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Unformatted text preview: other method of balancing a fuel mixture is to ﬁrst develop stoichiometry
relations for CH4 and H2 individually:
CH4 þ 2ðO2 þ 3:76N2 Þ ! CO2 þ 2H2 O þ 2 Á 3:76N2
H2 þ 0:5ðO2 þ 3:76N2 Þ ! H2 O þ 0:5 Á 3:76N2
Then, multiply the individual stoichiometry equations by the mole fractions of the
fuel components and add them: 2.2 Combustion Stoichiometry 19 0:95 Á fCH4 þ 2ðO2 þ 3:76N2 Þ ! CO2 þ 2H2 O þ 2 Á 3:76N2 g
0:05 Á fH2 þ 0:5ðO2 þ 3:76N2 Þ ! H2 O þ 0:5 Á 3:76N2 g
)0:95CH4 þ 0:05H2 þ 1:925ðO2 þ 3:76N2 Þ !
0:95CO2 þ 1:95H2 O þ 7:238N2 2.2.1 Methods of Quantifying Fuel and Air Content
of Combustible Mixtures In practice, fuels are often combusted with an amount of air different from the
stoichiometric ratio. If less air than the stoichiometric amount is used, the mixture is
described as fuel rich. If excess air is used, the mixture is described as fuel lean. For
this reason, it is convenient to quantify the combustible mixture using one of the
following commonly used methods:
FuelAir Ratio (FAR): The fuelair ratio, f, is given by
f¼ mf
;
ma (2.13) where mf and ma are the respective masses of the fuel and the air. For a stoichiometric mixture, Eq. 2.13 becomes
mf
Mf
¼
;
fs ¼
g
ma stoichiometric ða þ b À 2Þ Á 4:76 Á Mair
4 (2.14) where Mf and Mair (~28.84 kg/kmol) are the average masses per mole of fuel and air,
respectively. The range of f is bounded by zero and 1. Most hydrocarbon fuels have
a stoichiometric fuelair ratio, fs, in the range of 0.05–0.07. The airfuel ratio (AFR) is
also used to describe a combustible mixture and is simply the reciprocal of FAR, as
AFR ¼ 1/f. For instance, the stoichiometric AFR of gasoline is about 14.7. For most
hydrocarbon fuels, 14–20 kg of air is needed for complete combustion of 1 kg of fuel.
Equivalence Ratio: Normalizing the actual fuelair ratio by the stoichiometric fuelair ratio gives the equivalence ratio, f.
f¼ f
mas Nas
NO2s
¼
¼
¼
fs
ma
Na
NO2;a (2.15) The subscript s indicates a value at the...
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 Winter '14
 Physics, Energy, Heat

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