Weber Number

Effect of Weber Number


The Weber number (We) is the most important variable in controlling the drop size of the dispersed phase. The Weber number can be expressed in many ways depending on available data and units. For our purposes, the following equation applies:


Weber Number Equation

Where Q = Flow rate (m3/h)ρ = Density (kg/m3) σ = Interfacial tension (dynes/cm)

D = Pipe Inside Diameter (mm)

Weber Number Equation

Where Q = Flow rate (US gpm)sg = Specific Gravityσ = Interfacial tension (dynes/cm)

D = Pipe Inside Diameter (inch)

In general, the Weber number expresses the ratio between inertial forces trying to disperse the phases and the interfacial forces trying to keep them distinct. At a given viscosity ratio, the drop size therefore decreases as the Weber number increases.

Interfacial Tension


Interfacial tensions usually range from 20 to 70 dynes/cm.


Increasing values of interfacial tension decrease the Weber number and therefore increase the drop size. Fluids with lower interfacial tensions will therefore more readily produce smaller drop sizes.