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I've been studying the design of turbopumps and their impellers, all the design equations and what-not; this question concerns the contours of the impeller vanes. I'm going to attempt to enter aerospace engineering in the future, so I figured I might as well start getting exposure to the concepts now.

I'm currently attempting to design an impeller in CAD to test my knowledge of the equations as well as get better at CAD, but one of the facets of the design rather baffles me.

As you can see here (source, page 208):

Impeller radial contour

The calculation of the radial contour of the impeller (that is, concerning $\beta_1$ and $\beta_2$ should be relatively easy given the other content in the paper, but radial impellers in turbochargers and other such equipment almost always appear to have a "lip" or forward facing top edge near the eye of the volute or casing inlet.

Here is an example of what I'm referring to: CAD-modeled impeller

You can tell that the vanes are backwards-curved, but near the top of the impeller vane closer to the eye, the curvature reverses and flares forward to approach near-flat slope.

My questions are:

  • What is the purpose of this axial contour (forgive me if that isn't the proper term)?

  • Are there specific design parameters associated with that contour? The Rocketdyne text above didn't seem to make any mention of the axial profiles of the impeller, other than the diagram to the left describing the dimensioning of the shrouds.

  • Where can I find an explanation and source of design information for how to design the contour?

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I should have a little more overview than I have of pumps but I don't. What you refer to seems to be called an entrance vane or an impeller guide vane. The purpose is to help the water gain the required rotational velocity:

... in connection with a centrifugal impeller it is desirable to provide an axially extending entrance-vane section having vanes curved such that their radius of curvature gradually decreases in the direction of the fluid flow there through. Source [Patent, 1945]

The water approaches the impeller with a flow direction along the axis. Without curving it inwards, the water would have a very abrupt change from going straight to moving with the blade, with a considerable waste of energy.

However, by curving the blades inwards, you 'scoop' the water up. The water will be pushed along the the slope of the vain and will help turn the axial motion that it had to rotational, increasing the pump efficiency.

This design is not easy to machine and hence only used where necessary, like rocket engines where you need a good deal of pressure. Most applications are just not worth the trouble.

With this construction, the entrance vane section comprises a hub portion having a plurality of vanes extending radially therefrom, with each of the vanes having a rather complicated twisted profile. Obviously, such an entrance vane section is difficult to cast and/or machine. Accordingly, it is an object of this invention to provide a novel construction of such an impeller to facilitate its fabrication.

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  • $\begingroup$ Thank you for a clear and concise answer; it makes sense that it would be only used in scenarios demanding high efficiency; with some turbopumps reaching >90%, I suppose their development would justify the machining complexity. $\endgroup$ – ecfedele Apr 10 '15 at 15:55

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