Page 127 - Mechanical Engineers' Handbook (Volume 4)

116 Thermodynamics Fundamentals

• Expander or turbine with negligible heat transfer to the ambient (Fig. 5b)
˙
First law W ˙m (h h )
T
2
1
Second law S ˙ gen ˙m (s s ) 0
1
2
h h
Efficiency 1 2 1
T
h h 2,rev
1
• Compressor or pump with negligible heat transfer to the ambient (Fig. 5c):
˙
First law W ˙m(h h )
C
1
2
Second law S ˙ gen ˙m(s s ) 0
2
1
h h
Efficiency 2,rev 1 1
C
h h 1
2
• Nozzle with negligible heat transfer to the ambient (Fig. 5d):
1
2
First law – (V V ) h h 2
2
2
1
2
1
Second law S ˙ gen ˙m(s s ) 0
2
1
V V 2
2
Efficiency 2 1 1
N
V 2 2,rev V 2 1
• Diffuser with negligible heat transfer to the ambient (Fig. 5e):
2
First law h h 1 – (V V )
2
2
2
1
2
1
Second law S ˙ gen ˙m(s s ) 0
1
2
h 2,rev h 1
Efficiency 1
D
h h
2 1
• Heat exchangers with negligible heat transfer to the ambient (Figs. 5f and g)
First law ˙ m (h h ) ˙m (h h )
hot 1 2 cold 4 3
Second law S ˙ ˙m (s s ) ˙m (s s ) 0
gen hot 2 1 cold 4 3
Figures 5f and g show that a pressure drop always occurs in the direction of flow, in any
heat exchanger flow passage.
REFERENCES
1. A. Bejan, Advanced Engineering Thermodynamics, 2nd ed., Wiley, New York, 1997.
2. A. Bejan, Heat Transfer, Wiley, New York, 1993.
3. A. Bejan, Entropy Generation through Heat and Fluid Flow, Wiley, New York, 1982.
4. A. Bejan, Entropy Generation Minimization, CRC Press, Boca Raton, FL, 1996.
5. A. Bejan and S. Lorente, ‘‘The Constructal Law and the Thermodynamics of Flow Systems with
Configuration,’’ Int. J. Heat Mass Transfer 47, 3203–3214 (2004).
6. A. Bejan, Shape and Structure, from Engineering to Nature, Cambridge University Press, Cambridge,
UK, 2000.