Page 77 - ebook fluid mechanics_finalize

FLUID MECHANICS

4.4 Energy of a flowing fluid

A liquid may possess three forms of energy:

a. Potential energy

If a liquid of weight W is at a height of z above datum line

Potential energy = Wz

Potential energy per unit weight = z

The potential energy per unit weight has dimensions of Nm/N and is measured as a
length or head z and can be called the potential head.

b. Pressure energy

When a fluid flows in a continuous stream under pressure it can do work. If the

area of cross-section of the stream of fluid is a, then force due to pressure p on

cross-section is pa.

If a weight W of liquid passes the cross-section

W
Volume passing cross-section =


W
Distance moved by liquid =
 a

W p
Work done = force  distance = ap  = W
 a 

p p
pressure energy per unit weight = =
  g

Similarly the pressure energy per unit weight p/W is equivalent to a head and is

referred to as the pressure head.

c. Kinetic energy

If a weight W of liquid has a velocity v,

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