Chapter 2
Thermodynamics of Combustion
2.1
Properties of Mixtures
The thermal properties of a pure substance are described by quantities including
internal energy,
u
, enthalpy,
h
, specific heat,
c
p
, etc. Combustion systems consist of
many different gases, so the thermodynamic properties of a mixture result from a
combination of the properties of all of the individual gas species. The ideal gas law
is assumed for gaseous mixtures, allowing the ideal gas relations to be applied to
each gas component. Starting with a mixture of
K
different gases, the total mass,
m
,
of the system is
m
¼
X
K
i
¼
1
m
i
;
(2.1)
where
m
i
is the mass of species
i
. The total number of moles in the system,
N
, is
N
¼
X
K
i
¼
1
N
i
;
(2.2)
where
N
i
is the number of moles of species
i
in the system. Mass fraction,
y
i
,
and
mole fraction,
x
i
,
describe the relative amount of a given species. Their definitions
are given by
y
i
±
m
i
m
and
x
i
±
N
i
N
;
(2.3)
where
i
¼
1,2,
. . .
,
K
. By definition,
X
K
i
¼
1
y
i
¼
1
and
X
K
i
¼
1
x
i
¼
1
:
S. McAllister et al.,
Fundamentals of Combustion Processes
,
Mechanical Engineering Series, DOI 10.1007/978-1-4419-7943-8_2,
#
Springer Science+Business Media, LLC 2011
15