Environmental Engineering Reference
In-Depth Information
and CO using a variety of gaseous and liquid fuels (including biofuels) (Arghode
and Gupta
2010a
,
b
,
2011
; Khalil and Gupta
2011a
,
b
,
2013
,
2014a
,
b
; Arghode
et al.
2012
). To achieve reactions closer to the distributed regime and avoid the
presence of thin reaction zone and hot-spot zones in the
flames, controlled mixing
between the combustion air and product gases is necessary so as to form a hot and
diluted oxidant with rapid mixing with the fuel. High recirculation of hot recircu-
lated combustion gases and its fast mixing with the fuel leads to spontaneous
ignition of the fuel with distributed reaction conditions. The recirculated hot
products not only play a role in increasing the oxidant temperature while decreasing
oxygen concentration, but also the recirculated active radicals within the hot
products play an important role in the reaction kinetics. This results in avoidance of
thin reaction zone and hot-spot regions in the
fl
flame, which helps to minimize or
mitigate NO
x
emissions (thermal NO
x
) produced from the Zeldovich thermal
mechanism (Tsuji et al.
2003
; Correa
1992
).
In CDC, the reaction occurs in a distributed regime due to the volume distributed
nature of the mixture of combustion gases, fuel, and oxidizer in the combustion
chamber. Depending on the ignition delay time and mixing timescales, the reaction
zone is a distributed regime as compared to thin reaction
fl
fl
flame front in conventional
fl
flames. Such volume distributed combustion can be achieved by air injection at
high velocities to avoid stabilization with large thermal gradients in the
flame. This
may be accomplished by appropriate separation of air and fuel jets and internal
recirculation of a large amount of product gases to aid the spontaneous ignition of
the mixture with the evolution of distributed reaction zone. The concept of separate
injection of fuel and air at a high velocity with desirable and controlled amounts of
gas recirculation and mixing between the product gases and fresh reactants can be
applied to combustors operating at higher heat release intensities [5
fl
50 MW/m
3
-
atm (Vincent
1950
)] that are commensurable with the gas turbine application. These
requirements can be met with different con
-
gurations of fuel and air injection into
the combustor while using a carefully tailored
field in the combustor.
The importance of recirculation zone generation and good preparation of the air
fl
ow
-
fuel mixture for ignition cannot be overstated. One common practice used to create
recirculation and stabilize combustion is to utilize swirl
flow that entrains and re-
circulates a portion of the hot combustion products back to the root of the
fl
ame. For
such combustors, swirl characteristics play a major role in mixing and combustion
(Gupta et al.
1984
; Archer and Gupta
2004
; Leuckel and Fricker
1976
). Swirl
fl
ows
have been widely investigated for several decades because of their extensive use in
all kinds of practical combustion systems,
fl
including gas turbine combustion.
Numerous experiments in swirl
fl
flows have been carried out extending from very
fundamental isothermal
flows to those formed in complex swirl
combustor geometries (Gupta et al.
1984
). Experimental results have established the
general characteristics of swirl
fl
flows and reacting
fl
fl
flows that reveal the important effects of swirl on
promoting
ciency, and controlling emis-
sion of pollutants from combustion (Gupta et al.
1984
). Leuckel and Fricker (
1976
)
conducted a variety of measurements using a non-premixed single swirl burner
consisting of an annular swirling air jet and a centrally located non-swirling fuel jet.
fl
flame stability, increasing combustion ef
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