# Graetz Number

Written by Jerry Ratzlaff on . Posted in Dimensionless Numbers

Graetz number, abbreviated Ga, is 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.  The calculation and formula for the Graetz number is below.

 $$\large{ Gz = \frac {D_h}{l} Re Pr }$$          Where: $$\large{ Gz }$$ = Graetz number $$\large{ D_h }$$ = hydraulic diameter $$\large{ l }$$ = length $$\large{ Re }$$ = Reynolds number $$\large{ Pr }$$ = Prandtl number