Page 124 - DJJ20063- Thermodynamics 1

DJJ20063- Thermodynamics 1

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

v) Heat received or rejected

During its passage through the system the fluid can have direct reception or rejection of
heat energy through the system boundary. This is designated by Q. This must be taken in

its algebraic sense. Thus,

Q is positive when heat is received.
Q is negative when heat is rejected.

Q = 0 if heat is neither received nor rejected.

vi) External work done

During its passage through the system the fluid can do external work or have external
work done on it. This is usually designated by W. This also must be taken in its algebraic

sense. Thus if,

External work is done by the fluid then W is positive.
External work is done on the fluid then W is negative.

If no external work is done on or by the fluid then W = 0.

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