Single string cavitation and swirling flow in a nozzle and a hollow-cone spray

Abstract

Single string cavitation (SC) and a swirling flow around it may occur in a fuel injector of diesel engines at the moment of low needle lift, if the tip of the needle valve is close to the nozzle entrance. The swirling flow in a nozzle of the injector induces a hollow-cone spray with a very large spray angle and enhances spray atomization, which reduces the emission of particulate matters. However, the characteristics of the complicated gas-liquid two-phase swirling flow in a nozzle and its effect on spray angle have not been clarified yet. In this study, a simple one-dimensional model is developed to simulate the swirling flow, based on the mass and momentum conservation equations for the annular swirling liquid film around a single SC inside a nozzle and the hollow-cone liquid film in stagnant air. In order to obtain the boundary condition at the entrance and exit of a nozzle and to verify the validity of the swirling film flow model, we conduct a high-speed visualization of a single SC in a transparent mini-sac nozzle by visible light with refractive index matching for diesel as well as a high-speed X-ray phase contrast imaging of a hollow-cone spray to measure spray angle and the thickness of the hollow-cone liquid film along the axis. Then, we carry out a number of one-dimensional simulations to investigate the effects of the axial and azimuthal velocity components of the swirling liquid film flow in the nozzle on spray cone angle. As a result, we obtain the following conclusions. (1) The proposed one-dimensional model on the swirling film flow with single SC can quantitatively predict the motion of the swirling film flow within a nozzle and the hollow-cone spray. (2) We derived a correlation on spray angle based on the axial and azimuthal velocity components in the nozzles, and verified its validity.