Experimental and Numerical Evaluation of Diesel Spray Momentum Flux 2009-01-2772

In the present work, an experimental and numerical analysis of high pressure Diesel spray evolution is carried out in terms of spray momentum flux time history and instantaneous injection rate. The final goal of spray momentum and of injection rate analyses is the evaluation of the nozzle outlet flow characteristics and of the nozzle internal geometry possible influences on cavitation phenomena, which are of primary importance for the spray evolution. Further, the evaluation of the flow characteristics at the nozzle exit is fundamental in order to obtain reliable boundary conditions for injection process 3D simulation.

In this paper, spray momentum data obtained in ambient temperature, high counter-pressure conditions at the Perugia University Spray Laboratory are presented and compared with the results of 3D simulations of the momentum rig itself. The experimental tests, performed using a commercial common-rail injector, include high speed imaging of the spray during momentum tests and the measurement of instantaneous injection rate for a more detailed comparison with the numerical analysis. Numerical 3D simulations allowed to evaluate the actual contribution to global spray momentum given by the liquid phase and by the gaseous phase, as a function of time and target distance from nozzle. This analysis allowed to explain the observed influences of the measurement procedure on the spray momentum flux experimental data.

In particular, the present CFD analysis highlighted a significant contribution to the jet momentum flux provided by gaseous phase, which tends to become predominant with growing distances from the nozzle.

The findings suggested that the target design and position can affect significantly the spray momentum flux measurement. In some operating conditions, unexpected trends of the spray momentum time integral as a function of the target distance were obtained, suggesting that main hypotheses on which spray momentum measurement devices are based - i.e. orthogonal flow deviation - may not hold true for some system configurations.