Page 36 - Advanced Gas Turbine Cycles

Chapter 2

REWERSIBILITY AND AVAILABILITY

2.1. Introduction

In Chapter 1, the gas turbine plant was considered briefly in relation to an ideal
plant based on the Carnot cycle. From the simple analysis in Section 1.4, it was explained
that the closed cycle gas turbine failed to match the Carnot plant in thermal efficiency
because of
(a) the ‘6 effect’ (that heat is not supplied at the maximum temperature and heat is not
rejected at the minimum temperature) and
(b) the ‘u effect’ (related to any entropy increases within the plant, and the consequent
‘widening’ of the cycle on the T, s diagram).
Since these were preliminary conclusions, further explanations of these disadvantages
are given using the second law of thermodynamics in this chapter. The ideas of
reversibility, irreversibility, and the thermodynamic properties ‘steady-flow availability’
and ‘exergy’ are also developed.
In defining the thermal efficiency of the closed gas turbine cycle, such as the one shown
in Fig. 1.2, we employed the first law of thermodynamics (in the form of the steady-flow
energy equation round the cycle), which states that the heat supplied is equal to the work
output plus the heat rejected, i.e.

Here W is the net work output, i.e. the difference between the turbine work output ( WT) and
the work required to drive the compressor (W,), W = WT - W,.
For rhe open circuit gas turbine of Fig. 1.3, if the reactants (air Ma and fuel Mf) enter at
temperature To, and the exhaust products (Ma + Mf) leave at temperature T4, then the
steady-flow energy equation yields

where subscripts R and P refer to reactants and products, respectively, and it has been
assumed that there are no heat losses from the plant. If we now consider unit air flow at
entry with a fuel flow .f (= Mf/Ma) then the enthalpy flux HRo is equal to the sum of
the enthalpy (hat)) and the enthalpy of the fuel flow ,f supplied to the combustion
chamber (fhfo), both at ambient temperature To, and the enthalpy of the exhaust gas

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