TITLE.PM5

210 ENGINEERING THERMODYNAMICS

dharm

/M-therm/Th4-6.pm5

(b) Using Mollier chart.

Refer to Fig. 4.77.

Isentropic

2

1

15

ba

r

7.

5

ba

r

x = 0.95 1

Throttling

0.06 bar

(^3) x=1 3
Saturation
line
x (^2) =
(^0). (^90)
2692
2560
h (kJ/kg)
s=s s = 8.3 3
= 6.23
12
s (kJ/kg K)
Fig. 4.77
lLocate point 1 at an intersection of 15 bar pressure line and 0.95 dryness fraction line.
lDraw vertical line from point 1 intersecting 7.5 bar pressure line at point 2. Line 1-2 repre-
sents isentropic expansion.
lFrom point 2 draw a horizontal line intersecting at the saturation line at point 3. Line 2-3
then represents throttling expansion.
From Mollier chart :
h 1 = 2692 kJ/kg, h 2 = 2560 kJ/kg
s 1 = s 2 = 6.23 kJ/kg, s 3 = 8.3 kJ/kg K
∴ (i) Change in entropy = s 3 – (s 1 or s 2 )
= 8.3 – 6.23 = 2.07 kJ/kg K (increase). (Ans.)
(ii) Change in enthalpy = h 2 (or h 3 ) – h 1
= 2560 – 2692 = – 132 kJ/kg
= 132 kJ/kg (decrease). (Ans.)

4.15. Unsteady Flow Processes

In engineering practice, the variable flow process applications are as common as the steady
flow process. The rate of energy and mass transfer into and out of the control volume are not
same in the case of unstable (or variable or transient) flow process.
Following two cases only will be discussed :

1. Filling a tank.


2. Emptying a tank or tank discharge.