Modelling Suction Pressure Under Low Temperature Conditions 4

[KimWonGun]

Considering suction pressure of an oil pump in temperatures below -20F, test data show suction pressures that fall short of the 14.5 psi threshold. But my attempt to model this predicts cavitation because the suction pressure is calculated to be at 14.5 psi.

I am using the equations f = 64/Re and P = f*[(q/a)^2*rho*L]/(2*D). What am I overlooking?

 

[KimWonGun]

The viscosity at -20F is extrapolated.

 

[KimWonGun]

Someone in my office suggested that at high viscosities the Darcy-Weisbach equation is inadequate, ignoring fluid flow in the pipe center. Is that correct?

 

[ione]

If the fluid has a newtonian behaviour and it is incompressible I can’t see why the Darcy Weisbach equation should not be applicable. Viscosity has an effect for the determination of the particular flow regime you are dealing with through the Reynolds number, and in particular at high viscosity you can have a laminar flow.
If you treat the friction factor in the appropriate way, that is applying f = 64/Re, there’s nothing wrong in using the Darcy Weisbach equation.

 

[BigInch]

They are probably thinking that high viscosities, while not necessarily non-Newtonian, they can be a very good indicator of a non-Newtonian fluids.

Thermal effects on high viscosity fluids can also make the standard equations inaccurate, being that the centerline flow can hold original entry temperature while the temperature of the fluid in contact with the pipe wall could be much higher, or lower, often establishing a sharp temperature break, which could cause the core flow to be in a totally different regime than the fluid near the wall. It is possible for example to have laminar flow at the wall and turbulent flow in the core, or v/v, or still yet a cold fluid's very high viscosity at the center, and hot low viscosity at the wall, or again v/v.

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