Reynolds number

The Reynolds number $\mathrm{R}e$ is a dimensionless quantity that is used to help predict similar flow patterns in different fluid flow situations. The concept was introduced by George Gabriel Stokes in 1851, but the Reynolds number is named after Osborne Reynolds (1842–1912), who popularized its use in the 1880s.

The Reynolds number is defined as the ratio of inertial forces to viscous forces and consequently quantifies the relative importance of these two types of forces for given flow conditions.

$Re\mathrm{=}\frac{{\rho }vD}{{\mu }}$

(1)

Where:

• $D$ is the internal diameter of the pipe
• $v$ is the average velocity of the fluid
• $\rho$ is the density of the fluid
• $\mu$ is the viscosity of the fluid

The Reynolds number is used to characterize different flow regimes within a similar fluid, such as laminar or turbulent flow. Laminar flow occurs at low Reynolds numbers, where viscous forces are dominant. Laminar flow is characterized by smooth, constant fluid motion. Turbulent flow occurs at high Reynolds numbers and is dominated by inertial forces, which tend to produce chaotic eddies, vortices and other flow instabilities.

By defining the following non-dimensional variables

$\begin{array}{rl}{\mathbf{v}}^{\prime }& =\frac{\mathbf{v}}{v}\\ p^{\prime }& =\frac{p}{\rho v^2}\\ {\mathbf{g}}^{\prime }& =g\frac{D}{v^2}\\ {\mathrm{\nabla }}^{\prime }& =D\mathrm{\nabla }\\ \frac{\mathbf{D}}{\mathbf{D}{t}^{\prime }}& =\frac{D}{v}\left(\frac{\mathbf{D}}{\mathbf{D}t}\right)\end{array}$

(2)

and substituting into the Navier-stokes equation, we get

$\frac{\mathbf{D}{\mathbf{v}}^{\prime }}{\mathbf{D}{t}^{\prime }}=-{\mathrm{\nabla }}^{\prime }{p}^{\prime }+\frac{\mu }{\rho Dv}\mathrm{\nabla }{\text{'}}^2{\mathbf{v}}^{\prime }+{\mathbf{g}}^{\prime }$

(3)

or

$\frac{\mathbf{D}{\mathbf{v}}^{\prime }}{\mathbf{D}{t}^{\prime }}-\frac{1}{Re}{{\mathrm{\nabla }}^{\prime }}^2{\mathbf{v}}^{\prime }=-{\mathrm{\nabla }}^{\prime }{p}^{\prime }+{\mathbf{g}}^{\prime }$

(4)

This limit permits solutions in which backward flow is possible, which gives rise to the instabilities that lead to turbulent flow.

The Reynolds number is commonly used for fluid flow in a pipe. This can be represented by:

$Re\mathrm{=}\frac{{\rho }vD}{{\mu }}=\frac{uD}{{\nu }}=\frac{QD}{{\nu }A}$

(5)

Where

• $D$ is the internal diameter of the pipe
• $v$ is the average velocity of the fluid
• $\rho$ is the density of the fluid
• $\mu$ is the viscosity of the fluid
• $\nu$ is the kinematic viscosity of the fluid
• Q is the volumetric flow rate of the fluid
• A is the the cross-sectional area of the fluid

This could be used typically for a round pipe but also could be adjusted for different types of pipes by adjusting the diameter or $D$ This could be used for AC units in which you could adjust the $D$ to be defined as $D=\frac{4A}{P}$ Where

• P is the wetted parameter