I'm reading about how the stator vanes are diffusors, which diffuse the air flow in a axial compressor to a larger flow area to build up pressure BUT at the same time the compressor shows its overall flow area getting REDUCED at the same place. So, how does that work out? Or in other words: What is the causal chain between the air getting a higher velocity in the rotor blades and the higher pressure in the exhaust port?

  • $\begingroup$ If the area is reduced for the same mass flow rate then the velocity increases. Check continuity… $\endgroup$
    – Solar Mike
    Commented Jan 21, 2022 at 14:52
  • $\begingroup$ Well, that's not the problem to understand. My problem is that texts state a diffusion in the stator and at the SAME time a flow area decrease. Those 2 together is what's making me scratch my head. $\endgroup$
    – Riccardo
    Commented Jan 21, 2022 at 16:52
  • $\begingroup$ Suggest that you consult some of the better books around. Engineering Thermodynamics Work and Heat Transfer is one possibility, 3rd Ed Rogers & Mayhew,, starting at page 448. $\endgroup$
    – Solar Mike
    Commented Jan 21, 2022 at 16:58

1 Answer 1


The device has to be designed so that each stage does its job.

The stator reduces the high velocity out put of a stage to a lower velocity, increasing the pressure. But the subsequent rotary section needs a smaller area because the higher pressure gas takes up less volume. So the two have to be designed in tandem. This is really advanced flow design. My guess is that most of these design parameters have been arrived at emperically, not via math. I would expect gas turbine manufactureres to have some really advanced modeling software, but with the equations being developed emperically.

  • $\begingroup$ So, makes you wonder how they made the earlier ones... $\endgroup$
    – Solar Mike
    Commented Jan 21, 2022 at 15:54
  • $\begingroup$ Well Parsons for example used an annular flow with steps instead of a continuous increase in flow area (turbines increase flow area instead of compressors, which decrease it.). $\endgroup$
    – Riccardo
    Commented Jan 21, 2022 at 16:47
  • $\begingroup$ Tiger Guy, can you be more specific? Where exactly do all these things happen exactly. The pressure rise, the diffusion, the flow area decrease, the velocity de- and increase? I have answers for some of those, but the understanding is missing about WHERE some things happen. So, I'm still not able to understand it causally from inlet to outlet. $\endgroup$
    – Riccardo
    Commented Jan 21, 2022 at 16:51
  • $\begingroup$ @Riccardo all this is happening basically in the same place. $\endgroup$
    – Tiger Guy
    Commented Jan 21, 2022 at 17:03
  • $\begingroup$ Hmmh. Then we are back at the original question. How does diffusion and conflation work at the same time/in the same machine element? $\endgroup$
    – Riccardo
    Commented Jan 21, 2022 at 17:41

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