Emissions and Soot in Partially Premixed Combustion - Novel Combustion Concepts for Sustainable Energy Development - page 433

increase in the secondary air ow

rate, CO emissions decrease initially and later attain an almost constant value, at
around 40 % of premixing air. For lower premixing levels, increase in percentage of
secondary air

fl flow rate. Also, with increase in the premixing air fl

flow rate leads to steep reduction in CO concentrations. This may be
because of the increased availability of air and hence oxygen, which promotes the
oxidation of CO to CO 2 . The CO 2 concentration initially decreases with increasing
percentage of premixing air, for a particular secondary air

fl

flow rate, till about 40 %
of premixing air. On further addition of oxygen/air here, the CO 2 concentration
increases, perhaps due to the oxidation of CO to CO 2 . At a particular premixing
level, the CO 2 concentration increases with an increase in the percentage of sec-
ondary air

fl

flow rate, till 40 % of premixing air, and for higher levels of premixing,
the reverse trend is followed. The NO concentration is quite low, and a minimum is
observed around 40

fl

86 % of premixing air, NO is
seen to increase as the secondary air increases from 100 to 150 %, and then
decreases at 200 % of secondary air. These effects can be attributed to the combined
effects of increased oxygen content and the temperature in the combustor. They
concluded that for such a con

50 % of premixing air. For 50

- -

guration, a premixing level of 45 % is optimum, as it
gives minimum emissions and higher

ames.
Mishra et al. ( 2005 ) measured the concentrations of hydrocarbons like C 2 H 2
(ethyne) in methane-air partially premixed

fl flame stability for LPG-air fl

flames using gas chromatography. With
the addition of primary air, the flame colour is seen to change from orange of the
non-premixed

fl

fl flame to blue. A double fl flame structure occurs at a premixed

equivalence ratio (

U p) of about 2.13. The inner

fl flame height reduces with the

increase in the premixing air flow rate and its colour changes from blue to greenish

at higher premixing levels. The blue

fl

flame indicates the presence of CH radicals,
while the green colour indicates a rise in C 2 radical concentration. Initially, the
hydrocarbons are located within the inner

fl

flame and with the increase in premixing;
the non-fuel hydrocarbon concentration shows a decrease for most of the locations
within the inner

fl

flame. The peak centreline concentrations of ethyne show a
monotonic decrease with the increase in premixing, indicating a reduction in soot
formation; attributed to a combined effect of

fl

fluid mechanics and chemical kinetics.
Mungekar and Atreya ( 2007 ) studied the formation of NO, soot and OH radicals
and temperature distributions in counter-

fl

fl ow, partially premixed methane-air

fl cient levels of partial premixing.

As oxygen is progressively added to the fuel side, the measured NO concentration
increases signi

flames. A double fl flame structure is observed at suf

cantly and the NO pro le broadens, as compared to a non-premixed

fl flame. The effect of radiation from the soot particles also affects the NO formation

mechanisms and NO concentration. At higher premixing levels, two NO formation
zones and one NO destruction zone have been identi ed. As the premixing air

increases, the thermal NO formation increases with the increasing temperature, and
slowly the formation rate of NO overcomes its destruction rate, leading to a net rise
in the NO concentration. The EINO also

first increases and then decreases with

increasing partial premixing.
Alder et al. ( 1998 ) studied the trends of EINO x , for laminar ethane and turbulent
methane partially premixed

fl flames, with varying premixed (

U p) and overall