Annular hydraulic diameter

Double value of the gap between circular concentric pipes:

(1)	$\begin{array}{l}\displaystyle d_h = d_{out} - d _{in}\end{array}$

where

$\begin{array}{l}d_{out}\end{array}$	inner diameter of outer pipe
$\begin{array}{l}d_{in}\end{array}$	outer diameter of inner pipe

It equals to Hydraulic diameter of the annular area.

Derivation

(2)	$\begin{array}{l}\displaystyle d_h = \frac{4 \, A}{P} = \frac{\pi \, d_{out}^2 - \pi \, d_{in}^2}{\pi \, d_{out} + \pi \, d_{in}} = d_{out} - d_{in}\end{array}$

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