Environmental Engineering Reference
In-Depth Information
Table 1 Differences between premixed and non-premixed combustion
Premixed combustion
Non-premixed combustion
Fuel and air are premixed before entering the
combustion chamber
Air enters the combustion chamber
rst, and
fuel is injected into it separately
More complete combustion is expected due to
better mixing
Combustion is often incomplete due to
insuf
cient time for mixing
Flame front is thinner, broader and more
contained
Flames are less localized as combustion
occurs only at the air-fuel interface
Flames are broader and smaller in height
Longer, narrower flames
Flames are bluish in colour
Flames are mostly incandescent yellow
Faster burning, as fuel and air are already
mixed
Burning rate is slower as the combustion
depends on the contact of the fuel with air,
i.e. mixing time is a limiting factor
Dangerous as the premixed mixture is within
the
Safer to use, as the fuel is injected only when
ignition is desired
flammability limits before the desired
ignition, accident prone
fl
Emissions of NO
x
can be higher, because of
higher temperatures; CO, and UHC emissions
are minimal
Generally, NO
x
emissions are lesser, but CO
and unburnt hydrocarbon emissions are too
high, indicating incomplete combustion
Examples: spark ignition (SI) engines use
premixing
Examples: Includes most of the practical
combustion systems; compression ignition
(CI) engines, candle flame
make the overall mixture lean and to complete the combustion. The air, which is
premixed with the fuel, is termed as primary air or the premixed air, and the air that
is introduced at the reaction zone is the secondary air or the main combustion air or
the diffusion air. The idea is to have a little bit of premixing, which would keep the
mixture below the rich
flammability limit, while the secondary or diffusion air
would make the mixture globally extremely lean. This can help prevent accidental
explosions of the
fl
flammable premixed mixtures.
Partially premixed
fl
flames can also occur either by design or under conditions
arising due to different phenomena, like poor mixing, spray vaporization,
fl
fl
flame lift-
off and local extinction, followed by re-ignition occurring in turbulent
fl
ames (Xue
and Aggarwal
2003
). Local variations in the fuel
air ratio or the equivalence ratio
can occur due to imperfect and/or progressive mixing of the fuel and the air streams,
which can also be termed as partially premixed combustion (PPC) (Mitrovic and
Lee
1998
). Partially premixed
-
flames produce much less emissions, mainly as a
result of much better and improved combustion. They are also deemed to be safer
and more stable as compared to pure diffusion or pure premixed
fl
flames (Jeong et al.
2006
). Various studies have already been conducted on partially premixed
fl
fl
ames,
both in co-
flowing in the same direction in
the combustion chamber as the main combustion air outside, which gives a co-
annular con
fl
ow (the premixed fuel
-
air mixture is
fl
guration), and counter-
fl
ow con
guration, where the fuel
air premixed
-
mixture is in the opposite direction to the oxidizer flow direction.
Lique
ed petroleum gas (LPG) can be chosen as a fuel, mainly because of its
easy availability and widespread use in domestic, industrial as well as transport
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