Mixture Preparation Effects on Distributed Combustion for Gas Turbine Application - Novel Combustion Concepts for Sustainable Energy Development - page 274

Chen and Driscoll ( 1990 ) have examined the physical processes that occur within the
non-premixed fl flames by exploring the enhanced mixing characteristics in swirl

fl flows that emanate from the formation of a central toroidal recirculation zone. Gupta

et al. ( 1977 ) examined multi-annular swirl burner for improved mixing and com-
bustion with the injection of fuel in the shear layer of the surrounding swirling fl ow

to improve radial mixing and achieve compact combustion.
In a previous investigation, the role of swirling air injection into the combustion
chamber for distributed combustion reactions was explored (Khalil and Gupta
2011a , b , 2014a ). Air was injected tangentially into the combustion chamber at a
high air velocity to form swirling motion. This air jet entrains large amounts of
product gases forming a recirculation zone. The amount of this recirculation is
controlled so that it increases the temperature of the mixture containing air, product
gases, and fuel to a temperature that is above the auto-ignition temperature of the
fuel. In the non-premixed condition, the fuel is injected at some distance down-
stream to provide suf

cient mixing (desirable mixing time should be less than the
ignition delay time). The uniformly mixed fuel/air/product gas will then sponta-
neously ignite to result in a distributed reaction regime, instead of a thin concen-
trated reaction

flames. Hence, it may be noted
that the CDC cases discussed here differ from conventional gas turbine

fl flame front formed in conventional fl

fl flames in

that they do not require a flame stabiliza-

tion. This helps to reduce pressure drop across the combustor. The product gases
mix with the fresh mixture to increase the temperature of the mixture high enough
to cause spontaneous ignition in the entire zone as compared to only a small region
of the fresh mixture for

fl flow reversal or low-velocity region for fl

ames.
Swirl combustors with tangential air entry have been shown to exhibit high swirl
intensity, which helps to reduce NO x emission and enhance

fl flame stabilization as exhibited in conventional fl

flame stability (Yetter
et al. 2000 ). Also, ultra-low NO x emission along with low CO emission has been
demonstrated for swirling CDC combustor where emissions below 3 PPM NO have
been demonstrated for a high heat release intensity of 36 MW/m 3 -atm at a rather
high equivalence ratio of 0.7 (Khalil and Gupta 2011a ). Swirling CDC has also
been demonstrated using a recirculation ratio (recirculated mass

fl

fl flow rate to inlet

mass fl flow rate) that is three times higher than that for non-swirling con guration

(Khalil and Gupta 2014a ).
Lean premixed combustion has been inherently used in gas turbine combustors
to lower flame temperatures and to avoid stoichiometric non-uniformities that arise

due to incomplete mixing of fuel and air. In agreement with this, experimental
investigations performed on the test combustor show that premixed combustion
demonstrates lower pollutants emission as compared to non-premixed combustion
mode over the operational range (Arghode and Gupta 2010a , b , 2011 ; Khalil and
Gupta 2011a , b , 2013 , 2014a , b ; Arghode et al. 2012 ). However, operation of these
premixed systems can lead to undesired phenomenon of

fl

fl ame fl flashback. High

temperatures in a premixer caused by flashback can result in damage to or shutdown

of an engine. Other phenomena relating to premixed combustion have been studied
in detail. The initial un-mixedness in a fuel

fl

air premixer has been shown to make
the overall lean mixtures to auto-ignite sooner than might be expected based on the

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