor stage or HP compressor and therefore driven by the HP turbine.
The traditional view of a centrifugal being bigger dia than axial for same flow has to be modified when it is used in combination with axial stages as in the case you are interested in.
As the air goes thro the axial stages the density goes up and the volume goes down, so the final stage, the centrifugal, handles a lower specific volume (cu ft per lb) than the front, so it is significantly smaller than it would have to be as a first stage. How much smaller can be seen by looking at a cross section of a PW120 which has 2 centrifugals in series.
See my previous link.

The attributes of the centrifugal, high stage pressure ratio in short length, are no longer penalised by big dia. In fact if you look at the PW150 you see the impeller tip dia is the same as the first stage axial blade dia.
So a cf is a good way to bring up the rear of a compressor with a high stage pressure ratio and not much length.

Admittedly, the impeller is only half the centrifugal stage and the other half, the diffuser pipes, do increase the dia but this configuration of a few axials followed by a cf is very attractive not only for props. Many business jet fan engines have the same arrangement.