Identification and Characterization of Steady Spray Conditions in Convergent, Single-Hole Diesel Injectors

Reduced-order models typically assume that the flow through the injector orifice is quasi-steady. The current study investigates to what extent this assumption is true and what factors may induce large-scale variations. Experimental data were collected from a single-hole metal injector with a smoothly converging hole and from a transparent facsimile. Gas, likely indicating cavitation, was observed in the nozzles. Surface roughness was a potential cause for the cavitation. Computations were employed using two engineering-level Computational Fluid Dynamics (CFD) codes that considered the possibility of cavitation. Neither computational model included these small surface features, and so did not predict internal cavitation. At steady state, it was found that initial conditions were of little consequence, even if they included bubbles within the sac. They however did modify the initial rate of injection by a few microseconds. Though the needle was never stationary, the mass discharge by the nozzle remained constant for most of the injection. The momentum discharge was more sensitive to lower needle lifts than the mass flow rate. An annular jet, that may follow either the needle surface or the sac wall, forms at low needle lift. The presence of this jet corresponds to a loss of momentum through the nozzle exit. The coefficient of area remains remarkably consistent during the early/late needle transient and is an important discovery.