Emissions and Soot in Partially Premixed Combustion - Novel Combustion Concepts for Sustainable Energy Development - page 425

the world, global warming, extinction of various species, health problems, advent of
new diseases, loss of fertile land which ultimately is used to feed us humans and
animals, loss of aesthetic beauty of nature and so on. Thus, such alarming hap-
penings are indeed a sign for humans to try and undo the harm, which has already
been done to the environment.
It is also a fact, though, that through technological advancements, the quality of life
has improved. Talking from the perspective of Aerospace Engineering, travelling time
has been greatly reduced and areas presumed to inaccessible at one point of time are
now fully connected by air. But the increasing air traf

c across the globe has its own
set of concerns, and one of the primary issues to be addressed is that of air pollution.
While considering the process of gas turbine combustion (aircraft engines),
the emissions are already quite low, as the air-fuel mixture is extremely diluted. So, the
emissions per unit volume of exhaust are much reduced. Despite this fact, since
the airplanes travel at a high altitude, these incredibly small emissions can also affect
the atmospheric cycles, climatic patterns and cloud formation. Recently, many erratic
weather patterns are being experienced all over the world, part of which has been
attributed to the increased air traf c all across the globe. For example, the frequent

occurrence of hurricanes in the United States in the recent years has also been cor-
related with the increased number off

flying over the States. Thus,
there is an urgent need to further reduce the aircraft emissions, and still more stringent
norms are being applied. Another area of concern is that the environment in and
around airports has been found to be extremely toxic, mainly because of the pollutants
emitted while the aircrafts take off and land there. That poses direct hazard to the
passengers and the employees present in the vicinity. Apart from these issues, the
more complete the combustion attained in a gas turbine engine, lesser is the fuel
consumption, which translates into saving of energy, reduction in operation cost and
also corresponds to a reduction in the weight of the aircraft.
Conventional combustion systems use either premixed or non-premixed com-
bustion, which have their own pros and cons. Premixed

fl ights taking off and fl

flames occur when the fuel
and air are premixed before entering the reaction zone. Hence, the fuel

fl

air mixture
itself enters the reaction zone and ignites when subjected to a source of ignition, say
a spark plug. On the other hand, in the case of non-premixed or diffusion

-

fl ames,

first the air enters the combustion chamber, and whenever combustion is desired,
fuel is injected or sprayed into the air in the combustion chamber and ignited.
Relative merits and demerits of premixed and non-premixed combustion are listed
in Table 1 .
It can be seen that both kinds of combustion phenomena have their own pros and
cons. So, an alternative was sought after and partially premixed ames were

deemed to be a good compromise between completely premixed and completely
diffusion

fl

flames. It incorporates the advantages of both and overcomes both their
drawbacks. As the name suggests, partial premixing implies that the

fl

fl flame is only

' partial

partially premixed and in parts, it is also a diffusion fl ame. The term

premixing ' hence, basically refers to the condition when a small amount of air or

oxygen is mixed with the fuel (rich mixture), before it enters the reaction zone and
then further at the reaction zone, more quantities of oxidizer are added, so as to