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Starlionblue
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Axial Flow Vs Centrifugal Flow. What Does It Mean?

Mon Aug 09, 2004 10:23 pm

Just what the topic says. I know that engines today are axial flow, while engines in the dark ages (1930-50?) were centrifugal flow.

I wondered how exactly this is defined and what it means, because the authors of all my aviation books and lots of websites seem to take the knowledge for granted. I know how jet engines work, and that annular combustors have replaced cans, but this does not answer the question.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
airplay
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RE: Axial Flow Vs Centrifugal Flow. What Does It Mean?

Mon Aug 09, 2004 10:31 pm

There are two types of compressors in use in modern jet engines. Axial flow compressors move the air into ever smaller spaces using a series of axial compressor fans. Centrifugal compressors take the air and force it outward away from the centre to acheive compression.

Therefore the overal flow for axial flow engines is in a straight line, whereas the airflow through centrifugal engines changes directions through each stage.

Centrifugal compressors are not ancient or outdated, except when you consider some of the larger modern jet engines.

The successful (and still in production) PT-6 line uses centrifugal compressors. Also, some engines use a combination of centrifugal and axial stages.
 
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Starlionblue
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RE: Axial Flow Vs Centrifugal Flow. What Does It Mean?

Mon Aug 09, 2004 10:58 pm

Aaaaaaaaaaaaaaaaaaaaaaaaaaaah. Light bulb went off above my head. Thx Airplay!

How exactly are the fan blades shaped to achieve this?
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
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HAWK21M
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RE: Axial Flow Vs Centrifugal Flow. What Does It Mean?

Mon Aug 09, 2004 11:29 pm

.....while engines in the dark ages (1930-50?) were centrifugal flow.
Most APUs are Centrifugal types
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L-188
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RE: Axial Flow Vs Centrifugal Flow. What Does It Mean?

Mon Aug 09, 2004 11:38 pm

A lot of engines these days at still centrifugal.....The turbines can be made stronger in smaller engines that way. A lot of turboprop engines are built that way.

When you think Centrifugal....Think Centrifuge. The air is sucked into the middle and then the higher pressure air is forced out to the sides....Pretty much all auto turbochargers are of this design.

Axial flow....Think about the air moving along the axis....Front to back.
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Starlionblue
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RE: Axial Flow Vs Centrifugal Flow. What Does It Mean?

Mon Aug 09, 2004 11:50 pm

When you said turbochargers it clicked for me. Thx L-188. I guess like the difference between a fan and a blower in computer cooling circles.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
FredT
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RE: Axial Flow Vs Centrifugal Flow. What Does It Mean?

Mon Aug 09, 2004 11:51 pm

I thought I was doing good trying to avoid those airport hotels... and look at me now.
 
411A
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RE: Axial Flow Vs Centrifugal Flow. What Does It Mean?

Tue Aug 10, 2004 12:07 am

Centrifugal compressors are certinally nothing new to large piston engine as well.

ALL of the older large radial piston engines used centrifugal design superchargers (not turbochargers) to enhance power.
The fuel was routed from the pressure carbueretor to the center of the supercharger, where it was mixed with air and pressurized in the intake manifold, and delivered to cylinders.
With some engines, 59+ inches of manifold pressure was achieved for takeoff.
Many of these same engines had two speed superchargers.
At approximately 12,000msl, the superchargers (blowers) were shifted, thru a conical clutch arrangement, to a different gear drive ratio, in order to spin faster...for enhanced high altitude ops.

A very few large piston engines used both superchargers AND turbochargers.
One was the Pratt&Whitney R4360, and could be successfully operated up to 40,000 feet.
These particular engines were fitted to the Boeing Stratocruiser.
 
kalakaua
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RE: Axial Flow Vs Centrifugal Flow. What Does It Mean?

Tue Aug 10, 2004 12:14 am

The centrifugal flow compresor has a single or two stage unit using an impeller to accelerate the air and a diffuser to produce the required pressure rise. The axial flow compressor is a multi-stage unit using alternate rows of rotating (rotor) blades and stationary (stator) vanes, to accelerate and diffuse the air until the required pressure rise is reached. Particularly on small engines, an axial compresor is used to boost the inlet pressure to the centrifugal.

