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- How much room is needed between fan or turbine blades before the lose their effectivness? — Like suppose, on our CAD machine, we had a hub with 10 or 12 rotors on it, and we keep adding more identical rotors, moving all the rotors equally distant from each other. Eventually what we arrive at is not a hub with rotors so much as a disk with venturi-shaped slots around the edge. Is this better or worse, and do blisks (-oooooh!-) such as this posess any special properties? - MC
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I’m not a turbine engineer, but I believe that the answer depends on blade angle and rotation speed. Faster rotation speeds would generally allow less spacing. But to ensure good airflow at all air speeds, angles of attack, etc., they often have a twist to the blades; that is, they don’t have a constant angle from hub to tip. I know that in jet engines the blades on the compressor sections (the front part)are often set loosely into the hub to allow for temperature expansion. When the wind blows and turns the compressor rotors of turbofans at rest, you’ll sometimes hear s clink-clink sound as the rotor windmills.
IANAE either, but I think the reason the turbine blades are twisted is the same reason a propeller is: to provide similar airflow over the entire span of the blade. When a turbine or prop blade moves through the air it develops lift; but since the lift is in the horizontal plane instead of the vertical, we call it “thrust”. The tip of the rotating surface naturally moves faster than the root. If the angle of attack were constant, then it would develop more lift at the tip and cause undue stress to the component. By giving the root a high AoA and the tip low AoA, the blade develops approximately the same lift over it’s entire surface and reduces the flexing stress of the part. Of course the turbine blade or prop is most efficient at one speed. “Constant speed” propellers have variable pitch to make them more efficient over a greater range of speeds, but it would be an engineering nightmare to have “constant speed turbine blades”.
As for the OP – I don’t know.
I’m not sure I completely understand the question, but in the Straight Dope spirit I’ll try to answer it anyway.
First, Johnny is right, and the same principles apply to compressor and turbine blades as to propellers. The best performance is obtained when the entire span of the blade has the same pressure ratio, accounting for the twist. Since pitch of the blades on the rotating parts cannot be varied, the angles of the stator vanes between the rotor sections are varied to control flow vs. speed.
You’re right, it would be ideal aerodynamically and mechanically to have the rotor and stator vanes be as short axially as possible, but the strength and vibration frequencies, as well as the gaps needed for thermal growth and dimensional variation, are practical limits. Generally, the need to allow the wake from one row of airfols to die out somewhat before hitting the next row is the controlling factor, however.
The stator vanes are necessary, not only to control the airflow volume and pressure ratio for each rotor stage at various power settings, but simply to direct the discharge angle of the flow from each stage at the inlet side of the next rotor stage. This has to be done gradually because the compressor is trying to force air into increasingly-smaller volumes, against the pressure gradient, and the air wants to flow the other way (that’s called a “stall”, and produces loud popping noises and occasionally allows combustion flames to come out the front of the engine, giving the passengers some entertainment).
A “blisk” (lovely word, eh?) is a combination of the disk and blades in a single piece of metal, rather than having separate blades retained by dovetail joints. That reduces weight and size, and improves durability, but substantially increases machining cost and creates maintainability and repairability problems. They also do not have the inherent blade vibration damping that dovetails provide. Blisks are used only on the forward (larger) sections of axial compressors, because the blades at the aft end are too small to fit cutting tools between. They can’t be used at all on turbines that require internal blade cooling, because the cooling air requires dovetail joints to allow it to be introduced, and because the material requirements for turbine blades are so different from those for disks.
Some development is being done on various super-high-strength materials that will allow compressor stages to be made as integrally-bladed rings, or “blings” (how do ya like THAT one?).
If that isn’t already more than you wanted to know, please say so.