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Looking at the turbocharger picture below there is a compressor and a turbine side.

  • If you dont know the flow direction of the fluid, how can you distinguish which side is the compressor and which is the turbine? Normally for subsonic design this would mean increasing cross section area is a compressor and reverse for turbine. However, since the design is very different considering the shape of the blades between both sides there seems to be a better way to identify it. How?

  • Why is on the left side of the pictures every second blade shorter? What effect does it prevent/provoke?

  • Is there a special reason why the blades dont have a certain profile (for example NACA profiles) like in airplane turbines instead of (near) constant thickness? This part seems milled with a 5/6 axis machine, so price should not change with a profile shape design.

turbine http://www.glr.tu-darmstadt.de/media/glr/turbolader/DSC_6847_3.jpg

Stefan
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  • it would be great if you would comment on my reply. Otherwise it's hard to help you here. Did my explanations help you? – rul30 Sep 13 '15 at 14:44
  • From having seen these things I would guess that the near side in the image is the compressor; however my aerodynamics isn't good enough to fully explain why. Considering operating temperatures may also be useful to you, I would expect the turbine side to be operating at much higher temperatures and therefore use different materials/manufacturing techniques which may limit the shapes that can be constructed. – welf Sep 14 '15 at 09:58

1 Answers1

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Regarding point 1: To be honest I am not 100% sure. However, here are my thoughts: As with all profiles the turning of the flow is easier when it is accompanied with a pressure gradient (from high to low pressure). Easier is quite an unspecific word lumping together a lot of undesirable aerodynamic features like boundary layer growth and or boundary layer separation. The gradient between high pressure and low pressure can be reduced by increasing blade length (reducing the pressure-gradient acting on the boundary layer). Regarding your mention of subsonic flow and cross section: In the case of a transonic compressor the inflow is supersonic and the outflow is subsonic. Plus, the compressor flow is always from a lower to a higher radius (the turbine is the other way around). So both sides will feature a bigger area at the higher radius and a smaller area ant the smaller smaller radius. So I guess we look at the compressor with the turbine in the background. Because it features longer blades with reduced flow turning at the outer diameter and splitter blades (end-bend).

Regarding point 2: The design of a radial compressor is influenced by a lot of parameters one is manufacturability. I guess that the requested pressure rise of the compressor and the associated turning of the flow was not possible with just seven blades, however the machining and minimum blade thickness did not allow for 14 full blades. If you are able to provide a meridional view of pressure rise one would probably see that the first section of the compressor has a lower loading and the rear sections of the passage feature a higher loading because it can be distributed across a larger number of blades. So in short the answer is: The design depends on the requirements in terms of manufacturability and thermodynamic behaviour (e.g. off-design characteristics). And what we see seems to be the optimal trade-off.

Regarding point 3: You need to keep in mind that NACA profiles are based on constant free stream flow velocity. In an axial machine this assumption is somewhat valid because the difference in radii of inflow and outflow stream surface are

small. In an radial machine the free stream velocity changes due to the higher circumferential velocity (i.e. higher radius). NACA profiles do not account for centrifugal forces which play a huge role in radial machines.

rul30
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  • Very good insight, could you please further explain the effects of "longer blades" and the "reduced flow turning" mentioned under point 1? Why is this done? – Stefan Sep 13 '15 at 18:28
  • I am not sure if I get your reply, can you please elaborate more which concept you do not understand, and why? – rul30 Sep 14 '15 at 20:23
  • I don't understand your last sentence of Point 1. I dont see what "reduced flow turning" (especially the word turning) has to do with the longer blades / splitter blades. The only thing I can derive (from pure logic) is that it would mean that shorter blades have a higher flow turning, but what does that even mean? To be honest pretty much the entire last sentence is unclear to me. – Stefan Sep 15 '15 at 14:53