The centrifugal compressor is more rugged than the axial and is also easier to develop and manufacture. But the axial compresor consumes more air than a centrifugal compressor even with the same frontal area. Axials also can be designed to attain much higher pressure ratios.

Air flow is an important factor because it determines the amount of thrust. Axial compressor engine gives more thrust for the same frontal area of a centrifugal. Also, the axial has the ability to increase its pressure ratio by adding extra stages. But centrifugals are still used in smaller engines which are simple and rugged.

Since high pressuer ratios is a favourite, so axials are chosen because of the improved efficiency, which in turn leads to improved specific fuel consumption for a given thrust.
Gravity explains the motions of the planets, but it cannot explain who set the planets in motion.
 
doug_or
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RE: Axial Flow Vs Centrifugal Flow. What Does It Mean?

Tue Aug 10, 2004 2:04 am

You can actauly make a centrifugal turbojet engine out of an auto turbochrager. I think there was a thread in the hobby forum a while ago.
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EconoBoy
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RE: Axial Flow Vs Centrifugal Flow. What Does It Mean?

Tue Aug 10, 2004 4:20 am

An analogy to describe the differences in axial v centrifugal is to imagine pushing water up a slope with a broom. An axial compressor does it in a series of small, short sweeps whereas a centrifugal does it in one big shove. The biggest disadvantage of a centrifugal engine is its portly diameter.
 
LineMechQX
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RE: Axial Flow Vs Centrifugal Flow. What Does It M

Wed Aug 11, 2004 11:20 am

Yes centrifugal compressors are still very much alive. Pratt and Whitney Canada includes at least one centrifugal compressor in every engine it makes. Its one of their trademarks.

Late
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UAL Bagsmasher
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RE: Axial Flow Vs Centrifugal Flow. What Does It Mean?

Thu Aug 12, 2004 2:20 am

The compressor section of the GE CT7-9B that is used on the Saab 340B has 5 stages axial, 1 stage centrifugal. (See, I did learn something in the Gen Fam class Smile/happy/getting dizzy)
 
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Francoflier
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RE: Axial Flow Vs Centrifugal Flow. What Does It Mean?

Mon Aug 16, 2004 12:49 pm

The Garret engines (TPE 331) use a set of 2 centrifugal compressors.

The centrifugal compressor allow for a greater compression ratio per compressor stages, i.e. one centrifugal compressor wheel get a certain amount of air to a higher pressure than one axial compressor wheel could.

It is especially useful on small engines, like the turboprop engines. It saves space and weight. Ever seen how small a TPE 331 is?

Say about 2 and 1/2 times the lenght of the screen in front of you. And it puts out more than 1000 shp...

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MD-90
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RE: Axial Flow Vs Centrifugal Flow. What Does It M

Sun Aug 22, 2004 12:30 am

Didn't the Comet have axial-flow turbojets (at least at first)?


The R4360 was not a paragon of reliability, however impressive it was, technically (the most powerful piston aircraft engine ever put into production, as I understand it).
 
timz
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RE: The Comet

Sun Aug 22, 2004 8:36 am

The Comet I had DH Ghosts, didn't it? Pretty sure they were centrifugal only. The Comet 4's Avons were of course axial.
 
MD-90
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RE: Axial Flow Vs Centrifugal Flow. What Does It Mean?

Mon Aug 23, 2004 1:41 am

Well, I used to think that axial meant that the compressor rotated like a waterwheel in the water (like the paddlewheel of a steamboat). Similiar to a car's turbocharger.

It's usually not very well explained how the vanes are different yet similiar in the two types.
 
kalakaua
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RE: Axial Flow Vs Centrifugal Flow. What Does It Mean?

Mon Aug 23, 2004 4:24 am

Well, MD-90, sure... In an axial compressor the flow travels parallel to the axis of rotation. In the centrifrugal compressor, the airflow is turned perpendicular to the axis of rotation.
Gravity explains the motions of the planets, but it cannot explain who set the planets in motion.

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