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I understand what diffusion of mass or diffusion of heat means. I know that the conversion of the dynamic head of the flow into static pressure in a stator or diffuser is called 'diffusion'. But in the case of diffuser what is being 'diffused'? What is 'diffusion' in a stator/diffuser?

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"Diffusion" in this context means taking a flow of high-velocity gas and converting its kinetic energy into compression work, to compress that gas. In the specific case of a centrifugal compressor, as used for example in car engine turbos, an impeller forcefully slings air up to high speed and shoots it into a diffuser, which is a confined space where it the incoming air "piles up" , slows down, and gets squeezed up to higher pressure.

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A Diffusor in this context is the opposite of a nozzle. While a nozzle speeds up a fluid/air flow by diminishing flow diameter, diffusion does the opposite by giving it more room, thus slowing it down and increasing pressure.

This is often done to slow down air speed below mach in order to protect a rotor/stator from damage.

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  • $\begingroup$ Opposite of compression is expansion isn't it. Compression doesn't speed up the flow it increases its pressure and maybe also density. $\endgroup$ – GRANZER May 24 '20 at 16:48
  • $\begingroup$ Compression via tightening the flow. Maybe the wrong word, as it speeds up the flow, but actually lowers pressure. I'll have to look up the real term, English isn't my native language and I'm not used to writing about physics in English. $\endgroup$ – Sascha Rambeaud May 25 '20 at 13:03

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