TITLE.PM5

FIRST LAW OF THERMODYNAMICS 151

dharm

/M-therm/Th4-4.pm5

Fig. 4.30 shows a schematic flow process for an open system. An open system is one in
which both mass and energy may cross the boundaries. A wide interchange of energy may take
place within an open system. Let the system be an automatic engine with the inlet manifold at the
first state point and exhaust pipe as the second point. There would be an interchange of chemical
energy in the fuel, kinetic energy of moving particles, internal energy of gas and heat transferred
and shaft work within the system. From Fig. 4.30 it is obvious that if there is no variation of flow
of mass or energy with time across the boundaries of the system the steady flow will prevail. The
conditions may pass through the cyclic or non-cyclic changes within the system. As a result the
mass entering the system equals the mass leaving, also energy entering the system equals energy
leaving.

Z 2

l 2

l 1

Z 1

A 1

p 2 A 2

p 1

V= A 111 l

V= A 222 l

Boundary

System

Datum plane

Fig. 4.30

The steady flow equation can be expressed as follows :

u 1 + C^1

2

2 + Z^1 g + p^1 v^1 + Q = u^2 +

C 22

2 + Z^2 g + p^2 v^2 + W ...(4.45)

(u 1 + p 1 v 1 ) +

C 12

2 + Z^1 g + Q = (u^2 + p^2 v^2 ) +

C 22

2 + Z^2 g + W

h 1 +

C 12

2 + Z^1 g + Q = h^2 +

C 22

2 + Z^2 g + W [Q h = u + pv]

If Z 1 and Z 2 are neglected, we get

h 1 + C^1

2

2

+ Q = h 2 + C^2

2

2 + W ...[4.45 (a)]

where, Q = Heat supplied (or entering the boundary) per kg of fluid,

W = Work done by (or work coming out of the boundary) 1 kg of fluid,