Coalescence features of planar collision between particulate matters of same diameter from diesel engine
2016
It is usually assumed that collision means simultaneous coalescence
for particulate matters of diesel engine combustion process. However,
the limitation of this assumption is that the recovery phenomenon
in collision period cannot be taken into consideration, while compression
and recovery are two inevitable periods for collision of incomplete
elastic particulate matters, in which Van de Waals force and elastic
deformation force influence the coalescence features. In present research,
the planar collision between particulate matters of same diameter
in post-combustion period of diesel engine was studied and coalescence
features of it was investigated further according to compression and
recovery phenomena. In addition, the force analyses of particulate
matters was utilized to reveal force effect and incomplete elastic
collision function based on Van de Waals force and elastic deformation
force was solved. In this case, coalescence feature parameters, namely,
diameter, collision rate, incidence angle, compression distance and
coalescence efficiency, as well as the relation between each other
were investigated in terms of recovery phenomenon. The mathematic
analyses showed that, Van de Waals force and elastic deformation force
both gradually increased for particulate matters under any diameters
when particulate matters compressed; furthermore, elastic deformation
force was always larger than Van de Waals force with a trend of greater
difference considering invariant diameters; and the larger the diameter
was, the weaker the elastic deformation force became under the same
compression distance, but Van de Waals force followed the opposite
pattern of elastic deformation force. On the other hand, collision
frequency of 50 nm between 45 and 40 nm was the lowest because higher
diameter dispersivity caused greater collision frequency, for instance,
it was 1.2×10 16 and 6.2×10 15 m - 3 s - 1 for particulate
matters of 15 and 45 nm respectively. Besides this, the compression
distance increased with raising incidence angle under the same initial
relative velocity, which indicated that central collision had the
widest compression distance; increasing initial relative velocity
resulted in wider compression distance under the same incidence angle.
Recovery factor was an essential element both for critical velocity
and coalescence efficiency considering the central collision of 50
nm particulate matter; to be specific, both of critical velocity and
coalescence efficiency reduced with raising recovery factor; the coalescence
efficiency was approximately 50.05% when recovery factor was about
0.026 suggesting highly efficient collision and rapid coalescence;
however, the coalescence efficiency was almost close to zero as long
as the recovery factor was larger than 0.063, suggesting immediate
separation after collision and barely no coalescence; in summary,
a little variation of recovery factor could lead to the huge change
of coalescence efficiency.
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