Water jet

[geoffthehammer]

If water is flowing through an orifice under pressure and forms a jet. Is there a way of predicting at what distance from the orifice the jet will cease to be a coherent stream and break up into small droplets. In my particular case the water is pressurised at 6000psi, the orifice is 0.003" diameter and the jet is vertically downwards

 

[btrueblood]

It depends on the length of the orifice, edge conditions, and upstream pressure fluctuations; also the vapor pressure and temperature of the fluid play a role. And a lot of other stuff too, but those are the big drivers. There is some data in NASA/NACA tech reports, and a few other sources. The problem is intrinsic to the design of rocket engine fuel injection, among others.

Are you trying to maintain coherence for as long as possible, or cause more rapid stream breakup? To get best coherence, use a long piece of capillary tubing and upstream flow straighteners, and have the tip of the tube ground to a taper, with a nice, clean, square edge at the exit. To get more rapid breakup, use non-circular orifices, or orifices with an overhanging "lip" on one side, and pre-swirl the flow (although there are regimes of flow where very small amounts of preswirl can help maintain coherence; most of us males have a daily proof of this phenomenon . To really get rapid breakup, impinge two streams onto each other.

 

[radaes]

I've seen some nozzle stream descriptions in fluid-dyn texts, especially when they contain sections on atomizers. "The Mechanical Engineer's Handbook" has a section by Rolf Reitz in the Fluids chapter that goes into exhaustive detail.
Looking for desciptions of Rayleigh breakup vs 1st & 2nd wind-induced & atomization regimes might help you along.
("Rayleigh breakup: Drop diameters are larger than the jet diameter. Breakup occurs many nozzle diameters downstream of nozzle.")

Hope this helps,

radAES