Page 50 - Advanced Gas Turbine Cycles

Chapter 3

BASIC GAS TURBINE CYCLES

3.1. Introduction

In the introduction to Chapter 1 on power plant thermodynamics our search for high
thermal efficiency led us to emphasis on raising the maximum temperature T,, and
lowering the minimum temperature Tmi,, in emulation of the performance of the Carnot
cycle, the efficiency of which increases with the ratio (T,,,JTmin). In a gas turbine plant,
this search for high maximum temperatures is limited by material considerations and
cooling of the turbine is required. This is usually achieved in ‘open’ cooling systems, using
some compressor air to cool the turbine blades and then mixing it with the mainstream
flow.
Initially in this chapter, analyses of basic gas turbine cycles are presented by reference
to closed uncooled ‘air standard’ (ah) cycles using a perfect gas (one with both the gas
constant R and the specific heats c,, and c, constant) as the working fluid in an externally
heated plant. Many of the broad conclusions reached in this way remain reasonably valid
for an open cycle with combustion, i.e. for one involving real gases with variable
composition and specific heats varying with temperature. The a/s arguments are developed
sequentially, starting with reversible cycles in Section 3.2 and then introducing
irreversibilities in Section 3.3.
In Section 3.4, we consider the open gas turbine cycle in which fuel is supplied in a
combustion chamber and the working fluids before and after combustion are assumed to be
separate semi-perfect gases, each with c,(T), c,(T), but with R = [c,(T) - c,(T)]
constant. Some analytical work is presented, but recently the major emphasis has been on
computer solutions using gas property tables; results of such computations are presented in
Section 3.5.
Subsequently, in Chapter 4, we deal with cycles in which the turbines are cooled. The
basic thermodynamics of turbine cooling, and its effect on plant efficiency, are considered.
In Chapter 5, some detailed calculations of the performance of gas turbines with cooling
are presented.
We adopt the nomenclature introduced by Hawthorne and Davis [l], in which
compressor, heater, turbine and heat exchanger are denoted by C, H, T and X, respectively,
and subscripts R and I indicate internally reversible and irreversible processes. For the
open cycle, the heater is replaced by a burner, B. Thus, for example, [CBTXII indicates an
open irreversible regenerative cycle. Later in this book, we shall in addition, use subscripts

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