speedracer1407
Posts: 330
Joined: Fri Dec 24, 2004 4:19 pm

### Axial/centrifugal Engines

Although I feel comfortable with the basics of turbine engine parts/dynamics/characteristics (mostly thanks to a few years of reading informative posts on this forum), a recent thread prompted some questions about what I percieve to be "old" centrifugal jet engines.

Quite simply, what is a centrifugal turbine/jet engine/compressor, and how is it fundamentally different from an axial engine?

Details, no matter how technical, are welcome.

O
Dassault Mercure: the plane that has Boeing and Airbus shaking in their boots.

sovietjet
Posts: 2573
Joined: Sat Mar 15, 2003 12:32 am

### RE: Axial/centrifugal Engines

The basic difference is that in a centrigufal type the compressed air is thrown outward and then redirected parallel to the thurst axis. In a axial type the airflow is always in the direction of the thrust axis. The Mig-15/17s VK-1 engines were centrigufal...most engines today are axial-flow.

Lemurs
Posts: 1320
Joined: Sat Mar 12, 2005 5:13 am

### RE: Axial/centrifugal Engines

Well I'll start off with the simplest possible explanation and let others expand. You've almost certainly seen pictures of an axial jet engine at this point. It has multiple disks of compressor blades, each successive disk increasing the level of compression further until you get to the combustion chambers, and final the turbines to harness the power and run the compressors, etc...

Now, picture a centrifigual jet engine instead as a massively scaled up turbocharger off of a road vehicle engine. There is one central solid compressor disk with vanes that accelerate and compress the air outwards, where it the exits to the combusion chambers around the compressor, which then exhaust over a single turbine to run the whole thing. Dramatically less complex, hence why the first 10+ years or so of jet engine development was centered around centrifugal jets. They were wide, short, and not exactly efficient, but they did the job.

I could be missing something, but I've never heard of a multiple stage centrifigual engine before...I have no idea how that might even work.
There are 10 kinds of people in the world; those who understand binary, and those that don't.

jetmech
Posts: 2314
Joined: Wed Mar 29, 2006 2:14 am

### RE: Axial/centrifugal Engines

A centrifugal compressor works by taking air into the centre of the compressor disk and adding kinetic energy to the air by accelerating it outward (centrifuging) in a direction perpendicular to the spin axis of the compressor. This air is then diffused in a divergent duct which slows down the air, and converts some of the kinetic energy into pressure energy. The air enters a centrifugal compressor in a direction parallel to the spin axis, turns 90 degrees at the beginning of the centrifuging process to a direction perpendicular to the spin axis, and then turns 90 degrees again to a direction parallel to the spin axis at some point before or after the diffusion process.

http://en.wikipedia.org/wiki/Image:D...oblin_annotated_colour_cutaway.png
http://www.tpub.com/content/fc/14104/img/14104_90_1.jpg

An axial compressor works in a similar way, but with two fundamental differences. Firstly, the air travels parallel to the spin axis at all times, secondly, each stage of an axial compressor is made up of a set of rotating compressor blades that adds kinetic energy to the air by accelerating it axially, followed by a pair of stationary stator vanes that diffuses the air and converts some of the kinetic energy to pressure energy. Adjacent pairs of stator vanes form a divergent duct. A complete axial compressor is made up of many of these stages of compressor / stator pairs, thus the diffusion process in an axial compressor occurs at many points, which is in contrast to the centrifugal compressor where the diffusion happens more or less in one location.

http://en.wikipedia.org/wiki/Image:Axial-flow-compressor.png
http://ffden-2.phys.uaf.edu/212_fall...003.web.dir/Erik_Weflen/axcomp.JPG
http://www.aoxj32.dsl.pipex.com/Graphics/AxComp2.gif

A single stage of centrifugal compressor can achieve a pressure ratio as high as 4:1 or 5:1, which is much higher than the pressure ratio of a single stage of axial compressor which IIRC is about 1.5:1 at most. Despite this, axial compressors are used in modern commercial turbofans as they are more efficient than centrifugal compressors, have a smaller frontal area, and I seem to remember that they are more suitable for multi staging. The lower efficiency of the centrifugal compressor may be partly due to the two 90 degree turns made by the air, and the fact that all the diffusion takes place at once, which may be less thermodynamically efficient

 Quoting Lemurs (Reply 2): I could be missing something, but I've never heard of a multiple stage centrifigual engine before...I have no idea how that might even work.

