Page 109 - Advanced Gas Turbine Cycles

82 Advanced gas turbine cycles

5.6. A note on real gas effects

The real gas calculations with cooling as described above give indications of maxima in
the plots of thermal efficiency against T3 = T,,, for a given pressure ratio (e.g. Fig. 5.3).
These do not appear in air standard analysis such as that described in Chapter 3. The
calculations of Chapter 4 showed that such maxima can occur not only for cooled but also,
surprisingly, for uncooled calculations. Fig. 4.9 showed such graphs of qo against T,,, to
be very flat, but there was clearly a real gas effect independent of cooling at high T,,,.
Recent detailed investigations of these real gas effects by Wilcock et al. [3] have revealed
that this ‘turnover effect’ on uncooled efficiency at high values of T,,, is related to the
changes in real gas properties (cpg and yg) with both temperature and composition.

5.7. Other studies of gas turbine plants with turbine cooling
There are several studies in the literature which parallel the approach of Horlock et al.
[2] described above. Some of the more important are listed here and briefly discussed.
Perhaps the most comprehensive set of papers were those by El-Masri and his
colleagues in a series of publications in the 1980s. El-Masri describes his methods of
predicting cooling flow requirements in Ref. [4] for combined convection and film
cooling, and in Ref. 151 with thermal barrier coatings. The approach is similar but not
identical to that described above. Following initial cycle calculations with working fluids
with constant properties [6,7] El-Masri developed a computer code-GASCAN [SI-
embracing real gas properties and used this in the second law calculations of air-cooled
Brayton gas turbine cycles [9] and combined cycles [lo]. These calculations presented
details of exergy losses, work output and rational efficiency and gave some indication of an
optimum combustion temperature yielding maximum efficiency (for a given pressure
ratio), along the lines already described in this chapter.
Similarly, comprehensive calculations including turbine cooling were made by Lozza
and his colleagues [ 1 13. These calculations give results broadly similar to those described
in this chapter but an important feature of this work involved a degree of parameterisation
of the cooling methods--e.g. variation of the allowable blade temperature.
A third set of similar but simpler calculations were described by MacArthur [ 121 who
applied aero-engine cooling technology to obtain improved performance of industrial type
gas turbine power plants.

5.8. Exergy calculations
Once the state points are known round a cycle in a computer calculation of performance,
the local values of availability and/or exergy may be obtained. The procedure for
estimating exergy losses or irreversibilities was outlined in Chapter 2. Here we show such
calculations made by Manfrida et al. [ 131 which were also presented in Ref. [ 141.
Fig. 5.10 shows the exergy losses as a fraction of the fuel exergy (including the partial
pressure terms referred to in Section 2.4) for the General Electric LM 2500 [CBTIrc plant,