Number of compressor vanes

[matt512s]

Hi;
How does one determine the number of vanes for a
centrifugal compressor? Is it a function of rotor size?
Best Wishes;
Matt

[racketmotorman]

Hi Matt
Is that vanes on the wheel or in the diffuser
Cheers
John

[matt512s]

Hi;
I was asking about the wheel, but I should probably know about
the diffuser as well.
Thanks;
Matt

[racketmotorman]

Hi Matt
You can have anything from maybe 10 blades and upwards on a compressor wheel depending on size and the amount of work it has to do , a truck sized turbo comp wheel might have 6 full length blades and another 6 part length blades , slightly bigger turbos 7 of each , bigger turbo comps off stationary diesels 17 or 18 full length blades and I've got a comp wheel from an Allison 250 engine that has 36 blades on it but it has to spin fast enough for a 7:1 compression ratio so does a lot of work and the closer blade spacing is necessary .
There are simply too many variables to give a simple answer to "how many" .
As for the diffuser, the same thing can apply , I've seen huge 11inch dia comp supercharger diffusers off radial aircraft engines with only 7 diffuser blades yet there are small ones with twice that number, all depends on design criteria and personal preferences in some cases.
The best solution to your questions , is find an good engine thats doing what you want to do and copy the numbers :-))
Cheers
John

[skyfrog]

Hi

If you are designing a centrifugal compressor, use a number of vanes as large as you can, as it is helpful in getting bigger theoretical work done by the compressor. However vane itself has thickness and weight for providing adequate strength while operating in high speed, there is a compromise in number of vanes.

John has provided many nice examples as to existing compressors of engines. In addition, there is yet another example. My compressor has an OD of 128mm, and has 10 driving vanes, 10 flow dividing vanes, altogether 20 vanes were used. Mine may not be the best but I couldn't get any better in the number of vanes.

Check my website if you are interested.

[matt512s]

Hi Horace!
I was blown away by your website! What I am pondering is
the construction of a small turbine to turn an 18" ducted fan.
I have in mind an ultralight class aircraft that has at least an
hour of flight duration. Since a turbojet is a thirsty creature,
I would like most of the thrust to come from the fan. I have
considered exhausting pulse jets into a turbine that is con-
centric with the fan and using a Reynst jar combustor with a
compressor and turbine. I am limited by low funds, no welder,
and no metalworking experience. Can any turbine concepts
be adapted to run at lower temperatures and stresses? I
have access to molding materials and could build molds and
cast parts. I could even order high temp ceramic casting
material good to 3000 deg. F if it would take the mechanical
stress. Please tell me what is possible and impossible for
someone like mee to do.
Best Wishes;
Matt

[racketmotorman]

Hi Matt
I hope Horace doesn't mind me also helping to answer your query ,
Turbines work best on "high" rpm and unimaginable stress levels in combination with high temperatures , if you lower the rpm you also lower the compression ratio and by lowering the compression ratio the thermodynamics get degraded and you burn more fuel for less power output or you need to run at even higher temps in an attempt to restore the thermodynamics.
Having built two freepower turbined vehicles I can assure you its a very difficult task for a home tinkerer with limited tooling to work with .
Turbines are a fairly high tech sorta power plant and in comparison to an IC engine, very expensive , and for aircraft use need to be perfectly made , definitely something a home mechanic should not attempt , far too dangerous .
Cheers
John

[matt512s]

Hi John;
Thanks for your input. I guess maybe a lower tech approach
would be in order. Perhaps a Reynst jar exhausting into an
augmentor (to add mass flow and cool the gas), and then into
the concentric turbine/ducted fan I mentioned. I only need about
15-25 hp and I am trying to design the plane to be as small and
light as possible. It shouldnt take too much to achieve 55 kts at
full power in level flight. About how much oomph do I really need?
Best Wishes;
Matt

P.S. How about a low rpm compressor to lean out the Reynst mixture?

[racketmotorman]

Hi Matt
I'm currently working on putting a freepower turbine onto a pulsejet engine of around 50 lbs thrust to hopefully get around 20 hp at 15,000 turbine rpm , which is pretty slow for a turbine to run , but the karting chain and sprockets I'm using for the reduction are only safe for those sorta rpm, so thats whats she has to run at,.
I've got the 1100mm dia "fan" mounted along with its drive shaft and sprocket , the freepower turbine shaft is made and the housing is nearing completion .
Then just make up a freepower turbine wheel and pulsejet engine :-))

For a low speed aircraft you only need to move a lot of air slowly which requires relatively less horsepower than trying to get a high speed airflow which will only be wasteful on a low speed aircraft , so a large dia fan at slower rpm is what you need , but turbines need rpm to make power so a lot of reduction gearing usually required .
Try and stay clear of "compressors" , they need to run fast and consume huge amounts of power , once you start down the road of using compressors, you may as build a "normal" gas turbine engine .
Theres no simple way to get horsepower from turbines .
The best engine for what you want , is a small lightweight two stroke engine of up to 25 hp approx . driving a large dia prop thru a 2 or 3 :1 tooth belt reduction drive .
You'll get reliability and vastly better fuel efficiency than anything you can build .
If you look at what the ultralite guys are using , do the same , theres no point trying to "reinvent the wheel" :-)) if there was anything to be gained by using a different power plant , they'd be doing it .
Cheers
John

[matt512s]

Hi;
Oh, well; I guess it's back to the old pistons. I would have
liked the whine. Thanks for all your help.
Best Wishes;
Matt