Why do airplanes use an axial flow jet engine instead of a more compact centrifugal jet engine?

Question

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.

Answer 1

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, not more velocity.

Since they are 3D structures, in a basic solid design (that you'd find in early jets and small modern turbines), the volume 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 mass-to-area law is more complex. Still, it cannot eliminate the volume effect entirely. The end result is that the mass of centrifugal compressors grows considerably faster than their mass flow.

At the highest power levels, centrifugal compressors become prohibitively heavy even for fixed powerplant machinery, where durability otherwise trumps weight, so the largest base load powerplants run all-axial. With axial compressors, the flat design with a short air flow path allows mass to only grow in direct proportion to mass flow, and very powerful engines can be built within reasonable dimensions.

It's not pure cube-square law in either case, but it's something like k1*massflow^[2.5, 2.8] for centrifugal versus k2*massflow^[2.2, 2.4] for axial, where k2>k1, giving centrifugal compressors some advantage in small sizes. Centrifugals are also much cheaper to produce (at least small ones).

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 (also reduced somewhat in practice) 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, as power grows, 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. Newer diagonal compressors are in-between and quite good, offering an even more tailored compromise.

Answer 2

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.

.

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.

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.

Answer 3

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.

Answer 4

Centrifugal compressors are limited by the size and sharp angle of the diffuser duct that has to straighten out the outward flowing air flow from the impeller compressor and reroute it back to the combustion chambers. This restricts the amount of air that can flow through this engine. Axial flow engines are straight through design and don't have this limitation.

Answer 5

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.