Axial vs. Centrifugal flow compressors.

For those of you who wish to "build your own"

Axial vs. Centrifugal flow compressors.

Postby j79guy on Sun Jan 18, 2009 3:51 pm

I was recently asked on an engineering forum, about building an axial flow type compressor for an RC application. (Radio Controlled hobby aircraft.)
I recommended the fellow consider building his engine with a centrigual flow compressor for the following reasons:

When you are discussing propulsion turbine engines with a mass inlet airflow of 7-10 Lb/Sec or less, axial compressors have a tough time reaching the efficieny of a properly designed centrifugal compressor. Clearly, there are adventages to axial type, such as smaller frontal area/diameter of the engine, allowing for a lower drag airframe, however you indicate that you are interested in a hobby type application, not cutting edge military aircraft, thus I recommend building your first engine around a centrifugal compressor. Perhaps later when you have accumulated experience with building a few engines, you could logically move to an axial flow type. Without labouring the subject too much, the disadvantages to axial flow are:
- Low compression ratio per stage. Using modern wide-chord type compressor blades of NACA C4 profile, or better, will still only give 1.3:1 compression raise per stage, under ideal conditions. Most engines after running a bit and accumulating some fouling of the compressor blades, this compression ration perstage falls off significantly. Chances are that in building your first axial flow compressor, unless you have access to manufacturing equipment that we are not aware of, the compression ratio per stage will be somewhat more modest, perhaps 1.15:1. In any case, you will need more that just a few stages to get a reasonable total compression ratio with an axial flow. Centrifugal compressors on the otherhand, in a hobby type application, can draw on literally dozens of proven turbocharger wheels, which have over 3:1 compression ratio potential right out of the box.
- Tip losses. Every single axial flow compressor blade and stator vane requires a certain tip clearance, depending on materials used in the construction of your engine, rpm, and compression ratio. You can envision this tip clearance and the losses resulting therefrom. Centrifugals only require clearance to the wheel face, and in enclosed type wheels, this is eliminated as well.
- Toughness. Ingest the tiniest of objects into an axial flow compressor and you will deteriorate the performance, or worse, damage the blading/stators. Centrifugals you can throw the proverbial cat through, and come out unscathed.

The Rolls Royce/Allison 250 engine line comes to mind. The earlier maodels hade several stages of axial, and a single centrifugal as the last stage. Later models reduced the number of axial stages, and The latest and largest model of 250 has a single centrifugal flow only.

Robin.
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Re: Axial vs. Centrifugal flow compressors.

Postby TillamookTurbine on Mon Jan 19, 2009 10:05 am

Another factor to consider with regards to small compressors is the effect of the Reynolds number. I'm not an expert aerodynamicist but will try to explain.

The Reynolds number is the ratio of aerodynamic inertia forces divided by the viscous forces. For example an aircraft develops an inertia force when deflecting the airflow to produce lift. This is similar to an axial compressor accelerating the flow through the rotor. The aircraft "feels" drag from skin friction; a viscous force. Generally, a larger aircraft (or compressor) has a higher Reynolds number and better aerodynamic performance. A good example can be found in model sailplanes. Some high performance full scale sailplanes have Lift over Drag ratios on the order of 50 to 1. Exact smaller scale replicas will only have L/D's of about 30 to 1; mostly attributed to the lower Reynolds number.

Axial flow compressor are using aerodynamic lift for compression whereas a centrifugal compressor uses less lift and more drag to accelerate the airflow. Therefore, small axial flow compressors suffer lower performance because of their lower Reynolds number.

Terry Spath
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Re: Axial vs. Centrifugal flow compressors.

Postby AgentJayZ on Tue Feb 24, 2009 9:27 pm

A good description of Reynolds number.
I think that this is a reason you never see turbochargers with axial compressors; the other reasons being cost of manufacture and durability.
Plus, an RC sized axial flow engine would be an inch or so in diameter, spin at 150,000 rpm or so, and need tip clearances of less than one thousandth of an inch to even begin to pump that thick, gooey, low-Reynolds-number air through it.
And those blades would be so thin and delicate, they would not survive even the slightest contact with their casing shrouds.
Might take years to build, and run for a few seconds, if at all.
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