Optical diagnostics of Soot in optically dense sprays
Soot diagnostics in engines can be carried out with optical methods that have the potential
for high temporal and spatial resolution. Some of these methods were applied
to study diesel spray combustion in the Chalmers High Pressure, High Temperature
(HP/HT) spray rig. The conditions inside the combustion chamber were controlled to
achieve operational conditions similar to those in an internal combustion engine, reaching
up to 70 Bar and 560 C. Pressurised and preheated air flowed at a constant velocity
(0.1 m/s) through the combustion chamber, the conditions before each injection can be
considered quiescent. Fuel was injected into the combustion chamber using a common
rail system equipped with a solenoid injector and a vertically aligned single-hole nozzle.
Diesel fuel was used and injected at different injection pressures and for different
Spatial and time resolved spray combustion temperature measurements were made
at 20,000 frames per second using a monochromatic, non-intensified, high speed camera.
The system is based on a mirror arrangement together with two different interference
filters that transmit light at different frequency intervals in the visible range. Data were
analysed on the basis of the Mie-Planck theory, corrected for particles much smaller
than visible light wavelength. Temperature inhomogeneities were observed across the
combusting spray, along with a rapid increment of the temperature after the start of
ignition. The maximum registered temperatures were just below the adiabatic flame
temperature for each operational condition.
Soot formation and evolution were investigated with simultaneous planar laser induced
incandescence (LII), elastic scattering and extinction using a laser sheet and two
intensified CCD cameras. The laser pulse was split into two overlapping but time delayed
pulses, a low energy for elastic scattering and a high energy for LII. Due to the relation
between the size of the spray and the height of the laser sheet, the spray was divided
into four different regions, vertically translating the combustion chamber to study each
one them. The measurements were taken at different intervals after the start of ignition
in order to obtain information on soot evolution. At high pressures heat conduction
becomes dominant in the cooling of laser heated nanoparticles. Special attention was
thus paid to this term in the heat balance equation for the interpretation of the LII data
and a new non-continuum heat transfer model was developed. Calculations of size and
concentration of soot particles were made based on this model.