How to design spray nozzle (like in pulverizer) for 3d print?

Question

How do spray nozzles work? How do they convert liquid to aerosol? Is it possible to print tiny spray nozzles from metal powder?

Answer 1

Nozzle design can be fairly complex, but basically the nozzle speeds up the velocity of the fluid to the point where inertial forces exceed surface tension forces, and the fluid breaks up into drops. The Weber number, $We = \frac{\rho u^2 l}{\sigma}$ is a convenient parameter to do a basic analysis on a nozzle flow. In this equation, $\rho$ is the fluid density, $u$ is its velocity, $l$ is a characteristic length (usually droplet diameter), and $\sigma$ is the fluid surface tension. When $We > 1$, the fluid is likely to break up into smaller droplets.

You can use the Weber number equation to calculate how fast your fluid must be traveling to break up into the target droplet diameter. This velocity, combined with the flow rate of the fluid through the nozzle, can be used to estimate the orifice area: $A = \frac{\dot V}{u}$ (area is volumetric flow rate divided by velocity). Compare the orifice diameter to the resolution of your printer; if the diameter is too small, then you might have to drill the orifice separately, after the nozzle is printed.