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Lookid October 24, 2017 05:55

Flow from low to high pressure
 
Hello,

I was wondering. You have flows that are moving from low to high pressure, for example in a Venturi. In this case, you have a deceleration of the fluid with that increase of pressure.

On a centrifugal-radial pump, the flow goes from low to high pressure, so you have an increase of pressure and the fluid is accelerated. If I understood, it is because the path of the flow is curved and goes from a small radius to a higher one when the flow is going away from the eye. And according to the book of Gulich, Centrifugal Pump, the pressure is low on small radii paths and increases with higher radii.
(Please correct me if this is a wrong explanation)

So now, I was wondering if it is possible to have an increase of pressure with an accelerated flow on a straight flow (non-curved path) ?

LuckyTran October 28, 2017 19:06

Check out Fanno Flows and Rayleigh Flows or just compressible flows in general.

Lookid October 29, 2017 08:25

Quote:

Originally Posted by LuckyTran (Post 669553)
Check out Fanno Flows and Rayleigh Flows or just compressible flows in general.

I might misunderstand it but the same principles seem to apply :

Rayleigh
Subsonic flow: heating > Mach number increases > Velocity increases > Pressure decreases
Supersonic flow: heating > Mach number decreases> Velocity decreases> Pressure increases

Fanno
Friction causes entropy to rise until a maximum: Mach number = 1
Subsonic flow: Mach number increases > Velocity increases > Pressure decreases
Supersonic flow: Mach number decreases > Velocity decreases > Pressure increases

So no velocity and pressure increasing at the same time

e_cfd November 16, 2017 12:18

For the venturi, still flow is from high to low pressure (total pressure i. e. static+dynamic pressure). In fact, in this case by decreasing velocity, the static pressure increases (not the total pressure) and energy is conserved. (Bernoulli principle)

In centrifugal pumps same thing happens. In addition, in this case, the pumpīs blades deliver energy to the flow thus, from the inlet to the outlet the energy of the flow increases (total pressure as well)

FMDenaro November 16, 2017 12:41

Just consider the Fanno flow by means of a pysical sketch. The straight channel geometry is "modified" by the spatially evolving boundary layer that causes a restriction of the section. That changes the velocity depending on subsonic/supersonic flow.


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