Droplet Velocity Measurements in Direct-Injection Diesel Sprays Under High-Pressure and High-Temperature Conditions by Laser Flow Tagging

The droplet velocity is an important parameter for breakup, evaporation, and combustion of Diesel sprays, but it is very difficult to measure it by widely used laser diagnostic techniques like PDA, PIV and LCV under realistic high-pressure and high-temperature conditions. This is basically caused by laser beam steering and multiple scattering of light due to very high droplet densities, in particular close to the nozzle. It was demonstrated recently, that these problems can be greatly reduced by the laser flow tagging (LFT) technique. For this purpose, the model fuel is doped with a phosphorescent tracer. A number of droplet groups within the spray are tagged by illuminating them with focused beams of a pulsed laser, and their velocities are measured by recording the phosphorescence twice after each laser pulse using a double-frame ICCD. It is demonstrated in the present work that LFT measurements can also be conducted in direct injection Diesel sprays under quiescent high-pressure and high-temperature (50 bar and 800 K) conditions. N-decane was used as the fuel. The measurements were done in a combustion vessel. The accuracy of the averaged measurements turned out to be about 3% at a nozzle distance of 1mm, and 5% at a 4.5 mm, respectively.