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Pumping Basics

  By Mike Sondalini  

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HOW FLUIDS FLOW IN PIPES (Cont.)

VISCOSITY AND DENSITY EFFECTS

 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.

 VELOCITY EFFECTS

 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.

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