Internal and near nozzle flow characteristics in an enlarged model of an outwards opening pintle-type gasoline injector

The internal nozzle and near the nozzle exit flows of an enlarged transparent model of an outwards opening injector were investigated for different flow rates and needle lifts under steady state flow conditions. A high resolution CCD camera, high speed video camera and an LDV system were employed to visualize the nozzle flow and quantify the tangential velocity characteristics. The images of the internal flow between the valve seat and the square cross-section end of the needle guide revealed the presence of four separated jet flows and four pairs of counter-rotating vortices with each pair bounded in-between two adjacent jets. The counter-rotating vortices are highly unstable with a circumferential oscillatory motion which was transmitted to the spray outside the nozzle with almost the same frequency. The dominant circumferential frequencies at the nozzle exit were identified by FFT analysis of the tangential velocities. A linear relationship exists between the dominant frequencies and the flow Reynolds number based on injection velocity and needle lift. Magnified images of the flow just outside the nozzle exit showed formation of interconnecting streamwise strings on the liquid film as soon as it emerges from the annular exit passage. The interspacing between the strings was found to be linearly related to injection velocity and almost independent of the needle lift.