Bejan number
There are two Bejan numbers (Be) in use, named after Duke University professor Adrian Bejan in two scientific domains: thermodynamics and fluid mechanics.
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Thermodynamics
In the context of thermodynamics, the Bejan number is the ratio of heat transfer irreversibility to total irreversibility due to heat transfer and fluid friction:[1]
\[Be=\frac{\dot S'_{gen, \Delta T}}{\dot S'_{gen, \Delta T}+ \dot S'_{gen, \Delta p}}\]
where
\[\dot S'_{gen, \Delta T}\] is the entropy generation contributed by heat transfer \[\dot S'_{gen, \Delta p}\] is the entropy generation contributed by fluid friction.
Fluid mechanics and heat transfer
In the context of fluid mechanics and heat transfer. the Bejan number is the dimensionless pressure drop along a channel of length \(L\):[2]
\[Be=\frac{\Delta P . L^2} {\mu \alpha}\]
where
\[\mu\] is the dynamic viscosity \[\alpha\] is the thermal diffusivity
The Be number plays in forced convection the same role that the Rayleigh number plays in natural convection.
See also
References
Further reading
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