Empirical correlation on circulation and single string cavitation in a diesel nozzle

抄録

Single string cavitation (SC) and a swirling flow occur in a nozzle of diesel fuel injector under low needle lift conditions, when the distance Z_N between needle tip and nozzle entrance is short. The swirling flow induces a hollow-cone spray and enhances the atomization of fuel spray, by which the emission of particulate matter is reduced. Previous studies have shown that spray angle α can be predicted by the ratio of the azimuthal velocity v_θ to the axial velocity v_z in the nozzle. In order to evaluate v_θ and v_z, we have to predict the circulation Γ_en in a nozzle and the diameter D_sc of single SC. However, empirical correlations for Γ_en and D_sc have not been established yet. In this study, five transparent single-hole nozzles with different sac inlet widths and needle lifts were designed to change and measure the circulation Γ_sac at the sac upstream of the nozzle by particle image velocimetry (PIV). We estimate Γ_en using one-dimensional simulation model and measure D_sc by image analysis. As a result, we obtained the following conclusions. (1) Circulations Γ_sac in the sacs with various needle lifts and inflow velocities are proportional to the local inlet velocity v_in at the sac inlet. (2) The diameter D_sc of single string cavitation can be simply evaluated using Γ_sac in the sac. (3) The circulation Γ_en at the entrance of the nozzle can be evaluated by the correlation based on W_sac, Z_N, D and v_N. (4) By using the correlations on Γ_en and D_sc, we can evaluate v_θ, v_z, and the resulting spray angle α.