Physics:Graetz number
From HandWiki
In fluid dynamics, the Graetz number (Gz) is a dimensionless number that characterizes laminar flow in a conduit. The number is defined as:[1]
- [math]\displaystyle{ \mathrm{Gz} = {D_H \over L} \mathrm{Re}\, \mathrm{Pr} }[/math]
where
- DH is the diameter in round tubes or hydraulic diameter in arbitrary cross-section ducts
- L is the length
- Re is the Reynolds number and
- Pr is the Prandtl number.
This number is useful in determining the thermally developing flow entrance length in ducts. A Graetz number of approximately 1000 or less is the point at which flow would be considered thermally fully developed.[2]
When used in connection with mass transfer the Prandtl number is replaced by the Schmidt number, Sc, which expresses the ratio of the momentum diffusivity to the mass diffusivity.
- [math]\displaystyle{ \mathrm{Gz} = {D_H \over L} \mathrm{Re}\, \mathrm{Sc} }[/math]
The quantity is named after the physicist Leo Graetz.
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