IIRC, the Rolls-Royce Dart turboprop has a two stage centrifugal compressor.

http://ffden-2.phys.uaf.edu/212_fall...web.dir/Erik_Weflen/centrfcomp.JPG

[Edited 2006-09-26 09:22:27]

[Edited 2006-09-26 09:23:44]

[Edited 2006-09-26 09:36:30]

[Edited 2006-09-26 09:39:11]
JetMech split the back of his pants. He can feel the wind in his hair .

Molykote
Posts: 1239
Joined: Tue Aug 23, 2005 8:21 pm

### RE: Axial/centrifugal Engines

 Quoting JetMech (Reply 3):A single stage of centrifugal compressor can achieve a pressure ratio as high as 4:1 or 5:1, which is much higher than the pressure ratio of a single stage of axial compressor which IIRC is about 1.5:1 at most. Despite this, axial compressors are used in modern commercial turbofans as they are more efficient than centrifugal compressors, have a smaller frontal area, and I seem to remember that they are more suitable for multi staging. The lower efficiency of the centrifugal compressor may be partly due to the two 90 degree turns made by the air, and the fact that all the diffusion takes place at once, which may be less thermodynamically efficient

JetMech offers a great summary and I would like to expand on one point only.

One of the disadvantages to the centrifugal compressor is a relatively small mass flow rate compared to an axial design. This makes a centrifugal compressor more appealing for smaller applications and less so for larger ones.
Speedtape - The aspirin of aviation!

Starlionblue
Posts: 17632
Joined: Fri Feb 27, 2004 9:54 pm

### RE: Axial/centrifugal Engines

 Quoting Molykote (Reply 4):One of the disadvantages to the centrifugal compressor is a relatively small mass flow rate compared to an axial design. This makes a centrifugal compressor more appealing for smaller applications and less so for larger ones.

I presume this is why turbochargers tend to be centrifugal. High compression ratios, single stage, "small application".

In computer parlance, there are two types of air impellers:
- Fans, which blow air through perpendicular to the plane of the impeller. This is axial flow.
- Blowers, which take in air from the "front" and blow it out parallel to the plane of the impeller. This is centrifugal flow. Blowers are commonly used in laptops and such when there is no space to mount a fan "upright".
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo

A342
Posts: 4017
Joined: Sun Jul 31, 2005 11:05 pm

### RE: Axial/centrifugal Engines

I have also something to add to the discussion.

Centrifugal compressors are still extensively used in small turboshafts which power helicopters, in larger engines coupled with axial compressors in front of the centrifugal one.

As I have read in an article about turboshafts, the invention of new manufacturing techniques and 3D computer airflow modelling has led to a comeback of the centrifugals. They can be manufactured in just a single part, therefore making the engines simpler and easier to maintain.

I have also read that SNECMA and another firm will produce a new turbofan engine for business/regional jets. Its maximum thrust will be about 55kN, and it will feature a centrifugal compressor !
Exceptions confirm the rule.

Starlionblue
Posts: 17632
Joined: Fri Feb 27, 2004 9:54 pm

### RE: Axial/centrifugal Engines

Some computer pics to illustrate. These show clearly the difference between axial and centrifugal flow.

Fan with axial flow: http://www.newegg.com/Product/ShowIm...perFlo+Case+Cooling+Fan+%2D+Retail

Blower with centrifugal flow. There is an arrow bottom right showing where the air flows out:
http://www.newegg.com/Product/ShowIm...A+120mm+Power+Motor+Blower+%2D+OEM
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo

N231YE
Posts: 2620
Joined: Tue Jul 25, 2006 9:24 am

### RE: Axial/centrifugal Engines

In addition, I have read that an axial flow compressor is actually lighter than a centrifugal flow compressor, with all of it diffuser ducts, etc, added in. Kind of like the early can combustors were replaced by the more modern annular-type.

