Page 69 - Advanced Gas Turbine Cycles

Chapter 3. Basic gas turbine cycles 45

0 10 20 30 40 50 60
PRESSURE RATIO
Fig. 3.15. Overall efficiencies of several irreversible gas turbine plants (with T,, = 120O0C).

the simple recuperative plant. The highest efficiency (with a high optimum pressure ratio)
occurs for the most complex [CICBTBTXII~ plant, but the graph of efficiency (7)) with
pressure ratio is very flat at the high pressure ratios, of 30-55 (7) approaches the efficiency
of a plant with heat supplied at maximum temperature and heat rejected at minimum
temperature).
Finally, carpet plots of efficiency against specific work are shown in Fig. 3.16, for all
these plants. The increase in efficiency due to the introduction of heat exchange, coupled
with reheating and intercooling, is clear. Further the substantial increases in specific work
associated with reheating and intercooling are also evident.

3.5. Discussion

The discussion of the performance of gas turbine plants given in this chapter has
developed through four steps: reversible a/s cycle analysis; irreversible a/s cycle analysis;
open circuit gas turbine plant analysis with approximations to real gas effects; and open
circuit gas turbine plant computations with real gas properties. The important conclusions
are as follows:
The initial conclusion for the basic Joule-Brayton reversible cycle [CHTIR, that
thermal efficiency is a function of pressure ratio (r) only, increasing with t-, is shown to
have major limitations. The introduction of irreversibility in ah cycle analysis shows
that the maximum temperature has a significant effect; thermal efficiency increases
with (T3/T,), and so does the optimum pressure ratio for maximum efficiency.
The a/s analyses show quite clearly that the introduction of a heat exchanger leads to
higher efficiency at low pressure ratio, and that the optimum pressure ratio for the