Aleph Zero:
Sorry, I could not find anything published to the public domain as to how concepts of centrifugal compressor design is incorporated into axial compressor design, to produce a more efficient hybrid that looks like a regular axial compressor to the person who does not know the difference. I can tell you generally how this is done, but keep in mind that I’m also being careful not to violate any Intellectual Property (IP) agreements.
First, I’ll mention the two things are done different from conventional axial design. Then I’ll tell you generally how they work together to make the compressor more efficient. If you have a good understanding of the aerodynamic analysis that goes into both an axial and a centrifugal compressor, then you can derive much more from what I say. But if not, you will still understand the basics of what is being done.
The first of the two things that is done is only done to within the space constraints available. A flight engine may not have much space available because you have to keep the compressor small to permit the bypass flow from the fan. You are also somewhat space limited near the engine centerline because of the need to put a fat fan shaft out the front of the engine. But you don’t have these space constraints in most industrial gas turbines, so you can do this to a greater extent. The idea is to get the flow path at the front of the compressor to look like the flow path at the front of a centrifugal compressor. Basically, the air enters near the center and immediately finds itself directed to a flow path at a much greater radius.
If you Google images for the PW4000, which was designed about the time these things were learned and by the same engineers who first learned them, you will see where they did their best to make the front part of the flow path look this way, but were seriously limited by these space constraints.
The second thing they do is they make the bottom of the front blades look like a centrifugal impeller. They even call that the impeller section. The cross section of the blade in that area does not look like the regular air foil section of a compressor blade. The top of the blade looks like a regular airfoil section, and that is where nearly all the compression work is done.
I said two things, but I guess I meant three. Besides this, the vanes behind the blades are designed to perform a similar function as the diffuser section of a centrifugal compressor. But in my opinion, you can’t tell the difference by looking at them, even though the aero guys say they designed it that way. Mechanical designers like me make the blades to the section defined by the aero guy, and we don’t always understand all the details as to why they look that way.
As the air enters, it is spun to a great tangential velocity by the lower impeller section of the blades. It moves radically and is compressed as it does so, just like in a centrifugal compressor, until it gets to the section of the blade shaped like a regular air foil, which retains much of the tangential velocity, but also moves the air back to the vanes. The vanes act like a diffuser and straighten out the air flow and distribute it properly to repeat the process in the second stage of impeller/blades.
If you check out the Ramgen web site, you can see where they brag about a new compressor design that they are trying to develop. It is an idea that many people have tried to make work in the last five decades. All the aerodynamics professors believe that is should work and that it should give us a compressor that is both a whole lot more efficient and less expensive to build. Perhaps Ramgen will be successful. The idea is to take these principles to the max and shape the whole blade like an impeller, which results in supersonic airflow coming off the back of the stage. That much is easy. What is hard is the next step where they process the air to convert all that airspeed into pressure efficiently. If someone can figure out how to make this work, then they will be able to replace five or six normal axial stages with one impulse stage. If they are successful, it may very well make carbon sequestration economically viable, and then we can have a coal plant with zero emissions. How much would that change things?