This is a DeHavilland Goblin centrifugal-flow turbojet, from Wikipedia.

And an axial flow of a GP7000, from Pratt & Whitney:

Starlionblue
Posts: 17632
Joined: Fri Feb 27, 2004 9:54 pm

### RE: Axial/centrifugal Engines

 Quoting N231YE (Reply 8): This is a DeHavilland Goblin centrifugal-flow turbojet, from Wikipedia.

Nice pic. Looks like a Swedish AF Goblin.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo

sovietjet
Posts: 2573
Joined: Sat Mar 15, 2003 12:32 am

### RE: Axial/centrifugal Engines

There was also something about centrifugal jets that made them more reliable in terms of FOD damage. Maybe I'm wrong but I'm stuck with the impression that you can throw alot of stuff in a centrifugal engine without it breaking. I made this conclusion based on many stories I've read and heard about from actual pilots about the Mig-15/17 taking off from grass fields and such with foreign objects clearly seen sucked in and yet no damage at all. Maybe I'm wrong....

N231YE
Posts: 2620
Joined: Tue Jul 25, 2006 9:24 am

### RE: Axial/centrifugal Engines

This should solve all of the questions. I'm quoting Jet Engines, by Klaus Hünecke.

Page 89;

 Quote:A major benefit of the centrifugal compressor is a large pressure ratio per stage (of the order of 5:1), and a relatively low-cost manufacture-attractive features both for the small engine market and the automotive industry [turbochargers].

Page 90;

 Quote:Most present-day turbo engines for aircraft employ axial compressors...The principle advantage of the axial compressor is its ability to deliver high mass flow rates together with large pressure ratios at the same time-features which the centrifugal compressor, due to its method of compression, cannot provide. The axial flow compressor is also beneficial: -internally, because the air flows in a uniform direction which eliminates the need for turning the flow -externally, because the smaller-cross section reduces aerodynamic drag of the engine nacelle.

Hünecke, Klaus. Jet Engines. Motorbooks International Publishers & Wholesalers. Osceola: 1998.

A342
Posts: 4017
Joined: Sun Jul 31, 2005 11:05 pm

### RE: Axial/centrifugal Engines

 Quoting A342 (Reply 6):I have also read that SNECMA and another firm will produce a new turbofan engine for business/regional jets. Its maximum thrust will be about 55kN, and it will feature a centrifugal compressor !

Here we go. I was a bit wrong about the thrust level, but anyway:

http://www.ainonline.com/Issues/02_06/02_06_snecma_1.htm

 Quote:The company's primary target for the SM-X is business aircraft. "The typical opportunity will be a large cabin/long-range aircraft with a maximum takeoff weight between 50,000 and 60,000 pounds," it stated. "The engine design will also meet the needs of 40- to 60-seat regional jets," the company said. ... Unusually, the SM-X core will feature a single-stage centrifugal com- pressor coupled with a four-stage axial compressor. Snecma will no doubt consult sister company Turbomeca for its small engines centrifugal compressor experience from the helicopter powerplant field.
Exceptions confirm the rule.

Lemurs
Posts: 1320
Joined: Sat Mar 12, 2005 5:13 am

### RE: Axial/centrifugal Engines

I remember reading an article about a light jet startup from a few years ago that proposed to build their own engine too...they had a really radical 3 stage axial flow engine that they were working the bugs out of when they went under and it never made it to market. The whole thing was tiny...the size of a suitcase and not more than 90 pounds or so, but could pump out more than double or tripple the thrust of a similar size/weight centrifigual engine for that configuration, and lower fuel burn to boot.

Maintnance on it would have been a bit of a nightmare though...how many microscopic parts do you need to make such a tiny 3 stage axial flow engine? I have to see if I can find the article now, because I know this sounds insane otherwise. Anyone know the company I am talking about?
There are 10 kinds of people in the world; those who understand binary, and those that don't.

