enginehistory.org

[ckuhns]

I have noticed that some turbo shaft and turbo prop engines have both a axial and centrifugal compressor. Any reason for this type of construction? A few examples of this are the P & W PT6 series, Lycoming T53 and the Allison/ Rolls -Royce 250.

[kmccutcheon]

Please note that all of these are relatively small-diameter gas turbines. Centrifugal final stages are used because they can be relatively small and still achieve a 4X compression whereas an axial stage typically produces about 1.2X. This doesn't work with larger-diameter compressors because the large impeller's weight, structural considerations and tip speeds become prohibitive.
_________________
Kimble D. McCutcheon

[mksmith]

To piggyback off of what Mr. MCCutcheon has said, by virtue of the way a centrifugal compressor functions, sharps bends in the airflow, in some cases as much as 360 degrees, result in the loss of energy in the airstream. As the original poster pointed out, centrifugal compressors are used primarily on turboprop and turboshaft engines. The reason for this is that majority of the energy is extracted by the turbine in turboprop and turboshaft engines, and the loss of energy in the airstream isn't as important as it would be in the case of a turbojet or low bypass turbofan where the acceleration of hot gasses in the exhaust section is the primary propulsive force. I've conveniently not mentioned high bypass turbofan engines. In this case, axial compressors are used primarily because of the lower weight and the efficiency required for modern combustors. As Mr. McCutcheon mentioned, it would be weight prohibitive to use centrifugal compressors in succession. However, this method has been used to great effect in the Honeywell TPE331 as well as the RR Dart.

[avasko]

There were two large engines utilizing both axial and centrifugal stages. Bristol Engine Co. developed two such engines, only one of which achieved production status. The first engine vintage 1947 was the Theseus which had a front air intake feeding an eight-stage axial compressor. A centrifugal stage followed by a heat exchanger which was wrapped around the turbine stages. Here it was preheated by the exhaust flow. Sanity prevailed and the heat exchanger was abandoned in the later model. The engine was air tested but dropped for the Proteus.

The Proteus, while being produced in some numbers had a troubled life. Originally intended to be buried in the wings of the Brabazon land plane and the Princess flying boat. It is ironic that their only use was in the Bristol Britannia where the protruded out-front looking like a big radial. Tractor buried engines need to be short so as to minimize interference with wiung structure. Bristol, already familiar with reversing airflows in the Theseus, used the concept on the Proteus but somewhat differently.

In the Proteus, the air entered passages built into the cowling which conducted it aft. It then made an inward dive going through spaces between the outlet ducts of the combustion cans. It completed its 180-degree turn now going forward into the first stage of the axial compressor. The eight-stage axial compressor blew forward into the centrifugal stage. The air now made another 180-degree turn going outward and aft and entered the eight combustors. The combustors were wrapped around the axial compressor. Each combustor had a separate outlet duct leaving space between them for the inlet air to get through to the compressor. Effluent from the combustors the entered the three-stage turbine, two stages drove the compressors and the third drove a coaxial shaft forward to the prop reduction gear which, not being connected to the compressor drive, was a free turbine.

Drawing

The Achilles Heel of the Proteus was that first 180-defree turn. It could fly through rain or snow with no problem. The skin of the ducts was deiced by turbine bleed air. However, very cold high-altitude alto-cirrus clouds are composed of ice crystals. This first bit of ducting centrifuged the ice crystals out and they adhered to the wall. Eventually, enough accumulated to insulate the adhesion surface. The hot gas from the turbine doing the deicing removed the adhesion and the ice, now in chunks, loosened in chunks. These proceeded to the axial compressor. If the compressor successfully ground them, the heat of compression turned the water to steam. This would put the fires out in the combustors. Continuous ignition, as used in later engines was not available. Instead, the installed glow plugs which hopefully relit the burners with the unburned fuel lighting off with a perceptible "bump", hence the name "Bump Stalls."

If it didn't relight, the Flight Crew used HP Cock to turn off the fuel. It was allowed to motor to clear out fuel. Air-start procedures were then used. The "Whispering Giant", the nickname for the Brittania. It could get quieter. A customer airline of ours once lost all four engines due to bump stalls, Simultaneously I mean.

Even worse though were to times when the ice severely damaged the compressor leading to the term "Corn-cobbing". The problem was never actually eliminated but flight manuals required pilots to avoid high altitude clouds that were composed of ice crystals. Our customer airline however was forced to fly as directly as possible across the Gulf of Mexico since it was a model 302 with only four fuel tanks.

As a "by the way", the Proteus did have a successful carreer in marine use with never a bump stall.

So, back to the thread, two large engines did indeed use axial and centrifugal compressors. Revised to clarify wording and correct bad spelling, 12/02/23. url][/url][url][/url]turbine bleed air.

[kmccutcheon]

Good discussion, avasco.
_________________
Kimble D. McCutcheon

[ckuhns]

I was wondering what type of compressors are installed in the APU's of biz jets and ailiners and what is their HP?


















apu

[kmccutcheon]

[admin02]

[kmccutcheon]

[ckuhns]

The History of North American Small Gas Turbine Aircraft Engines.On page 163 is the answer to the question that I proposed to start this thread. I quote it here."In addition,using a centrifugal stage for the high pressure portion of the compressor avoided the use of small delicate blades that would have been required in the high pressure part of a small all-axial flow compressor"