Liquid leaving a nozzle first remains a sheet or a jet, then becomes unstable and breaks up (primary atomisation); the coarse droplets formed are broken down further by aerodynamic forces (secondary atomisation).
The result is not a single diameter but a distribution — and the success of the application depends on the shape of that distribution.
Why the distribution matters
- Evaporation scales with surface area; small droplets evaporate quickly, coarse ones reach the target still wet [1].
- Penetration scales with mass; very small droplets are carried away before reaching the target.
- Drift is a problem with small droplets: it creates environmental constraints in agricultural spraying and in cooling towers.
These three requirements conflict; the design seeks a narrow optimum. An analysis run with a single mean diameter cannot see that conflict at all.
Nozzle types
Pressure swirl (simplex). The liquid is given swirl to form a fine cone. Simple and common; but atomisation quality degrades quickly as the flow falls — the turndown range is narrow.
Air-assisted / twin-fluid. High-velocity air breaks up the liquid. Gives good atomisation over a wide load range; the price is compressed air consumption.
Flat jet and fan. For coating applications, where pattern quality is critical.
Modelling
Resolving the primary atomisation directly is expensive and unnecessary for most industrial problems. The practical route is to define a measured size distribution at the nozzle exit and track the droplets as a discrete phase. See the multiphase note.
Points to watch:
- The drag coefficient must be computed as a function of Reynolds number [2].
- Droplet-wall interaction must be defined: stick, splash, form a film. The default "sticks" assumption misleads in coating and cooling problems.
- An evaporating droplet is both a mass and a latent heat source; both change the gas phase.
References
- Ranz, W. E. & Marshall, W. R. (1952). Evaporation from drops. Chemical Engineering Progress, 48, 141–146 (Part I) and 173–180 (Part II).
- Morsi, S. A. & Alexander, A. J. (1972). An investigation of particle trajectories in two-phase flow systems. Journal of Fluid Mechanics, 55(2), 193–208. DOI: 10.1017/S0022112072001806