Compressor matching

[lilrex]

Hey I am designeing an axile flow engine, and have a lot of questions that you guys should be able to help me with. :)

what is the most effecient matching for a compressor to a turbine. now I know this will involve some math. I am a little lost on this one so any help would be apreciated!

I know how to calculate the ideal flow useing vector math, now the pressure rise per stage is found by calculateing the speed increase per stage then using brunellies equation to fing the pressure rise at the diffuser, am I correct?

about blade dynamics, what is the typical blade loading in psi? what airfoil do you suggest? what angle do you suggest as a good traid off between blade stall at starting and good running characteristics?

I have a meager idea for most of the questions however there are a few importent ones:
Blade loading, blade angle, compressor matching.

thanks,

[lilrex]

I thought I should clarify the blade angle question, I know the blade angle changes depending on what stage it is and there is a "twist" to the blades to correct for the possition realitive to its radius. however I would like to know its absolute angle realitive to the incomeing fluid vector at operating velocity.

like should I insure that the blade does not stall when the flow is at a near zero? or should I go ahead and design the blades to impell the fluid in the most effciant way at operating velocity?

thanks

[racketmotorman]

Hi ??
Axial compressors , even in their simpler forms, are notoriously complex creatures with very narrow operational rpm and air flow ranges compared with a modern radial compressor .
I'd suggest you get hold of a couple of good books specifically on axial compressor design and read about their complexity before you try making anything .
Your questions requires many thousands of words of explanation . far too difficult to give simple answers , so many variables .

Cheers
John

[lilrex]

I realize that it is a chalenging project, I wouldent be interested if it werent, I will be more specific as to my questions, perhaps you start with a simple question like recomended blade loading? this will allow me to calculate the effectiveness of the impeller, and will provide the maximum mass flow through the impeller and give me a base for the blade angle and RPM. this sort of thing is found experimentally which is why I am asking here :)

[racketmotorman]

Hi
Work on a maximum inlet air velocity of 500 ft/sec , this velocity will have a reduced static pressure (for density calcs ) of ~12.7psi .
Mean blade speed ~600 ft/sec , and work on a max pressure ratio across the stage of 1.2 :1
Cheers
John

[lilrex]

Cool, a place to start. thank you!

[Jason]

Hey any ideas how to mold an axial copmressor fan..............?

[Ash Powers]

Jason,

You could probably get away with building the axial fans along the same construction technique used to build turbine wheels from sheet metal; cutting, grinding, etc etc. That would be the most economical method to construct one - if you are prepared to dump a few thousand dollars into it, you could have molds CNC produced to create wax patterns, ceramic slurry them and melt out the wax, and then have them vacuum cast in aluminum. Casting parts like this isn't the best method IMHO given that casting does have imperfections - any small air cavities in the casting wont be seen and will present a serious failure mode.

Again, on the high-dollar approach, you could simply draw up the parts in CAD and take them to a CNC machine shop to have them machine the parts directly from billet.

Last thing you want is shrapnel flying in your direction, so if it were me, I think I would take the approach of the steel plate/cut/grind method for reliability and low-cost. Not exactly the lightest method but likely a lot safer.

Producing the actual parts is more like putting the icing on the cake though - your biggest obstacle will be in the design of the parts rather than construction.

[lilrex]

Jason,

Casting a fan out of aluminum should not be very difficult. The surface should even improve flow separation. All it would take is some meticulous molding but should not be much of a problem.

To make the pattern:

1 start with a disk of plywood or MDF (the thickness that the finished blades will take) cut 5 deg. bevels from the parting line so it will release properly. Use a lathe to thin the disk profile and make the hub leaving a ring along the periphery.

2 cut slots along the periphery of the disk used to place the individual blades, make sure the solidity of the fan is not so great that the blades overlap each other otherwise the mold will not release the pattern using this method of casting.

3 make the individual blades with the correct profile and glue them into the slots.

4 prepare sand and place the pattern on the plane board with your drag over the pattern and ram it up.

5 turn drag over and use a putty knife to clear a parting line between the blades extending from the top edge of the first blade to the bottom of the second and so forth.

6 ram up the coupe with a riser in the center of the hub and a sprue on the periphery with a sprue horn to the center riser.

7 put the flask together and pour the aluminum.

Seems extensive but it’s really not that difficult.

I use oil based sand from budget casting supply, and my furnace is made out of a popcorn can lined with kaowool and a 220V resistance element. The furnace will melt 6 lbs of aluminum in a hurry for about 75 cents. The whole setup probably cost me about 150 bucks.

I would be happy to discuss casting in detail if you wish.

You can measure the density of the part to find any voids and you can use a fiberglass screen in the mold to filter slag out of the melt.

You can try and heat treat it yourself but it might be good to send it to a professional as it is a relatively precise process. Try and stay with in the strength limits of the material and your safety factor which should be high enough to include the fatigue stress properties of the alloy you are using (this is important and will determine the life of your fan).