**Hydraulic Diameter**

1.
General formula

2. Formula for rectangular duct

3. Formula for annular flow

What is the hydraulic diamter ? The hydraulic diameter, defined as
the ratio in between 4 times the flow section over the wetted
perimeter, allows, for non circular flows, to define an equivalent
diameter which allows to perform fluid dynamic calculation.

**The hydraulic diameter is especially useful when having to
calculate a Reynolds
number in order to determine the flow regimes (laminar,
intermediary, turbulent).**

## 1. Calculation of hydraulic diameter

The hydraulic diameter is used for non-circular flow and is calculated
thanks to the following general formula : D

_{H}=4*(flow
section)/(wetted perimeter)

D_{H}=Hydraulic Diameter

## 2. Hydraulic diameter for rectangular duct

The Hydraulic Diameter in
rectangular pipe is calculated thanks to the following formula : D_{H}=(2.e.l)/(l+e)

e=Height of the flow section (m)

l=Width of the flow section (m)

## 3. Hydraulic diameter for annular flow

The Hydraulic Diameter in annular
flow is calculated thanks to the following equation : D_{H}=2.R.(1-K)

R=Outside diameter of the tube (m)

K=R_{inside}/R

R_{inside}=Inside diameter of the tube (m)

**Source**
Mecanique et Rheologie des fluides en genie chimique, Noel Midoux,
Lavoisier Tec et Doc, page 197