Yikes!
Posts: 284
Joined: Sat Oct 13, 2001 4:51 pm

### RE: Axial/centrifugal Engines

If I recall, the Garrett's on the BA41 (piece of ... crap) ((I can't use the phrase BA41 without adding some description of what I think of it...)) had dual centrifugal compressors. Makes for a tight engine. Couldn't knock the power output from such a small power plant. Too bad it was attached to such a miserable airframe!!

Problems arise when you mix centrifugal and axial compressors on the same engine (PT6). Axial flow compressors have a relatively straight line performance from low RPM to max RPM. Centrifugal compressors have a relatively low performance at low RPM and logarithmic increase in performance with RPM. The PT6 engines require interstage bleed valves to get rid of/boost pressure differentials between the two compressors.

Of note, there are no centrifugal turbines!

jetmech
Posts: 2314
Joined: Wed Mar 29, 2006 2:14 am

### RE: Axial/centrifugal Engines

 Quoting Yikes! (Reply 14):Of note, there are no centrifugal turbines!

The turbochargers used in cars usually have a centrifugal turbine. But yes, I am not too sure if a centrifugal turbine has ever been used on an aircraft engine. It is interesting to note that many of the huge turbochargers used on very large ship engines have a centrifugal compressor driven by an axial turbine.

http://static.howstuffworks.com/gif/turbo-parts.gif
JetMech split the back of his pants. He can feel the wind in his hair .

Moose135
Posts: 2668
Joined: Mon Oct 04, 2004 11:27 pm

### RE: Axial/centrifugal Engines

 Quoting Sovietjet (Reply 1):The Mig-15/17s VK-1 engines were centrigufal...most engines today are axial-flow.

The T-37 is still flying with their old centrifugal compressor engines.

 Quoting Yikes! (Reply 14):Centrifugal compressors have a relatively low performance at low RPM and logarithmic increase in performance with RPM.

The Tweet has thrust attenuators in the exhaust stream - they deploy with gear down & full flaps (I think) forcing you to keep a higher rpm on approach so you have enough throttle response for a go-around.
KC-135 - Passing gas and taking names!

Lemurs
Posts: 1320
Joined: Sat Mar 12, 2005 5:13 am

### RE: Axial/centrifugal Engines

 Quoting JetMech (Reply 15):The turbochargers used in cars usually have a centrifugal turbine. But yes, I am not too sure if a centrifugal turbine has ever been used on an aircraft engine. It is interesting to note that many of the huge turbochargers used on very large ship engines have a centrifugal compressor driven by an axial turbine.

That has more to do with the logistics of how you get the hot gas to the turbine though, don't you think? In a turbocharger you're feeding in the hot exhaust gasses from entirely external combusion areas (cylinders) through a manifold, and also sending your freshly compressed outside air to another distribution manifold for said external burning. I don't see how you could really design an axial assembly without all kinds of bends and kinks to keep the compression path and exhaust path out of each other ways...and bends and kinks tend to be bad for free flowing turbopump usage. It just seems to way more practical this way. Then again, I am not paid to solve these problems, so who knows...

There are 10 kinds of people in the world; those who understand binary, and those that don't.

Fly2HMO
Posts: 7187
Joined: Sat Jan 24, 2004 12:14 pm

### RE: Axial/centrifugal Engines

 Quoting N231YE (Reply 11): This should solve all of the questions. I'm quoting Jet Engines, by Klaus Hünecke.

Ah yes. Good book. Although I prefer "The Jet Engine" published by RR themselves. Has amazing illustrations and is in plain english too.

