Environmental Engineering Reference
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overall fuel
flow of liquid fuel
over metal surfaces, for example, as found in fuel injectors and swirl cups, has been
described as a function of temperature and surface composition (Correa et al.
1996
).
Hence, it is advantageous for fuel and air to be injected into the combustor sepa-
rately and then allowed to mix rapidly before they burn. The main means of
achieving this is through careful design of the aerodynamics of burners and
enclosures, which allows control over the mixing mechanisms and residence time.
Under CDC conditions, it is believed that optimum fuel injection and mixing should
result in pollutants emission that are lower than premixed case if adequate control
on the mixture preparation can be made to achieve the distributed thermal
air ratio. Also, carbon deposition arising from the
fl
-
eld.
flames has been successfully demon-
strated to reduce NO
x
emission levels in hydrocarbon
High-velocity coaxial air in jet diffusion
fl
flames (Kim et al.
2000
).
Enhanced shear produced by high-velocity coaxial air results in enhanced mixing
between fuel and air before combustion and also lowers the residence times
resulting in lower emission levels. Mixing in coaxial jets has been studied in details,
where the main characteristics of these jets were described (Villermaux and Rehab
2000
). Coaxial jet has been also used in gas turbine model combustors demon-
strating low pollutants emission over their range of operation (Bobba et al.
2008
).
However, it is anticipated that mixing between air, fuel, and hot product gases to
achieve CDC conditions does not depend solely on the air and fuel injection
method, but rather on the interaction between fresh streams, introduction method
combined with
fl
field developed inside the combustor.
In this paper, the effect of different fuel introduction scenarios on the combustor
fl
ow
fl
field and on pollutants emission is investigated to evaluate the key features
associated with each fuel injection scenario on the combustor performance under
various operational conditions. A combustor comprising swirling air injection and a
helical path for product gases inside the combustor was adopted. A comparison
between using premixed air/fuel jet, coaxial air/fuel jets, and separate air and fuel jets
was established to identify the most suitable con
ow
guration for the previously
developed combustor (Khalil and Gupta
2011a
,
b
,
2014a
). The effects of burner exit
Reynolds number on the relative effects of burner geometry (circular and elliptic
with an aspect ratio 2:4) have been investigated (Kamal and Gollahalli
2001
).
Results show that an increase in Re decreases the bene
ts of increased air entrain-
ments into the
flame from the elliptic burner geometry. Similarly, the effects of
changes in NO and CO emission tend to level off at higher burner Re. Consequently,
circular inlet jets associated with high inlet velocities have been used in this study
As the impact of fuel injection location is established, fuel dilution effects with
different amounts of air (and also air dilution effects with different amounts of fuel)
are investigated for enhanced mixture preparation with emphasis on achieving ultra-
low emissions comparable to or less than those from novel premixed combustion in
distributed combustion without encountering any
fl
flashback or combustion insta-
bilities associated with premixed combustion. Previously, different dilution sce-
narios were examined where the NO emissions were shown to be as low as
premixed combustion case (Khalil et al.
2013
). It is worth noting that in these
dilution cases, the mixture formed (fuel added to air or air added to fuel) had an
fl
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