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Diffuser: pressure drop or rise?

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Old   November 20, 2013, 17:39
Default Diffuser: pressure drop or rise?
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Hello all,
there's a really basic question that has been bothering me for a few days now.

Let's consider a diffuser, ie a conical portion of a duct for which the inlet area is smaller than the outlet area. Now, due to the Bernoulli equation and mass conservation, at the outlet I should have a lower velocity than at the inlet which in turn results in a higher pressure at the outlet so, along the pipe, my pressure is actually increasing.

Now, looking at every hydraulics manual, it is stated that in such a pipe I should have a local pressure loss instead. Such a pressure drop is proportional to the massflow and to the area reduction.

What's the wrong bit here? Am I missing something?

Thanks for your help!

Giovanni
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Old   November 21, 2013, 01:50
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Hamid Zoka
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Bernoulli equation does not consider the viscosity and therefore the skin friction between fluid and pipe walls.
In case of diverging pipes, there is an increase in outlet static pressure due to the increase in section area. But on the other hand there is also a pressure loss due to skin friction between fluid and pipe wall which degrades the total energy of the flow.
In other words, diverging pipe not only does not give the energy to the flow (it just convert the velocity to pressure!) but also decreases the energy content of the flow due to skin friction. Therefore the delta pressure will be the algebraic sum of these two.

I hope it helps!

Regards
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Old   November 21, 2013, 03:30
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Helps a lot! Thanks!
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