In our engines classroom at school we have an Allyson turboshaft engine in cutaway display form from a Bell Jetranger. It is a very unique engine in that it has a hybrid compressor (5(?) axial, one centrifugal), and a single combustion chamber. Combustion takes place 180 deg. opposite of compression, and exhaust is again 180 deg opposite of the cumbostion chamber flow. Makes for an interesting looking engine. And the compressor intake is only about 8" dia

We also have a Garret turboprop engine with a two stage centrifugal compressor. It is a very small engine, found on King-Airs:

Edit: Found a better illustration of the Allyson:

]

[Edited 2006-09-27 05:07:46]

G4Doc2004
Posts: 120
Joined: Thu Feb 19, 2004 7:49 am

### RE: Axial/centrifugal Engines

Actually, most APU's, such as the Garrett GTCP36-150 series and the Sunstrand T-62 series have a "centrifugal" turbine, which is actually called a radial inflow turbine. For many apparent reasons, this type of turbine would be highly inefficient in any other application than this.
"Failure to prepare is preparing to fail"--Benjamin Franklin

Molykote
Posts: 1239
Joined: Tue Aug 23, 2005 8:21 pm

### RE: Axial/centrifugal Engines

 Quoting FLY2HMO (Reply 18):Ah yes. Good book. Although I prefer "The Jet Engine" published by RR themselves. Has amazing illustrations and is in plain english too.

I'll second this recommendation. Absolutely stunning graphics/photos are included with a down to earth (but still sufficiently detailed) overview of jet engines. The book is valuable for the casual enthusiast as well as the experienced engineer.
Speedtape - The aspirin of aviation!

jetmech
Posts: 2314
Joined: Wed Mar 29, 2006 2:14 am

### RE: Axial/centrifugal Engines

 Quoting Lemurs (Reply 17):That has more to do with the logistics of how you get the hot gas to the turbine though, don't you think?

G'day Lemurs  . Ummm..... I'm not really sure whether your post is referring to the feasibility and or the desirability of centrifugal (radial inflow) or axial turbines. My post was merely attempting to clarify Yikes statement that there are no centrifugal turbines

 Quoting Yikes! (Reply 14):Of note, there are no centrifugal turbines!

But yes, for many situations, packaging constraints will have a major influence on the type of turbine used for any particular application. The complex piping needed to use an axial compressor and axial turbine under the limited space of a car bonnet would definitely lead to many pressure and flow losses making the arrangement inefficient.

With ships though, there is much more space to play with. For this reason, many of the turbochargers used on these engines use an axial turbine. The reason for this is that "axial turbines are more efficient in most operational ranges". (M. P. Boyce, 2002, "Gas Turbine Engineering Handbook" pp 337)
JetMech split the back of his pants. He can feel the wind in his hair .

Yikes!
Posts: 284
Joined: Sat Oct 13, 2001 4:51 pm

### RE: Axial/centrifugal Engines

Yeah, yeah, yeah, you're all right

But I was tunnel visioned on aircraft propulsion power plants.

Guess I need to think outside the box!!

 Quoting JetMech (Reply 15):Quoting Yikes! (Reply 14): Of note, there are no centrifugal turbines! The turbochargers used in cars usually have a centrifugal turbine. But yes, I am not too sure if a centrifugal turbine has ever been used on an aircraft engine. It is interesting to note that many of the huge turbochargers used on very large ship engines have a centrifugal compressor driven by an axial turbine

vc10
Posts: 1376
Joined: Thu Nov 15, 2001 4:13 am

### RE: Axial/centrifugal Engines

The Bristol Proteus turbo prop engine had 12 axial flow stages and one centrifugal stage and the low pressure end of the compressor faced rearwards with the hot engine exhaust pointing rearwards too. The engine had one or two problems during developement, which prompted an american to say

" what do you expect from an engine that breathes through it'd arse"

Some people do have a way with words

littlevc10

DH106
Posts: 598
Joined: Wed Jun 15, 2005 5:32 pm

### RE: Axial/centrifugal Engines

 Quoting VC10 (Reply 23):" what do you expect from an engine that breathes through it'd arse"

Wouldn't that be "breathed through it's ass" if it was an American comment ?

Is it me, or does the British version embody much more 'feeling' and depth of meaning than the somewhat blander US equivalent?
...I watched c-beams glitter in the dark by the Tanhauser Gate....

