Page 56 - Advanced Gas Turbine Cycles

Chapter 3. Basic gas htrbinc cycles 33

qI3

\
isentropic
turbines
isentropic
compressors

S
Fig. 3.6. T, diagram for 'ultimate' reversible gas turbine cycle [CICIC.. .BTBT.. .XIR.
s
3.2.2. Irreversible air standard cycles
3.2.2.1. Component pelformance
Before moving on to the als analyses of irreversible gas turbine cycles we need to define
various criteria for the performance of some components, all of which have been assumed
to be perfect (reversible) in the analyses of Section 3.2.1. The criteria used are listed in
Table 3.1.
In addition to the irreversibilities associated with these components, pressure losses
(Ap) may occur in various parts of the plant (e.g. in the entry and exit ducting, the
combustion chamber, and the heat exchanger). These are usually expressed in terms of
non-dimensional pressure loss coefficients, t= AP/@)~, where @)m is the pressure at
entry to the duct. (Mach numbers are assumed to be low, with static and stagnation
pressures and their loss coefficients approximately the same.)
As alternatives to the isentropic efficiencies for the turbomachinery components, and
qc, which relate the overall enthalpy changes, small-stage or polytropic efficiencies ( qpT and
qK) are often used. The pressure-temperature relationship along an expansion line is then
p/Tz = constant, where z = ['y/(y - l)T)pTI,
and the entry and exit temperatures are related by T3/T4 = r!") = xT.
Table 3.1
Performance criteria
Component Criterion of performance
Turbine Isentropic efficiency
% = Enthalpy drophentropic enthalpy drop
Compressor Isentropic efficiency
= Isentropic enthalpy riselenthalpy rise
Heat exchanger Effectiveness (or thermal ratio)
E = Temperature rise (cold side)/maximum temperature difference between entry
(hot side) and entry (cold side)