Multi-staging radial compressors

[cudabean]

Any one know why there aren't many multi-stage radial or centrifugal compressors out there? I think I've seen two stagers, but even those are rare. It seems to me like you could reduce complexity of a high-efficiency fanjet by eliminating the ten or more axial flow stages and replace them with, perhaps, three radial stages. This, of course also eliminates ten stages of stators at the same time.

cudabean

[Peter Hanely]

Radial flow compressors give more pressure per stage, but at greater frontal area than an axial compressor. Long and skinny vs. short and fat. Also, a radial flow pump is not without stators.

[Mike Kirney]

It seems axial compressors are easier to manufacture and maintain, as their impellers are constructed of individual milled blades that are mounted on solid metal discs, which are then keyed to a shaft. Radial compressors often have oddly-shaped impellers cast as single pieces of metal, so that if only one blade gets cracked or worn, then you have to replace the whole thing, as opposed to one tiny part. I wouldn't doubt that axial compressors dissipate heat better because of this lack of complex castings, and generally it's easier to precisely machine little parts than it is to precisely cast bigger parts, which will probably need machining anyway. Airflow is more direct in an axial set-up. In a radial set-up, air is pumped from the centre of the scroll to the edges, a 90-degree change in direction, so daisy-chaining them would require two more 90-degree changes of direction per stage, which might impact efficiency. These changes in direction also hinder the ability of the compressor to take advantage of ram-air effects. I have been told that precision-balancing of radial compressor impellers is a nightmare, especially at high rpms, because of their odd shape.

[cudabean]

Mike,

Thanks for the response. I wasn't expecting such a good one in this somewhat dormant forum. I never thought of the dynamic balancing issues of radial compressors. However, the auto manufacturers and small gas turbine builders seem to favor the radial compressor for their small units. A larger radial compressor wouldn't have to be one piece and this could simplify dynamic balancing as well, for example, you could start with a flat plate with relieved channels for the impeller blades, though I'm not sure how challenging it would be to succesfully fasten them in considering the forces involved.

Also I acknowledge that you'd have inevitable losses transitioning from one stage to the next.

Another challenge would be designing the successive stages so that they operate at the same RPMs, but still operate efficiently at the different Reynolds numbers as the air gets compressed more and more. Seems like there'd be more than a little bit of cut-and-try to eventually get it right.

cudabean

[Hank]

The reason centrifugal compressors are rarely run in series are surge effects between the units causing pressure losses. The controls required are more of a complexity then running an axial flow unit.

The US invested a considerable chunk of change in developing a two-stage cetrifugal compressor for the version of the Allison V-1710 engine that was to power the P-39 Bell Interceptor. The costs were considered and the engine was installed with a single-stage unit. All the performance benefits were lost and America went to war with a fighter that lost power with rapidity above 16,000 ft. The P-39 was relegated to the backwaters in the middle of 1942.

Hank

[Mike Everman]

There are several multistage centrifugal blowers with electric motors you might be able to tear apart and use, go to http://www.aaaim.com/cgi-local/shop991/ ... tm#ACB9804
Or search through http://www.candhsales.com
Good surplus resource for this kind of stuff. Cheap, too. The 5-stage (can that be right?) 50cfm above is only $65. Been planning on tearing one apart forever for just this reason.
Happy hunting,
Mike
And please, if you get one, post a photo of it's guts for us, OK?

[sormlord]

garret or air research just produced a maltistage radial turbo
to meet tighter emissions laws for dessel tractor's stated the
inharent componding affect of to radial compresor on the same shaft.

[Joe]

Have you seen the AFRL Propulsion Directorate work of late? I think they had some project to simplify gas turbines and make them more reliable, etc., and they used four axial stages followed by a single centrifugal wheel in one of the test engines.