Experimental and Numerical Study of Swirling ... - Solid Mechanics

Experimental and Numerical Study of Swirling Flow in Scavenging Process for 2-Stroke
Marine Diesel Engines
Chapter 1
for the scavenging process. Moreover, it not only serves to scavenge the
cylinder exhaust gases but also as to fill the cylinder with fresh air charge for
the next cycle which, in case of four stroke engine, is also accomplished in a
separate piston stroke. This makes scavenging process very important for
engine performance and efficiency both in terms of fuel and emissions. This
is because scavenging process not only removes exhaust gases and provides
fresh air to the engine but also provides the necessary and initial swirl to the
fresh air in which the diesel fuel is to be injected. As diesel combustion is a
mixing enhanced combustion process, a proper and good mixing of fuel with
air ensures better/ complete combustion of the fuel injected. Further, the
resulting heat release defines the production of different pollutants like NOx
and Soot etc. The heat release during combustion also defines the
temperature distribution in the cylinder fluid mixture (injected fuel, fresh
air, combustion products) and heat transfer through cylinder walls, cylinder
head and piston. This in turn defines the distribution thermal stresses
generated in the aforementioned solid materials and the requirement of their
cooling.
From the point of view of engine research and development, for example in
computational modeling of in-cylinder diesel combustion, an initial flow
field is to be defined in the compressed air pocket in which the diesel fuel is
to be injected. This flow field then governs the simulation of fuel
evaporation and mixing, consequent combustion and pollutant formation
and heat generation and transfer through the cylinder enclosure walls. An
improper definition of this flow field gives erroneous simulation results and
hinders the efforts for understanding the overall process and identifying the
key aspects required to be improved in order to develop the so called future
engines. The knowledge about the initial flow field can only be understood
by understanding the scavenging process. During the compression stroke,
when the piston closes the scavenging ports, the in-cylinder flow field at that
moment is the initial condition for the final piston-compressed flow field in
which the fuel is to be injected.
Practically, in the current marine diesel engines, the amount of air supplied
during the scavenging process is more than what is required. This makes
some amount of fresh air to enter and leave the cylinder through exhaust
valve. This is called as Short Circuiting (Pulkrabek, 2003). In addition to
having not well understanding of the in-cylinder fluid dynamics during
scavenging process (flow field and interaction of fresh air with exhaust gases),
another aspect of supplying more air is cooling the exhaust valve.
Efficient scavenging improves the combustion performance of the 2-stroke
diesel engines. Thus understanding, optimizing and developing an efficient
scavenging process is one of the key aspects of developing efficient and
environmentally friendlier marine diesel engines.
4
Introduction