Pumping
Basics
By Mike Sondalini
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Liquids
do not all behave the same. Blood
has different flow characteristics than water.
Paint flows differently to gasoline petrol.
Liquids are categorized by their behaviors when undergoing shear.
Those liquids that have a constant shear rate with change of velocity
(like water) are called Newtonian (
Newton
first developed the mathematical
explanation for the phenomenon). Those
with shear rates that vary with changing velocity (like paint and blood) are
Non-Newtonian. The shear rate is a
measure of a fluid’s viscosity or slipperiness.
The density of a fluid affects its viscosity.
Fluids with more mass per unit volume are heavier and require more energy
to move them and shear less easily. A
temperature rise decreases the viscosity and density of liquids.
The more viscous, or less slippery, a fluid the harder it is to get
shearing between layers. The high
viscosity prevents rapid velocity changes occurring between layers.
The sub layer in viscous fluids is thicker than in low viscosity fluids.
At
low speeds the whole flow across a pipe is laminar and the fluid slides over
itself. As the speed becomes faster
eddies start to form and cross the fluid layers.
A transition from laminar to turbulent flow develops.
At still higher velocities the flow in the core of the pipe becomes
turbulent with swirling eddies throughout. Figure
2 shows where the various flow regions occur at a tank nozzle.
Figure 2 Flow regimes at a tank nozzle.
The laminar sub layer is
always present against the pipe wall. But
as the velocity rises the energetic swirling eddies begin to impact more deeply
and the sub layer begins to thin. At
still higher velocities the sub layer thins further and the taller roughness
peaks stick into the turbulent region. Where
the sub layer covers the roughness projections the wall is considered
‘smooth’. When the wall
roughness pokes out of the sub layer the wall is considered ‘rough’.
This means the same wall can be both smooth and rough depending on the
fluid’s velocity.
Experiments
have proven the pressure loss along a pipe with laminar flow is proportional to
the velocity (p µ
V) where as for turbulent flow the pressure loss is proportional to the square
of the velocity (p µ V2). A
slower flow permits a thicker sub layer and creates a ‘smooth’ pipe wall.
This minimizes the losses along the pipe.
There is a very much greater loss of pressure in turbulent flow.
The
pipe system designer has to strike a practical balance between increasing the
pipe diameter to reduce energy loss and keeping the diameter small to lower
installation costs.
Continued in Pumping Basics E-Book ...
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