N231YE
Posts: 2620
Joined: Tue Jul 25, 2006 9:24 am

### RE: Axial/centrifugal Engines

 Quoting FLY2HMO (Reply 18):Ah yes. Good book. Although I prefer "The Jet Engine" published by RR themselves. Has amazing illustrations and is in plain english too.

I never knew such a book existed. Guess I'll have to find it. Thanks for bringing it up!

Fly2HMO
Posts: 7187
Joined: Sat Jan 24, 2004 12:14 pm

### RE: Axial/centrifugal Engines

 Quoting N231YE (Reply 25):I never knew such a book existed. Guess I'll have to find it. Thanks for bringing it up!

This is the new edition, although I have the older (5th) paperbook edition:

http://shop.keypublishing.com/acatal...ls_Royce__The_Jet_Engine.html#a108

You can find the 5th edition for download as an ebook, though it wouldn't be entirely, uhm, legal, i guess is the word....

speedracer1407
Posts: 330
Joined: Fri Dec 24, 2004 4:19 pm

### RE: Axial/centrifugal Engines

Ah, the turbocharger. I so should have put two and two together to answer my own question about centrifugal compressors, as I'm a long-time car nut. Funny how the basic concept is so, well basic, and yet I never knew centrifugal compressors were essential compenents of many early jet engine designs.

Btw, to all, thanks for your excellent, informative posts. It's always nice to get a big, informed turnout on a thread.
Dassault Mercure: the plane that has Boeing and Airbus shaking in their boots.

Dougloid
Posts: 7248
Joined: Tue Jul 05, 2005 2:44 am

### RE: Axial/centrifugal Engines

 Quoting Moose135 (Reply 16):he T-37 is still flying with their old centrifugal compressor engines. Quoting Yikes! (Reply 14): Centrifugal compressors have a relatively low performance at low RPM and logarithmic increase in performance with RPM. The Tweet has thrust attenuators in the exhaust stream - they deploy with gear down & full flaps (I think) forcing you to keep a higher rpm on approach so you have enough throttle response for a go-around.

Yes sir, the Two Ton Dogwhistle.....my ears have stopped bleeding.

 Quoting FLY2HMO (Reply 18):We also have a Garret turboprop engine with a two stage centrifugal compressor. It is a very small engine, found on King-Airs:

You're looking at my old office there...some versions could deliver 1,000 shp all day long if you had the money for jet fuel. A lovely engine.
If you believe in coincidence, you haven't looked close enough-Joe Leaphorn

N231YE
Posts: 2620
Joined: Tue Jul 25, 2006 9:24 am

### RE: Axial/centrifugal Engines

 Quoting Speedracer1407 (Reply 27):Ah, the turbocharger. I so should have put two and two together to answer my own question about centrifugal compressors, as I'm a long-time car nut. Funny how the basic concept is so, well basic, and yet I never knew centrifugal compressors were essential compenents of many early jet engine designs.

People can build homemade turbojet engines from old automobile turbochargers:

http://gp3.co.uk/

http://www.asciimation.co.nz/beer/

Dougloid
Posts: 7248
Joined: Tue Jul 05, 2005 2:44 am

### RE: Axial/centrifugal Engines

 Quoting N231YE (Reply 29):Quoting Speedracer1407 (Reply 27): Ah, the turbocharger. I so should have put two and two together to answer my own question about centrifugal compressors, as I'm a long-time car nut. Funny how the basic concept is so, well basic, and yet I never knew centrifugal compressors were essential compenents of many early jet engine designs. People can build homemade turbojet engines from old automobile turbochargers: http://gp3.co.uk/ http://www.asciimation.co.nz/beer/

Doncha just love it? One of these days I'm gonna have my own jet motor.

When I worked for Garrett we had the motor shop in half of a warehouse in Torrance for a while. The other half was a turbocharger castings receiving and inspection facility. They'd take samples of the incoming castings, slice them, acid etch them and look at the grain under a scope. The turbocharger rotors thus bisected made great paperweights after they did this and I have a couple of them on my desk.
If you believe in coincidence, you haven't looked close enough-Joe Leaphorn

mrocktor
Posts: 1391
Joined: Mon Jan 24, 2005 12:57 am

### RE: Axial/centrifugal Engines

One interesting bit of information that ties together a lot of the information already posted on axial vs centrifugal compressors:

Why do centrifugal compressors achieve higher pressure ratios than axial compressors? Because the pressure gradient is not aligned with the mechanical gap between rotor and casing.

