Page 308 - Advanced thermodynamics for engineers

296 CHAPTER 13 EFFECT OF DISSOCIATION ON COMBUSTION PARAMETERS

causes of the production of NO x from fuels which do not contain nitrogen (through the combining of
the nitrogen and oxygen in the air), and they also increase the amount of CO during weak and slightly
rich combustion. Processes which include chemical kinetics are not equilibrium processes, but they
attempt to reach equilibrium if there is sufficient time. The results in this chapter do not include the
effects of rate kinetics: these are introduced in Chapter 14.

13.1 CALCULATION OF COMBUSTION BOTH WITH AND WITHOUT
DISSOCIATION
It is possible for the pressure, temperature and equilibrium composition (i.e. neglecting rate kinetics) of
various mixtures to be calculated using computer programs, based on the principles outlined in
Chapter 12. A computer program, called EQUIL2, based on the enthalpy coefficients given in
Chapter 9 has been used to evaluate the results presented in this chapter. The program, which has a
range of fuels programmed into it, can be run in a dissociation and non-dissociation mode: it can be
accessed by visiting http://booksite.elsevier.com/9780444633736. A series of calculations was
undertaken using octane (C 8 H 18 ) and methane (CH 4 ) as the fuels. The equivalence ratio was varied
from a weak value of f ¼ 0.5 to a rich one of f ¼ 1.25. It was assumed that the initial pressure and
temperature at entry to the combustion chamber were 1 bar and 300 K respectively, and that the fluid
was compressed through a volumetric compression ratio of 12, with an index of compression
(k ¼ c p /c v ) of 1.4. Combustion then took place at constant volume. This is essentially similar to the
combustion that would take place in a reciprocating engine with constant volume combustion at top
dead centre (tdc): which is an ‘Otto cycle’ in which the heat addition process associated with the air-
standard cycle is replaced by a realistic combustion process. Combustion in this Otto cycle is adiabatic
and occurs at constant volume: this is a constant internal energy process, as shown on Fig. 13.1. The
principles introduced here can be developed to study the effect of equilibrium on a realistic (finite rate
of combustion) cycle, and this is done in Chapter 16.
The computer program was written in a manner that enabled it to also be used for constant pressure
combustion, e.g. like that occurring in a gas turbine combustion chamber. In this case the enthalpy of
both the reactants and products would be equal.

13.2 THE BASIC REACTIONS
The stoichiometric combustion of methane is defined by the chemical equation:

CH 4 þ 2ðO 2 þ 3:76N 2 Þ0CO 2 þ 2H 2 O þ 7:52N 2 (13.1)
while that for octane is

C 8 H 18 þ 12:5ðO 2 þ 3:76N 2 Þ08CO 2 þ 9H 2 O þ 47N 2 : (13.2)
If the mixture is weak, i.e. there is more oxygen than required for complete combustion of the fuel,
then the equation for methane becomes

2 1 7:52
CH 4 þ O 2 þ 3:76N 2 0CO 2 þ 2 1 O 2 þ 2H 2 O þ N 2 (13.3)
f f f