Page 121 - Entrophy Analysis in Thermal Engineering Systems

114 Entropy Analysis in Thermal Engineering Systems

As the entropy generation terms in Eqs. (8.40)–(8.42) and (8.44) are per
unit flowrate of the fuel, the specific entropy generation of the cycle is deter-
mined by adding these terms as well as the entropy generation of the fuel
compressor, Φ fc . Hence,

(8.48)
SEG ¼ Φ c + Φ t + Φ com + Φ L + Φ fc

8.4.3 Illustrative example

Fig. 8.3 depicts illustrative numerical results where the thermal efficiency
and SEG are plotted versus the pressure ratio at three different values of
the turbine inlet temperature. The main observation is that a higher
thermal efficiency corresponds to a lower SEG. The maximum thermal
efficiency and the minimum SEG occur at the same pressure ratio for
each value of TIT. The pressure ratios at which the thermal efficiency
is a maximum and SEG is a minimum are 18, 23.7, and 31 for a TIT
of 1173, 1273, and 1373K, respectively. The respected maximum
thermal efficiency is 0.343, 0.370, and 0.395 at the above values of TIT.
The contribution of each irreversible process to the minimum SEG is
shown in Fig. 8.4 for three values of TIT. The largest source of the SEG

Fig. 8.3 Variation of the thermal efficiency and specific entropy generation of the gas
turbine cycle with the pressure ratio at three different values of TIT (η c ¼η fc ¼0.85,
η t ¼0.90, T 0 ¼298.15 K, p 0 ¼1 bar).