# What does the actual path of air within a turbojet engine look like?

While Wikipedia describes the flow path as axial, I wonder if the path could be helical instead. Wikipedia explanation is described first in section 1, section 4 is for what could be the actual flow path, and sections 2 and 3 list the reasons for that. Section 5 is my question.

1. Usual high-level explanation

Wikipedia explains air flow in a jet engine this simplified way:

Flow path, Wikipedia, author: Jeff Dahl

Not obvious on the picture, there is a stator with vanes after each rotor, to create the pressure increase on vanes pressure side, and to straighten the flow for the next stage of the cascade, else there would be no compression and the engine would be an expensive fan. See this question for a picture showing the stators.

2. Taking into account blade and vanes

Usual depiction of the path, taking into account blades and vanes, is zigzagging, similar to:

The path above seems valid for motionless blades and vanes, ignoring blades are in rotational motion. If we dive into a bit more detailed explanation, the path would be refined like this:

Vanes pressure side is upward while blades pressure side is downward.

3. Taking into account blade rotation

With blades rotation, blades of the second stage do move while air is traveling across the first stage. While rotating, second stage blades deflect older air, which now exits the second stage using a vane which is lower than the vane it exited the first stage (during this time air of the first stage has reached the second stage, but is deflected by an upper blade, etc):

Air path is therefore spiraled a bit, the exact amount of the helical displacement depending of the ratio between air axial speed and blades angular speed. This is not necessarily the exact vane angular spacing (or a multiple of it).

4. Seemingly more consistent depiction of air path

If this was the correct way it works, then the overall flow would rather be helical than axial, something more like this:

5. Question: How blades and vanes impact overall air path? How the air path can be better explained, taking into account blades are not fixed?

I'm looking for some details, perhaps blade/vanes geometry and airspeed, not an over-simplified view.

• What did you use to create the air flow path in your second image? – par Nov 11 '16 at 20:08
• @isanae The OPs image "looks nice", but unfortunately it is completely wrong, because, as another answer said, it ignores the effect of the stator vanes (which are not very obvious in the either illustration, but absolutely essential to how the compressor and turbine stages work). – alephzero Nov 11 '16 at 20:55
• @alephzero Fair enough. I have zero knowledge, I just found this on the HNQ and I was hoping we could improve the Wikipedia article. Many people discuss articles on SE without ever making corrections to them. – isanae Nov 11 '16 at 20:58
• As @alephzero says, it's hard to see on those images but there is a static blade between each compressor stage called a stator blade. This removes the circumferential velocity and makes it axial again, ready for the next stage. – Notts90 supports Monica Nov 11 '16 at 21:22
• Ok, so there will be some rotation as per your image due to the stators not putting the flow back to its original circumferential position but I think it would be a lot milder. Rotating a blade pitch or two per stage. Bearing in mind you could have anything from around 28-30 blades to 50+ depending on the stage, the pitch isn't huge. – Notts90 supports Monica Nov 11 '16 at 22:20

## 3 Answers

(Source) Image shows CFD simulation for a jet engine carried out by Northern Arizona University.

Pretty much as your illustration, but more turbulent and not as many complete turns.

Let's take one rotor disk at a time.

Each blade on each disk is like a wing. Air is rushing in at it. And it, is moving down on the air (or up from the other side). Giving the blade an angle of attack.

In a wing, air is pushed down.

So, one disk will push down on the air on one side, up the other, left and right top and bottom—the stator disk will straighten it out for the following rotor. And so on, i.e., a stepping swirl.

If the stator reversed the flow, it will be an immense angle of attack for the receiving rotor, i.e., the rotor will stall. So, a stepping swirl makes more sense.

While a propeller, an axial compressor, and a turbine are different in what they do, I don't see why the shape of the corkscrew of the propeller I asked about here—would change for any rotating disk carrying airfoil blades—

(Source)

—except in magnitude and thermodynamic properties.

To complement the earlier CFD image, here are two more:

(Original image) Three stage transonic compressor.

(Source) Analysis of Rotor-Stator-Interaction and Blade-to-Blade Measurements in a Two Stage Axial Flow Compressor

• This may be of interest youtu.be/kYSfLXAjLzE – DeltaLima Nov 11 '16 at 21:12
• @mins this may help as well. youtu.be/jjT3aBOvrXM – DeltaLima Nov 11 '16 at 21:43
• @DeltaLima: It's basically the answer. Very clever. There is actually no axial flow at all, contrary to what has been assumed in other answers. I'm just beginning to understand how an engine works. – mins Nov 11 '16 at 21:57
• @mins I will try to compose an answer today or tomorrow. Problem is I only have a mobile phone to work from this weekend, so there will be little inkscaping – DeltaLima Nov 12 '16 at 6:11

You should do a little bit of research on blade velocity triangles. You'll see that

• Stators swirl the air
• Blades de-swirl the air

So if you look at several rows of stator-blade pairs, the air flow will look like a stepped helix. This video has an alright explanation of the theory:

If you want to have a good look inside engines, I recommend the AgentJayZ channel on youtube, he does excellent "show and tell" of components in an engine though doesn't cover all theory very well, for example:

• +1 that first video is exactly what is needed to answer this question. – Notts90 supports Monica Nov 12 '16 at 9:26
• AgentJayZ is great, one of few possibilities to see engines details (and be surprised). – mins Nov 12 '16 at 15:08

Blades and vanes impact the flow such that the flow follows the paths of the cascades. The principle of a compressor is that the blades increase the angular momentum of the flow. Thus, the flow somehow resembles the screw in your drawing, but only for a few clockings. The vanes remove the angular momentum and concert it into inner energy of the fluid. Ideally the flow is in axial direction at the outlet of a vane. Blade and vane cascades are installed intermittendly, such that the flow goes axial - screw -axial - screw.... The turbine works the other way around. The vanes create angular momentum that is removed in the blade cascades.

• "- screw -axial - screw....": So what is the overall path, isn't it a "sheared" helix? How many turns on the whole length of the core? – mins Nov 11 '16 at 21:33