Numerical simulation of a pure water jet inside an orifice : jet stability and effects of droplets collisions

The aim of the present work is to perform a CFD numerical simulation of a pure water jet in order to investigate its creation and stability achieving a better understanding of the process and its disturbances. A few works in the previous literature have been carried out on these topics which can be an helpful mean to gain knowledge on the process mechanisms and improve the control capabilities. The internal geometry of the orifice plays an important role during the first instants of the jet creation affecting the jet break-up and the creation of droplets which remain inside the orifice sticking or rebounding on the walls of the orifice exit tube. A CFD analysis is carried out to study the effect of the droplet collision with the main jet: the jet break-up, early presence of water, condensed humidity or jet disturbances can create these water droplets which then can be dragged by the high velocity air field created inside the orifice tube by the main water jet. Droplets can later collide along the main jet or be sucked up towards the capillary (the upper small orifice hole where the jet is created) causing local disturbances and loss of the hydraulic flip condition which is crucial for the coherence of the jet. This random process effectively explains the instabilities which can usually be noticed by a naked-eye observation during the water jet formation and later on; the study of this phenomenon can lead to new instruments for an improved design of water jet cutting head components on the way to high precision applications. The results of simulations are validated by means of a high-speed camera.