Reynolds number

Reynolds number:

Reynolds number is a dimensionless number, from that one can get a good idea of how turbulent a flow is. To be more precise, it is the ratio of inertial forces 

(resistant to change or motion)

 to viscous forces 

(heavy and gluey).

The Reynolds number, in a sense, represents a ratio of pressure to shear forces:

ρ V l / μ = ρ V² / μ (V/l) 
ρ V² is related to the pressure while μ V/l is related to μ dU/dy, the shear stress

The range of Reynolds number -- from McMasters.

The formula for Reynolds number (N_Re) can be written as follows,

N_Re=Diameter of the geometry * velocity of the fluid * Density of the fluid/Viscosity of the fluid

If the geometry is non circular, then one should use hydraulic (equivalent) diameter in place of normal diameter.

Hydraulic diameter= 4 * Area/Wetted perimeter

Fluid means, it can be either liquid or gas.

Physical interpretation of Reynolds number:

If Reynolds numbers of a flow is less than 2300, that flow is considered laminar flow. If Reynolds number is more than 2300 and less than 4000, then that flow is considered transition flow. If Reynolds number is more than 4000, then the flow is considered turbulent flow.

Reynolds number needs to be calculated to find heat and mass transfer coefficients through empirical formulas.

Also to calculate pressure drop in pipes, Reynolds number needs to be calculated to find the friction factors of the pipes.

Reynolds number is the most important dimensionless number used in chemical engineering.