Looking at a cross section of an axial compressor disk, the gap between the rotating blades and the casing is aligned with the pressure gradient - so you have a tendency for high pressure air (the air after the disk) to bleed back to the low pressure area before the disk.

Looking at a cross section of a centrifugal compressor, the path for high pressure air on the edge to bleed back to the center low pressure area via the mechanical gap between rotor and casing is much longer.

This effect is proportional to the ratio of mechanical tolerance (the gap between rotor and casing) to the size of the rotor. On a GE90 fan, the <1mm gap on a 10m diameter rotor is 1/10000 basically irrelevant - on a model airplane jet engine, the same gap on a 2cm diameter rotor is very relevant.

Basically, mechanical tolerances do not scale down past a certain point. As your rotor becomes smaller the contribution of this loss becomes greater (there is no way to make the gaps tighter after a certain point).

This is the reason why on very small jet engines (airplane model scale) you will see only centrifugal compressors. On small turboshaft applications you will see a centrifugal compressor as the last stage (smallest disk!).

Since centrifugals can't be conveniently cascaded (requires extensive rerouting of the airflow) multi-stage centrifugals are not used (the loss by rerouting is greater than the gain due to gaps). On the other hand a centrifugal "last stage" functions very well with an annular combustion chamber.

mrocktor

[Edited 2006-09-28 21:30:51]

[Edited 2006-09-28 21:42:02]

Dougloid
Posts: 7248
Joined: Tue Jul 05, 2005 2:44 am

### RE: Axial/centrifugal Engines

The mating surface for the impellers on a Garrett is a plasma sprayed aluminum coating. A few hard landings will destroy your clearance and your compressor efficiency goes out the window. Likewise the interstage seals are phenolic wafers easily damaged by an out of balance prop. It's all about getting the air to go where you want it to and not where it wants to.

But they are very damage tolerant. One of my customers took a bird down the gullet and the pilot saw the bird go in and a simultaneous 40 degree EGT rise....the bird was atomized and it munged up the internal airflow. I saw one that tried to swallow a seagull one time. It bent one of the impeller blades-no small task as it is made from a solid block of titanium. I took that engine apart and sent the impeller to Garrett. They cut off the bent part, welded new material on it, finished and rebalanced it...schweet!
If you believe in coincidence, you haven't looked close enough-Joe Leaphorn

lowrider
Posts: 2542
Joined: Wed Jun 30, 2004 3:09 am

### RE: Axial/centrifugal Engines

The PT6 uses various numbers of axial flow stages (depending on the dash number) followed by a centrifugal compressor. Not as fuel efficient as the garrett grenade, but reliable.
Proud OOTSK member

2H4
Posts: 7960
Joined: Tue Oct 19, 2004 11:11 pm

### RE: Axial/centrifugal Engines

Where does Garrett (and other powerplant manufacturers) source their titanium?

2H4

Intentionally Left Blank

greasespot
Posts: 2956
Joined: Sat Apr 24, 2004 10:48 am

### RE: Axial/centrifugal Engines

 Quoting Sovietjet (Reply 1):...most engines today are axial-flow.

The pw120 series engines use two Centrifigal compressors. This series is the PW118/120/120A/121/121A/124/125 and 127 engines.

GS
Sometimes all you can do is look them in the eye and ask " how much did your mom drink when she was pregnant with you?"

