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Axial/centrifugal Engines  
User currently offlineSpeedracer1407 From United States of America, joined Dec 2004, 333 posts, RR: 0
Posted (7 years 10 months 1 week 21 hours ago) and read 15113 times:

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


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39 replies: All unread, showing first 25:
 
User currently offlineSovietjet From Bulgaria, joined Mar 2003, 2577 posts, RR: 17
Reply 1, posted (7 years 10 months 1 week 20 hours ago) and read 15125 times:
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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.

User currently offlineLemurs From United States of America, joined Mar 2005, 1439 posts, RR: 4
Reply 2, posted (7 years 10 months 1 week 20 hours ago) and read 15117 times:

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.
User currently offlineJetMech From Australia, joined Mar 2006, 2684 posts, RR: 53
Reply 3, posted (7 years 10 months 1 week 19 hours ago) and read 15109 times:

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.
User currently offlineMolykote From United States of America, joined Aug 2005, 1340 posts, RR: 29
Reply 4, posted (7 years 10 months 1 week 18 hours ago) and read 15075 times:

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 asprin of aviation!
User currently onlineStarlionblue From Greenland, joined Feb 2004, 17000 posts, RR: 67
Reply 5, posted (7 years 10 months 1 week 15 hours ago) and read 15043 times:

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."
User currently offlineA342 From Germany, joined Jul 2005, 4680 posts, RR: 3
Reply 6, posted (7 years 10 months 1 week 14 hours ago) and read 15036 times:

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.
User currently onlineStarlionblue From Greenland, joined Feb 2004, 17000 posts, RR: 67
Reply 7, posted (7 years 10 months 1 week 14 hours ago) and read 15027 times:

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."
User currently offlineN231YE From , joined Dec 1969, posts, RR:
Reply 8, posted (7 years 10 months 1 week 13 hours ago) and read 15017 times:

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.
http://upload.wikimedia.org/wikipedia/en/thumb/e/e3/DH_Goblin_annotated_colour_cutaway.png/800px-DH_Goblin_annotated_colour_cutaway.png

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



User currently onlineStarlionblue From Greenland, joined Feb 2004, 17000 posts, RR: 67
Reply 9, posted (7 years 10 months 1 week 11 hours ago) and read 14994 times:

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."
User currently offlineSovietjet From Bulgaria, joined Mar 2003, 2577 posts, RR: 17
Reply 10, posted (7 years 10 months 1 week 10 hours ago) and read 14964 times:
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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....

User currently offlineN231YE From , joined Dec 1969, posts, RR:
Reply 11, posted (7 years 10 months 1 week 8 hours ago) and read 14949 times:

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.


User currently offlineA342 From Germany, joined Jul 2005, 4680 posts, RR: 3
Reply 12, posted (7 years 10 months 1 week 7 hours ago) and read 14926 times:

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.
User currently offlineLemurs From United States of America, joined Mar 2005, 1439 posts, RR: 4
Reply 13, posted (7 years 10 months 1 week 5 hours ago) and read 14889 times:

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.
User currently offlineYikes! From Canada, joined Oct 2001, 284 posts, RR: 1
Reply 14, posted (7 years 10 months 1 week 4 hours ago) and read 14877 times:

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!


User currently offlineJetMech From Australia, joined Mar 2006, 2684 posts, RR: 53
Reply 15, posted (7 years 10 months 1 week ago) and read 14836 times:

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
http://content.answers.com/main/cont.../thumb/d/d8/350px-Turbocharger.jpg



JetMech split the back of his pants. He can feel the wind in his hair.
User currently offlineMoose135 From United States of America, joined Oct 2004, 2301 posts, RR: 10
Reply 16, posted (7 years 10 months 1 week ago) and read 14835 times:

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!
User currently offlineLemurs From United States of America, joined Mar 2005, 1439 posts, RR: 4
Reply 17, posted (7 years 10 months 1 week ago) and read 14823 times:

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...
 Smile



There are 10 kinds of people in the world; those who understand binary, and those that don't.
User currently offlineFLY2HMO From , joined Dec 1969, posts, RR:
Reply 18, posted (7 years 10 months 1 week ago) and read 14823 times:

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:

http://www.motorflug.aero/cms/projekt01/media/history/Allison-250-C18-gross.jpg]

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

User currently offlineG4Doc2004 From United States of America, joined Feb 2004, 123 posts, RR: 0
Reply 19, posted (7 years 10 months 1 week ago) and read 14817 times:

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
User currently offlineMolykote From United States of America, joined Aug 2005, 1340 posts, RR: 29
Reply 20, posted (7 years 10 months 6 days 20 hours ago) and read 14798 times:

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 asprin of aviation!
User currently offlineJetMech From Australia, joined Mar 2006, 2684 posts, RR: 53
Reply 21, posted (7 years 10 months 6 days 19 hours ago) and read 14790 times:

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  Smile. 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.
User currently offlineYikes! From Canada, joined Oct 2001, 284 posts, RR: 1
Reply 22, posted (7 years 10 months 6 days 14 hours ago) and read 14761 times:

Yeah, yeah, yeah, you're all right  Wink

But I was tunnel visioned on aircraft propulsion power plants.

Guess I need to think outside the box!!

Quoting JetMech (Reply 15):



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


User currently offlineVC10 From United Kingdom, joined Nov 2001, 1407 posts, RR: 16
Reply 23, posted (7 years 10 months 6 days 13 hours ago) and read 14748 times:

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


User currently offlineDH106 From United Kingdom, joined Jun 2005, 626 posts, RR: 1
Reply 24, posted (7 years 10 months 6 days 13 hours ago) and read 14744 times:

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?  Big grin



...I watched c-beams glitter in the dark near the Tanhauser Gate....
25 N231YE : I never knew such a book existed. Guess I'll have to find it. Thanks for bringing it up!
26 Post contains links FLY2HMO : 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
27 Speedracer1407 : 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. F
28 Dougloid : Yes sir, the Two Ton Dogwhistle.....my ears have stopped bleeding. You're looking at my old office there...some versions could deliver 1,000 shp all
29 Post contains links N231YE : People can build homemade turbojet engines from old automobile turbochargers: http://gp3.co.uk/ http://www.asciimation.co.nz/beer/
30 Dougloid : " target=_blank>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 w
31 Mrocktor : One interesting bit of information that ties together a lot of the information already posted on axial vs centrifugal compressors: Why do centrifugal
32 Dougloid : 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 compr
33 Lowrider : 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 ga
34 2H4 : Where does Garrett (and other powerplant manufacturers) source their titanium? 2H4
35 Post contains images Greasespot : The pw120 series engines use two Centrifigal compressors. This series is the PW118/120/120A/121/121A/124/125 and 127 engines. GS
36 Dougloid : I do not know that, but I believe a lot of it comes from Russia.
37 Post contains images JetMech : 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 p
38 2H4 : Ahh. Maybe they get it from Sandvik, then... 2H4
39 Post contains images Mrocktor : I don't know, and fluid dynamics being as complex as they are I doubt it Beyond a certain tolerance there is nothing you can do with regards to seali
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