Page 49 - BASIC CONCEPTUAL OF THERMOFLUID

EXAMPLE 6

The specific internal energy of a fluid is increased from 120 kJ/kg to 180 kJ/kg during a constant
volume process. Determine the amount of heat energy required to bring about this increase for 2 kg
of fluid.
EXAMPLE 7

3
2.25 kg of fluid having a volume of 0.1 m is in a cylinder at a constant pressure of 7 bar. Heat energy
3
is supplied to the fluid until the volume becomes 0.2 m . If the initial and final specific enthalpies of
the fluid are 210 kJ/kg and 280 kJ/kg respectively, determine:
a) the quantity of heat energy supplied to the fluid

b) the change in internal energy of the fluid

SOLUTION : EXAMPLE 6

U1 = 120 kJ U2 =180 kJ

Q = ? W= 0 (Constant volume process)

ΣU = Q - W
U - U = Q - 0
1
2
180 -120 = Q - 0
Q = 60 kJ / kg ´2 kg =120 kJ

n
Polytropic process (pV = C)

• If a compression or expansion is performed slowly, and if the piston cylinder assembly is
cooled perfectly, then the process will be isothermal. In this case the index n = 1.
• If a compression or expansion is performed rapidly, and if the piston cylinder
assembly isperfectly insulated, then the process will be adiabatic. In this case the
index n = g.

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