Dougloid
Posts: 7248
Joined: Tue Jul 05, 2005 2:44 am

### RE: Axial/centrifugal Engines

 Quoting 2H4 (Reply 34):Quoting Dougloid (Reply 32): It bent one of the impeller blades-no small task as it is made from a solid block of titanium. Where does Garrett (and other powerplant manufacturers) source their titanium? 2H4

I do not know that, but I believe a lot of it comes from Russia.
If you believe in coincidence, you haven't looked close enough-Joe Leaphorn

jetmech
Posts: 2314
Joined: Wed Mar 29, 2006 2:14 am

### RE: Axial/centrifugal Engines

 Quoting Mrocktor (Reply 31): Why do centrifugal compressors achieve higher pressure ratios than axial compressors? Because the pressure gradient is not aligned with the mechanical gap between rotor and casing.

G'day Mrocktor  ! Very interesting reply you wrote. Is that the sole reason why a single stage of centrifugal compressor can achieve such a higher pressure ratio than a single stage of axial compressor? I have no doubt the reason presented in your post is correct, and I am in no way questioning your knowledge, but it seems that a major leap in axial compressor pressure ratios could be achieved with proper attention to sealing   .

Perhaps the relative amounts of acceleration achieved by each of these compression methods also has something to do with it. I can certainly picture a single stage of centrifugal compressor adding more kinetic energy to the incoming air than a single stage of axial compressor  ?

Once again, I am not questioning your post or your knowledge, it just seems to me that something more intrinsic about the relative designs of axial and centrifugal compressors would also have an influence on the pressure ratios achieved.

BTW, I can't wait for the chance to visit Brazil. It looks like a fascinating country!

[Edited 2006-09-29 03:57:56]
JetMech split the back of his pants. He can feel the wind in his hair .

2H4
Posts: 7960
Joined: Tue Oct 19, 2004 11:11 pm

### RE: Axial/centrifugal Engines

 Quoting Dougloid (Reply 36):I do not know that, but I believe a lot of it comes from Russia.

Ahh. Maybe they get it from Sandvik, then...

2H4

Intentionally Left Blank

mrocktor
Posts: 1391
Joined: Mon Jan 24, 2005 12:57 am

### RE: Axial/centrifugal Engines

 Quoting JetMech (Reply 37):Is that the sole reason why a single stage of centrifugal compressor can achieve such a higher pressure ratio than a single stage of axial compressor?

I don't know, and fluid dynamics being as complex as they are I doubt it

 Quoting JetMech (Reply 37):but it seems that a major leap in axial compressor pressure ratios could be achieved with proper attention to sealing

Beyond a certain tolerance there is nothing you can do with regards to sealing. Making a part (such as the disk and the casing) to a 1mm tolerance is dirt cheap, to a .1mm tolerance is much more expensive and after a certain point it just ceases to be possible. Don't forget that the whole rotor assembly is in motion and is flexible. A minor defletcion of the axis due to flexibility could ruin your tolerance, a minor thermal expansion as well.

 Quoting JetMech (Reply 37):Perhaps the relative amounts of acceleration achieved by each of these compression methods also has something to do with it.

It would have to relate to the amount of work done on the fluid by the compressor. Maybe someone versed in fluid dynamics can fill us in.

mrocktor

### Who is online

Users browsing this forum: aeromoe, raggi and 1 guest

### Popular Searches On Airliners.net

Top Photos of Last:   24 Hours  •  48 Hours  •  7 Days  •  30 Days  •  180 Days  •  365 Days  •  All Time

Military Aircraft Every type from fighters to helicopters from air forces around the globe

Classic Airliners Props and jets from the good old days

Flight Decks Views from inside the cockpit

Aircraft Cabins Passenger cabin shots showing seat arrangements as well as cargo aircraft interior

Cargo Aircraft Pictures of great freighter aircraft

Government Aircraft Aircraft flying government officials

Helicopters Our large helicopter section. Both military and civil versions

Blimps / Airships Everything from the Goodyear blimp to the Zeppelin

Night Photos Beautiful shots taken while the sun is below the horizon

Accidents Accident, incident and crash related photos

Air to Air Photos taken by airborne photographers of airborne aircraft

Special Paint Schemes Aircraft painted in beautiful and original liveries

Airport Overviews Airport overviews from the air or ground

Tails and Winglets Tail and Winglet closeups with beautiful airline logos