Page 71 - Advanced Gas Turbine Cycles

Chapter 4

CYCLE EFFICIENCY WITH TURBINE COOLING (COOLING
FLOW RATES SPECIFIED)

4.1. Introduction

It was pointed out in Chapter 1 that the desire for higher maximum temperature (Tmm)
in thermodynamic cycles, coupled with low heat rejection temperature (Tmin), is
essentially based on attempting to emulate the Carnot cycle, in which the efficiency
increases with (TmJTmi,,).
It has been emphasised in the earlier chapters that the thermal efficiency of the gas
turbine increases with its maximum nominal temperature, which was denoted as T3.
Within limits this statement is true for all gas turbine-based cycles and can be sustained,
although not indefinitely, as long as the optimum pressure ratio is selected for any value of
T3; further the specific power increases with T3. However, in practice higher maximum
temperature requires improved combustion technology, particularly if an increase in
harmful emissions such as NO, is to be avoided.
Thus, the maximum temperature is an important parameter of overall cycle
performance. But for modem gas turbine-based systems, which are cooled, a precise
definition of maximum temperature is somewhat difficult, and Mukhejee [l] suggested
three possible definitions. The first is the combustor outlet temperature (Tcot) which is
based on the average temperature at exit from the combustion chamber. However, in a
practical system, this does not take into account the effect of cooling flows that are
introduced subsequently (e.g. in the first turbine row of guide vanes). So a second
definition involving the rotor inlet temperature (T",) has tended to be used more widely
within the gas turbine industry. T", is based on the averaged temperature taken at the
exit of the first nozzle guide vane row, NGV (ie. at entry to the first rotor section), and
this can be calculated assuming that the NGV cooling air completely mixes with the
mainstream. A third definition, the so-called IS0 firing temperature, Trso, can be
calculated from the combustion equations and a known fuel-air ratio, but this definition
is less frequently used (it should theoretically yield the same temperature as Tcot).
T,, and T", are both important in the understanding of relative merits of candidate
cooling systems, and we shall later emphasise the difference between T,, and Tfit. Without
improvements in materials and/or heat transfer, it is doubtful whether much higher T",
values can be achieved in practice; as a result, a practical limit on plant efficiency may be
near, before the stoichiometric limit is reached. Below we refer to T,,, as T3, the maximum