# Graetz Number

Written by Jerry Ratzlaff on . Posted in Dimensionless Numbers

Graetz number, abbreviated as Ga, a dimensionless number, that characterizes laminar flow with heat transfer in a conduit.  This number is used to determine the thermal development on the entrance to ducts.  A Graetz number of 1000 and below is usually where a flow is considered thermally developed.

## Graetz number formula

 $$\large{ Gz = \frac {D_h}{l} \; Re \;Pr }$$

### Where:

$$\large{ Gz }$$ = Graetz number

$$\large{ D_h }$$ = hydraulic diameter

$$\large{ l }$$ = length

$$\large{ Re }$$ = Reynolds number

$$\large{ Pr }$$ = Prandtl number