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Axial flow jet engines take up a lot of space. Centrifugal jet engines create high pressure. Can you create a centrifugal air pump that pushes as much air as a axial jet engine. Centrifugal air engines reach higher speeds from my research. What are the main disadvantages of centrifugal air engines from the standard axial engines.

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Centrifugal compressors only produce a more compact engine at low mass flow, which means low thrust.

The amount of thrust an engine can produce is proportional to its intake area times exhaust velocity. Increasing the latter is undesirable, as energy and thus fuel consumption is proportional to velocity squared. So engine designers target mass flow to gain more thrust.

Since they are 3D structures, in a naive solid design (that you'd find in early jets and microturbines), the mass of a centrifugal compressor grows in cubic proportion to its diameter, while frontal area, which limits its mass flow and thus its thrust, increases only as diameter squared. This creates a cube-square law.

Large real-life parts are filled with lightening and cooling channels, so the law is more complex, Still, the end result is that the mass of centrifugal compressors grows faster than power, and at the highest power levels they become prohibitively heavy even for fixed powerplant machinery, where durability otherwise trumps weight. With axial compressors, the flat design with short air flow path allows mass to only grow in direct proportion to power, and very powerful engines can be built within reasonable dimensions.

High-performance engines, such as those in commercial aircraft, also need to pack more thrust into the smallest cross-section they can, while maintaining efficiency, so as to reduce drag and also fit under the wings, enabling heavier jets. Axial compressors offer a lot more intake area for any given cross-section - thus more thrust.

The smallest jets, where thrust requirements are small and the engine's cross-section is very small compared to the fuselage, can afford the extra diameter of a centrifugal or diagonal flow compressor. Yes, it's the same cube-square law that keeps the engine cross-section to total cross-section ratio increasing as aircraft go up in size. Small centrifugal compressors are simpler, easier to build, and more robust than small axial ones.

So in every industry there is a crossover point from centrifugal to axial. For aircraft where drag is critical, it's just above small bizjets, mobile ground and helicopter turbines stay centrifugal or mixed up to a few MW, and in the tens of megawatts even fixed powerplants switch from axial/centrifugal to all-axial. Engines close to that crossover point typically combine axial and centrifugal stages, and newer diagonal compressors offer an even more tailored compromise.

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    $\begingroup$ Thank you, a very thorough yet easy to follow answer. $\endgroup$ – Mast May 25 at 16:29
  • $\begingroup$ "Centrifugal compressors are only more compact at low mass flow" no they aren't, they are more compact at any mass flow. A large part of the frontal area cannot be used for airflow, is the issue in larger engines. $\endgroup$ – Koyovis May 26 at 2:31
  • $\begingroup$ @Koyovis Define "compact" then. Inlets have to be sized for required airflow, so, for airliner engines that are only as wide as their inlets, a wider compressor will mean a wider and heavier engine. Centrifugal compressors are compact in smaller engines, but supersizing them doesn't maintain that - axials are better at ingesting as much air as possible in given engine size. $\endgroup$ – Therac May 26 at 7:44
  • $\begingroup$ "The necessary mass of a centrifugal compressor grows in cubic proportion to its diameter" Not necessarily: The length of an engine might even shrink when increasing the diameter. In this case the volume (and therefore the weight) would grow less than squared. $\endgroup$ – Martin Rosenau May 26 at 15:37
  • $\begingroup$ Mass of the centrifugal compressor grows exponentially compared to the mass of three axial stages? The centrifugal compressor can be hollowed out, the main stress bearing material is at the impeller tip. $\endgroup$ – Koyovis Jun 11 at 1:43
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Axial turbine engines take up a lot of space...lengthwise. Centrifugal compressors are shorter and wider, and are very often used in turboprop and turboshaft engines, for instance the Rolls Royce Dart is a single axis turboprop engine. The photo demonstrates the compactness of the engine...lengthwise.

Wiki.

Can you create a centrifugal air pump that pushes as much air as a axial jet engine.

Yes you can. It will have two main issues:

  1. The losses in the centrifugal compressors will be higher than for an axial configuration reaching the same compression ratio.
  2. Part of the frontal area cannot be used for airflow.

What are the main disadvantages of centrifugal air engines from the standard axial engines.

Centrifugal compressors:

  • Are slightly less efficient than axial compressors (for a given compression ratio).
  • Expel the airflow perpendicular to free stream, so that if multiple stages are required the airflow must be guided into quite a bendy pathway, with additional negative impact on efficiency.
  • Result in an engine with a larger frontal area.

Advantages are:

  • Centrifugal compressors achieve higher compression ratios than axial compressors - per stage. Compression ratios of 4 - 6, while an axial stage can only do 1.4 - 1.6.
  • They are of a more robust and often less costly construction.

Larger engines (high mass flow, high compression ratio and minimal frontal area) use axial compressors almost exclusively: the internal losses are lowest. Centrifugal compressors are used for design cases where other factors than efficiency are of interest, such as cost and limitation of length for helicopter turboshafts.

An example of the use of a centrifugal compressor in a turbofan engine is the Garrett AiResearch ATF3. A 3-shaft engine, with the fan on shaft 1, five axial stages on shaft 2, and a centrifugal stage on shaft 3.

enter image description here

Picture source

The hot exhaust stream is deflected back into the fan bypass, which cools it down and results in a low IR signature. This engine is used in the Dassault Falcon. Note that there is no principal technical difficulty to scale this engine up to A380 level, other than the decreased efficiency due to the multiple deflections in the airstream. The wider frontal area is not a problem in the compressor part, and the total compression ratio increases a lot using this final stage.

As in the ATF3, centrifugal compressors are often combined with an axial stage, which pre-whirls the air stream into the centrifugal compressor and increases both efficiency and max compression ratio per single stage, as compared to a flat single stage.

From an old uni book

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While a centrifugal compressor can acheive greater compression than a single compressor stage in an axial flow engine, the axial flow design allows for multiple compressor stages, achieving higher overall compression of the air and consequently greater efficiency. Centrifugal flow jet engines are, however, tough and reliable.

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  • $\begingroup$ Centrifugal compressors can also be multi-staged. $\endgroup$ – Koyovis May 26 at 20:06
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All existing centrifugal-compressor aircraft engines have been turbojets or turboprops.

Aside from their low power compared with modern turbofans (the Rolls-Royce Dart mentioned in another answer produced about 1 MW, compared with 50MW for a modern large turbofan engine core) the larger diameter of a centrifugal compressor would make it hard to design an efficient turbofan, by restricting the area of the fan duct.

Increasing the outer diameter of the fan blades to enlarge the bypass duct is not a practical option, since it is limited by the ground clearance of the aircraft. Trying to avoid that issue with very large diameter tail-mounted engines would just create a different set of design problems.

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    $\begingroup$ Not all; the Pratt & Whitney Canada PW600 is a turbofan (albeit a tiny one) with a centrifugal core compressor. $\endgroup$ – Sean May 25 at 22:05
  • $\begingroup$ The RR Dart powers a propeller. Talk about a large diameter fan... $\endgroup$ – Koyovis May 26 at 1:17

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