Optical diagnostics of Soot in optically dense sprays

Licentiate thesis, 2006

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 durations. 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.