Reacting Boundary Layers in Homogeneous Charge Compression Ignition (HCCI) Engine

An experimental and computational study of the nearwall
combustion in a Homogeneous Charge
Compression Ignition (HCCI) engine has been conducted
by applying laser based diagnostic techniques in
combination with numerical modeling. Our major intent
was to characterize the combustion in the velocity- and
thermal boundary layers. The progress of the combustion
was studied by using fuel tracer LIF, the result of which
was compared with LDA measurements of the velocity
boundary layer along with numerical simulations of the
reacting boundary layer.
Time resolved images of the PLIF signal were taken and
ensemble averaged images were calculated. In the fuel
tracer LIF experiments, acetone was seeded into the fuel
as a tracer. It is clear from the experiments that a proper
set of backgrounds and laser profiles are necessary to
resolve the near-wall concentration profiles, even at a
qualitative level. Partial resolution of the velocity
boundary layer was enabled by using a slightly inclined
LDA probe operated in back-scatter mode. During these
conditions, it was possible to acquire velocity data within
0.2 mm from the wall. A one-dimensional model of the
flow field was devised to make the connection between
the thermal and the velocity boundary layer.
The investigations suggest that wall interaction is not the
responsible mechanism for the rather high emissions of
unburned hydrocarbons from HCCI engines. It is
believed that the delayed oxidation, indicated by the fuel
tracer LIF experiments and numerical simulations, is due
to the thermal boundary layer. From the data at hand, it
is concluded that the thermal boundary layer is on the
order of 1 mm thick. In this boundary layer the reactions
are delayed but not quenched.