Droplet size and velocity measurements for the characterization of a DI-diesel spray impinging on a flat wall

摘要

In small high-speed, direct-injection diesel engines the injected fuel spray impinges on the walls of the piston bowl. The mixture formation process is influenced considerably by the spray-wall interaction. Stringent exhaust gas emission regulations and growing demands for fuel economy are leading to the application of high-pressure, fuel-injection systems, e.g., common-rail. The trend towards downsized engines with smaller piston displacements leads to reduced distances between nozzle and wall. Higher injection pressures and smaller nozzle-wall distances both increase the significance of spray-wall interaction and near-wall mixture formation. In the present study, the influence of governing parameters like injection pressure and wall temperature on the characteristics of the impinged spray was investigated. The measurements have been performed in an optical-accessible, high-pressure, high-temperature combustion chamber, in which pressures and air temperatures up to 5 MPa and 500°C can be realized under steady-state conditions. A flat wall with adjustable wall temperature was placed at a distance of 20 mm from the nozzle hole. A passenger-car, common-rail injector with a mini-sac-hole nozzle was applied to the combustion chamber. The spray axis of the investigated nozzle hole was oriented perpendicular to the wall. Commercially available diesel fuel was used. Locally resolved simultaneous measurements of droplet velocity (axial and radial velocity component) and droplet diameter at several positions 2 mm above the wall inside the impinged and deflected spray were performed with phase doppler velocimetry (PDV). By the experiments effects of the variations on the temporal and spatial development of the near-wall spray, e.g., wall jet, wall jet vortex and droplet size distribution were determined.