Page 73 - Advanced Gas Turbine Cycles

Chapter 4. Cycle eficiency with turbine cooling (cooling flow rates specified) 49

remain substantially valid for open cycles with combustion, i.e. for those involving
real gases with variable composition and specific heats varying with temperature.
The arguments of this section are developed sequentially, starting with internally
reversible cycles and then considering irreversibilities. Here we concentrate on the gas
turbine with simple closed or open cycle (CHT, CBT).

4.2.1. Cooling of intentally reversible cycles

4.2.1.1. Cycle [CHTIRC, with single step cooling
Consider first a cycle with reversible compression and expansion, but one in which,
after a unit flow of the compressed gas has been heated externally, it is cooled by mixing
with the remaining compressor delivery air (+) before entering the turbine in the internal
cycle, which is otherwise reversible (Fig. 4.1 shows the T,s chart). This single step of
cooling is representative of cooling the nozzle guide vanes of the first stage in a real gas
turbine plant, reducing the rotor inlet temperature from T3 = T,, to T5 = Tfit.
We assume low velocity (constant pressure) mixing of the ‘extra’ cooling gas mass flow
(+) at absolute temperature T2 with the gas stream (of unit mass flow), which has been
heated to the maximum temperature T3 = TB. From the steady flow energy equation, if
both streams have the same specific heat (c,), it follows that
(4.1)

where T5 is the resulting temperature in the mixed stream, before it is expanded through
the turbine. The turbine work output is now WT = (1 + +)cpT5[(l - (l/x)], and the

QB I

s

Fig. 4.1. Temperature-entropy diagram for single-step cooling-reversible cycle [CHTI, (after Ref. [5]).