Page 110 - Entrophy Analysis in Thermal Engineering Systems

CHAPTER EIGHT

Irreversible engines—Open cycles

8.1 Introduction
The conventional methods for production of power require com-
bustion of fuel in air. The energy content of the fuel is then released in the
form of heat, which is transferred to the working substance either directly
(e.g., internal combustion engines) or indirectly (e.g., steam power cycle).
Combustion-driven powergenerationsystems(e.g.,gasturbinescycles,steam
or combined cycle power plants, and internal combustion engines) have one
feature in common in that fuel and air are supplied through two different
streams, which are then mixed and reacted. A portion of the combustion heat
is wasted and discharged to the surrounding. For example, in a steam power
cycle (Fig. 8.1), the waste heat is rejected to the environment through the
condenseraswellasthefluegasesthatleavethestackoftheboileratamoderate
temperature (e.g., 413K).
As discussed in Chapter 7, minimizing entropy generation rate may only
be equivalent to maximizing thermal efficiency under certain design condi-
tions such as fixed rate of heat input or fixed power output. To rectify this
issue, we introduce a new parameter referred to as specific entropy gener-
ation (SEG) that is defined as the rate of total entropy generation per unit
flowrate of the fuel combusted, and has units of J/(molK). It will be shown
that optimizing a power system for maximum thermal efficiency is always
equivalent to optimizing for minimum SEG. The SEG method will then
be applied to typical combustion-driven power cycles, including gas turbine
and combined cycles.

8.2 Specific entropy generation
Consider a power-generating system, which takes in a hydrocarbon
fuel. Assume that both fuel and air are supplied at the ambient temperature
_
and pressure (T 0 , p 0 ). Denoting the net power production by W net and the
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