Page 83 - DJJ20063- Thermodynamics 1

DJJ20063- Thermodynamics 1

b) Kinetic energy

A fluid in motion possesses kinetic energy. If the fluid flows with velocity C, then,

for unit mass of fluid,

C 2
Kinetic energy =
2

c) Internal energy
All fluids store energy. The store of energy within any fluid can be increased or

decreased as a result of various processes carried out on or by the fluid. The

energy stored within a fluid which results from the internal motion of its atoms

and molecules is called its internal energy and it is usually designated by the letter

U. If the internal energy of the unit mass of fluid is discussed this is then called the
specific internal energy and is designated by u.

d) Flow or displacement energy
In order to enter or leave the system, any entering or leaving volume of fluid must

be displaced with an equal volume ahead of itself. The displacing mass must do

work on the mass being displaced, since the movement of any mass can only be

achieved at the expense of work.

Thus, if the volume of unit mass of liquid (its specific volume) at entry is v1 and its

pressure is P1, then in order to enter a system it must displace an equal specific

volume v1 inside the system. Thus work to the value P1v1 is done on the specific
volume inside the system by the specific volume outside the system. This work is

called flow or displacement work and at entry it is energy received by the system.

Similarly, at exit, in order to leave, the flow work must be done by the fluid inside
the system on the fluid outside the system. Thus, if the pressure at exit, is P2 and

the specific volume is v2 the equation is then,

Flow or displacement work rejected = P2v2

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