Page 154 - Advanced thermodynamics for engineers

140 CHAPTER 6 FINITE TIME (OR ENDOREVERSIBLE) THERMODYNAMICS

Explain why this value of h differs from that of the Otto cycle, and discuss the significance of
T 3 T 2
the term . Evaluate the entropy change required at the high-temperature reservoir
T 3 lnðT 3 =T 2 Þ
to supply the Otto cycle in terms of the entropy span, s 3 –s 2 , of the Otto cycle.
h . i
Ds H ¼ c v T 2 e Ds=c v 1 T H

P6.6 It is required to specify an ideal closed cycle gas turbine to produce electricity for a process
plant. The first specification requires that the turbine produces the maximum work output
possible between the peak temperature (1200 K) and the inlet temperature (300 K). The
customer then feels that the efficiency of the turbine could be improved by
1. incorporating a heat exchanger;
2. introducing reheat to the turbine by splitting the turbine pressure ratio such that the pressure
ratio of each stage is the square root of the compressor pressure ratio.
Evaluate the basic cycle efficiency, and then evaluate separately the effects of the heat
exchanger and reheat. If these approaches have not increased the efficiency, propose another
method by which the gas turbine performance might be improved (between the same
temperature limits), without reducing the work output; evaluate the thermal efficiency of the
proposed plant. The ratio of specific heats may be taken as k ¼ 1.4.
[0.5; 0.4233; 0.5731]