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This seems like a very basic level question but I'm really trying to wrap my head around it and understand the problem.

When looking through the literature/articles on pressure recovery in a diffuser nobody seems to mention (or I'm too thick to understand the obvious conclusion) why would we want to do it?

To sum up my understanding of the problem; fluid enters the reduced cross section, because of conservation of mass, the velocity is increases. Because of Bernoulli's principle, increase of velocity causes/is accompanied by the reduction of pressure (static pressure). When the cross section increases again (in a diffuser), velocity decreases and pressure increases. The increase in pressure after contraction is called pressure recovery. I added the image bellow.

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Now my question is, why would we want to do it? Theoretical answer is that we increase the enthalpy of fluid and this directly increases the isentropic efficiency. I get the theory but I cannot understand the real-life behaviour.

The only conclusion I got is that when velocity decreases, friction and turbulent losses are lower.

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The example you furnish is not a useful application of pressure recovery, it just illustrates how it happens. The principle of pressure recovery is important in the design of for example radial flow jet engines, as follows.

In a radial jet engine, a fast-spinning disc with radial vanes on it takes in outside air and accelerates it to great speed by flinging the air radially outwards. At the rim of the disc is an annular tube which surrounds the disc rim and the high-speed air is forced into this tube, which is called a diffuser. In the diffuser, pressure recovery occurs which converts some of the kinetic energy of the fast-moving air into potential energy of compressed air. The compressed air is then ducted out of the diffuser and into the combustor cans where heat is added to the flow by burning fuel.

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  • $\begingroup$ if I understand correctly; fast moving air at the exit of the fast-spinning disc is "less useful" for the combustion than the compressed air at the exit of diffuser (higher density of oxygen molecules etc)? $\endgroup$
    – user2882635
    Commented Jul 19, 2022 at 11:39
  • $\begingroup$ @user2882635, the point is that you want the air compressed to maximize the efficiency of the subsequent burn. this is why you want the diffuser to convert kinetic into potential energy. $\endgroup$
    – niels nielsen
    Commented Jul 19, 2022 at 16:16
  • $\begingroup$ thanks for the clarification. what I don't really understand is why the pressure recovery is talked about in valves and pipe flows - what is it's purpose. $\endgroup$
    – user2882635
    Commented Jul 20, 2022 at 6:55
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A pump (liquids) or compressor (gases) must provide enough pressure to overcome the pressure drop in the total length of piping that the flow is traveling through. It is desirable to minimize pressure drop in order to minimize the pump or compressor size and to minimize the energy needed to operate that equipment. Pressure recovery is desirable because it helps to do that.

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