# Bejan number

There are two different Bejan numbers (Be) used in the scientific domains of thermodynamics and fluid mechanics. Bejan numbers are named after Duke University professor Adrian Bejan.

## Thermodynamics

In the field of thermodynamics the Bejan number is the ratio of heat transfer irreversibility to total irreversibility due to heat transfer and fluid friction:

$\mathrm {Be} ={\frac {{\dot {S}}'_{\mathrm {gen} ,\,\Delta T}}{{\dot {S}}'_{\mathrm {gen} ,\,\Delta T}+{\dot {S}}'_{\mathrm {gen} ,\,\Delta p}}}$ where

${\dot {S}}'_{\mathrm {gen} ,\,\Delta T}$ is the entropy generation contributed by heat transfer
${\dot {S}}'_{\mathrm {gen} ,\,\Delta p}$ is the entropy generation contributed by fluid friction.

## Fluid mechanics, heat transfer and mass transfer

In the field of fluid mechanics the Bejan number is the dimensionless pressure drop along a channel of length $L$ :

$\mathrm {Be} ={\frac {\Delta PL^{2}}{\mu \nu }}$ where

$\mu$ is the dynamic viscosity
$\nu$ is the momentum diffusivity

The above formulation of Bejan number has a large importance on fluid dynamics, because it is directly related to fluid dynamic drag D by the following expression of drag force

$D=\Delta _{p}A_{w}={\frac {1}{2}}C_{D}A_{f}{\frac {\nu \mu }{l^{2}}}Re^{2}$ which allows expressing the drag coefficient $C_{D}$ as a function of Bejan number and the ratio between wet area $A_{w}$ and front area $A_{f}$ :

$C_{D}={\frac {A_{w}}{A_{f}}}{\frac {Be}{Re_{L}^{2}}}$ where $Re_{L}$ is the Reynold Number related to fluid path length L.

In the context of heat transfer. the Bejan number is the dimensionless pressure drop along a channel of length $L$ :

$\mathrm {Be} ={\frac {\Delta PL^{2}}{\mu \alpha }}$ 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.

In the context of mass transfer. the Bejan number is the dimensionless pressure drop along a channel of length $L$ :

$\mathrm {Be} ={\frac {\Delta PL^{2}}{\mu D}}$ where

$\mu$ is the dynamic viscosity
$D$ is the mass diffusivity

For the case of Reynolds analogy (Le = Pr = Sc = 1), it is clear that all three definitions of Bejan number are the same.

Also, Awad and Lage: obtained a modified form of the Bejan number, originally proposed by Bhattacharjee and Grosshandler for momentum processes, by replacing the dynamic viscosity appearing in the original proposition with the equivalent product of the fluid density and the momentum diffusivity of the fluid. This modified form is not only more akin to the physics it represents but it also has the advantage of being dependent on only one viscosity coefficient. Moreover, this simple modification allows for a much simpler extension of Bejan number to other diffusion processes, such as a heat or a species transfer process, by simply replacing the diffusivity coefficient. Consequently, a general Bejan number representation for any process involving pressure-drop and diffusion becomes possible. It is shown that this general representation yields analogous results for any process satisfying the Reynolds analogy (i.e., when Pr = Sc = 1), in which case the momentum, energy, and species concentration representations of Bejan number turn out to be the same.

Therefore, it would be more natural and broad to define Be in general, simply as:

$\mathrm {Be} ={\frac {\Delta PL^{2}}{\rho \delta ^{2}}}$ where

$\rho$ is the fluid density
$\delta$ is the corresponding diffusivity of the process in consideration

In addition, Awad: presented Hagen number vs. Bejan number. Although their physical meaning is not the same because the former represents the dimensionless pressure gradient while the latter represents the dimensionless pressure drop, it will be shown that Hagen number coincides with Bejan number in cases where the characteristic length (l) is equal to the flow length (L). Also, a new expression of Bejan number in the Hagen–Poiseuille flow will be introduced. This expression is

$\mathrm {Be} ={{32\mathrm {Re} L^{3}} \over {d^{3}}}$ where

$\mathrm {Re}$ is the Reynolds number
$L$ is the flow length
$d$ is the pipe diameter

The above expression shows that the Bejan number in the Hagen–Poiseuille flow is indeed a dimensionless group, not recognized previously.