Optical diagnostics of Soot in optically dense sprays

Sammanfattning: Soot diagnostics in engines can be carried out with optical methods that have the potentialfor high temporal and spatial resolution. Some of these methods were appliedto study diesel spray combustion in the Chalmers High Pressure, High Temperature(HP/HT) spray rig. The conditions inside the combustion chamber were controlled toachieve operational conditions similar to those in an internal combustion engine, reachingup 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 beconsidered quiescent. Fuel was injected into the combustion chamber using a commonrail 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 differentdurations.Spatial and time resolved spray combustion temperature measurements were madeat 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 interferencefilters that transmit light at different frequency intervals in the visible range. Data wereanalysed on the basis of the Mie-Planck theory, corrected for particles much smallerthan visible light wavelength. Temperature inhomogeneities were observed across thecombusting spray, along with a rapid increment of the temperature after the start ofignition. The maximum registered temperatures were just below the adiabatic flametemperature for each operational condition.Soot formation and evolution were investigated with simultaneous planar laser inducedincandescence (LII), elastic scattering and extinction using a laser sheet and twointensified CCD cameras. The laser pulse was split into two overlapping but time delayedpulses, a low energy for elastic scattering and a high energy for LII. Due to the relationbetween the size of the spray and the height of the laser sheet, the spray was dividedinto four different regions, vertically translating the combustion chamber to study eachone them. The measurements were taken at different intervals after the start of ignitionin order to obtain information on soot evolution. At high pressures heat conductionbecomes dominant in the cooling of laser heated nanoparticles. Special attention wasthus paid to this term in the heat balance equation for the interpretation of the LII dataand a new non-continuum heat transfer model was developed. Calculations of size andconcentration of soot particles were made based on this model.