Power Number

Written by Jerry Ratzlaff on . Posted in Dimensionless Numbers

Power number, abbreviated as $$N_p$$, is a dimensionless number that represents the proportional  drag force and inertial force. It is used in momentum transfer and power consumption by fans, mixers and pump impellers at a specified rotational speed. The calculation and formula for the Power Number are shown below.  Note: it may be referenced as the Newton Number.

formulas that use Power number

 $$\large{ N_p = \frac {P}{\rho \; n^3 \; d^5} }$$

Where:

$$\large{ N_p }$$ = Power number

$$\large{ \rho }$$  (Greek symbol rho) = density of fluid

$$\large{ d }$$ =  diameter of stirrer

$$\large{ P }$$ = power

$$\large{ n }$$ = rotational speed